Reduction of C=N bonds

The first example of an asymmetric reduction of C=N bonds proceeding via DKR was reported in 2005 by Lassaletta et al. In this process, the transfer hydrogenation of 2-substituted bicyclic and monocyclic ketimines could be accomplished via DKR by using a HCO2H/TEA mixture as the hydrogen source and a chiral ruthenium complex including TsDPEN ligand,... 

Spindler F, Blaser HU (1998) Enantioselective reduction of C=N bonds and enamines with hydrogen. In Beller M, Bolm C (eds) Transition metals for organic synthesis, vol 2. Wiley-VCH, Weinheim, p 69... 

Many of the comments related to C=0 reduction in this section also apply to the homogeneous reductions of C=N bonds, a process for which heterogeneous methods predominate192. This section will therefore concentrate on recent developments and, in particular, on asymmetric transformations. Transfer hydrogenations of C=N bonds by homogeneous catalysts have been reported264. 

Many stereoselective reductions of C=N bonds have been reported, and in the case where the original chiral group is removable (i.e. a chiral auxiliary) this method may be... 

Inevitably there is some overlap with other chapters, particularly those dealing with reduction of C=N bonded groups (Chapters 1.2-1.8, this volume) and that on the reduction of heterocycles (Chapter 3.8, this volume). For completeness, most of the important reduction methods for the appropriate groups are also referred to here. An attempt has been made to cover the literature to late 1989 a few later references are also included. 

Enantiospecific reduction of C=N bonds is of interest for the synthesis of a-amino acids and derivatives such as amines. While nonenzymatic reductive amination has been known since 192711, only recently have enzymatic procedures to L-amino acids became established. The reduction can be achieved by different enzymes following different mechanisms, e.g. by pyridoxalphosphate (PLP)-dependent transaminases (E.C. 2.6.1, discussed in Chapter 12.7) or by amino acid dehydrogenases (E.C. 1.4.1) using NADH or NADPH as the cofactor. The synthetic usefulness of the transaminase reaction is diminished by the location of the equilibrium (Keq often is close... 

Hydrogenation and reduction of C=N bond. Chiral Bronsted acids possessing a bulky backbone such as VAPOL derivative 9 attract and hold imine molecules in the concave space, and this reasoning has led to successful development of a protocol for the s3mthesis of a-amino acid derivatives from imino precursors by transfer hydrogenation (from Hantzsch ester). ... 

Reduction of C=N bonds. A simple method for the synthesis of a-stannyl-amines containing a secondary amino group involves borohydride reduction of the corresponding imidoylstannanes. 3-Unsubstituted 4-isoxazolines are available by a reductive elimination of 3-thioisoxazolium salts with NaBH. ... 

Reduction of C=N bonds. A convenient preparation of camphorsultam involves... 

Group 4 metallocene complexes can also be used as catalysts in the reduction of C=N bonds. Willoughby and Buchwald employed the titanium-based Brintzinger catalyst (3.54) for the asymmetric reduction of imines. The catalyst is activated by reduction to what is assumed to be the titanium(III) hydride species (3.55). The best substrates for this catalyst are cyclic imines, which afford products with 95-98% ee. Various functional groups including alkenes, vinyl silanes, acetals and alcohols were not affected under the reaction conditions. For example, the imine (3.56) was reduced with excellent enantioselectivity, without reduction of the alkene moiety. 

The reduction of C=N bonds and immonium ions was suggested to be involved in the TV-alkylation of amines by CO + H2O with different rhodium catalysts like RhCU, [Rh(COD)Cl]2, RhCl(PPh3)3, and... 

Guizzetti S, Benaglia M (2010) Trichlorosilane-Mediated Stereoselective Reduction of C=N Bonds. Fur J Org Chem 2010 5529... 

Reductions of C-N Bonds. An ethanolic mixture of PMHS and Pd/C will reduce oximes (eq 18) to amines and reduc-tively open aziridines (eq 19). Either (a) PMHS plus catalytic w-butyltin tris(2-ethylhexanoate) (eq 20) or (b) PMHS plus ZnCl2 reduces imines. The PMHS/DBATO combination reduces azides, while PMHS/Ti(0-i-Pr)4 can be applied to reductive aminations (eq 21). Asymmetric imine reductions via chiral titanium complexes and PMHS are also viable, but very substrate dependent with nonaromatic imines working best (69-99% ee vs. 6-97% ee for aromatic imines). ... 

Reduction of C=N Bonds Catalyzed by Recoverable Lewis Base Catalysts... 

Especially in recent years, the interest of both academic and industrial research has significantly increased, focusing study on the discovery of new catalytic systems that are active under low hydrogen pressures and would be able to provide high-to-excellent enantioselectivities, extending in parallel also the scope of the more challenging substrates for the asymmetric reduction of C=N bonds. 

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