Rhenium and ruthenium catalyzed processes involving organosilanes 2

CHAPTER 1 Introduction Transition metal complexes are one of the most important compounds in synthetic chemistry due to their ability to catalyse many reactions. Of these many reactions, organic transformations have been the focus of many research groups in the past decades. Transition metals have incomplete d-shells which are easily accessed energetically, which allow them to exhibit a variety of oxidation states and coordination numbers. This property makes these metal complexes ideal candidates for catalysts. A catalyst provides an alternative pathway with lower activation energy to accelerate a reaction. In theory, it should also remain chemically unchanged at the end of the reaction. Catalysis allows reactions to proceed efficiently and usually reduces the wastage of chemicals, thus providing atom economy in most cases. Catalysis can be classified into two main groups which are homogeneous and heterogeneous catalysis. In homogeneous catalysis, all substrates, including the catalyst, are in the same phase. However, in heterogeneous catalysis, at least one of the substrates is not in the same phase. One of the most common classes of catalysts is organometallic compounds. Organometallic compounds are defined as complexes containing metal-carbon bonds, of which a group of such compounds are the metal-carbonyl complexes. Metal1 carbonyl complexes are useful catalysts as the carbonyl ligand can be easily characterized using spectroscopic techniques such as Infrared and 13 C-NMR. Both techniques allow the identification of the symmetry, arrangement of ligands, as well as the functional groups present in the complex. Metal-carbonyl complexes have distinct carbonyl stretching frequencies within the range of 1700 – 2100 cm-1, which is a region usually free of most other molecular vibrations. By monitoring the shifting of the carbonyl stretching frequencies, one is able to deduce if processes such as ligand association and dissociation have taken place, or if the symmetry of the complex has been changed. IR spectroscopy is hence a very useful tool in the investigation of reaction intermediates, and eventually the mechanism, of a metal-carbonyl-catalyzed reaction. 1.1. Organometallic Compounds in Catalysis 1.1.1. Organometallics An organometallic compound is usually defined as a substance that contains metalcarbon bonds. Nowadays, this definition is often relaxed to include other non-carbon ligands such as nitrosyls, cyanates, hydrides and metals with ligands of groups 15 and 16.1-3 Besides that, main group metals such as silicon, boron, germanium, and tellurium have also been included into the vast family of organometallics. Organometallics can be sub-divided into various categories such as catalysis, synthetic organometallic chemistry and bioorganometallic chemistry, with many 2 established works done in these fields. Industrial processes such as the Suzuki Reaction,4 Reppe Syntheses,5 Fischer-Tropsch Reactions.6,7 the Sonogashira Reaction,8 and olefin metathesis9 are but a few examples of successful application using organometallic compounds as catalysis. It is thus the focus of this work to concentrate on silane transformations catalyzed by rhenium and ruthenium organometallic complexes. 1.1.2. Organo-Rhenium Complexes In the periodic table, rhenium is the last natural occurring stable element to be found in 1925 by Noddack, Tacke and Berg.10 Of the variety of rhenium complexes that were synthesized, rhenium carbonyls make up a significant part of these known complexes. Re(CO)5Cl, Re(CO)5Br, Re2(CO)10 and CpRe(CO)3 are but a few examples of commercially available rhenium carbonyl complexes. Due to their cost, many rhenium complexes have not been as well studied in terms of their catalytic abilities.11 However, catalytic reactions involving oxo-rhenium complexes (MeReO3)12,13 have been reported by a few research groups (Scheme 1). R" R R'" MeReO3 R' Oxidizing agent R" O R R'" R' Scheme 1. Olefin Epoxidation using MeReO3 as catalyst Recent work done by various groups explored the catalytic capabilities of rhenium carbonyl complexes towards organic reactions such as C-C bond cleavage and 3 formation reactions. Kusama et. al.14 reported the use of Re(CO)5Br as a catalyst towards Friedel-Crafts acylation (Scheme 2). In normal Friedel-Crafts reactions, the Lewis Acid is usually added in stoichiometric amounts with respect to the substrates. However, with Re(CO)5Br as a catalyst, the reaction proceeded smoothly albeit with a mixture of regio-isomers. It is only until recently that Kuninobu et. al.15 reported the use of Re2(CO)10 as catalyst for regioselective alkylation of phenols. CH3 O O + Ph CH3 Re(CO)5Br (10 mol %) Cl reflux, 2hr Ph 91% Solvent Scheme 2. Friedel-Crafts acylation catalyzed by Re(CO)5Br Besides Friedel-Crafts reactions, reactions involving nucleophilic addition to unsaturated bonds were also explored. Re(CO)5Br was reported16 to be able to catalyze the condensation of aldehydes and activated methylene complexes with high stereoselectivity (Scheme 3). The stereoselectivity of the reaction decreases as unsymmetrical ketones are introduced as the reactant. Besides addition onto carbonyl compounds, the nucleophilic addition onto C-C double and triple bonds were also reported.17 NC O Ph R 1 + NC R 2 R 2 Re(CO)5Br (0.5mol %) neat Ph R 1 Scheme 3. Nucleophilic addition of activated nitrile onto a carbonyl substrate substrate Of more interest are the reported work of Chen et. al.18 and Kuninobu et. al.19 on CH bond activation catalyzed by rhenium (I) carbonyl complexes. The work by Chen et. 4 al. involving the coupling of an alkene with borane complexes (Scheme 4) has since then been modified by various groups using other transition metal catalysts; whereas the work by Kuninobu et. al. provided different insights to the reaction pathways and mechanistic studies of the insertion of unsaturated molecules into a C-H bond (Scheme 5). Other interesting reactions involving rhenium (I) catalysts like C-Si bond formation,20 C-N bond formation21 and C-O bond formation22 were also reported. All the reactions stated above show that rhenium (I) carbonyl complexes have huge potential in catalyzing a variety of interesting reactions. t t Bu Bu NH N H + [Re(CO)3Br(thf)]2 (3mol%) Ph Ph toluene, reflux, 24hr Scheme 5. Insertion of unsaturated molecules into a C-H bond In this project, the following commercially-available, air-stable rhenium (I) compounds, Re(CO)5Br and Re2(CO)10 were chosen to catalyze the hydrosilylation of a wide range of carbonyls including aldehydes, ketones and esters. The products formed from these reactions have wide applications in industrial processes like the production of polymeric materials.23 Rhenium (I) complexes in the form Re(CO)5X (X = Br, Re(CO)5) were hence chosen because it was reported to be capable of activating Si-H bonds. However, their activity towards carbonyl hydrosilylation has not been fully explored.24 5 1.1.3. Organo-Ruthenium Complexes Over the past two decades, organo-ruthenium catalysts have provided vast contributions to developing new efficient organic syntheses such as olefin metathesis and carbon-carbon bond formation.25 Organo-ruthenium complexes such as [Ru(3allyl)Br(CO)3] 26 are usually used as catalysts for organic transformation reactions as the ruthenium metal centre is able to adopt a wide range of oxidation states.27 As such, they have a very good ability to promote catalytic cycles (example of silane hydrolysis28 depicted in Scheme 6). CO Ph3P Br Br Br Ph3P Ru Ru OC CO CO CO Br Br Br Ru Ru OC PPh3 Br CO CO Br PPh3 THF CO CO Br Ph3P Et3SiH OC THF Br CO Br Br Ph3P Ru Ru OC -Et3SiBr Br Ph3P Ru H Si Et3 OC H H2O THF -Et3SiOH CO CO Br Ph3P Ru OC HO Br Et3SiBr Si Et3 -H2 Br Ph3P Ru OC H HO H Scheme 6. Silane hydrolysis catalyzed by a ruthenium dimer Due to their ability to accommodate different oxidation states and hence, different coordination number, organo-ruthenium complexes containing many different ligands such as phosphines, cyclopentadienyl, arenes and dienes have been synthesized and their applications towards catalytic transformations investigated.29 Among these transformations include bond activation and formation involving C-C,30 C-H31 and Cheteroatom32 (Scheme 7), oxidation reactions33 and hydrogenation.34 6 [Cp*RuCl(Y Me)]2 Ph NH4BF4 + O OH Ph CH3 reflux, 3hr + H2O O Si(OEt) 3 O O RuH2(CO)(PPh3)3 + Si(OEt)3 toluene, 125°C NMe 2 NMe 2 R'SeSeR' + RuCl3 2 R" X Zn, DMF, 60-100 °C 2 R'Se R" Scheme 7. Examples of C-C, C-H, C-heteroatom bond activation and formation catalyzed by organoruthenium complexes Recent interest have been shown in synthesizing ruthenium complexes via oxidative addition of the allyl halide onto Ru3(CO)1235 to obtain products of the form [Ru(3allyl)(CO)3X] (X = halide). Such complexes were reported to be useful catalysts for regioselective and steoreospecific allylation reactions under mild conditions.26 Modification of the ligands around the catalyst have also been done to better understand the mechanism of such allylation reactions towards nucleophilic attack.36 Besides allylruthenium complexes, polynuclear-ruthenium complexes have also been widely synthesized and their catalytic capabilities towards many organic reactions were extensively explored. One of the most classic examples of a polynuclear-ruthenium catalysed reaction would be the Water Gas Shift Reaction, in which Ru3(CO)12 was utilised as the catalyst.37 Other interesting reactions include the use of a ruthenium-phosphine dimer as catalyst for the hydrogenation of nitriles to the 7 corresponding amines38 and also oxidation of alkanes to alcohols and ketones.39 Of more recent interest would be the use of ruthenium dimers containing bridging halides to catalyze the Kharasch reaction with high efficiency.40 As ruthenium metal has the ability to adopt a variety of oxidation states, many modifications have been made to organo-ruthenium catalysts to further fine tune their electronic and steric properties. Ruthenium-NHCs41 have recently been synthesized and they were shown to be able to catalyze reactions such as Ring-Opening Metathesis Polymerization,42 and cyclopropanation reaction41 (Scheme 8). [RuCl2(p-cymene)]2 imidazol(in)ium salt KO-t-Bu or NaH n PhCl, 2hr, 60 °C Visible light n EtO2C n EtO2C [RuCl2(p-cymene)]2 imidazol(in)ium salt KO-t-Bu PhCl, 24hr, 60 °C + + CO 2Et Scheme 8. Examples of ROMP and Cyclopropanation reaction catalyzed by Ru-NHCs All the above reported organo-ruthenium catalyzed reactions showed the versatility of ruthenium chemistry in the progression of various fields of research. In this project, the commercially available and air-stable ruthenium carbonyl, Ru3(CO)12 has been utilised as a catalyst in the investigation of its efficiency towards catalytic cross coupling between silanes and other heteroatoms. It was previously reported43 that ruthenium was able to catalyse the cross coupling between C-S, C-Se and C-Te. 8 + N2 However, little has been done in this area to further understand the mechanism and applications of using ruthenium for cross coupling reactions. 1.2. Organosilane Chemistry Organosilane compounds contain mainly a carbon-silicon bond and its chemistry is similar to that of normal organic compounds containing carbon-carbon bonds. But as the silicon atom being more electropositive, the carbon-silicon bond is actually polar. This leads to certain distinctions from normal organic compounds, ie, nucleophilic attack usually occurs at the silicon atom, silicon-oxygen, silicon-nitrogen and siliconhalide bonds are stronger than their carbon-heteroatom counterpart, and addition to olefins occur in an anti-Markovnikov manner, with the ability to hydride transfer.44 Organosilanes are formed initially by reacting elemental silicon with chlorine gas or hydrogen chloride. Subsequently, reaction with alkali metal alkyl compounds (alkyl lithium, alkyl sodium etc), with Grignard reagents45 or hydrosilylation of an olefin under catalytic conditions would yield the desired organosilane (Scheme 9). Si 2Cl2  Si 3HCl,  RLi SiCl4 HSiCl3 -LiCl +RMgCl RHSiCl2 -MgCl2 -SiCl4, -H2 Si RSiCl3 RHC=CH2 RCH 2CH 2SiCl3 catalyst Scheme 9. Formation of organosilanes using different methods 9 Of these methods, the more popular method to derive the required organosilane is to perform a Grignard reaction with a suitable Grignard reagent while hydrosilylation of olefins under catalytic conditions would afford very efficient yields. Chloro derivatives of organosilanes are perhaps one of the most useful classes of organosilanes as they are frequently utilized as precursors for silicon-heteroatom compounds such as Si-N and Si-OH. These silicon-heteroatom compounds have great application in industrial processes and one of the most important processes involves silicone manufacturing, which requires the reaction of water with chlorosilanes to form a silanol precursor.44 Of the many reactions involving organosilane compounds, the hydrosilylation reaction is the only reaction that provides the most application in inorganic, organometallic, organic, bioorganic, materials and polymer chemistry.46 Especially in organic chemistry, hydrosilylation provides the most efficient and direct route of introducing silyl functional groups into unsaturated substrates. The resultant products are useful intermediates that can participate in a variety of synthetically valuable organic reactions, with some of them capable of participation in one-pot processes.47 Some of these organic reactions include the Tamao-Fleming protocol,48 which involves the steoreospecific oxidation of an organosilane into an alcohol (Scheme 10), and the Hiyama coupling,49 which involves the cross-coupling of organosilanes with organo halides to form carbon-carbon bonds (Scheme 11). H R 1 SiR'2X R 2 H2O2, KHF2 DMF, r.t. H OH 1 R R 2 H3C HBF4.Et2O m-CPBA, Et3N, Et2O H Si CH3 R 1 R 2 X=F,Cl,OR,H,NR2 Scheme 10. Reaction scheme depicting the Tamao oxidation (left to right) and the Fleming Oxidation (right to left). 10 FR SiR'3 + R" X R R" Pd cat. R = Aryl, Alkenyl, Alkynyl R' = Cl, F, Alkyl R" = Aryl, Alkenyl, Alkynyl, Alkyl X = Cl, Br, I, OTf Scheme 11. The Hiyama Cross-Coupling reaction. Besides hydrosilylation reactions, cross-coupling reactions involving organosilanes also provide many useful applications in organic and bioorganic chemistry. Transition metal-catalyzed silane hydrolysis and alcoholysis involving the reaction between organosilanes and water/alcohol produces silanols, silyl ethers and siloxanes,50 which provide useful starting materials for the manufacture of pharmaceuticals51 and also as excellent research examples for bioactivity.52 In addition, such reactions also provide a good source of hydrogen (Scheme 12),50 which can be utilized in a variety of reactions, with the most classic example being the hydrogenation of unsaturated compounds catalyzed by palladium (II)53 or iridium (I)54 complexes. R4-xSi Hx + xH 2O oxorhenium catalyst xH 2 + R4-xSi(OH)x R = alkyl or aryl x = 1-3 Solv = CH3CN or H2O Scheme 12. Silane hydrolysis producing hydrogen as the by-product. Besides silicon-oxygen coupling, silicon coupling with other heteroatoms such as sulfur and nitrogen also provide many uses. Although organosilanes are useful 11 protecting groups for carbonyl compounds, organosulfur derivatives of these silanes provide more selectivity in protecting certain functional groups like aldehydes and ketones (Scheme 13).55 However, the methods of synthesizing these compounds require harsh conditions or air-sensitive catalysts such as B(C6H5)3 (Scheme 14).56 Also, there are cases whereby sulfur poisoning of the catalyst is a problem as it caused the drop in efficiency in the system, and eventually the catalytic yield of these organosulfur derivatives of silanes.57 R O R' + R" 3Si R OSiR"3 R' SR"' R"'S Scheme 13. Protecting groups containing a Silicon-Sulfur bond used for protecting aldehydes and ketones. Other than silicon-sulfur compounds, silicon-nitrogen compounds also provide many useful applications in organic chemistry. The formation of iminosilanes58a, silazanes58b and diaminosilanes58c has been reported recently by coupling silanes such as trichlorosilane and trimethylsilane with various amino substrates like aminofluorosilanes. Besides employing expensive or dificult to synthesize substrates, harsh reducing agents like butyl lithium were required. However, these reactions were significant as they provide much insight into the role of amine ligands in processes such as alkene polymerisation59 and alkene metathesis.60 1.3. Main Objectives 1.3.1. Catalytic hydrosilylation of carbonyls via Re(CO)5Cl photolysis It was reported that the following commercially-available rhenium (I) complexes, Re(CO)5X (X = Br, Cl, Re(CO)5) are capable of activating Si-H bonds. However, their 12 catalytic capabilities towards organosilane-related reactions such as hydrosilylation has not been explored.18 As it was also reported19 that the air and moisture-stable Re(CO)5Cl dissociates a CO ligand readily upon photolysis (quantum yield of 0.06 to 0.44 from 366nm to 313nm), it shows that Re(CO)5X compounds have the capability to generate a vacant site for ligand/substrate association. As it is desirable for any catalyst to be able to perform its catalytic task in the presence of air and moisture, it is the aim of this part of the project to photoactivate Re(CO)5Cl and other rhenium carbonyl derivatives in order to carry out hydrosilylation reactions on a variety of carbonyl substrates at room temperature. The relative efficiency of the catalysts towards hydrosilylation of different carbonyls will also be investigated and identification of possible key catalytic intermediates will be done. Eventually, the mechanism of the hydrosilylation reaction will also be elucidated using a combination of IR and NMR spectroscopy as well as computational studies. 1.3.2. Ruthenium carbonyl-catalysed Si-X coupling (X = S, O, N) In this part of the project, the following air-stable, commercially-available ruthenium carbonyl compound, Ru3(CO)12, will be used as a catalyst for the coupling reaction between silanes and various substrates such as thiols, amines and alcohols. It was previously reported that Ru3(CO)12 can be used to activate Si-H bonds31 and in the process form dimeric ruthenium silyl species of the form (R3Si)Ru-Ru(SiR3). The silanes used in this case range from chlorosilanes to alkylsilanes. However, their catalytic capabilities were not explored. Ru3(CO)12 was also known to be able to 13 activate benzylic C-H bonds in the presence of silanes to afford benzylsilanes,32 while silylation of silanols with vinylsilanes catalyzed by Ru3(CO)12 to afford alkenes and siloxanes was also reported. 33 It is hence the aim of this part of the project to utilize Ru3(CO)12 as a coupling catalyst for Si-heteroatom (S, N and O) coupling. The scope of the catalysis as well as the kinetics and detection of reactive intermediates will be investigated using FTIR, NMR and ESI. Computational chemistry using the Gaussian suite of programs will also be employed for further elucidation of the reaction mechanism. 14 1.4 References [1] Elschenbroich, C. Organometallics, 3rd ed.; Wiley-VCH, Weinheim, 2005 [2] Crabtree, R. H. The Organometallic Chemistry of the Transition Metals, 4th ed.; Wiley-WCH, Weinheim, 2005 [3] Hunt, L. B. Platinum Metals Rev. 1984, 28, 76 [4] Chemler, S. R.; Trauner, D.; Danishefsky, S. J. Angew. Chem., Int. Ed. 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M.; Toste, D.; Pinkerton, A. B. Chem. Rev. 2001, 101, 2067 [32] Zhao, X.; Yu, Z.; Yan, S.; Wu, S.; Liu, R.; He, W.; Wang, L. J. Org. Chem. 2005, 70, 7338 16 [33] Murahashi S. -I.; Komiya, N.; Hayashi, Y.; Kumano, T. Pure Appl. Chem. 2001, 73, 311 [34] Blaser, H. -U.; Malan, C.; Pugin, B.; Spindler, F.; Steiner, H.; Studer, M. Adv. Synth. Catal. 2003, 345, 103 [35] Sbrana, G.; Braca, G.; Piacenti, F.; Pino, P. J. Organomet. Chem. 1968, 13, 240 [36] Mbaye, M.D.; Demerseman, B.; Renaud, J.-L.; Toupet, L.; Bruneau, C. Angew. Chem. Int. Ed., 2003, 42, 5066 [37] Ford, P. C. Acc. Chem. Res. 1981, 14, 37 [38] Grey, R. A.; Pez, G. P.; Wallo, A. J. Am. Chem. Soc.1981, 103, 7536 [39] Neumann, R.; Khenkin, A. M.; Dahan, M. Angew. Chem., Int. Ed. Engl. 1995, 34, 1587 [40] Quebatte, L.; Solari, E.; Scopelliti, R.; Severin, K. Organometallics 2005, 24, 1404 [41] Delaude, L.; Demonceau, A.; Noels, A. F. Cur. Org. Chem. 2006, 10, 203 [42] Maj, A. M.; Delaude, L.; Demonceau, A.; Noels, A. F. J. Organomet. Chem. 2007, 692, 3048 [43] Beletskaya, I. P.; Ananikov, V. P. Chem. Rev. 2011, 111, 1596 [44] Auner, N.; Weis, J. Organosilicon Chemistry IV, Wiley-VCH: Weinheim 2000 [45] Arkles, B. Grignard Reagents and Silanes, reprinted from: Silverman, G.S.; Rakita P.E. Handbook of Grignard Reagents, Marcel Dekker: New York 1996 [46] Roy, A. K. Adv. Organomet. Chem. 2008, 55, 1 17 [47] Marciniec, B.; Maciejewski, H.; Pietraszuk, C.; Pawluć, P. Hydrosilylation, A comprehensive review on recent advances, Springer Science+Business Media B. V. 2009 [48] Jones, G. R.; Landais, Y. Tetrahedron 1996, 55, 7599 [49] Hatanaka, Y.; Hiyama, T. J. Org. Chem. 1988, 53, 918 [50] Ison, E. A.; Corbin, R. A.; Abu-Omar, M. M. J. Am. Chem. Soc. 2005, 127, 11938 [51] Srimani, D.; Bej, A.; Sarkar, A. J. Org. Chem. 2010, 75, 4296 [52] Dai, X.; Strotman, N. A.; Fu, G. C. J. Am. Chem. Soc. 2008, 130, 3302 [53] Tour, M. J.; Cooper, J. P.; Pendalwar, S. L. J. Org. Chem. 1990, 55, 3452 [54] Wang, D. W.; Wang, D. S.; Chen, Q. A.; Zhou, Y. G. Chem. Eur. J. 2010, 16, 1133 [55] Evans, D. A.; Truesdale, L. K.; Grimm, K. G.; Nesbitt, S. L. J. Am. Chem. Soc. 1977, 99, 5009 [56] Harrison, D. J.; Edwards, D. R.; McDonald, R.; Rosenberg, L. Dalton Trans. 2008, 26, 3401 [57] Chauhan, B. P. S.; Boudjouk, P. Tetrahedron Lett. 2000, 41, 1127 [58] a) Jendras, M.; Klingebiel, U.; Niesmann, J. Organosilicon Chemistry V, Wiley VCH, 2000, 264; b) Abele, S.; Becker, G.; Eberle, U.; Oberprantacher, P.; Schwarz, W. Organosilicon Chemistry V, Wiley VCH, 2000, 270; c) Oprea, A.; Mantey, S.; Heinicke, J. Organosilicon Chemistry V, Wiley VCH, 2000, 277 [59] Bolton, P. D.; Mountford, P. Adv. Synth. Catal. 2005, 347 [60] Hoveyda, A. H.; Schrock, R. R. Chem. Eur, J. 2001, 7, 945 18 Chapter 2 Catalytic hydrosilylation of carbonyls via Re(CO)5Cl photolysis 2.1 Introduction The reaction of silanes with a wide range of carbonyl compounds remains important in the generation of useful protecting groups for organic synthesis. For example, hydrosilylation across the C=O bond of aldehydes and ketones produces protected alcohols in a single step.1 In the presence of a catalyst, these reactions could be carried out under mild conditions in contrast to using harsh reducing agents such as lithium aluminium hydride. Hydrosilylation catalysis has been reported for various transition metal complexes containing iron,2-3 tungsten,4 molybdenum,5 manganese,6-7 rhenium,1,8 rhodium,9 silver10 and most recently ruthenium.11 Recent works were also done on the mechanistic studies of hydrosilylation using oxo-rhenium complexes.12-13 While Berke et al.8 has reported the use of different rhenium (I) catalysts for the hydrosilylation of aldehydes and ketones, one of the more interesting processes is the CpFe(CO)2Me-catalysed reaction between silanes and dimethylformamide (DMF) reported by Pannell et al.14 As shown in Scheme 1, hydrosilylation across the C=O bond of DMF first afforded a silyl ether which further reacted with another molecule 19 of silane to give siloxane and trimethylamine as final products. Cutler et al.15 has used a manganese complex (CO)5MnC(O)CH3 to reduce the much less reactive ester to ether via a silyl acetal intermediate. Silyl esters are another important class of siliconcontaining compounds as they find wide use in the production of polymeric materials.16 Mizuno et al.16 reported that a range of ruthenium catalysts enables the conversion of carboxylic acids to silyl esters to take place. Furthermore, Yamamoto’s group showed that a boron catalyst, B(C6F5)3, reduces carboxylic acids further to the corresponding alkanes as depicted in Scheme 1.17 However, most of the reported carbonyl hydrosilylation catalysts require careful handling as they are relatively unstable under ambient conditions. Although the following commercially-available rhenium (I) complexes, Re(CO)5X (X = Br, Cl, Re(CO)5) have been reported to activate Si-H bonds, their activity towards carbonyl hydrosilylation has not been explored fully.18 It was previously reported19 that the air and moisture-stable Re(CO)5Cl dissociates a CO ligand readily upon photolysis (quantum yield of 0.06 to 0.44 from 366nm to 313nm). In light of efforts in utilizing energy from light rather than from fossil fuel sources, much work would have to be carried out to further explore light-driven catalytic processes. As it is also desirable for the catalysts to be air and moisture-stable and easy to handle, we have photoactivated Re(CO)5Cl and other rhenium carbonyls in order to carry out hydrosilylation reactions on a variety of carbonyl substrates at room temperature. The objectives of the work are to determine the relative efficiency of the catalysts towards different carbonyls, identify possible key catalytic intermediates and explore the mechanism of hydrosilylation using a combination of IR and NMR spectroscopy as well as computational studies. 20 O O + R3SiH Me 2N OSiR3 [CpFe(CO)2Me] H r.t., h Me 2N R = H, Alkyl, Aryl O PhSiH3 + O R 3% (L)(CO)4MnC(O)CH3 C6D6 OEt O Et3SiH OH H B(C6F5)3, r.t. R R3SiH Me 2N OSiEt 3 PhSiH3 SiR3 + H CH3 O(SiH 2Ph)2 OEt H + OEt OSiEt3 Et3SiH OSiEt3 R3Si 2 Et3SiH R CH3 R H OSiEt3 - 2O(SiEt3)2 Scheme 1: Catalytic hydrosilylation of carbonyl compounds 2.2 Results and Discussion 2.2.1 Hydrosilylation of silanes with aldehydes and ketones Preliminary experiments have been carried out to assess the efficiency of UV photolysis of Re(CO)5Cl towards carbonyl hydrosilylation using Et3SiH and acetone as substrates (Table 1, Entry 1). The quantum yields for Re(CO)5Cl dissociation at 313 nm and 366 nm have been determined to be 0.44 and 0.06 respectively19. The light source used in our work is broadband, ranging from 300nm to 800 nm. Since Re(CO)5Cl absorbs in the 300-350 nm region, the quantum yield would also be expected in the similar range as quoted. 1H NMR analysis of the mixture after 4 hours of photolysis showed isopropoxytriethylsilane, Et3SiOCH(CH3)2 (1a) as the sole 21 product (Figure 1), with a turnover number determined to be 70 to 80 with respect to Re(CO)5Cl (1% mol loading). OSiEt3 HB C HA CH B Figure 1. 1H NMR peaks of (1a) Peaks Assignment: HA - 3.99ppm (septet) HB - 1.15ppm (doublet) The reaction appears to proceed most efficiently if an excess of silane is used to achieve full conversion of acetone to the product. A ratio of silane : carbonyl of 3:1 has been used for further comparison with other carbonyl substrates. However the silyl ether (1a) does not undergo further reaction with the excess silane. Such is also the case in the Re(CO)5Cl-catalysed reactions of Et3SiH with other carbonyl substrates (Table 1, Entries 2-6). When the mixture was heated from 30 to 120°C in the absence of light, the yield of the resultant silyl ether was less than 10%. No hydrosilylation product was observed when the mixture was photolysed in air or upon removal of the rhenium catalyst. Re2(CO)10 has been found to be an effective catalyst as well (Table 1, Entries 8-9) with similar TON ratio to Re(CO)5Cl. However hydrosilylation proceeded only sluggishly when HRe(CO)5 was used (Table 1, Entries 10-11). Compare to previously reported catalysts7 such as [Re(CH3CN)3Br2(NO)] (TON = 99 for aldehydes/ketones), similar efficiency has been observed for Re(CO)5Cl. However, hydrosilylation with 22 substrates such as 1,3-diketones, anthraquinones and acetylacetone did not occur even after extended photolysis of up to 12 hours. Entry Substrate Product[a] Catalyst used TOF[b](hr1 ) OSiEt3 O Re(CO)5Cl 1 (1a) 18 (1b) 20 H O 2 Re(CO)5Cl H O OSiEt3 O 3 H H Re(CO)5Cl OSiEt3 Et3SiO 20 (1c) O 4 Re(CO)5Cl OSiEt3 (1d) 21 (1e) 23 (1f) 11 (1g) 4 H OSiEt3 O 5 Re(CO)5Cl OSiEt3 O 6 Re(CO)5Cl OSiEt3 O 7 Re(CO)5Cl O 8 Re2(CO)10 (1a) 15 Re2(CO)10 (1b) 18 O 9 H 23 O 10 HRe(CO)5 (1a) HRe(CO)5 (1b) 3 O 11 H 5 When the substrate was changed from acetone to benzaldehyde, an increase in the amount of product formed over the same period was observed (Table 1, Entries 1 and 2). This can be attributed to the increased reactivity of aldehydes when compared to ketones. When different silanes such as Ph2SiH2 and Ph3SiH were used, a decrease in the corresponding silyl ether yield was observed (Table 2). The relative lower rates towards hydrosilylation of acetone appear to bear a strong relation to the steric hindrance of the silane. In addition, when the carbonyl substrate was changed from aliphatic to a hindered aromatic carbonyl, i.e. benzophenone, acetophenone and benzaldehyde, a decrease in the TOF was observed. 2.2.2 Hydrosilylation of esters and carbonates In the presence of Re(CO)5Cl, esters and carbonates have been reduced with silanes, although at a lower rate compare to the aldehydes. Under our experimental conditions, the reaction of ethyl acetate and Et3SiH gave ethoxytriethylsilane as the main product with trace amount of diethyl ether. Acetaldehyde has been detected as an intermediate whereby its signal intensity decreases upon further photolysis while that of ethoxytriethylsilane continues to increase (Figure 2). Similar results have been Table 1. The reactions of Et3SiH with various aldehydes and ketones. [a] Product obtained after 4hrs of photolysis in vacuo, with 1% mol loading of catalyst. obtained forwas methyl phenylacetate and methyl formate. Ratio of silane to carbonyl substrate is 3:1. [b] TOF calculated based on 4hrs of photolysis in vacuo. 24 Entry Substrate Silane Time Product TOF[b](hr1 [a] ) 25 1 OSiPh3 O Ph3SiH 10hrs (2a) 1 H OSiPh3 O 2 H Ph3SiH 10hrs (2b) 1.5 OSiPh2H O 3 Ph2SiH2 5hrs (2c) 8 (2d) 10 (2e) 8 (2f) 14 H OSiPh2H O 4 H Ph2SiH2 5hrs OSiMe2Ph O Me2PhSiH 5 5hrs H O 6 OSiMe2Ph H Me2PhSiH 5hrs Table 2. The reactions of Acetone and Benzaldehyde with various silanes in ratio of silane : carbonyl = 3:1 [a] TON calculated based on 1% mol loading of Re(CO) 5Cl. Yield of product determined based on the integration of the 1H NMR spectra using toluene as standard. 26 The reaction of Et3SiH with diethyl carbonate afforded a variety of products with Et3SiOEt (1) and EtOCH2OSiEt3 (2) being the major ones. A trace amount of ethyl formate EtOCHO (3) has been detected. When the reaction mixture was left overnight, 1 H NMR peaks belonging to (EtO)2CH2 (4) were detected along with a decrease in (1) and (2). Figure 2. 1H NMR intermediate studies on the amount of intermediates and products produced during the reaction of ethyl acetate and Et3SiH 2.2.3 Comparison of hydrosilylation rates for different carbonyls By comparing the ratio of the products within the same photolysis period, the following general relationship has been determined for the relative rate using Re(CO)5Cl and Et3SiH : Aliphatic Aldehydes > Aromatic Aldehydes > Aliphatic Ketones > Aromatic ketones ~ Esters (Table 3). Comparing within each functional group, the alkyl analogues are more reactive than their aryl counterparts. Using pyruvic acid which has both C=O and O-H groups, it was found that Et3SiH preferentially reacted with the O-H group to produce the silyl ester. 27 Entry Substrate 1 Ethanal 2 Propanal 3 Benzaldehyde Ratio[a],[b] Product OSiEt3 OSiEt3 OSiEt3 Ph 90 90 9 OSiEt3 4 Acetone 1 OSiEt3 5 Acetophenone 0.5 Ph OSiEt3 6 Benzophenone 0.03 Ph 7 Ph Ethyl Acetate OSiEt 3 0.25 O 8 Pyruvic Acid OSiEt 3 480 O Table 3. Ratio of hydrosilylation products formed. Relative rates were determined by adding 3:1:1 ratio of Et3SiH with various carbonyl substrates and acetone. [a] Ratio with respect to Acetone. [b] Yield calculated on the basis of the integration of the 1H NMR spectra using toluene as standard 2.2.4 Mechanism The identification of the organic intermediates and products suggests that the first step of hydrosilylation proceeds via addition of R3SiH across the carbonyl group except when an O-H group is present. The formation of both H2 and a strong Si-O bond provides much of the driving force behind the reaction between the alcohol and 28 silane. However our focus is on the addition reaction, hence the complex reaction pathways for the diethylcarbonate case are highlighted as an example (Scheme 2). The products of the reaction between Et3SiH and diethylcarbonate were observed and identified in triethylethoxysilane 1 H NMR (Experimental Section) and were attributed to (1), ethylformate (2), ethoxymethoxytriethylsilane (3), diethoxymethane (4) and hexaethylsiloxane (5). The formation of these products led us to postulate the following mechanism: An unstable silylacetal intermediate is first formed, which undergoes ‘condensation’ to afford (1) and (2). With excess silane, the ethylformate (2) can be further reduced to (3). This explains the relatively little amount of formate left in the mixture. In addition, the experimental data also suggested that compound (1) undergoes further reaction with (2) to generate the ether (4) and siloxane (5). O O 2 Et3SiH Et3SiOEt (1) [Re] + EtO SiEt3 OEt Step 1 EtO O OEt (2) EtO Step 2 SiEt3 Et3Si (5) + OEt EtO (4) Et3SiOEt (1) [Re] Et3SiH OSiEt3 [Re] O H (3) EtO H Scheme 2: Proposed reaction scheme of the step-wise reaction As one of the main objectives of this work is to elucidate the mechanism of hydrosilylation, FTIR spectroscopy has been utilised for the detection of any metal carbonyl intermediates present in the catalytic mixture. Upon photolysis of Et3SiH 29 with Re(CO)5Cl, a dimeric rhenium carbonyl species with a bridging hydride (henceforth known as complex (A)) has been identified in the mixture (Figure 3). The identification of the complex was based on the IR spectral resemblance to rhenium complexes20 of formula HRe2(CO)9(SiR3). Its bridging hydride signal at -9.03ppm has also been recorded in the 1H NMR spectrum. The diphenylsilyl analogue, complex (B) has also been prepared upon Re(CO)5Cl photolysis in the presence of diphenylsilane. Other than (A), another hydride signal at -5.77 ppm found in the reaction mixture has been attributed to HRe(CO)5. 1.2 Abs 1.05 0.9 0.75 0.6 0.45 0.3 0.15 -0 2115 2100 2085 2070 2055 2040 2025 2010 1995 1980 Cy 1965 1950 1/cm 2100 2080 2060 2040 2020 2000 1980 Figure 3. IR and 1H Hydride NMR Spectrum of (A) obtained from photolysis of Re(CO)5Cl with Et3SiH. IR peaks (cm-1): 2102, 2094, 2085, 2030, 2026, 2020, 2007, 2000, 1978 Lit Values13 for HRe2(CO)9SiCl3(cm-1): 2150, 2095, 2085, 2047, 2019, 2012, 1999, 1978 1 H NMR peaks (ppm): -9.03 The formation of complex (A) can be rationalised as follows (Scheme 3). Upon UV photolysis of Re(CO)5Cl, a CO ligand dissociates to form the 16-electron Re(CO)4Cl intermediate followed by Et3SiH coordination. Upon reductive elimination, either HCl or Et3SiCl would be formed together with the corresponding Et3SiRe(CO)4 or HRe(CO)4 species. A free CO molecule coordinates back to HRe(CO)4 to give 30 HRe(CO)5. The bridging of its hydride ligand to Et3SiRe(CO)4 would then result in the formation of (A). When (A) was isolated and tested for aldehyde hydrosilylation, the silylether was generated about two to three times faster compared to Re(CO)5Cl. UV irradiation is still essential for the catalysis to occur. The IR signals of (A) persisted even after the end of catalysis, with a recovery of about 70-80%. SiEt3 CO OC Re CO Et3SiH - HCl CyH, h Re(CO) 5Cl - CO CO + Re(CO) 4Cl - Et 3SiCl Et3SiH H CO OC Re CO CO CO CO OC SiEt3 CO OC Re CO Re H OC CO (A) CO CO Scheme 3: Formation of (A) from Re(CO)5Cl photolysis in Et3SiH Interestingly, complex (A) can also be generated upon Re2(CO)10 photolysis in triethylsilane, which would explain the similarity in the reaction rate to Re(CO)5Cl. From these observations, there are reasons to believe that complex (A) acts as a resting state in carbonyl hydrosilylation. One of the crucial steps in the catalysis involves the photocleavage of (A) to form HRe(CO)5 and a 16-electron rhenium 31 carbonyl Et3SiRe(CO)4 . As it was shown earlier that catalysis with HRe(CO)5 is sluggish, we believe that the rhenium silyl species is most likely the active catalytic species instead. A mechanism for the hydrosilylation of carbonyl compounds is proposed to account for the experimental observations (Scheme 4). Upon photolysis, (A) dissociates to afford Et3SiRe(CO)4. Then the carbonyl substrate undergoes coordination onto the vacant site and facilitates the silyl ligand shift onto the oxygen atom. This process results in the formation of an alkyl ligand bound to the Re centre (Steps 1-2). Another silane undergoes coordination via a 2 silyl-complex4,7 or a sigma (σH) silyl-complex (Step 3). The H atom migrates from the silane to the alkyl group, thus regenerating the catalyst and releasing the silyl ether product (Step 4). When either the carbonyl or silane has been depleted, the R3SiRe(CO)4 would coordinate back to HRe(CO)5 and becomes part of the resting state (A). The activity of (A) in the catalytic cycle can be tested by using its Ph2SiH2 analogue, (B) to catalyse the hydrosilylation of acetone and Et3SiH. Upon completion of catalysis, 1H NMR analysis showed the presence of iPrO(SiEt3) as the expected main product with a small amount of iPrO(SiHPh2). More importantly, the IR spectrum has changed from that of (B) to (A). These observations show that silyl exchange has occurred between (B) and the free Et3SiH during catalysis and lent support to (A) participating in the catalysis (Scheme 5). 32 CO OC SiEt3 CO OC CO Re CO + Re CO CO CO (A) OC CO OC h Re H OC CO SiEt3 CO Re OC H CO CO SiEt3 O R - RCH 2COSiEt3 CO OC Re H CO CO Step 1 Step 4 SiEt3 OC H CO OC CO R H Et3SiO Re O CO R SiEt3 CO Re CO CO H Step 2 CO OC Step 3 Re R H H SiEt3 CO Et3SiO CO Scheme 4: Proposed mechanism of the hydrosilylation reaction OC SiPh2H CO Re CO CO O OC O SiPh2H CO Re CO CO CO OC Re CO CO HPh2SiO H SiEt3 H SiEt3 CO OC - iPrOSiPh2H SiEt3 OC Re CO Re CO CO CO Et3SiO CO Scheme 5: Silane exchange from (B) to (A) during reaction 33 2.2.5 Calculations Computational studies have been carried out to show the thermodynamic feasibility of the catalytic cycle in Scheme 4 and to provide a general explanation for the relative rates of hydrosilylation among different carbonyl substrates. The energetics and structures of the complexes together with any transition states have been calculated using mp2/lanl2dz level of theory and basis set in Gaussian 03.21 Vibrational frequencies were calculated for the optimised structures. In order to save computational time, formaldehyde CH2O and silane SiH4 have been used as the substrates as shown in Scheme 6. No solvent effect has been computed as well. O H SiR3 CO OC H - H 3COSiR3 Re CO I -96.5 CO IV -11.61 SiR3 OC H CO OC CO H H R3SiO Re Intermediate 1 O CO H SiR3 CO Intermediate 3 Re CO CO H 74.27 II CO OC H Re CO H R3SiO -45.50 III H SiR3 CO Intermediate 2 Scheme 6: Calculated catalytic cycle for SiH4 and CH2O. Values quoted are for free energies in kJ/mol 34 Table 4 shows the relevant thermodynamic parameters and co frequencies for the intermediates (restricted to singlet states only) and transition states found for the cycle while the calculated structures of the relevant intermediates and transition states are found in Figure 4. The calculations showed that Steps 1 and III in Scheme 6 are exogernic due to coordination of substrates to the relevant rhenium complexes. In contrast Step II is endoergic as activation is required for the silyl transfer to the carbonyl ligand in order to form Intermediate II. The transition state TSI for this step has been found and optimized with an activation energy of 74.3 kJ/mol relative to Intermediate I or 9.6 kJ/mol relative to CH2O and Re(CO)4SiH3. The reaction coordinate correctly depicts the migration of the silyl to the O atom of formaldehyde at an imaginary frequency i of 281 cm-1. Step IV turns out to be exogernic with a transition state TSII at only 26.0 kJ/mol relative to Intermediate 3. The reaction coordinate for this transition state shows an almost linear migration of an H atom from silane to the alkyl group at an imaginary frequency of 798 cm-1. The formation of the strong C-H bond is the driving force behind the reaction and accounts for the relatively lower activation barrier. By comparison of the energies, Step II represents the rate-determining step in the catalytic cycle. The computations show that this proposed catalytic cycle is indeed feasible as the highest point in the catalytic cycle represented by TS1 is only 9.6 kJ/mol (or 2.3 kcal/mol) above Re(CO)4SiH3 and CH2O, the starting pair of the cycle. Thus, once the rhenium silyl species has been generated, the catalysis should proceed readily even at room temperature. 35 Complex Structures for CH2O substrate H3SiRe(CO)4 Intermediate I Intermediate II Intermediate III TS I Enthalpy H Free energy G CO or i -535.751262 -535.806047 1767, 1807, 1809, 1908 -649.819307 -649.791523 -655.891354 -649.880743 -649.85245 -655.960371 -649.783078 -649.840337 -655.882012 -655.950457 1775, 1784, 1818, 1908 1779, 1792, 1820, 1916 1767, 1784, 1808, 1921 1789, 1799, 1817, 1900 i = 281 1797, 1820, 1832, 1921 i = 798 -763.804788 -763.868593 -727.961207 -728.02476 -879.689834 -879.758904 TS II Transition states for different substrates TS I for HCOOCH3 TS I for (CH3)2CO TS I for PhCHO 1781, 1793, 1821, 1907 i = 291 1777, 1793, 1805, 1900 i = 279 1780, 1792, 1809, 1902 i = 265 Ea 9.6 67.7 24.0 2.0 Table 4. Calculated enthalpies (Hartree), free energies (Hartree), carbonyl vibrational and imaginary frequencies (cm-1), and the activation barrier to TS1 (kJ/mol, relative to Re(CO)4SiH3 and respective carbonyl) for the rhenium intermediates and transition states of the catalytic cycle. The silane used is SiH4. Method and basis set used : mp2/lanl2dz. The relative hydrosilylation rates can be explained by comparing the activation barrier of the rate-determining step for the different substrates. We have focused on optimising the respective transition states TS1 of four representative carbonyls; formaldehyde, benzaldehyde, acetone and methyl formate. The value of the activation barrier is calculated relative to the starting pair in the cycle i.e. Re(CO)4SiH3 and the corresponding carbonyl. The various transition states together with the essential molecular parameters have been depicted in Figure 5. Comparing the values of Ea in Table 4, it can be seen that the barriers for the aldehydes are lower than the ketone which itself is much lower than the ester. In the 36 ester case, electronic effects due to the OR group destabilise the transition state much more than the aldehyde or ketone counterpart. Interestingly, the calculated activation energy for the benzaldehyde system is the lowest despite experimental kinetic studies suggesting a faster reaction for aliphatic aldehydes. Although the difference is slight (~7 kJ/mol), the inclusion of solvent effects may be able to reverse this trend. 37 2.61A 2.21A Intermediate 1 2.09A 1.64A 1.79A Intermediate 2 2.26A Intermediate 3 Figure 4. Calculated structures of intermediates and transition states for Scheme 6. Essential bond lengths (Ao) are shown. 38 2.61A Re(CO)4(SiH3) 2.80A 2.05A 2.24A TS 1 2.31A 2.63A 1.66A 2.39A TS 2 Figure 4 continued. 39 2.05 A 2.80 A 2.24 A 2.83 A TS (Formaldehyde) 2.01 A 2.26 A TS (Acetone) 2.82 A 2.04 A 2.23 A 2.84 A 2.00 A 2.26 A TS (Methyl Formate) TS (Benzaldehyde) Figure 5. Optimised transition states for the rate-determining steps of different substrates. Bond lengths in Ao 40 2.3 Conclusion Re(CO)5Cl has been found to catalyse the hydrosilylation reaction between various silanes and carbonyl substrates such as aldehyde, ketone, ester and carbonate effectively with a turnover frequency of 20 to 25 hr-1 for aldehydes. The rates of hydrosilylation are highest for the least sterically-hinderd silanes with aldehydes, followed by aliphatic ketone. Aromatic ketone, ester and carbonate react slowest with the silanes. A rhenium dimer complex HRe2(CO)9 has been detected using IR spectroscopy and is believed to be the resting state for the catalysis. Upon its photolysis, the active catalyst Re(CO)4SiH3 is released and participate in the catalytic cycle. Computational studies have shown that the proposed cycle can be carried out readily at room temperature and are able to explain the relative hydrosilylation rates among the various carbonyl substrates. 2.4 Experimental Section All chemicals were used as purchased. All reactions were carried out under vacuum conditions unless otherwise stated. Proton nuclear magnetic resonance (1H NMR) spectra were obtained on a Bruker Avance 500 (AV500) spectrometer at room temperature. The chemical shifts were recorded relative to tetramethylsilane for spectra taken in CDCl3. Infrared (IR) spectra were obtained on a Shimadzu IR Prestige-21 Fouriertransformed infrared spectrometer (1000-4000cm-1, 1cm-1 resolution, 4 scans co- 41 added for spectra averaging) using a 0.05mm path-length CaF2 cell. Mass spectra of the organic products are recorded with a Finnigan Mat 95XL-T spectrometer. Catalysis using Re(CO)5Cl: Re(CO)5Cl (0.011mmol) was mixed with Et3SiH (0.33mmol) and the carbonyl substrate (0.11mmol) in a quartz tube. The mixture was photolysed in vacuo, using a Legrand Broadband Lamp (200-800nm, 11W) for 4 hours. Products were identified using 1H NMR. Catalytic reduction of diethylcarbonate: Re(CO)5Cl (0.011mmol) was mixed with Et3SiH (0.33mmol) and diethylcarbonate (0.11mmol) in a quartz tube. The mixture was photolysed in vacuo, using a Legrand broadband Lamp (200-800nm, 11W) for 4 hours. 1H NMR (CDCl3): Triethylethoxysilane (1) - 0.60(m), 0.98(m), 1.20(m), 3.65(m); Ethylformate (2) - 1.31 (t), 4.24 (q), 8.04 (s); Diethoxymethane (4) - 0.60(m), 0.98(m), 1.20(m), 3.65(m), 4.86(m). Synthesis of HRe(CO)5: HRe(CO)5 was synthesized via a modification of reported methods.22-24 0.011mmol of Re(CO)5Cl was dissolved in methanol and the mixture was stirred for 1hr after which the mixture was cooled down using liquid nitrogen. Excess Zinc and acetic acid were then added to the cooled mixture before evacuation of the air inside the rbf was done. The reaction was then left to stir for 24hr at room temperature. The HRe(CO)5 formed was extracted using 3x3mL of hexane and recrystallised using CHCl3. CO(Cyclohexane): 2015(vs), 2005(m), 1983(w); 1H NMR: -5.77ppm 42 Comparison of rate of hydrosilylation reaction between different substrates: Re(CO)5Cl (0.011mmol) was mixed with Et3SiH (0.33mmol) and two different carbonyl substrates (0.11mmol each) in a quartz tube. The mixture was photolysed in vacuo, using a Legrand Broadband Lamp (200-800nm, 11W) for 4 hours. Products were identified using 1H NMR and yields were calculated with toluene (0.011mmol) as standard. Synthesis of HRe2(CO)9(SiR3) from Re(CO)5Cl: Re(CO)5Cl (0.011mmol) and R3SiH (0.022mmol) were dissolved in cyclohexane in a quartz tube. The mixture was photolysed in vacuo, using a Legrand Broadband Lamp (200-800nm, 11W) until all (CO) of Re(CO)5Cl have disappeared (2-4hours). The yellow solution was concentrated and passed through a column using a 1:1 mixture of hexane and chloroform. The resultant yellow band was then collected and the solvent removed using vacuum and recrystallised in hexane solution to afford a yellow powder (Yield with respect to Re(CO)5Cl is 65%). The products were identified based on comparing their IR and 1H NMR spectra to reported values.15 HRe2(CO)9(SiEt3) CO(Cyclohexane): 2030(m), 2021(s), 2016(sh), 2008(m), 2000(m), 1982(sh), 1978(s); 1H NMR: -9.03ppm; UV(nm): 200-400 nm (broad with max = 218 nm), ESI mass spectrum : m/z = 742 HRe2(CO)9(SiPh2H) CO(Cyclohexane): 2029(m), 2020(s), 2014(sh), 2008(m), 2000(m), 1981(sh), 1976(s) ; 1H NMR: -9.71ppm Synthesis of HRe2(CO)9(SiEt3) from Re2(CO)10: Re2(CO)10 (0.011mmol) and Et3SiH (0.022mmol) were dissolved in cyclohexane in a quartz tube. The mixture was photolysed in vacuo, using a Legrand Broadband Lamp (200-800nm, 11W) until all 43 (CO) of Re(CO)5Cl have disappeared (2-4hours). The yellow solution was concentrated and passed through a column using a 1:1 mixture of hexane and chloroform. The resultant yellow band was then collected and the solvent removed using vacuum and recrystallised in hexane solution to afford a yellow powder (Yield with respect to Re(CO)5Cl is 68%). Silyl exchange between HPh2SiRe(CO)4 and Et3SiH: Et3SiH (0.22mmol) and Acetone (0.11mmol) were added to a cyclohexane solution containing HRe2(CO)9(SiR3) (0.011mmol) in a quartz tube. The mixture was photolysed in vacuo and an IR spectrum of the reaction was taken at every 30 minute interval. 2.5 References [1] Abu-Omar, M. M.; Du, G. Organometallics 2006, 25, 4920 [2] Morris, R.H. Chem. Soc. Rev. 2009, 38, 2282 [3] Shaikh, N. S.; Junge, K.; Beller, M. Org. Lett. 2007, 9, 5429 [4] Fernandes, A.C.; Fernandes, R.; Romão, C.C; Royo, B. Chem. Comm. 2005, 213 [5] Gądek, A.; Szymańska-Buzar, T. Polyhedron 2006, 25, 1441 [6] Son, S. U.; Paik, S.; Chung, Y. K. J. Mol. Catal. A: Chem. 2000, 151, 87 [7] Cavanaugh, M.D.; Gregg, B.T.; Cutler, A.R. Organometallics 1996, 15, 2764 [8] Dong, H.; Berke, H. Adv. Synth. Catal. 2009, 351, 1783 [9] Ojima, I.; Nihonyanagi, M.; Kogure, T.; Kumagai, M.; Horiuchi, S.; Nakatsugawa, K. J. Organomet. Chem. 1975, 94, 449 44 [10] Wile, B. M.; Stradiotto, M. Chem. Commun. 2006, 4104 [11] Do, Y.; Han, J.; Rhee, Y. H.; Park, J. Adv. Synth. Catal. 2011, 353, 3363 [12] Chung, L. W.; Lee, H. G.; Lin, Z.; Wu, Y. J. Org. Chem. 2006, 71, 6000 [13] Du, G.; Fanwick, P. E.; Abu-Omar, M. M. J. Am. Chem. Soc. 2007, 129, 5180 [14] Sharma, H. K.; Pannell, K. H. Angew. Chem. Int. Ed. 2009, 48, 7052 [15] Mao, Z.; Gregg, B. T.; Cutler, A. R. J. Am. Chem. Soc. 1995, 117, 10139 [16] Ojima, Y.; Yamaguchi, K.; Mizuno, N. Adv. Synth. Catal. 2009, 351, 1405 [17] Gevorgyan, V.; Rubin, M.; Liu, J.; Yamamoto, Y. J. Org. Chem. 2001, 66, 1672 [18] Hua, R.; Jiang, J. Cur. Org. Synth. 2007, 4, 151 [19] Wrighton, M S.; Morse, D. L.; Gray, H. B.; Ottessen, D. K. J. Am Chem. Soc. 1976, 98, 1111 [20] Hoyano, J. K.; Graham, W. A. G. Inorg. Chem. 1972, 11, 1265 [21] Frisch, M. J.; et al. Gaussian 03, ReVisions B. and B.05; Gaussian Inc.: Wallingford, CT, 2004. [22] Braterman, P. S.; Harrill, R. W.; Kaesz, H. D. J. Am. Chem. Soc. 1967, 89, 2851 [23] Byers, B. H.; Brown, T. L. J. Am. Chem. Soc. 1977, 99, 2527 [24] Li, N.; Xie, Y.; King, R. B.; Schaefer III, H. F. J. Phys. Chem. A. 2010, 114, 11670 45 Chapter 3 Ruthenium carbonyl-catalyzed Si-X coupling (X = S, O, N) 3.1 Introduction Organosilanes containing Si-H bonds have been used extensively in synthetic organic chemistry for many years. Hydrosilylation of unsaturated bonds such as alkenes1, alkynes2 and carbonyls3 is one of the more important processes involving organosilanes. Transition metal complexes containing iron,4-5(re paper) tungsten,6 molybdenum,7 manganese,8-9 rhenium,10-11 rhodium,12 silver,13 nickel,14 and most recently ruthenium.15-16, 2 have been used to catalyze such processes. Silane hydrolysis and alcoholysis forming silanol, silyl ether and siloxane17 demonstrate further the versatility of organosilanes as valuable substrates. Ruthenium carbonyl complexes have also been reported to generate hydrogen from a silane/water mixture17-18 and from formic acid.19 The hydrogen gas produced can be used in a variety of reactions, of which one of the most well-studied reaction involves the hydrogenation of unsaturated compounds catalyzed by palladium (II)20 or iridium (I)21 complexes. These transformations are made possible because of the relatively weaker Si-H bonds available for activation as compared to C-H bonds. Silicon coupling with other heteroatoms such as sulfur and nitrogen are, however, less widely studied. Besides organosilanes, organosulfur derivatives of silanes can also be used to protect carbonyl groups selectively.22 However, methods of their 46 preparation require air-sensitive catalysts such as B(C6H5)3,23 and in some cases, sulfur poisoning is observed leading to much lower efficiency achieved in the catalytic system.24 The various couplings of Si-N bonds from silanes and amines resulting in the production of iminosilanes16a, silazanes16b and diaminosilanes16c have also been reported. However, these reactions were not extensively explored although they provide much insight into the role of amine ligands in processes such as alkene polymerisation25 and alkene metathesis26. Of recent interest are the formation of direct Si-N bonds via the addition of silanes to metal-imido27-30 (M=NR) complexes, which provides novel pathways to synthesize potential catalysts for metathesis reactions. Ruthenium dodecacarbonyl Ru3(CO)12 has been reported to activate Si-H bonds31 to form dimeric ruthenium silyl complex of formula (R3Si)Ru-Ru(SiR3). Chatani et. al.32 reported the activation of benzylic C-H bonds by Ru3(CO)12 in the presence of silanes to afford benzylsilanes, while Madalska et. al.33 was able to utilise Ru3(CO)12 to catalyse the silylation of silanols with vinylsilanes to afford alkenes and siloxanes. In this work, we have found that Ru3(CO)12 acts as a versatile and efficient catalyst for Si-heteroatom (S, N and O) coupling with high turnover numbers. The scope of the catalysis as well as the kinetics and detection of reactive intermediates have been investigated using FTIR and NMR spectroscopy. A ruthenium dimer, [Ru(CO)4(SiEt3)] 2, has been identified as the likely resting state of the catalyst and a mechanism for silane-thiol coupling based on this dimer has been proposed. 47 Computational chemistry using the Gaussian suite of programs has also been employed for elucidating the reaction mechanism. 3.2 Results and Discussion A preliminary study using a 1:1 mixture of Et3SiH and dodecanethiol as the substrates has been carried out in order to optimise the experimental conditions for efficient coupling (see Table 1). A temperature of 80oC over 4 hours using 1% mol loading of Ru3(CO)12 was sufficient for a full conversion (turnover number TON = 100, turnover frequency = 25 hr-1) of the substrates into the silylthioether, C12H25S– SiEt3 (Table 1, Entry 1). Complex Experimental conditions TON TOF/hr-1[a] Ru3(CO)12 80oC, 4 hrs 100 25 80oC, 2 hrs 65 33 80oC, 8 hrs 100 25 80oC, 4 hrs, vacuum 100 25 25oC, 4 hrs 32 8 80oC, 8 hrs, 0.5% mol loading 180 23 25oC, 4 hrs, 300-800 nm photolysis 22 6 80oC, 4 hrs, silane/thiol = 1.5 100 25 80oC, 4 hrs, thiol/silane = 1.5 50 13 Ru2(CO)4(PPh3)2Br4 80oC, 4 hrs 55 15 Ru2(CO)6Br4 80oC, 4 hrs 52 13 52 13 Ru2(CO)6I4 o 80 C, 4 hrs Table 1: Optimization of the experimental conditions for the coupling of triethylsilane with dodecanethiol using [Ru] catalyst. Unless stated otherwise, the reactions were carried out in air with 1% catalyst mol loading using 1:1 silane/thiol mol ratio in toluene solvent. 48 No coupling was observed in the absence of Ru3(CO)12 while heating the reaction mixture to higher than 80°C did not increase the coupling rate significantly. Full conversion can only be achieved after 24 hrs under lower temperature conditions ( 35 hr-1 compared to the reverse case of using more thiol to silane. As depicted in Scheme 1, hydrogen gas is expected to be generated and indeed, on-line mass spectrometric detection of the headspace above the solution mixture has recorded its presence at m/z = 2 (Figure 1). By calibration with a known H2 pressure, the yield of the gaseous product has been determined to be 85%±20% compared to the silylthioether. 49 Figure 1. Gaseous mass spectra taken of the headspace content of the coupling reaction between Et3SiH and dodecanethiol catalysed by Ru3(CO)12. The peak labelled 'After reaction' corresponds to m/z = 2. As haloruthenium carbonyls such as Ru2(CO)4(PPh3)2Br4, Ru2(CO)6Br4 and Ru2(CO)6I4 are known to activate Si-H bonds18, we have prepared these complexes using reported methods34 for the Si-S coupling reaction. However the coupling is found to proceed less effectively for each of them compared to Ru3(CO)12. Based on the optimization studies, the catalytic conditions have been fixed at 80oC, 4 hours and 1% mol loading of Ru3(CO)12 for further studies with different substrates. 50 Entry Substrate [a] Product TON[b] TOF/ hr1[c] C12H25SH C12H25S – SiEt3 100 25 2 C8H17SH C8H17S – SiEt3 100 25 3 PhCH2SH PhCH2S – SiEt3 100 25 4 p-Me(C6H4)SH p-Me(C6H4)S – SiEt3 100 25 5 C2H5OH C2H5O – SiEt3 100 25 6 iPrOH iPrO – SiEt3 100 25 7 C5H11OH C5H11O – SiEt3 100 25 8 C6H5OH C6H5O – SiEt3 100 25 9 C8H17NH2 C8H17NH – SiEt3 100 25 10 PhNH2 PhNH – SiEt3 100 25 11 PhMeNH PhMeN – SiEt3 100 25 C12H25SH C12H25S–SiHPh2 78 13 13 PhCH2SH PhCH2S–SiHPh2 69 12 14 p-Me(C6H4)SH 15 C2H5OH C2H5O–SiHPh2 70 12 16 C5H11OH C5H11O–SiHPh2 65 11 17 PhNH2 PhNH–SiHPh2 58 10 C12H25SH C12H25S–SiPh3 65 9 19 PhCH2SH PhCH2S– SiPh3 57 8 20 p-Me(C6H4)SH p-Me(C6H4)S– SiPh3 62 9 21 C2H5OH C2H5O– SiPh3 55 8 22 C5H11OH C5H11O– SiPh3 51 7 23 PhNH2 PhNH– SiPh3 35 5 1 12 18 Et3SiH Ph2SiH2 Ph3SiH p-Me(C6H4)S– SiHPh2 73 12 Table 2. Coupling reactions between different silanes and various substrates catalysed by 1 mol% Ru3(CO)12 at 80oC, 4 hrs. [a] Products identified using 1H NMR. [b] TON based on NMR integration. [c] TOF based on reaction time: Et3SiH (4 hrs), Ph2SiH2(6 hrs) and Ph3SiH (7 hrs). 51 Table 2 shows the various coupling reactions of triethylsilane that have been carried out with thiol, amine and alcohol substrates. In general, all three types of coupling Si-S, Si-N and Si-O take place with similar TOF efficiency under the same conditions. Based on the TOF, it was found that thiols undergo silane coupling slightly more effectively, followed by alcohols and amines, and with the aliphatic substrates being more reactive than aromatic substrates. When bulkier silanes were used, a 2-3 times decrease in the rate of coupling with all three types of substrates was observed. Full conversion can still be achieved for all substrates upon prolonging the reaction time to 6 hrs for Ph2SiH2 and a minimum of 7 hours for Ph3SiH. 3.2.1 Reactive intermediates and Mechanism The detection and identification of reactive intermediates using FTIR, NMR and mass spectroscopy have been carried out in order to provide some insights into the mechanism of the coupling reactions. We have chosen the Si-S coupling with dodecanethiol and triethylsilane substrates as the model reaction. For this purpose, the concentration of the ruthenium catalyst has been scaled up in order to generate more intermediates for characterization. The FTIR spectrum taken of the reaction mixture containing 5%mol Ru3(CO)12 and a 1:1 silane:thiol mixture heated for 3 hours at 80oC is shown in Figure 2.  0.9 Abs 0.8  0.7 0.6 52 53 Two new sets of peaks have appeared at the expense of the initial Ru3(CO)12 signals at 2061, 2029 and 2009cm-1. The first set consisting of a strong CO band at 2017cm-1 and 2 weaker peaks at 2041cm-1 and 2006cm-1 has been assigned to a dimeric ruthenium silyl carbonyl complex, [Ru(CO)4(SiEt3)] 2 based on previous literature.31 Each ruthenium atom carries a silyl ligand which is oriented trans to each other (Scheme 2). The reduced number of carbonyl bands observed in the spectrum is indicative of the highly-symmetrical nature of the complex. An ESI mass spectrum recorded of the mixture shows a cluster of peaks centred at m/z=659 which matches the molecular ion mass of the ruthenium dimer. OC OC Ru 3(CO) 12 + Et3SiH Et3Si OC Ru OC OC CO Ru CO CO SiEt3 CO + OC OC Ru CO CO Ru CO CO Ru OC H CO H Scheme 2. Ru2(CO)8(SiEt3)2 and H2Ru3(CO)10 complexes The second set of CO peaks (2082cm-1, 2067cm-1, 2031cm-1, 2013cm-1) are due to a ruthenium carbonyl hydride cluster H2Ru3(CO)10.35 A 1H NMR spectrum recorded of the mixture showed a hydride signal at -14.3ppm, which is attributed to the equivalent bridging hydrides of the cluster. Similarly the mass spectrum showed the expected molecular ion cluster of peaks at m/z=559 together with the fragmentation patterns consistent with sequential losses of the CO ligands. Indeed, both the [Ru(CO)4(SiEt3)] 2 and H2Ru3(CO)10 complexes can also be generated in the reaction between Ru3(CO)12 with silane only, independent of any thiols in the solution. This result also implied that the hydrogen atoms of the hydride 54 CO cluster originate from the silane substrate. The detailed mechanism of the formation of both of these complexes from is probably very complicated but most likely involves the initial dissociation of Ru3(CO)12 into reactive Ru(CO)4 monomeric units. Both complexes could represent the eventual thermodynamically-stable ruthenium silylated and hydride products upon reactions of Ru(CO)4 with the silane. While the [Ru(CO)4(SiEt3)] 2 can be purified in small yields ([...]... HRe(CO)5 1 .2 Abs 1.05 0.9 0.75 0.6 0.45 0.3 0.15 -0 21 15 21 00 20 85 20 70 20 55 20 40 20 25 20 10 1995 1980 Cy 1965 1950 1/cm 21 00 20 80 20 60 20 40 20 20 20 00 1980 Figure 3 IR and 1H Hydride NMR Spectrum of (A) obtained from photolysis of Re(CO)5Cl with Et3SiH IR peaks (cm-1): 21 02, 20 94, 20 85, 20 30, 20 26, 20 20, 20 07, 20 00, 1978 Lit Values13 for HRe2(CO)9SiCl3(cm-1): 21 50, 20 95, 20 85, 20 47, 20 19, 20 12, 1999,... 1908 1779, 17 92, 1 820 , 1916 1767, 1784, 1808, 1 921 1789, 1799, 1817, 1900 i = 28 1 1797, 1 820 , 18 32, 1 921 i = 798 -763.804788 -763.868593 - 727 .96 120 7 - 728 . 024 76 -879.689834 -879.758904 TS II Transition states for different substrates TS I for HCOOCH3 TS I for (CH3)2CO TS I for PhCHO 1781, 1793, 1 821 , 1907 i = 29 1 1777, 1793, 1805, 1900 i = 27 9 1780, 17 92, 1809, 19 02 i = 26 5 Ea 9.6 67.7 24 .0 2. 0 Table 4... in vacuo 24 Entry Substrate Silane Time Product TOF[b](hr1 [a] ) 25 1 OSiPh3 O Ph3SiH 10hrs (2a) 1 H OSiPh3 O 2 H Ph3SiH 10hrs (2b) 1.5 OSiPh2H O 3 Ph2SiH2 5hrs (2c) 8 (2d) 10 (2e) 8 (2f) 14 H OSiPh2H O 4 H Ph2SiH2 5hrs OSiMe2Ph O Me2PhSiH 5 5hrs H O 6 OSiMe2Ph H Me2PhSiH 5hrs Table 2 The reactions of Acetone and Benzaldehyde with various silanes in ratio of silane : carbonyl = 3:1 [a] TON calculated... Organomet Chem 20 05, 690, 3774 [22 ] Hua, R.; Tian, X J J Org Chem 20 04, 69, 57 82 [23 ] Ojima, Y.; Yamaguchi, K.; Mizuno, N Adv Synth Catal 20 09, 351, 1405 [24 ] Hua, R.; Jiang, J Cur Org Synth 20 07, 4, 151 [25 ] Murahashi, S -I Ruthenium in Organic Synthesis, Wiley-VCH:Weinheim 20 04 [26 ] Renaud, J -L.; Demerseman, B.; Mbaye, M D.; Bruneau, C Cur Org Chem 20 06, 10, 115 [27 ] Bruneau, C.; Dixneuf, P.H Ruthenium. .. Organometallics 20 08, 27 , 24 90 [9] Hong, S H.; Sander, D P.; Lee, C W.; Grubbs, R H J Am Chem Soc 20 05, 127 , 17160 [10] Kuninobu, Y.; Takai, K Chem Rev 20 11, 111, 1938 [11] Hua, R.; Jiang, J -L Maitlis, P M Curr Org Synth 20 07, 4, 151 [ 12] Kühn, F E.; Santos, A M.; Herrmann, W A Dalton Trans 20 05, 24 83 [13] Espenson, J H Adv Inorg.Chem 20 03, 54, 157 [14] Kusama, H.; Narasaka, K Bull Chem Soc Jpn 1995, 68, 23 79... intermediates and explore the mechanism of hydrosilylation using a combination of IR and NMR spectroscopy as well as computational studies 20 O O + R3SiH Me 2N OSiR3 [CpFe(CO)2Me] H r.t., h Me 2N R = H, Alkyl, Aryl O PhSiH3 + O R 3% (L)(CO)4MnC(O)CH3 C6D6 OEt O Et3SiH OH H B(C6F5)3, r.t R R3SiH Me 2N OSiEt 3 PhSiH3 SiR3 + H CH3 O(SiH 2Ph )2 OEt H + OEt OSiEt3 Et3SiH OSiEt3 R3Si 2 Et3SiH R CH3 R H OSiEt3 - 2O(SiEt3 )2. .. Catalysts and Fine Chemistry, Springer: Berlin, 20 04 [28 ] Tan, S T.; Kee, J W.; Fan, W Y.; Organometallics 20 11, 30, 4008 [29 ] Naota, T.; Takaya, H.; Murahashi, S -I Chem Rev 1998, 98, 25 99 [30] Nishibayashi, Y.; Uemura, S Cur Org Chem 20 06, 10, 135 [31] Trost, B M.; Toste, D.; Pinkerton, A B Chem Rev 20 01, 101, 20 67 [ 32] Zhao, X.; Yu, Z.; Yan, S.; Wu, S.; Liu, R.; He, W.; Wang, L J Org Chem 20 05, 70,... iron ,2- 3 tungsten,4 molybdenum,5 manganese,6-7 rhenium, 1,8 rhodium,9 silver10 and most recently ruthenium. 11 Recent works were also done on the mechanistic studies of hydrosilylation using oxo -rhenium complexes. 12- 13 While Berke et al.8 has reported the use of different rhenium (I) catalysts for the hydrosilylation of aldehydes and ketones, one of the more interesting processes is the CpFe(CO)2Me-catalysed... cross-coupling reactions involving organosilanes also provide many useful applications in organic and bioorganic chemistry Transition metal -catalyzed silane hydrolysis and alcoholysis involving the reaction between organosilanes and water/alcohol produces silanols, silyl ethers and siloxanes,50 which provide useful starting materials for the manufacture of pharmaceuticals51 and also as excellent research... K J Am Chem Soc 20 09, 131, 9914 15 [16] Zuo, W -X.; Hua, R.; Qiu, X Synth Commun 20 04, 34, 321 9 [17] Kuninobu, Y.; Kawata, A.; Yudha, S S.; Takata, H.; Nishi, M.; Takai, K Pure Appl Chem 20 10, 82, 1491 [18] Chen, H.; Hartwig, J F Angew Chem Int Ed 1999, 38, 3391 [19] Kuninobu, Y.; Kawata, A.; Takai, K J Am Chem Soc 20 05, 127 , 13498 [20 ] Zhao, W -G.; Hua, R Eur J Org Chem 20 06, 5495 [21 ] Ouh, L L.; Müller, ... 38 2. 61A Re(CO)4(SiH3) 2. 80A 2. 05A 2. 24A TS 2. 31A 2. 63A 1.66A 2. 39A TS Figure continued 39 2. 05 A 2. 80 A 2. 24 A 2. 83 A TS (Formaldehyde) 2. 01 A 2. 26 A TS (Acetone) 2. 82 A 2. 04 A 2. 23 A 2. 84 A 2. 00... Re(CO)5Cl with Et3SiH IR peaks (cm-1): 21 02, 20 94, 20 85, 20 30, 20 26, 20 20, 20 07, 20 00, 1978 Lit Values13 for HRe2(CO)9SiCl3(cm-1): 21 50, 20 95, 20 85, 20 47, 20 19, 20 12, 1999, 1978 H NMR peaks (ppm):... attributed to HRe(CO)5 1 .2 Abs 1.05 0.9 0.75 0.6 0.45 0.3 0.15 -0 21 15 21 00 20 85 20 70 20 55 20 40 20 25 20 10 1995 1980 Cy 1965 1950 1/cm 21 00 20 80 20 60 20 40 20 20 20 00 1980 Figure IR and 1H Hydride NMR