Hydrogenation metal-promoted imine

Moreover, the occurrence of such a Os-H —H-N interaction on the surface of a metallic cluster has prompted us to pursue the synthesis of a novel class of terminally bound imine derivatives.[44] In fact it has been shown that the reactions of (H)( i-H)Os3(CO),o(NH3) with several aldehydes in chloroform or methylene chloride readily afford the (H)(p-H)Os3(CO),o(HN=CHR) (R = Me, Ph, CH2Ph, i-Bu) derivatives, where the stabilization of the coordinated imine is promoted by the occurrence of the unconventional hydrogen bond described above (scheme 8). 

The mechanism of this hydrogenation has not been revealed in detail, but several aspects are noteworthy in the context of the mechanistic discussion earlier in this chapter. First, this reaction almost certainly occurs by the insertion of an imine into a metal hydride (see Chapter 9), rather than by the transfer of a hydride and a proton by a metal-ligand bifunctional system. This iridium catalyst does not contain a protic ligand. Second, acid and iodide are needed as promoters to obtain the fast rates. The iodide is thought to help stabilize an iridium(III) species and the acid is thought to help in the release of the amine product from the metal. 

Transfer hydrogenation is particularly good for the reduction of ketones and imines that are somewhat more difficult to reduce with Hj than are C—C bonds. BSckvall and co-workers have shown how RuCl fPPhj) is effective at 80°C with added base as catalyst promoter. The role of die base is no doubt to form the isopropoxide ion. which presumably coordinates to Ru and by elimination forms a hydride and acetone. Noyori and co-workers have has a remarkable asymmetric catalytic hydrogen transfer that goes without direct coordination of the C=0 bond to the metal. Instead, the metal donates a hydride to the C=0 carbon while the adjacent RU-NH2R group donates a proton to the C=0 oxygen. 

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