Transfer Hydrogenation of Imines and Alkenes and Direct Reductive Amination of Aldehydes Catalyzed by Triazole-Derived Iridium(I) Carbene Complexes Dinakar Gnanamgari, ² Audrey Moores, ² Edward Rajaseelan, ‡ and Robert H. Crabtree* ,² Departments of Chemistry, Yale UniVersity, 225 Prospect Street, P.O. Box 208107, New HaVen, Connecticut 06520-8107, and MillersVille UniVersity, P.O. Box 1002, MillersVille, PennsylVania 17551-0302 ReceiVed October 11, 2006 A series of new iridium(I) triazole-based NHC complexes [(cod)Ir(NHC)L]BF 4 (L ) PPh 3 , pyridine) were prepared and showed good activity for transfer hydrogenation on CdO, CdN, and CdC double bonds in 2-propanol with K 2 CO 3 . The phosphine series was shown to be more active than the pyridine series in the case of imine transfer hydrogenation. A neopentyl wingtip substituent on the NHC gave the best catalytic activity with the following competitive order: aldehyde > ketone > imine. In a substrate containing both aldehyde and ketone functionalities, only the aldehyde was reduced. Of great interest, the transfer hydrogenation of polarized and nonpolarized CdC bonds was also proved possible. In a useful organic synthetic application, direct, one-pot reductive amination of RCHO with R′NH 2 to give RCH 2 NHR′ was shown for a variety of cases. Introduction N-heterocyclic carbenes (NHCs) have sometimes been con- sidered alternatives to phosphines as spectator ligands in homogeneous catalysis and share with them the possibility of tuning catalyst activity by varying the substitution scheme of the ligand. 1-7 Steric tuning of NHCs is possible by changing the R 1 and R 2 substituents at nitrogen, while electronic properties are mainly governed by the nature of the azole ring. The recent work of Hermann et al. focuses on changing the azole and comparing the σ-donor ability of several NHC ligands. 8 Tria- zole-based NHCs (X ) N; Figure 1) appear to have an electron donor power that lies between that of the conventional imida- zole-2-ylidene (X ) CH) NHCs and typical phosphines. 1,2,4- Triazolium salts with various substitution patterns are very accessible through the easy functionalization of N-alkyltriazole with alkyl bromide (Scheme 1). Triazolylidene ligands, relatively little studied so far, are thus very promising for catalytic applications. We decided to try triazolylidene complexes of Ir- (I) for transfer hydrogenation of CdC, CdN, and CdO bonds, where they prove to be very active. Transfer hydrogenation of unsaturated bonds is a reaction of great interest. On CdO double bonds, it has been extensively studied, leading to important applications such as racemization 9 of chiral alcohols and asymmetric reduction. 10 The synthetic power of this method has been extended to the production of amines, a family of molecules of great current interest, especially in biochemistry and the pharmaceutical industry. 11 Synthesis of amines can be achieved by reduction of a previously synthesized imine or by a one-pot reductive procedure (reductive amination) directly from an aldehyde and an amine. In the latter case a common method is to use sodium cyanoborohydride as a stoichiometric reductant, because it is selective for imine reduction. 12 However, catalytic reduction is preferred for large- scale industrial use in the hope of developing a greener chemistry by reducing waste production and energy use and lowering toxicity. 13 Excellent examples of catalytic imine hydrogenation, particularly the asymmetric variant using H 2 as * To whom correspondence should be addressed. E-mail: robert.crabtree@yale.edu. ² Yale University. ‡ Millersville University. (1) Lappert, M. F. J. Organomet. Chem. 1988, 358, 185-214. (2) Arduengo, A. J.; Dias, H. V. R.; Harlow, R. L.; Kline, M. J. Am. Chem. Soc. 1992, 114, 5530-5534. (3) Herrmann, W. A.; Kocher, C. Angew. Chem., Int. Ed. 1997, 36, 2163-2187. (4) Bourissou, D.; Guerret, O.; Gabbai, F. P.; Bertrand, G. Chem. ReV. 2000, 100, 39-91. (5) Stauffer, S. R.; Lee, S. W.; Stambuli, J. P.; Hauck, S. I.; Hartwig, J. F. Org. Lett. 2000, 2, 1423-1426. (6) Sanford, M. S.; Love, J. A.; Grubbs, R. H. J. Am. Chem. Soc. 2001, 123, 6543-6554. (7) Scott, N. M.; Nolan, S. P. Eur. J. Inorg. Chem. 2005, 1815-1828. (8) Herrmann, W. A.; Schutz, J.; Frey, G. D.; Herdtweck, E. Organo- metallics 2006, 25, 2437-2448. (9) Yamaguchi, K.; Koike, T.; Kotani, M.; Matsushita, M.; Shinachi, S.; Mizuno, N. Chem. Eur. J. 2005, 11, 6574-6582. (10) Gladiali, S.; Alberico, E. Chem. Soc. ReV. 2006, 35, 226-236. (11) Kobayashi, S.; Ishitani, H. Chem. ReV. 1999, 99, 1069-1094. (12) Carey, F. A.; Sundberg, R. J. AdVanced Organic Chemistry, Part B: Reactions and Synthesis, 4th ed.; Kluwer Academic/Plenum Publish- ers: New York, 2001; p 965. (13) Anastas, P. T.; Kirchhoff, M. M.; Williamson, T. C. App. Catal. A: Gen. 2001, 221,3-13. Figure 1. General structure of ligands used in this work (X ) CH, N). Scheme 1. Synthesis of Triazolium Salts 1a-c 1226 Organometallics 2007, 26, 1226-1230 10.1021/om060938m CCC: $37.00 © 2007 American Chemical Society Publication on Web 01/26/2007