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APPLICATION OF AMMONIA BORANE AND METAL AMIDOBORANES IN ORGANIC REDUCTION XU WEILIANG (B.Sci., Soochow University) A THESIS SUBMITTED FOR THE DEGREE OF DOCTOR OF PHILOSOPHY DEPARTMENT OF CHEMISTRY NATIONAL UNIVERSITY OF SINGAPORE 2012 ACKNOWLEDGEMENTS I would like to express my sincere gratitude to Prof. Chen Ping. As my Ph.D. supervisor, Prof. Chen taught me both basic and advanced techniques in chemistry with great patience. She also led me to the right direction with her experience and knowledge at every critical point of this thesis. Her assistance and supervision are great treasures to me and this thesis work. I also appreciate the help from my co-supervisor, Asst. Prof. Wu Jishan. Dr Wu gave me great suggestions on my research work and inspired me in every discussion with him. In addition, I need to warmly acknowledge Prof. Fan Hongjun and Prof. Zhou Yonggui from Dalian Institute of Chemical Physics, Chinese Academy of Sciences. The help from Prof. Fan in theoretical calculation improves the understanding of my research topic. The discussion with Prof. Zhou on research topic helps me achieve several additional insights into this topic. A very special recognition needs to be given to my research group members such as Prof. Xiong Zhitao and Prof. Wu Guotao for their extensive help and support during research. Finally, a special thanks to my family for their uncontional love and support in every way possible throughout the process of my Ph.D. course. i THESIS DECLARATION The work in this thesis is the original work of Xu Weiliang, performed independently under the supervision of Assoc Prof. Chen Ping, Chemistry Department, National University of Singapore, between 2007 and 2011. The content of the thesis has been published in: 1. Xu, W.; Zhou, Y.; Wang, R.; Wu, G.; Chen, P., Lithium amidoborane, highly chemoselective reagent for reduction of -unsaturated ketones to allylic alcohols. Organic & Biomolecular Chemistry, 2012,10, 367-371. 2. Xu, W.; Wang, R.; Wu, G.; Chen, P. Calcium amidoborane, a new chemoselective reagent for reduction of ,-unsaturated aldehydes and ketones to allylic alcohols. RSC Advances, DOI: 10.1039/C2RA01291J. 3. Xu, W.; Zheng, X; Wu, G.; Chen, P. Reductive amination of aldehydes and ketones with primary amines by using lithium amidoborane. Chinese Journal of Chemistry, DOI: 10.1002/cjoc.201200132. 4. Xu, W.; Fan, H.; Wu, G.; Chen, P., Comparative study on reducing aromatic aldehydes by using ammonia borane and lithium amidoborane as reducing reagents. New Journal of Chemistry, DOI: 10.1039/c2nj40227k. Name Signature Date ii Table of contents Acknowledgements………………………………………………………… i Publication list…………………………………………………… viii Summary………………………………………………………………………. ix List of Tables………………………………………………………………… xi List of Figures…………………………………………………………………. xii Abbreviation List…………………………………………………………… xiv Chapter 1. Introduction 1.1 Review on methods for organic reduction……………………………… 1.1.1 Catalytic hydrogenation………………………………………………. 1.1.2 Electroreduction and reduction with metals………………………… 1.1.3 Transfer hydrogenation…………………………………………… . 1.1.4 Reduction with hydrides and complex hydrides……………………… 1.2 Reducing reactivity of some typical borohydride compounds………… 10 1.2.1 Sodium borohydride (NaBH4)……………………………………… 10 1.2.2 Diborane (B2H6), tetrahydrofuran-borane complex (BH3-THF) and dimethyl sulfide Borane (BMS) ………………………………………………. 13 1.2.3 Amine borane ………………………………………………………… 19 1.2.4 Sodium aminoborohydrides (NaNRR’BH3) ………………………… 25 1.2.5 Lithium aminoborohydrides (LiNRR’BH3, LAB) ………………… . 28 1.3 Mechanistic interpretations on borohydride reduction……………………. 31 1.4 Review on ammonia borane and metal amidoboranes for hydrogen iii storage ……………………………………………………………… . 35 1.4.1 Ammonia borane (AB)……………………………………………… 35 1.4.2 Metal amidoborane (MAB)………………………………………… 38 1.5 Research gaps and aims…………………………………………………… 39 1.5.1 Research gaps………………………………………………………… 39 1.5.2 Research aims………………………………………………………… 40 Chapter 2. Methodology 2.1 Synthesis of metal amidoboranes…………………………………………. 42 2.1.1 Introduction………………………………………………………… 42 2.1.2 Synthetic procedure of metal amidoboranes. ……………………… 43 2.2 Synthesis of deuterated ammonia borane and deuterated metal amidoboranes 45 2.2.1 Introduction…………………………………………………………. 46 2.2.2 Synthetic procedure of deuterated ammonia borane and deuterated metal amidoboranes……………………………………………………… 46 2.3 Characterization methods………………………………………………. 47 Chapter 3. Reducing aldehydes and ketones by ammonia boranes 3.1 Introduction……………………………………………………………… 48 3.2 Results and discussion ……………………………………………………. 49 3.2.1 Reaction process and reactivity study……………………………… . 49 3.2.2 Kinetic study………………………………………………………… 53 3.2.3 Theoretical study……………………………………………………. 55 iv 3.3 Conclusion……………………………………………………………… 58 3.4 Experimental section………………………………………………………. 58 3.4.1 General Remarks………………………………………………… . 58 3.4.2 General experimental procedure for reducing aldehydes and ketones with AB 59 3.4.3 Products characterization . 60 Chapter 4. Reducing aldehydes, ketones and imines by metal amidoboranes 4.1 Introduction………………………………………………………………… 64 4.2 Results and discussion ……………………………………………………. 65 4.2.1 Reducing ketones by MAB……………………………………… . 65 4.2.2 Reducing imines with MAB……………………………………… . 71 4.2.3 Theoretical Study………………………………………………… 77 4.2.4 Reducing aromatic aldehydes with MAB…………………………… 79 4.3 Conclusion………………………………………………………………. 82 4.4 Experimental section……………………………………………………… 83 4.4.1 General Remarks…………………………………………………… 83 4.4.2 Synthesis of imines . 83 4.4.3 General experimental procedure for reducing ketones with LiAB, NaAB or CaAB . 84 4.4.4 General experimental procedure for reducing imines with LiAB, NaAB or CaAB 84 4.4.5 Products characterization 85 v Chapter 5. Chemoselectively reducing -unsaturated aldehydes and ketones into allyic alcohols by metal amidoboranes 5.1 Introduction……………………………………………………………… 92 5.2 Results and discussion …………………………………………………… 94 5.2.1 Reactivity study……………………………………………… 94 5.2.2 Mechanism study………………………………………………… 97 5.2.3 Reducing -unsaturated aldehydes with MAB………………… . 98 5.2.4 Explanation on 1,2-reduction property of MAB………………… 100 5.3 Conclusion………………………………………………………………… 100 5.4 Experimental section……………………………………………………… 101 5.4.1 General remarks………………………………………………… . 101 5.4.2 Synthesis of -unsaturated ketones………………………… . 101 5.4.3 General experimental procedure for reducing -unsaturated ketones or aldehydes with CaAB………………………………………………………. 102 5.4.4 Products characterization……………………………………… 103 Chapter 6. Reductive amination of aldehydes and ketones with primary amines by using lithium amidoborane 6.1 Introduction………………………………………………………………. 109 6.2 Results and discussion ……………………………………………………. 111 6.2.1 Choice of Lewis acid…………………………………………… . 111 6.2.2 Reactivity study………………………………………………… 112 vi 6.3 Conclusion………………………………………………………………… 114 6.4 Experimental section………………………………………………………. 115 6.4.1 General remarks…………………………………………………… 115 6.4.2 General experimental procedure for reducing amination by LiAB 115 6.4.3 Products characterization……………………………………… . 116 Chapter 7. Conclusion and Future work 7.1 Conclusion ……………………………………………………………… 121 7.2 Future work……………………………………………………………… 124 Reference……………………………………………………………………… 125 vii PUBLICATION LIST 1. Xu, W.; Zhou, Y.; Wang, R.; Wu, G.; Chen, P., Lithium amidoborane, highly chemoselective reagent for reduction of ,-unsaturated ketones to allylic alcohols. Organic & Biomolecular Chemistry, 2012,10, 367-371. 2. Xu, W.; Wang, R.; Wu, G.; Chen, P. Calcium amidoborane, a new chemoselective reagent for reduction of ,-unsaturated aldehydes and ketones to allylic alcohols. RSC Advances, DOI: 10.1039/C2RA01291J. 3. Xu, W.; Zheng, X.; Wu, G.; Chen, P. Reductive amination of aldehydes and ketones with primary amines by using lithium amidoborane. Chinese Journal of Chemistry, DOI: 10.1002/cjoc.201200132. 4. Xu, W.; Fan, H.; Wu, G.; Chen, P., Comparative study on reducing aromatic aldehydes by using ammonia borane and lithium amidoborane as reducing reagents. New Journal of Chemistry, DOI: 10.1039/c2nj40227k 5. Xu, W.; Fan, H.; Wu, G.; Wu, J.; Chen, P., Metal Amidoboranes, Superior Double Hydrogen Transfer Agents in Reducing Ketones and Imines. Chemistry- a European Journal, under revision. 6. Zheng, X.; Xu, W.; Xiong, Z.; Chua, Y.; Wu, G.; Qin, S.; Chen, H.; Chen, P., Ambient temperature hydrogen desorption from LiAlH4-LiNH2 mediated by HMPA. Journal of Material Chemistry. 2009, 19 (44), 8426-8431. 7. Xiong, Z.; Wu, G.; Chua, Y. S.; Hu, J.; He, T.; Xu, W.; Chen, P., Synthesis of sodium amidoborane (NaNH2BH3) for hydrogen production. Energy & Environmental Science 2008, (3), 360-363. 8. Xiong, Z. T.; Chua, Y. S.; Wu, G. T.; Xu, W. L.; Chen, P.; Shaw, W.; Karkamkar, A.; Linehan, J.; Smurthwaite, T.; Autrey, T., Interaction of lithium hydride and ammonia borane in THF. Chemical Communications. 2008, (43), 5595-5597. viii SUMMARY Ammonia borane (NH3BH3, AB) and metal amidoboranes (M(NH2BH3)n, MABs) are attractive materials for hydrogen storage due to their high hydrogen capacities and mild dehydrogenation temperature. One of the driving forces for releasing hydrogen from those materials is the co-existence of protic and hydridic hydrogens in their structures. On the other hand, although AB and MAB belong to borohydrides, their applications in organic reductions have not yet been extensively explored. Moreover, few investigations were given to the participation of protic hydrogens of amine boranes in organic reductions. The objectives of this study were to explore AB and MABs as reducing agents in organic reduction and to study the reduction mechanism involved. Our experimental results show that AB possesses high reactivity in reducing aldehydes at ambient temperature and in reducing ketones at 65oC. Based on the in-situ FT-IR and NMR characterizations, we found that not only the hydridic hydrogens of AB transfer to carbonyl groups, but the protic hydrogens of AB also participate in reaction. Furthermore, kinetic study and density functional theory (DFT) calculations indicate that the reaction between AB and carbonyl obeys a second-order rate law, being first order of each reactant. In addition, concerted double hydrogen transfer pathway is the dominant path in the reduction. In another part of this study, MABs were utilized to reduce unsaturated functional groups. Interestingly, MABs has higher reducibility towards unsaturated functional groups than AB. Moreover, the protic hydrogens of MABs are also proved to ix was treated with HCl (4ml, 2M) and stirred for an additional hour. Then, NaOH (2M) solution was added to adjust the pH value to 8. Next, the solution was extracted with 10 ml diethyl ether for times. The combined diethyl ether extracts were washed with brine, dried with NaSO4 overnight and concentrated in vacuum. In the final step, the residue was purified by silica gel flash chromatography to obtain the desired product. The product was characterized by FTIR, 1H NMR, 13C NMR and GC-MS. 6.4.3 Products characterization N-benzylaniline (entry 1,Table 6.2): 1H NMR (500 MHz, CDCl3, 25 oC; TMS): δ = 4.00 (s, 1H; N-H), 4.32 (s, 2H; CH2), 6.64-6.72 (m, 3H; Ar-H), 7.16-7.36 ppm (m, 7H; ArH).13C NMR (126 MHz, CDCl3, 25oC; CDCl3): δ = 48.36, 112.87, 117.59, 127.23, 127.52, 128.64, 129.27, 140.51 ppm. FT-IR (KBr): νmax = 3419,3052, 3026, 2920, 2841, 1602, 1505, 1452, 1324, 750, 693 cm-1.MS (EI): m/z (%) 182 [M-H]+ (100), 91 (70), 106 (12), 77 (10), 65 (9). N-benzyl-4-methylaniline (entry 2, Table 6.2): 1H NMR (500 MHz, CDCl3, 25 oC; TMS): δ = 2.25 (s, 3H; CH3), 3.89 (s, 1H; N-H), 4.31 (s, 2H; CH2), 6.57-7.00 (m, 4H; Ar-H), 7.28-7.36 ppm (m, 5H; Ar-H).13C NMR (126 MHz, CDCl3, 25oC; CDCl3): δ = 20.39, 48.67, 113.02, 126.75, 127.15, 127.50, 128.60, 129.75, 139.70, 145.96 ppm. FT-IR (KBr): νmax =3416, 3027, 2918, 2863, 1617, 1521, 807, 742, 697cm-1. MS (EI): m/z (%) 196 [M-H]+ (100), 91 (78), 120 (18), 65 (11). N-benzyl-4-methoxyaniline (entry 3, Table 6.2): 1H NMR (500 MHz, CDCl3, 25 oC; TMS): δ = 3.74 (s, 3H; CH3), 3.77 (s, 1H; N-H), 4.28 (s, 2H; CH2), 6.60-6.62 (m, 2H; Ar-H), 6.78-6.79 (m, 2H; Ar-H), 7.28-7.37 ppm (m, 5H; Ar-H). 13C NMR (126 MHz, CDCl3, 25oC; CDCl3): δ = 49.27, 55.83, 114.14, 114.97, 127.17, 127.55, 128.60, 139.75, 142.52, 152.26 ppm. FT-IR (KBr): νmax = 3392, 3060, 3028, 2998, 2906, 2833, 1624, 1512, 1245, 1034, 820, 742, 694 cm-1.MS (EI): m/z (%) 212 [M-H]+ (100), 122 (53), 91 (47), 195 (43), 167 (18). 116 N-benzyl-4-chloroaniline (entry 4, Table 6.2): 1H NMR (500 MHz, CDCl3, 25 oC; TMS): δ = 4.04 (s, 1H; N-H), 4.30 (s, 2H; CH2), 6.54-6.57 (m, 2H; Ar-H), 7.09-7.10 (m, 2H; Ar-H), 7.27-7.34 ppm (m, 5H; Ar-H). 13 C NMR (126 MHz, CDCl3, 25oC; CDCl3): δ = 48.36, 113.93, 122.13, 127.37, 127.41, 128.70, 129.07, 138.96, 146.67 ppm. FT-IR (KBr): νmax = 3427, 3062, 3028, 2922, 2852, 1600, 1500, 1321, 1177, 815, 733, 698 cm-1. MS (EI): m/z (%) 216 [M-H]+ (82), 91 (100), 65 (9), 139 (9). N-benzyl-4-nitroaniline (entry 5, Table 6.2): 1H NMR (500 MHz, CDCl3, 25 oC; TMS): δ = 4.41 (s, 2H; CH2), 4.86 (s, 1H; N-H), 6.54-6.56 (m, 2H; Ar-H), 7.31-7.35 (m, 5H; Ar-H), 8.05-8.07 ppm (m, 2H; Ar-H). 13 C NMR (126 MHz, CDCl3, 25oC; CDCl3): δ = 47.70, 111.34, 113.238, 126.37, 127.35, 127.87, 128.96, 137.38, 153.04 ppm. FT-IR (KBr): νmax = 3373, 2929, 1605, 1519, 740 cm-1. MS (EI): m/z (%) 227 [M-H]+ (100), 106 (40), 89 (24), 181 (21), 77 (19). Dibenzylamine (entry 6, Table 6.2): 1H NMR (500 MHz, CDCl3, 25 oC; TMS): δ = 1.62 (s, 1H; NH), 3.80 (m, 4H; CH2), 7.25-7.33 ppm (m, 10H; Ar-H). 13C NMR (126 MHz, CDCl3, 25oC; CDCl3): δ = 53.18, 58.72, 126.94, 128.15, 128.39, 128.80, 128.97, 129.58, 134.42, 140.35 ppm. FT-IR (KBr): νmax = 3308, 3195, 3062, 3027, 2920, 2837, 1495, 1454cm-1. MS (EI): m/z (%) 197 [M]+ (100), 91 (78), 120 (18), 65 (11). N-benzylpropan-1-amine (entry 7, Table 6.2): 1H NMR (500 MHz, CDCl3, 25 oC; TMS): δ = 0.90 (t, 3JHH = 7.41 Hz, 3H; CH3), 1.23 (s, 1H; NH), 1.49-1.53 (m, 2H; CH2), 2.58 (t, 3JHH = 7.24 Hz, 2H; CH2), 3.76 (s, 2H; CH2), 7.29-7.31 ppm (m, 5H; ArH). 13 C NMR (126 MHz, CDCl3, 25oC; CDCl3) : δ = 11.74, 23.14, 51.32, 54.00, 126.85, 128.10, 128.35, 129.11ppm. FT-IR (KBr): νmax = 3306, 3063, 3028, 2959, 2928, 2873, 2817, 1494, 1454 cm-1. MS (EI): m/z (%) 149 [M]+ (10), 91(100), 106 (5), 120 (60), 65 (15), 77 (5). N-(2-methylbenzyl)aniline (entry 8, Table 6.2): 1H NMR (500 MHz, CDCl3, 25 oC; TMS): δ = 2.41 (s, 3H; CH3), 3.83 (s, 1H; N-H), 4.30 (s, 2H; CH2), 6.67-6.77 (m, 5H; Ar-H), 7.23-7.37 ppm (m, 4H; Ar-H). 13C NMR (126 MHz, CDCl3, 25oC; CDCl3): δ = 18.96, 46.44, 112.76, 117.51, 126.21, 127.46, 128.30, 129.32, 130.37, 136.37, 137.08, 117 148.37 ppm. FT-IR (KBr): νmax = 3416, 3050, 3019, 2969, 2919, 2859, 1602, 1505, 1332, 747cm-1. MS (EI): m/z (%) 197 [M]+ (50), 105 (100), 77 (40), 93 (20) N-(3-methylbenzyl)aniline (entry 9, Table 6.2): 1H NMR (500 MHz, CDCl3, 25 oC; TMS): δ = 2.38 (s, 3H; CH3), 3.99 (s, 1H; N-H), 4.31 (s, 2H; CH2), 6.67-6.75 (m, 3H; Ar-H), 7.12-7.26 ppm (m, 6H; Ar-H). 13C NMR (126 MHz, CDCl3, 25oC; CDCl3): δ = 21.46, 48.40, 112.89, 117.56, 124.63, 128.03, 128.33, 128.57, 129.29, 138.45, 139.45, 148.31ppm. FT-IR (KBr): νmax = 3418, 3050, 3021, 2919, 2860, 1602, 1505, 1323, 749cm-1. MS (EI): m/z (%) 197 [M]+ (50), 105 (100), 77 (40), 93 (30). N-(4-methylbenzyl)aniline (entry 10, Table 6.2): 1H NMR (500 MHz, CDCl3, 25 oC; TMS): δ = 2.36 (s, 3H; CH3), 3.96 (s, 1H; N-H), 4.30 (s, 2H; CH2), 6.64-6.73 (m, 3H; Ar-H), 7.17-7.27 ppm (m, 6H; Ar-H). 13C NMR (126 MHz, CDCl3, 25oC; CDCl3): δ = 21.10, 48.11, 112.87, 117.51, 127.53, 129.26, 129.32, 136.41, 136.87, 148.27 ppm. FT-IR (KBr): νmax = 3419, 3049, 3020, 2920, 2860, 1603, 1505, 1325, 1266, 806, 748 cm-1. MS (EI): m/z (%) 196 [M-H]+ (85), 105 (100), 77 (18). N-(4-methoxybenzyl)aniline (entry 11, Table 6.2): 1H NMR (500 MHz, CDCl3, 25 o C; TMS): δ = 3.81 (s, 3H; CH3), 3.93 (s, 1H; N-H), 4.26 (s, 2H; CH2), 6.65-6.90 (m, 5H; Ar-H), 7.19-7.30 ppm (m, 4H; Ar-H). 13C NMR (126 MHz, CDCl3, 25oC; CDCl3): δ = 47.83, 55.31, 112.89, 114.89, 117.53, 128.82, 129.27, 131.49, 148.27, 158.92 ppm. FT-IR (KBr): νmax = 3398, 3047, 2962, 2836, 1604, 1514, 1425, 1302, 1253, 1175, 1034, 818, 748, 694cm-1. MS (EI): m/z (%) 212 [M-H]+ (50), 121 (100), 77 (13). N-(4-chlorobenzyl)aniline (entry 12, Table 6.2): 1H NMR (500 MHz, CDCl3, 25 oC; TMS): δ = 4.02 (s, 1H; N-H), 4.30 (s, 2H; CH2), 6.60-6.73 (m, 3H; Ar-H), 7.17-7.30 ppm (m, 6H; Ar-H). 13C NMR (126 MHz, CDCl3, 25oC; CDCl3): δ = 47.63, 112.91, 117.82, 128.69, 128.75, 129.29, 132.89, 138.00, 147.85 ppm. FT-IR(KBr): νmax = 3419, 3052, 3022, 2923, 2852, 1701, 1603, 1088, 1014, 817, 750, 692cm-1. MS (EI): m/z (%): 216 [M-H]+ (98), 125 (100), 90 (17), 77 (13), 106 (10), 181 (13). N-cinnamylaniline (entry 13, Table 6.2): 1H NMR (500 MHz, CDCl3, 25 oC; TMS): δ = 3.83(s, 1H; NH), 3.95 (s, 2H; CH2), 6.33-6.36 (m, 1H; CH), 6.70 (d, 3JHH = 15.80 Hz, 1H; CH), 6,74-7.76 ppm (m, 3H; ArH) 7.21-7.39 ppm (m, 7H; ArH). 13 C NMR 118 (126 MHz, CDCl3, 25oC; CDCl3): δ = 46.24, 113.10, 117.60, 126.37, 127.12, 127.56, 128.60, 129.31, 131.56, 136.93, 148.10 ppm. FT-IR (KBr): νmax = 3414, 3052, 3023, 2917, 2834, 1602, 1504, 1322, 966, 748, 692cm-1. MS (EI): m/z (%) 208 [M-H]+ (60), 117 (100), 91 (20), 77 (19). N-cinnamyl-4-methylaniline (entry 14, Table 6.2): 1H NMR (500 MHz, CDCl3, 25 o C; TMS): δ = 2.23 (s, 3H; CH3), 3.68 (s, 1H; NH), 3.90 (s, 2H; CH2), 6.32-6.34 (m, 1H; CH), 6.57-6.59 ppm (m, 3H; ArH) 7.00-7.36 ppm (m, 7H; ArH). 13C NMR (126 MHz, CDCl3, 25oC; CDCl3): δ = 20.36, 46.61, 113.28, 126.32, 126.88, 127.34, 127.46, 128.54, 129.75, 131.42, 136.96, 145.81 ppm. FT-IR (KBr): νmax = 3414, 3052, 3023, 2917, 2834, 1602, 1504, 1322, 966, 748, 692cm-1. MS (EI): m/z (%) 223 [M]+ (80), 117 (100), 91 (40), 77 (19). N-benzyl-3-phenylprop-2-en-1-amine (entry 15, Table 6.2): 1H NMR (500 MHz, CDCl3, 25 oC; TMS): δ = 1.55 (s, 1H; NH), 3.44 (s, 2H; CH2), 3.84 (s, 2H; CH2) 6.29-6.34 (m, 1H; CH), 6.53 (d, 3JHH = 15.80 Hz, 1H; CH), 7.21-7.36 ppm (m, 10H; ArH). 13C NMR (126 MHz, CDCl3, 25oC; CDCl3): δ = 51.22, 53.36, 126.28. 126.98, 127.34, 128.20, 128.44, 128.48, 128.54, 131.41, 137.18, 140.29 ppm. FT-IR (KBr): νmax = 3311, 3059, 3025, 2918, 2816, 1494, 1452, 966, 734 cm-1. MS (EI): m/z (%) 223 [M]+ (40), 117 (100), 91 (20), 77 (19). 3-phenyl-N-propylprop-2-en-1-amine (entry 16, Table 6.2): 1H NMR (500 MHz, CDCl3, 25 oC; TMS): δ = 0.92 (t, 3JHH = 7.40 Hz, 3H; CH3), 1.48-1.56 (m, 2H; CH2), 1.75 (s, 1H; NH), 2.60 (t, 3JHH = 7.24 Hz, 2H; CH2) 3.38-3.40 (m, 2H; CH2), 6.26-6.31 (m, 1H; CH), 6.51 (d, 3JHH = 15.87 Hz, 1H; CH), 7.18-7.35 ppm (m, 5H; ArH). 13 C NMR (126 MHz, CDCl3, 25oC; CDCl3): δ = 11.76, 23.20, 51.35, 51.87, 126.24, 127.29, 128.50, 128.59, 131.18, 137.17 ppm. FT-IR (KBr): νmax = 3307, 3059, 3025, 2958, 2929, 2872, 2813, 1494, 1448, 966, 742 cm-1. MS (EI): m/z (%) 175[M]+ (20), 117 (100), 84 (25), 146 (20), 77 (5). N-(3-(furan-2-yl)-2-methylallyl)aniline (entry 17, Table 6.2):1H NMR (500 MHz, CDCl3, 25 oC; TMS): δ =2.03 (s, 3H; CH3), 3.80 (s, 2H; CH2), 3.98 (s, 1H; NH), 2.60 (t, 3JHH = 7.24 Hz, 2H; CH2), 6.22 (s, 1H; CH), 6.32-6.69 (m, 5H; ArH), 7.14-7.35 119 ppm (m, 3H; Furan-H). 13C NMR (126 MHz, CDCl3, 25oC; CDCl3) : δ = 16.78, 52.08, 108.42, 111.06, 112.86, 114.21, 117.50, 129.23, 134.67, 141.04, 148.13, 153.24 ppm. FT-IR (KBr): νmax = 3420, 3051, 3030, 2911, 2851, 1602, 1507, 1310, 1266 cm-1. MS (EI): m/z (%) 213 [M]+ (90), 121 (100), 93 (85), 198 (20), 77 (90). N-cyclohexylaniline (entry 18, Table 6.2): 1H NMR (500 MHz, CDCl3, 25 oC; TMS): δ = 1.10-2.05 (m, 10H; CH), 3.24 (s, 1H; CH), 3.48 (s, H; NH) 6.58-6.64 (m, 3H; ArH), 7.13 (m, 2H; ArH). 13 C NMR (126 MHz, CDCl3, 25oC; CDCl3): δ = 25.00, 25.94, 33.50, 51.69, 113.144, 116.82, 129. 23, 147.42 ppm. FT-IR (KBr): νmax = 3404, 3052, 3019, 2958, 2929, 2859, 1601 cm-1. MS (EI): m/z (%) 175 [M]+ (20), 132 (100), 106 (10), 93 (15), 77 (10). N-benzylcyclohexanamine (entry 19, Table 6.2): 1H NMR (500 MHz, CDCl3, 25 o C; TMS): δ = 1.10-1.89 (m, 13H; CH), 2.46 (s, 1H; NH), 3.79 (s, 2H; CH2), 7.32-7.29 (m, 5H; ArH). 13C NMR (126 MHz, CDCl3, 25oC; CDCl3): δ = 25.00, 26.19, 33.56, 51.04, 56.17, 126.75, 128.06, 128.36, 141.01ppm. FT-IR (KBr): νmax = 3404, 3052, 3019, 2958, 2929, 2859, 1601 cm-1. MS (EI): m/z (%) 189 [M]+ (30), 91 (100), 146 (90), 160 (10), 77 (1). N-(hexan-2-yl)aniline (entry 20, Table 6.2): 1H NMR (500 MHz, CDCl3, 25 oC; TMS): δ = 0.90-1.34 (m, 11H; CH), 3.44 (s, 1H; NH), 6.57-6.6 (m, 3H; ArH), 7.15 (m, 2H; ArH). 13C NMR (126 MHz, CDCl3, 25oC; CDCl3): δ =14.06, 20.80, 22.76, 28.36, 36.96, 48.46, 113.08, 116.75, 129.25, 147.76 ppm. FT-IR (KBr): νmax = 3404, 3052, 3019, 2958, 2929, 2859, 1601, 1505, 1318 cm-1. MS (EI): m/z (%) 177 [M]+ (20), 120 (100), 162 (10), 106 (5), 77 (10). N-benzylhexan-2-amine (entry 21, Table 6.2): 1H NMR (500 MHz, CDCl3, 25 oC; TMS): δ = 0.88-1.28 (m, 13H; CH), 2.62 (s, 1H; NH), 3.70-3.82 (m, 2H; CH2), 7.22-7.29 (m, 5H; ArH). 13C NMR (126 MHz, CDCl3, 25oC; CDCl3): δ =14.04, 20.29, 22.88, 28.19, 36.79, 51.39, 52.54, 126.77, 128.10, 128.35, 140.89 ppm. FT-IR (KBr): νmax = 3312, 3063, 3027, 2958, 2957, 3858, 1454, 1376 cm-1. MS (EI): m/z (%) 191 [M]+ (5), 117 (100), 84 (80), 91 (70), 175 (60). 120 Chapter 7. Conclusion and future work 7.1 Conclusion The motivation of this research is to study the properties of AB and MABs in organic reductions. Therefore, the objectives are to utilize AB and MABs in reducing typical organic functional groups, to examine reactivities of these materials toward reduction, and to investigate the reduction mechanism. In the first part of this study, AB was found to possess high reactivities in reducing aldehydes at ambient temperature and in reducing ketones at 65 oC. Based on the in-situ FT-IR and NMR measurements, we found that not only the hydridic hydrogens of AB transferred to carbonyl groups, but also the protic hydrogens of AB participated in reaction. This finding provides a new perspective in defining the role of AB in organic reduction. In 1980, AB was first reported as a reducing reagent but only contributed its hydridic hydrogen in the reduction.[103] The reduction was via a two-step process including hydroboration and the follow-up hydrolysis or solvolysis. However, our experimental results challenge such a commonly accepted explanation in that AB is not only a hydride transfer agent but also a double hydrogen transfer agent. In order to understand the mechanism on how AB transfers two different hydrogens to unsaturated functional groups, kinetic study and DFT calculations have been carried out. Those results show that 1) the reaction between AB and carbonyl obeys a second-order rate law, being first order of each reactant; 2) the dissociations of both N-H and B-H bonds are involved in the rate determining step; 3) concerted double-H-transfer pathway is more kinetic favorable than step-wised pathway and 121 agrees with the kinetic results. Therefore, it should be the dominant path in the reduction. The simulation results are similar to the pathway proposed by Berke et al on AB reducing imines.[164] However, there are several limitations in this part of study. Firstly, only aldehydes and ketones were utilized as substrates to react with AB. Other unsaturated functional groups such as ester and amides were not considered in this thesis. It should be noted that this is not a critical issue since the results of reducing other unsaturated functional groups can be deduced from the present results. The reactions between AB and aldehydes or ketones are simpler than reactions of reducing esters or amides. Therefore, the simulation results of those reactions may be more accurate. A second limitation is that the difference of energy barrier between concerted and step-wised pathway is only 3.1 kcal/mol. Therefore, the dominated pathway cannot be clearly distinguished. However, the limitation is also not a critical one because both pathways show that double hydrogen transfer procedure is applicable. The overall process may be the combination of both pathways. In the second part of this study MABs, including LiAB, NaAB and CaAB, were utilized to react with compounds of unsaturated functional groups. It was found that MABs had higher reactivity toward unsaturated functional groups than AB: carbonyl compounds and imines can be reduced by MABs within 1hr at ambient temperature. Such a high reactivity can be attributed to the weaker B-H bond in MABs than that in AB. Moreover, the protic hydrogens of MABs participated in the reduction and transferred to unsaturated functional groups as evidenced by in situ FT-IR and NMR characterizations. This finding is significant because MABs are regarded as novel 122 hydrogen storage materials recently due to their high hydrogen contents. Few literatures on their reducing reactivity were reported. Therefore, this work provides a new perspective in the application of MABs in organic reduction. In order to understand the mechanism of how MABs transfer two different hydrogens to unsaturated functional groups, kinetics study and DFT calculations were also carried out. In addition, LiAB was used as representative of MABs. These results show that 1) the reaction between LiAB and carbonyl or imines obeys a first-order rate law, being first order of LiAB；2) the rate-determining step of reduction is the elimination of LiH from LiAB followed by the transfer of H(Li) to C site of unsaturated bond.[168] In addition, MABs were also found to be highly chemoselective reagents for the reduction of -unsaturated ketones to allylic alcohols and to be reducing reagents for reductive amination. These two applications evidence that MABs are attractive reagents for organic reductions. However, it should be pointed out that there are still some limitations in this part of work. Firstly, the MABs studied in this thesis are restricted to LiAB, NaAB and CaAB. Other MAB such as KAB and YAB are excluded. It should be noted that this is not a critical issue since LiAB, NaAB and CaAB are three representatives for MABs in hydrogen storage research and these three compounds are stable. The second limitation is that solid residues of MABs after reaction are unknown. The reason is that those residues are amorphous, insoluble in most aprotic solvents and sensitive to air and moisture. Therefore, the products are difficult to be characterized by XRD and/or NMR. 123 7.2 Future work There are several interesting directions for future work and applications in areas of research presented in this thesis: One possible avenue for future work is to extend the application of MABs in other organic reductions. Since MABs are demonstrated to be strong reducing reagents in this study, they may be used to reduce other organic unsaturated functional groups such as olefin, nitrile, amide and ester. The research on using borohydrides in those reductions has been carried out over one century. Therefore, the future work on application of MABs in those reductions should be feasible and straightforward based on the previous experiences. In addition, the instability of MABs should be taken into consideration in the experiments. Another interesting area for future research is to utilize AB and MABs as hydrogen donor in transfer hydrogen reaction. Generally speaking, there are three commonly used hydrogen donors: 2-propanol, formic acid and its salts, and Hantzsch ester. Although they are stable and inexpensive, they transfer double hydrogen under vigorous condition or with the aid of catalysts. However, AB and MABs can release hydrogen without any catalyst at temperature below 100 oC. Therefore, these two materials may be good alternatives for tradition hydrogen donors in transfer hydrogen reaction. 124 Reference [1] H. C. Brown, H. I. Schlesinger and A. B. Burg, Journal of the American Chemical Society 1939, 61, 673. [2] R. F. Nystrom and W. G. Brown, Journal of the American Chemical Society 1947, 69, 1197. [3] A. Staubitz, A. Robertson, M. Sloan and I. Manners, Chemical Reviews 2010, 110, 4023. [4] R. Hutchins, K. 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Clark, Tetrahedron Letters 2010, 51, 3167. 131 [...]... for the application of new borohydrides in organic reductions In the following sections of this chapter, the traditional methods in organic reduction , the applications of various typical borohydrides in reducing reactions and its corresponding reaction mechanisms, and the developments & applications of AB and MABs in hydrogen storage research will be reviewed 1.1 Review on methods for organic reduction. .. Dissolving Metal Reduction Dissolving metal reductions is one of the first reductions of organic compounds discovered hundred years ago.[17-19] This reduction is defined as acceptance of electrons The reaction of reducing carbonyl is illustrated in scheme 1.1 as an example to explain the mechanism[20-21]: when a metal is dissolved in a solvent such as liquid ammonia, it gives away electrons and becomes... the reduction of -unsaturated ketones to allylic alcohols and reducing agents for reductive amination These two applications provide strong evidences that MABs are promising candidates for organic reduction In conclusion, this study has achieved a ready entry to investigate the reducing capabilities of AB and MABs in organic reaction The results of this thesis may provide guidelines for utilizing... but reduction occurs rapidly in the presence of trimethyl borate.[86] 1.2.3 Amine borane In 1937, the first amine borane, Me3N-BH3, was reported by Schlesinger and his 19 co-workers[87] This complex was formed by the direct reaction of trimethylamine and diborane (scheme 1.20) This initial discovery paved an innovative way to synthesize numerous amine boranes by treating primary, secondary, and. .. amine with diborane.[88] In general, stable amine borane complexes will form if the pKa of the amine is above 5.0-5.5.[4] This means that ammonia and nearly all aliphatic amines form stable complexes with BH3 The major exceptions are branched chain tertiary amines, such as tri-isobutylamine, where steric hindrance of the alkyl groups prevents stable bonding.[4] Amine boranes are capable of reducing... because of their high solubility in organic solvents and reduced sensitivity to acid.[89-90] Furthermore, the reducing ability of amine borane is greatly dependent on the base strength of the amine moiety: the lower the pKa of the amine, the stronger the reducing agent.[91] For example, in aliphatic amine boranes, the reducing capabilities decrease in the order of NH3BH3> RNH2BH3> R2NHBH3> R3NBH3.[3] In. .. R3NBH3.[3] In addition, the activity of amine borane is always enhanced under acidic conditions.[3] Applications of amine boranes in reducing various functional group are discussed below 2 Me 3N + B2 H 6 2 Me3 N BH 3 Scheme 1.20 Formation of Me3N-BH3 by the direct reaction of trimethylamine and diborane 1.2.3.1 Reducing olefins to organoboranes The use of amine borane has attracted considerable attention... high solubility in a series of organic solvents and low sensitivity to acid.[3] Therefore, amine boranes are widely utilized in reducing reaction Related works have been systematically reviewed by Hutchins and his co-workers in 1984.[4] In addition, with the recent rapid development of hydrogen storage research, many researchers show their keen interests in amine boranes, such as ammonia borane (NH3BH3,...participate in the reduction and transfer to the unsaturated functional groups In addition, kinetic study and DFT calculations reveal that the reaction between MAB and carbonyl or imines obeys a first-order rate law, being first order of MAB The rate-determining step of reduction is the elimination of MH from MAB followed by the transfer of H(M) to C site of unsaturated bond MABs are... AB for short)[5], and cationic modified amine boranes, such as metal amidoborane (M(NH2BH3)n, or MAB for short) due to their high hydrogen capacities and low hydrogen releasing temperatures.[6] However, the research on AB and MABs is somehow limited in 1 hydrogen storage field Therefore, it would be an interesting topic to investigate the properities of AB and MAB in reducing organic compounds, . APPLICATION OF AMMONIA BORANE AND METAL AMIDOBORANES IN ORGANIC REDUCTION XU WEILIANG (B.Sci., Soochow University) A THESIS SUBMITTED FOR THE DEGREE OF DOCTOR OF PHILOSOPHY. methods in organic reduction , the applications of various typical borohydrides in reducing reactions and its corresponding reaction mechanisms, and the developments & applications of AB and. Dissolving Metal Reduction Dissolving metal reductions is one of the first reductions of organic compounds discovered hundred years ago. [17-19] This reduction is defined as acceptance of electrons.