Mechanistic aspects of hydrogen addition during enantioselective rhodium-catalysed reduction of CC double bonds with formic acid/triethylamine or molecular hydrogen

Abstract

Deuterium labelling experiments reveal a remarkably different hydrogen addition mode during homogeneously catalysed CC bond reduction of itaconic acid derivatives 1a–d using molecular hydrogen (hydrogenation) or formic acid/triethylamine (transfer hydrogenation). The expected vicinal addition of two hydrogen atoms across the double bond prevails for all substrates in conventional hydrogenation, whereas the deuterium pattern depends largely on the nature of the carboxyl group in the β (or allylic) position during transfer hydrogenation. Vicinal addition is observed only in case of itaconic acid 1a and α-methylitaconate 1c, while 1,3-addition is preferred with dimethylitaconate 1b and β-methylitaconate 1d. Significant amounts of polydeuterated products are formed also during hydrogenation and transfer hydrogenation. Monitoring the deuterium pattern as a function of time reveals that deuterium scrambling is responsible for polydeuteration, but not for the change of the addition mode. The use of monodeuterated formic acid isotopomers shows that the incorporation from the hydridic formyl position occurs preferentially at the terminal end of the double bond (C-3) whereas the protic hydrogen is directed either in the higher substituted olefinic (C-2) or the methylene (C-1) position. Control experiments using mesaconic (2) and citraconic (3) acids demonstrate that double bond migration in 1a–d is negligible under the reaction conditions. These results are best rationalised on the basis of a common mechanism for hydrogenation and transfer hydrogenation that involves (i) the generation of Rh–H intermediates, (ii) reversible hydride transfer to coordinated substrate to form two isomeric σ-alkyl intermediates, and (iii) irreversible product liberation through protolytic Rh–C cleavage. The key intermediates are similar if not identical for hydrogenation and transfer hydrogenation. The change of the hydrogen transfer pattern can be explained on the basis of the relative rates of the individual steps within the catalytic cycle as compared to the rate of isomerisation of the σ-alkyl intermediates.