Coupling via acceptorless dehydrogenation (AD), known as dehydrogenative coupling, is emerging as a powerful, atom-economic, and environmentally benign synthetic methodology for organic synthesis via C-C, C-N, and C-O couplings.1 Kempe et al.2 and Milstein et al.3 have recently advanced the dehydrogenative coupling methodology to synthesize pyrroles from secondary alcohols (e.g. 3 ) and b-amino alcohols (e.g. 4 ), using PNP-Ir (1 ) and PNN-Ru (2 ) pincer complexes, respectively. Continuing our interest in AD reactions and AD-based coupling reaction,4 we performed a DFT study to characterize the catalytic mechanism of these reactions.5 The study showed that, after precatalyst activation giving active 1A /2A , the transformation proceeds via the four stages: (stage 1) 1A /2A -catalyzed alcohol (3 ) dehydrogenation to give ketone (11 ), (stage 2) base-facilitated C-N coupling 11  and 4  to form an imine-alcohol intermediate (18 ), (stage 3) base-promoted cyclization of 18 ,and (stage 4) catalyst regeneration via H2 release from 1R/2R .For alcohol dehydrogenations, the bifunctional double hydrogen transfer pathway is more favorable than that via b-hydride elimination. Generally, proton transfer (H-transfer) shuttles facilitate various H-transfer processes in both systems. Notwithstanding, H-transfer shuttles play much more crucial role in PNP-Ir system than in PNN-Ru system. Without H-transfer shuttles, the key barriers up to 45.9 kcal/mol in PNP-Ir system are too high to be accessible, while the corresponding barriers (<32.0 kcal/mol) in PNN-Ru system are not unreachable. Another significant difference between the two systems is that the addition of alcohol to 1A  giving an alkoxo complex is endergonic by 8.1 kcal/mol, whereas the addition to 2A  is exergonic by 8.9 kcal/mol. The thermodynamic difference could be the main reason for PNP-Ir system requiring lower catalyst loading than PNN-Ru system. We discuss how the differences are resulted in terms of electronic and geometric structures of the catalysts and how to use the features in catalyst development.References:(1) (a) Gunanathan, C.; Milstein, D. Science 2013 , 341, 249. (b) Marr, A. C. Catal. Sci. Technol. 2012 , 2, 279. (c) Gunanathan, C.; Milstein, D. Acc. Chem. Res. 2011 , 44, 588. (d) Choi, J.; MacArthur, A. H.; Brookhart, M.; Goldman, A. S. Chem Rev. 2011 , 111, 1761. (e) Bower, J. F.; Krische, M. J. Top. Organomet. Chem. 2011 , 34, 107. (f) Watson, A. J.; Williams, J. M. Science 2010 , 329, 635. (g) Dobereiner, G. E.; Crabtree, R. H. Chem. Rev. 2009 , 110, 681.(2) Michlik, S.; Kempe, R. Nat. Chem. 2013 , 5, 140.(3) Srimani, D.; Ben-David, Y.; Milstein, D. Angew. Chem. Int. Ed. 2013 , 52, 4012.(4) (a) Li, H.; Lu, G.; Jiang, J.; Huang, F.; Wang, Z.-X. Organometallics 2011 , 30, 2349. (b) Li, H.; Jiang, J.; Lu, G.; Huang, F.; Wang, Z.-X. Organometallics 2011 , 30, 3131. (c) Li, H.; Wen, M.; Wang, Z. X. Inorg Chem 2012 , 51, 5716. (d) Li, H.; Wang, X.; Huang, F.; Lu, G.; Jiang, J.; Wang, Z.-X. Organometallics 2011 , 30, 5233. (e) (e) Li, H.; Wang, X.; Wen, M.; Wang, Z.-X. Eur. J. Inorg. Chem. 2012 , 2012, 5011. (f) Li, H.; Wang, Z. Sci. China-Chem. 2012 , 55, 1991.(5) Qu, S. L.; Dang, Y. F.; Song, C. Y. Wen, M.; Huang K.-W. Wang, Z.-X. J. Am. Chem. Soc. 2014 , DOI: 10.1021/ja411568a

Catalytic mechanisms of direct pyrrole synthesis via dehydrogenative coupling mediated by PNP-Ir or PNN-Ru pincer complexes: Crucial role of proton transfer shuttles in the PNP-Ir system

248th National Meeting (2014)

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