Hydrosilylation of Imines and Nitrones

Although there has been little research into the hydrosilylation of functionality other than carbonyls (and alkenes), what work has been published provides some exceptionally good examples of enantioselective catalysis.  

Some recent work has been directed towards the use of organocatalysts, in particular Lewis basic pipecolinic formamides, in the asymmetric hydrosilylation of N-arylimines. These catalysts function by activating the silane and exhibit broad substrate scope. For example formamide (3.186) effects enantioselective hydrosilylation of aryl-derived ketimines along with isopropyl-substituted imine (3.187) and a,P-unsaturated imine (3.188). 

Imines derived from aryl/alkyl ketones can be reduced enantioselectively using the CuH-based catalytic system developed by Lipshutz. In general, high ees can be obtained (94—99% ee) using this procedure, but it is limited to the hydrosilylation of N-dixylylphosphinylimines such as (3.189). 

The hydrosilylation of nitrones provides a route to N,N-disubstituted hydroxy-lamines. The use of an Ru/ToIBINAP complex was reported to give the best results in the asymmetric hydrosilylation of nitrones. Nitrone (3.198) was converted into hydroxylamine (3.199) in reasonable yield and good enantioselectivity. 

For reviews see (a) R. Noyori, Asymmetric Catalysis in Organic Synthesis, John Wiley 8c Sons, Inc., New York, 1994, 56. (b) T. Ohkuma and R. Noyori, in Comprehensive Asymmetric Catalysis, Vol. 1, ed. E. N. Jacobsen, A. Pfaltz and H. Yamamoto, Springer-Verlag, Berlin, 1999,199. 

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