Homogeneous deuteriation of alkenes using [RhX(PR3)3](X = halide, R = aryl) catalysts

Abstract

Catalytic deuteriation of cycloalkenes using [RhX(PR₃)₃](X = halide, R = aryl) catalysts frequently gives rise to some polydeuteriation and scrambled addition of deuterium to the cycloalkene substratrate. It is shown that these undesirable side reactions can be minimized by using small, electronegative aniono ligands or electron-withdrawing triarylphosphine ligands. Both these ligands inhibit the decomposition of the 16-electron intermediate rhodium(III) alkyl complex [RhD(X)(alkyl)(PR₃)₂]. Its decomposition by β-hydride abstraction brings about both polydeuteriation and scrambled addition of deuterium to the alkene. Acyclic, alk-1-enes give the best yields of [²H₂] products when [Rhl(PPh₃)₃] is the catalyst, since the formation of the less stable 2-alkyl intermediate is sterically inhibited.