NHC-osmium-catalysed reactions

Osmium is the least studied element of the iron triad and its main use consists in the preparation of stable models for reactive intermediates involved in reactions catalysed by ruthenium analogues. Although NHC-Os compounds were first synthesised by Lappert in the late 19708, their catalytic properties were investigated only very recently. 

The first report of NHC-containing osmium compounds acting as catalysts came from Esteruelas and co-workers in 2005. Thus, cationic benzyli-dene complexes 44 were prepared by reaction of the corresponding 16-electron precursors [(NHC)OsCl(p-cymene)][OTf] (NHC = IMes or IPr) with phe-nyldiazomethane, and their potential as initiators for olefin metathesis was probed in the RCM of diethyl diallylmalonate, the ROMP of cyclooctene, and a variety of self- and cross-metathesis reactions (Equation (7.10)). Although they were not as efficient as standard ruthenium-benzylidene metathesis initiators, compounds 44 displayed, nevertheless, a fairly decent activity. More importantly, in addition to being the first NHC-Os catalytic application, this study constituted a rare example of osmium catalysed C-C bond formation. 

In a subsequent work, Castarlenas and Esteruelas disclosed the synthesis of another cationic complex [(IPr)Os(OH)(p-cymene)][OTf] (45), which was an efficient pre-catalyst for transfer hydrogenation (Equation (7.11)). Several aliphatic and aromatic aldehydes could be reduced in high yields and high turnover numbers, whereas under these same experimental conditions, acetophenone reacted only very slowly. Regarding the activation of the pre-catalyst, the authors provided strong NMR evidence that, in the presence of isopropanol, compound 45 was converted into a cationic acetone-hydride complex [(IPr)0s(H)(0=C(CH3)2)][0Tf], which most likely acted as the active species for transfer hydrogenation. 

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