SYNTHESIS, STRUCTURE AND CATALYTIC APPLICATION OF NOVEL CARBENE COMPLEXES WITH BENZOTHIAZOLIN 2 YLIDENE LIGANDS 2

Chapter two: Results and Discussion Introduction Chapter Two Results and Discussions 2.1 Introduction This chapter is divided into three parts. Part I (Section 2.2-2.4) describes the synthesis and coordination chemistry of Pd(II)-NSHC complexes as well as their applications in catalysis. Part II (Section 2.5-2.6) focuses on the synthesis and characterization of a range of mixed-ligand carbene complexes with phosphine, aromatic N-heterocycle, pyridyl and azole ligands. The coordination chemistry and catalytic activities of N,S- and N,N-NHC Pd(II) complexes are compared. Part III presents the formation of ring-opening of benzothiazolium salts (Section 2.10), the coordination of Pt(II)-NSHC-(C^N) complexes (C^N = 2-phenyl-pyridine) (Section 2.11) and the formation of 5-member and 6-member fused ring imidazolium salts via oxidative addition and reductive elimination (Section 2.12). In the following section, the synthesis and characterization of benzothiazolium salts will be discussed. 33 Synthesis, Structure and Catalytic Application of Novel Carbene Complexes with Benzothiazolin-2-ylidene Ligands Chapter two: Results and Discussion Part I Part I Section 2.2: Synthesis and Characterization of Benzothiazolium Salts – Solvent Free Synthesis Section 2.3: Synthesis of Mono-, Bis- and Dinuclear Pd(II) Complexes with 3-benzylbenzothiazolin-2-ylidene ligand and their Activities toward Mizoroki-Heck Coupling Section 2.4: Synthesis and Characterization of Pd(II) Complexes of NSHCs with Pendant and Coordinated Allyl Functionality and their Suzuki Coupling Activites 34 Synthesis, Structure and Catalytic Application of Novel Carbene Complexes with Benzothiazolin-2-ylidene Ligands Chapter two: Results and Discussion Part I 2.2. Synthesis and Characterization of Benzothiazolium Salts – Solvent Free Synthesis The preparation of 3-benzylbenzothiazolium bromide (A) (in 65% yield) from the reaction of benzothiazole with benzyl bromide in DMF at 95 °C has been reported in the literature.32 However, only moderate yield was obtained with the use of dry and high-boiling point DMF as solvent. In this work, when neat benzothiazole is treated with benzyl bromide in the absence of a solvent at 60 °C (Scheme 2.1), the desired product A readily precipitates from the liquid mixture, which solidifies toward the end of the reaction. Washing with Et2O gives pure A as an off-white powder in quantitative yield. S Temperature + H N Salt A B C D S R-X N R R X X Temperature Br 60 °C Br 60 °C Br 120 °C I3 100 °C Scheme 2.1 Synthesis of benzothiazolium salts A-D. 35 Synthesis, Structure and Catalytic Application of Novel Carbene Complexes with Benzothiazolin-2-ylidene Ligands Chapter two: Results and Discussion Part I Table 2.1 Selected 1H, 13C NMR and ESI data for A-D. 13 C NMR (ppm) of SCHN-carbona 165.1 ESI [M-X]+ (m/z) 226 A H NMR (ppm) of SCHN-protona 12.27 B 11.87 165.0 176 C 12.09 164.9 178 D 11.52 163.1 178 Salts a: measured in CDCl3. Fig. 2.1 ORTEP representation of the cation of benzothiazolium salt C14H12BrNS (A) with 50% thermal ellipsoids and labeling scheme; hydrogen atoms are omitted for clarity. The 1H NMR spectrum of A shows a characteristic downfield resonance at 12.27 ppm for the SCHN proton (SCHN = benzothiazolium proton), indicating the formation of an azolium salt (Table 2.1). The formation of A is also supported by a signal at 165.1 ppm in the 13C NMR spectrum for the SCHN carbon (SCHN = carbon 36 Synthesis, Structure and Catalytic Application of Novel Carbene Complexes with Benzothiazolin-2-ylidene Ligands Chapter two: Results and Discussion Part I carbene precursor) and a base peak at m/z 226 for the azolium cation in the positive mode ESI mass spectrum (Table 2.1). The identity of A was further confirmed by its molecular structure (Fig. 2.1 and Table 2.2) elucidated by single-crystal X-ray diffraction. Compound A is described as a carbene precursor in Section 2.3. Fig. 2.2 ORTEP representation of the cation of benzothiazolium salt C10H10BrNS (B) with 50% thermal ellipsoids and labeling scheme; hydrogen atoms are omitted for clarity. 3-(2-Propenyl)benzothiazolium bromide B can be prepared by stirring neat benzothiazole with a slight excess of allyl bromide at ambient temperature for a few days.13 The reaction time can be significantly shortened if the reaction is carried out at 60 ◦C, affording 81% yield in only 12 h (Scheme 2.1). However, higher temperature should be avoided in order to prevent thermal decomposition. The characteristic downfield resonance at 11.87 ppm indicates the SCHN proton of the azolium salt 37 Synthesis, Structure and Catalytic Application of Novel Carbene Complexes with Benzothiazolin-2-ylidene Ligands Chapter two: Results and Discussion (Table 2.1). The 13 Part I C NMR signal for this SCHN carbon (SCHN = carbon carbene precursor) is at 165.0 ppm (Table 2.1). The positive mode of ESI mass spectrum molecular ion is 176 m/z. The crystal structure of B is shown in Fig. 2.2. The use of compound B as a carbene precursor will be described in Section 2.4. Fig. 2.3 ORTEP representation of the cation of benzothiazolium salt C10H12BrNS (C) with 50% thermal ellipsoids and labeling scheme; hydrogen atoms are omitted for clarity. 3-Propylbenzothiazolium bromide C was prepared in 78 % as a yellow powder from 1-bromopropane and benzothiazole at 120 ◦C without solvent (Scheme 2.1). The 1H NMR spectrum of C in CDCl3 shows a pseudo-sextet at 2.17 ppm and two sets of triplets at 5.12 and 1.08 ppm characteristic of the propyl substituent. The downfield resonance at 12.09 ppm for the SCHN proton (Table 2.1) indicates the formation of a benzothiazolium salt. This is further supported by a 13C signal at 164.9 ppm for the SCHN carbon (Table 2.1). The positive mode ESI mass spectrum shows a base peak at m/z = 178 corresponding to the [M − Br]+ cation (Table 2.1). The 38 Synthesis, Structure and Catalytic Application of Novel Carbene Complexes with Benzothiazolin-2-ylidene Ligands Chapter two: Results and Discussion Part I analysis of C by X-ray single-crystal diffraction confirms the benzothiazolium structure with N-propyl substituent (Fig. 2.3). The unit cell contains two independent molecules, which show disorder of the propyl-substituent. Fig. 2.4 ORTEP representation of the molecule of benzothiazolium salt C10H12BrNS (D) with 50% thermal ellipsoids and labeling scheme; hydrogen atoms are omitted for clarity. 3-Isopropylbenzothiazolium tri-iodide D forms readily from the reaction of benzothiazole in neat 2-iodopropane (used in excess). Unlike the related 1,3diisopropylbenzimidazolin-2-ylidene analogues, iPr2-bimyH+I-,215 it is isolated in its tri-iodide form, presumably from iodide and iodine addition reaction. The formation of iodine, which notably appears as a purple solid on the wall of the condenser at the end of the reaction, could be traced to photo-activation (from stray light) of alkyl iodide giving alkyl radical and iodine.216 The somewhat unsatisfactory yield (32%) is attributed to base-assisted Hofmann elimination of 3-isopropylbenzothiazolium iodide to propene and benzothiazole (Scheme 2.2). The yield of D can be raised to 44% when iodine is added to the reaction. The product salt is soluble in common organic 39 Synthesis, Structure and Catalytic Application of Novel Carbene Complexes with Benzothiazolin-2-ylidene Ligands Chapter two: Results and Discussion Part I solvents (e.g. halogenated solvents, methonol, THF, CH3CN, DMSO, DMF) and water, and it is generally more soluble than A-C. I + I2 (a) hv S + S I H N N S (b) I S + H N I2 (c) H N I I3 D S - I- H N I H H H S + (d) N Base Scheme 2.2 Proposed of benzothiazolium salt D and side products. The benzothiazolium proton (SCHN) resonance is characteristically downfield (11.52 ppm) (Table 2.1). It is also more deshielded compared with 1,3diisopropylbenzimidazolium iodide, iPr2-bimyH+I- (10.79 ppm),215 which could be due to the replacement of nitrogen by a more electropositive sulfur and that there is only one exocyclic substituent on the heterocycle. The thiazolium carbon (δC = 163.1 ppm) is downfield shifted by ~20 ppm with reference to the azolium carbon in 1,3iPr2-bimyH+X- (X = I, 139.5;215 X = Br, 140.7 ppm).217 The positive-mode ESI mass spectrum shows a principal peak at m/z = 178 corresponding to the thiazolium cation. 40 Synthesis, Structure and Catalytic Application of Novel Carbene Complexes with Benzothiazolin-2-ylidene Ligands Chapter two: Results and Discussion Part I Salt D is used as a carbene precursor in preparing Pd(II) mononuclear and dinuclear NSHC- complexes. The results will be discussed in Section 2.7 and 2.8. X-ray crystal structure diffraction analysis of D has confirmed the identity of 3-isopropyl substituted benzothiazolium cation with the linear tri-iodide anion (Fig. 2.4). Under solventless conditions, benzothiazole readily reacts with alkyl halides give A-D, which are air-stable that can be used conveniently. 41 Synthesis, Structure and Catalytic Application of Novel Carbene Complexes with Benzothiazolin-2-ylidene Ligands Chapter two: Results and Discussion Part I Table 2.2 Selected bond lengths [Å] and angles [deg] for A-D. Bond Lengths [Å] S1-C1 A B 1.690 (2) 1.684(3) Molecule a 1.683(4) C D N1-C1 N1-C8 C8-C9 1.314(2) 1.483(2) - 1.315(3) 1.481(3) 1.500(3) 1.317(4) 1.479(5) 1.550(8) N2–C11 N2–C18 C18-C19 1.325(4) 1.473(4) 1.556(8) 1.311(4) 1.498(3) 1.519(4) C9-C10 - 1.309(3) 1.541(9) C19-C20 1.529(8) - C8-C10 Angles [deg] N1-C1-S1 - - - - - 1.514(5) 114.14(14) 114.58(17) 114.6(3) N2–C11–S2 114.6(2) 115.0(2) C1-N1-C8 123.63(16) 123.95(18) 123.2(3) C11–N2–C18 122.8(3) 124.4(2) C9-C8-C10 - - - - - 112.9(3) N1-C8-C9 - 110.59(16) 109.6(4) N2-C18-C19 108.2(4) 109.1(2) N1-C8-C10 - - 111.3(2) I2-I1-I3 - - 177.95(8) Molecule b S2-C11 1.678(3) 1.681(3) 42 Synthesis, Structure and Catalytic Application of Novel Carbene Complexes with Benzothiazolin-2-ylidene Ligands Chapter two: Results and Discussion Part III Table 2.26 Selected bond lengths [Å] and angles [deg] of 2.39(a)-2.39(b) and 2.40. Bond Lengths [Å] 2.39(a) 2.39(b) 2.40 Pt1-C1 1.939(4) 1.946(4) Molecule a 2.053(7) Molecule b Pt1-N2 2.074(4) 2.080(3) - - - Pt1-N3 - - 2.022(6) Pt2-N6 2.024(6) Pt1-C21 2.000(4) 1.985(4) - - - Pt1-C32 - - 2.037(6) Pt2-C64 2.025(8) Pt1-Br1 2.521(5) 2.525(5) 2.443(9) Pt2-Br4 2.434(9) S1-C1 1.722(4) 1.725(4) - - - S1-C7 1.736(5) 1.747(4) - - - N1-C1 1.346(5) 1.340(5) 1.377(8) N4-C33 1.346(8) N1-C2 1.403(5) 1.401(5) 1.391(9) N4-C34 1.402(10) N1-C8 1.480(5) 1.471(5) 1.444(9) N4-C40 1.470(9) N2-C1 - - 1.344(9) N5-C33 1.351(9) N2-C7 - - 1.411(8) N5-C39 1.396(9) N2-C11 1.342(5) 1.344(6) - - - N2-C15 1.365(5) 1.368(6) - - - N3-C22 - - 1.334(8) N6-C54 1.367(10) N3-C26 - - 1.379(8) N6-C58 1.341(9) Pt2-C33 2.035(8) 144 Synthesis, Structure and Catalytic Application of Novel Carbene Complexes with Benzothiazolin-2-ylidene Ligands Chapter two: Results and Discussion Part III C14-C15 1.379(6) 1.389(6) - - - C15-C16 1.456(6) 1.472(6) - - - C16-C17 1.399(5) 1.392(6) - - - C16-C21 1.408(5) 1.413(6) - - - C20-C21 1.393(5) 1.404(6) - - - C25-C26 - - 1.404(10) C57-C58 1.379(11) C26-C27 - - 1.454(9) C58-C59 1.485(11) C27-C28 - - 1.409(10) C59-C60 1.412(11) C27-C32 - - 1.394(10) C59-C64 1.394(11) C1-Pt1-C21 93.71(16) 93.29(17) - - - C1-Pt1-C32 - - 178.2(3) C33-Pt2-C64 175.9(3) C1-Pt1-N2 174.76(14) 170.87(15) - - - C1-Pt1-N3 - - 98.1(2) C33-Pt2-N6 96.2(3) C21-Pt1-N2 81.08(15) 80.85(16) - - - C32-Pt1-N3 - - 80.7(3) C64-Pt2-N6 80.0(3) C1-Pt1-Br1 89.58(11) 90.02(12) 86.86(19) C33-Pt2-Br4 88.03(19) C21-Pt1-Br1 176.65(11) 173.56(11) - - - C32-Pt1-Br1 - - 94.2(2) C64-Pt2-Br4 95.7(2) N2-Pt1-Br1 95.62(9) 96.51(11) - - - Angles [deg] 145 Synthesis, Structure and Catalytic Application of Novel Carbene Complexes with Benzothiazolin-2-ylidene Ligands Chapter two: Results and Discussion Part III N3-Pt1-Br1 - - 173.60(16) N6-Pt2-Br4 175.41(19) C1-N1-C8 122.5(4) 121.2(3) 126.2(6) C33-N4-C40 123.4(7) C11-N2-C15 118.4(4) 118.8(4) - - - C22-N3-C26 - - 119.2(6) C54-N6-C58 118.5(7) C11-N2-Pt1 127.2(3) 125.9(3) - - - C22-N3-Pt1 - - 125.3(5) C54-N6-Pt2 124.0(6) C15-N2-Pt1 114.3(3) 114.4(3) - - - C26-N3-Pt1 - - 115.4(5) C58-N6-Pt2 117.3(5) N1-C1-S1 109.2(3) 109.7(3) - - - N1-C1-N2 - - 106.8(6) N4-C33-N5 104.8(7) N1-C1-Pt1 128.8(3) 126.6(3) 125.5(5) N4-C33-Pt2 128.8(6) S1-C1-Pt1 121.9(2) 123.7(2) - - - N2-C1-Pt1 - - 127.4(5) N5-C33-Pt2 126.3(5) N1-C8-C9 112.3(3) 113.6(3) 115.1(6) N4-C40-C41 113.7(6) N2-C11-C12 123.2(4) 122.2(5) - - - N3-C22-C23 - - 122.9(7) N6-C54-C55 122.2(9) C21-C16-C15 116.8(3) 115.8(4) - - - C32-C27-C26 - - 115.8(6) C64-C59-C58 114.8(7) C20-C21-C16 118.0(3) 117.1(4) - - - C31-C32-C27 - - 117.4(6) C63-C64-C59 116.6(8) C20-C21-Pt1 128.5(3) 127.9(3) - - 146 Synthesis, Structure and Catalytic Application of Novel Carbene Complexes with Benzothiazolin-2-ylidene Ligands Chapter two: Results and Discussion Part III C31-C32-Pt1 - - 129.1(6) C63-C64-Pt2 129.0(7) C16-C21-Pt1 113.6(3) 114.8(3) - - - C27-C32-Pt1 - - 113.5(5) C59-C64-Pt2 114.4(6) 147 Synthesis, Structure and Catalytic Application of Novel Carbene Complexes with Benzothiazolin-2-ylidene Ligands Chapter two: Results and Discussion Part III 2.12 Catalytic Annulation of Heterocycles via a Redox Process Involving the Imidazolium Salt N-Heterocyclic Carbene A catalytic azolium-alkene coupling reaction in which imidazolium salts act as substrate has been reported by Cavell, et. al The reaction proceeds via a redox process involving a carbene-M-hydride intermediate.147-148 Bergman and co-workers have reported the catalytic 2-substitution of N-heterocycles, including the annulation of alkenyl substituted azoles via a rhodium-catalyzed intramolecular coupling reaction in the presence of PCy3·HCl.283 A Rh(I) catalyst is used to convert benzimidazoles, thiazoles, oxazoles, pyridines, and pyrimidines into 2-substituted heterocycles and fused-ring heterocyclic compounds. The proposed mechanism for this reaction consists of activation of a heterocyclic C-H bond to generate a coordinatively unsaturated Rh(I)-NNHC intermediate, which was shown by X-ray crystallography to be a square-planar Rh(I) NNHC/hydride complex. This species undergoes insertion of the coordinated alkene into the Rh-carbene bond, followed by elimination to give the final product.283(b), 283(k) In this section, the use of N-vinyl or other N-alkenyl-substituted azolium salts in the reaction which would lead to the intramolecular formation of fused-ring or annulated heterocycles will be presented. The successful use of zerovalent Ni and Pd complexes with both NNHC and phosphine spectator ligands to produce novel fusedring imidazolium salts is discussed. The reaction also demonstrates the direct, in situ formation of catalytically active carbene-metal-hydride complexes. 148 Synthesis, Structure and Catalytic Application of Novel Carbene Complexes with Benzothiazolin-2-ylidene Ligands Chapter two: Results and Discussion Part III 2.12.1 The Synthesis of Azolium Salts and as Substrates in Catalysis The synthesis of mono- or disubsituted azolium salts was achieved by a simple alkylation of the corresponding imidazoles and thiazoles (Fig. 2.40). R N Br H N S H N 2.41(a): R = CH3 2.41(b): R = Butyl 2.41(c): R = Mes 2.41(d): R = Dipp N Br H N 2.41(e) N Br H N 2.41(m) N Br 2.41(n) R N Br H N 2.41(f): R = CH3 2.41(g): R = Butyl 2.41(h): R = Mes 2.41(i): R = Dipp N Br H N 2.41(o) R1 S R2 N Br H 2.41(j): R1 = H, R2 = H 2.41(k): R1 = H, R2 = CH3 2.41(l): R1 = CH3, R2 = CH3 N Br H N 2.41(p) Br H N 2.41(q) Fig. 2.40 Imidazolium and thiazolium salts used in the catalytic reaction. The various imidazolium salts studied here were characterized by NMR, highresolution electrospray MS, and/or elemental analysis. IMes (1,3-bis-(2,4,6trimethylphenyl)-imidazol-2-ylidene) acts as a σ-donor ligand and has been shown to facilitate the oxidative addition reaction of imidazolium salts to Ni(0).141 Stirring a solution of IMes and Ni(COD)2 in DMF gave a purple solution that immediately 149 Synthesis, Structure and Catalytic Application of Novel Carbene Complexes with Benzothiazolin-2-ylidene Ligands Chapter two: Results and Discussion Part III changed to yellow on addition of a DMF solution of the N-allyl-substituted imidazolium salt 2.41(a), indicating that the positive chemical reaction has occurred. However, despite heating at 70 °C for an extended period, no product was obtained; the reaction mixture contained only unreacted 2.41(a) (Scheme 2.15). The imidazolium salts 2.41(a-d) and the thiazolium salt 2.41(e) also failed to give the desired fused ring products. However, the inclusion of a further methylene group between the nitrogen and the alkene group, giving the N-but-3-enyl-substituted substrates 2.41(f-p), allows the formation of five-membered fused rings. When the temperature was raised to 70 °C for 16 h, the C2-annulated products were formed. Products 2.42(f, g, l, n, p) (Scheme 2.16, Fig. 2.41 and Table 2.27, Entries 2, 3, 8, 9, and 10) were isolated and fully characterized by 1H and 13 C NMR spectroscopy, high-resolution electrospray mass spectrometry, and/or elemental analysis. The addition of equiv of IMes (spectator ligand, L) was necessary for effective catalytic performance. Scheme 2.17 shows the proposed catalytic cycle for coupling reaction of N-but-3-enyl imidazolium/thiazolium. R E Br H N cat. [Ni(COD)2/Ligand] (1: 2.1) R E Br + 2.41(a): E = N, R = CH3 2.41(b): E = N, R = Butyl 2.41(c): E = N, R = Mes 2.41(d): E = N, R = Dipp 2.41(e): E = S 70oC 16 h DMF N 0% 2.42(a): E = N, R = CH3 2.42(b): E = N, R = Butyl 2.42(c): E = N, R = Mes 2.42(d): E = N, R = Dipp 2.42(e): E = S Scheme 2.15 No coupling reaction of 2-substituted imidazolium and thiazolium. 150 Synthesis, Structure and Catalytic Application of Novel Carbene Complexes with Benzothiazolin-2-ylidene Ligands Chapter two: Results and Discussion Part III cat. [Ni(COD)2 / Ligand] (1: 2.1) R1 Br E H N R2 R3 R1 E R2 70 oC 16h DMF R2 R3 N R3 2.41(f-p) Br 2.42(f-p) Scheme 2.16 The coupling reaction of N-but-3-enyl substituted imidazolium and thiazolium salts. H L Ni (R1) E R2 L migratory insertion N R3 L L Ni L (R1) E N R2 oxidative addition of imidazolium salt R3 reductive elimination of carbene/ alkyl ligands R2 R3 (R1) Br E H N L-Ni(0)-L catalyst R2 R3 (R1) Br E N Scheme 2.17 Proposed catalytic cycle for the coupling reaction of N-but-3-enyl imidazolium/thiazolium. N Br N 2.42(f) N Br S Br Br N N N N N N 2.42(g) 2.42(l) 2.42(n) Br 2.42(p) Fig. 2.41 Isolated fused-ring azolium salts. 151 Synthesis, Structure and Catalytic Application of Novel Carbene Complexes with Benzothiazolin-2-ylidene Ligands Chapter two: Results and Discussion Part III It is noted that formation of 2.41(f-h, p) occurs via migration of the hydride to the terminal position of the alkene, whereas 2.41(n) is formed via hydride migration to the 2-position of the alkene. It is likely that steric factors prevent the formation of the six-membered fused-ring product in which the highly substituted carbon would be attached to the imidazole C2. The influence of the substitution on the second nitrogen atom is apparent in the result which is summarized in Table 2.27. Table 2.27 Catalytic coupling results with different substrates.a Entry Substrate Conversion (%)b 2.41(a-e) 2.41(f) 100c 2.41(g) 100c 2.41(h) 44 2.41(i) 2.41(j) dipp) where steric bulk of the N substituent plays a part in the reaction efficacy. The N-but-3-enyl-substituted thiazolium salts 2.41(j-l) are also investigated as substrates for the catalytic reaction. Less than 5% conversion 152 Synthesis, Structure and Catalytic Application of Novel Carbene Complexes with Benzothiazolin-2-ylidene Ligands Chapter two: Results and Discussion Part III is noted for substrates 2.41(j,k) (Table 2.27, Entries and 7); some decomposition of the catalyst is observed. However, 3-(3-butenyl)-4,5-dimethylthiazolium bromide 2.41(l) showed 50% conversion to the annulated product (Table 2.27, Entry 8), as determined by 1H NMR of the mixture (the product was isolated and characterized). The poor conversion of 2.41(j,k) may be due to an interaction of the S atom and the Ni catalyst, which is hindered by the presence of the methyl group in the 4-position of 2.41(l).156(b) The effect of different spectator ligands was investigated using the cyclization of 2.41(f) to 2.42(f) as the test reaction. A variety of NNHC ligands and phosphines were investigated (Fig. 2.42), and the results are recorded in Table 2.28. NNHC ligands bearing a 2,4,6-trimethylphenyl (IMes and SIMes) (Table 2.28, Entries 1-2) are found to give comparable results to those with 2,6-diisopropylphenyl ring (Dipp) on both nitrogen (i.e. IPr, SIPr, Me2IPr) (Table 2.28, Entires 3-5). It would thus seem that, in the case of NNHCs, these two supporting ligands are beneficial for the reaction, which in turn suggests that reductive elimination or olefin insertion is the rate determining step. For phosphines, a balance between bulk and electron-donating ability seems necessary, as neither PPh3 (least basic and least bulky phosphine) nor PEt3 (most basic and bulky phosphine) is very effective in promoting the reaction (Table 2.28, Entries and 9). This balance would seem to have been achieved with the Buchwald ligand PCy2(Bip) (Table 2.28, Entry 7). These results suggest a different mechanism for phosphines compared to carbenes as spectator ligands (e.g. involving phosphine dissociation equilibria). Lower loadings of Ni(COD)2 were also investigated. With mol% of Ni(COD)2 relative to substrate, 15% conversion to the fused ring product 2.42(f) is obtained after 16 hrs (Table 2.28, Entry 10). The reaction is significantly slower than when 10 mol% of catalyst is used; however, extending the reaction time to 65 hrs 153 Synthesis, Structure and Catalytic Application of Novel Carbene Complexes with Benzothiazolin-2-ylidene Ligands Chapter two: Results and Discussion Part III resulted in 100 % conversion, without decomposition of catalyst (Table 2.28, Entry 11). This result implies that longer time is needed to obtain 2.42(f) when using mol% of catalyst. The amount of catalyst loading can be decreased to mol% with the requirement of longer time reaction. When the catalyst loading was further reduced to mol%, 6% conversion to 2.42(f) is obtained after 16 hrs (Table 2.28, Entry 12). Increasing the reaction time to 45 hrs, 66% conversion is obtained (Table 2.28, Entry 13) by using mol% loading of catalyst. After 65 hours with mol% catalyst loading, conversion to 2.42(f) is less than 2% (Table 2.28, Entry 14). Table 2.28 Catalytic coupling reactions with different supporting ligands.a Cat. Loading (%) 10 10 SIMes 16 100 10 IPr 16 100 10 SIPr 16 100 10 Me2IPr 16 100 10 PPh3 16 100 10 PCy2(Bip) 16 100 10 PCy3 16 55 10 PEt3 16 100 10 IMes 16 15 11 IMes 65 100 12 IMes 16 13 IMes 45 66 14 IMes 65 [...]... S-heterocyclic Carbene Complexes with Aromatic N-Heterocycle Section 2. 7: Formation and Structures of Pd(II) NSHC-Pyridyl Mixed-Ligand Complexes Section 2. 8: Benzothiazolin- 2- ylidene and Azole Mixed-Ligand Complexes of Pd(II) Section 2. 9: Structures and Suzuki-Coupling of NNHC Complexes of Pd(II) with Coordinated Solvate and PPh3 68 Synthesis, Structure and Catalytic Application of Novel Carbene Complexes with Benzothiazolin- 2- ylidene. .. NSHCs and two bromo ligands in a cis-arrangement (Fig 2. 5) Table 2. 3 Selected bond lengths [Å] and angles [deg] for complexes 2. 1 -2. 4 2. 1 2. 2(a) 2. 3 2. 4 Pd(1)-C(1) 1.971(5) 1.936(7) 1.936(3) 1. 921 (2) Pd(1)-C(15) 1.976(4) - - - Pd(1)-Br(1) 2. 454(6) 2. 458(1) 2. 423 (4) 2. 438(4) Pd(1)-Br (2) 2. 469(6) 2. 539(1) 2. 448(4) 2. 424 (4) Pd(1)-Br(3) - 2. 389(1) - - Bond Lengths [Å] 45 Synthesis, Structure and Catalytic Application. .. the 43 Synthesis, Structure and Catalytic Application of Novel Carbene Complexes with Benzothiazolin- 2- ylidene Ligands Chapter two: Results and Discussion Part I S 4 H + 2 Pd(OAc )2 N Br CH3CN S S 2 N N reflux - 2 HOAc Pd Br Br 2. 1 A DMSO, 70 oC - 2 HOAc S N Br Br Br Pd Pd Br + N 2 N OAc H S S 2. 2 (a) DMF CH3CN S 2 N 2. 3 Br Pd NCCH3 Br S 2 N Br Pd DMF Br 2. 4 Scheme 2. 3 Synthesis of Pd(II) carbene complexes. .. 22 100 100 51 Synthesis, Structure and Catalytic Application of Novel Carbene Complexes with Benzothiazolin- 2- ylidene Ligands Chapter two: Results and Discussion Part I 4 2. 1 0.05 4-bromobenzaldehyde DMF 22 100 95 5 2. 1 1 4-bromobenzaldehyde Toluene 17 100 18 6 2. 1 1 4-bromoacetophenone DMF 21 110 98 7 2. 1 1 4-bromoanisole DMF 21 110 79 8 2. 1 1 4-bromophenol DMF 21 110 75 9 2. 1 1 DMF 21 110 77 10 2. 2(a)... bromides and activated aryl chlorides giving good conversions 67 Synthesis, Structure and Catalytic Application of Novel Carbene Complexes with Benzothiazolin- 2- ylidene Ligands Chapter two: Results and Discussion Part II Part II Section 2. 5: Pd(II) Complexes of Mixed Benzothiazolin- 2ylidene and Phosphine Ligands and their Catalytic Activities toward C-C Coupling Reactions Section 2. 6: Mono- and Dinuclear... Addition of compound B to Pd(OAc )2 eliminates HOAc and affords the bis (carbene) complexes cis-[PdBr2(NSHC )2] (cis2.5, NSHC = 3- (2- propenyl )benzothiazolin- 2- ylidene) and trans-[PdBr2(NSHC )2] (trans -2. 5) along with the monocarbene complexes [PdBr2(NSHC)] (2. 6) and trans[PdBr2(benzothiazole-ĸN)(NSHC)] (2. 7) as minor side products A metathesis of cis2.5 with AgO2CCF3 yields the mixed dicarboxylato-bis (carbene) ... 4-chlorobenzonitrile DMF 42 100 6 52 Synthesis, Structure and Catalytic Application of Novel Carbene Complexes with Benzothiazolin- 2- ylidene Ligands Chapter two: Results and Discussion Part I 39 2. 4 1 4-chlorobenzaldehyde DMFd 20 110 17 40 2. 4 1 4-chloroacetophenone DMFd 20 110 5 d 20 110 10 41 2. 4 1 4-chlorobenzonitrile DMF a 1 mmol of Aryl halide, 1.5 mmol of base, 1 .2 mmol of tert-butyl acrylate,... [PdBr2(NSHC) ]2 (2. 2) Complex 2. 2 undergoes bridge cleavage reactions with CH3CN and DMF to give the mononuclear and solvated monocarbene complexes trans-[PdBr2(NSHC)(Solv)] [Solv = CH3CN (2. 3) and DMF (2. 4)] The catalytic activities of 2. 1 -2. 4 toward Mizoroki-Heck coupling reactions of aryl bromides with tert-butyl acrylate are described and compared 55 Synthesis, Structure and Catalytic Application of. .. cis -2. 5 2. 6 2. 7 Pd(1)-C(1) 1.976 (2) 1.958(3) 1.945(11) Pd(1)-C(9) - 2. 188(3) - Pd(1)-C(10) - 2. 181(3) - Pd(1)-C(11) 1.977 (2) - - Pd(1)-Br(1) 2. 470(3) 2. 460(4) 2. 419 (2) Pd(1)-Br (2) 2. 4 82( 3) 2. 475(4) 2. 422 (19) Pd(1)-N (2) - - 2. 093(9) S(1)-C(1) 1.715 (2) 1.707(3) 1.7 12( 13) S (2) -C(11) 1.710 (2) - 1.731( 12) N(1)-C(1) 1. 328 (3) 1.330(4) 1.314(15) N(1)-C(8) 1.456(3) 1.4 62( 4) 1.444(18) N (2) -C(11) 1.317(3) - 1 .27 8(17)... that monocarbene complexes may be involved as catalytically active species (Table 2. 8, Entries 2 and 3) .21 5 This result is in agreement with the optimum Pd : salt ratio of 1 : 1 reported for a similar imidazolium based catalyst .22 2 When deactivated aryl bromides 66 Synthesis, Structure and Catalytic Application of Novel Carbene Complexes with Benzothiazolin- 2- ylidene Ligands Chapter two: Results and Discussion . Results and Discussion Part I Synthesis, Structure and Catalytic Application of Novel Carbene Complexes with Benzothiazolin- 2- ylidene Ligands 42 Table 2. 2 Selected bond lengths [Å] and angles. Results and Discussion Part I Synthesis, Structure and Catalytic Application of Novel Carbene Complexes with Benzothiazolin- 2- ylidene Ligands 43 2. 3 Synthesis of Mono-, Bis- and Dinuclear. I Synthesis, Structure and Catalytic Application of Novel Carbene Complexes with Benzothiazolin- 2- ylidene Ligands 46 Pd(1)-N (2) - - 2. 063(3) - Pd(1)-O(1) - - - 2. 101(18) Pd (2) -C(15)