Role of CO in Molybdenum-Catalysed Asymmetric Alkylations

Molybdenum-catalysed asymmetric alkylation of, for example, allylic carbonates by malonates has developed into a powerful synthetic method. It has recently been shown that unsymmetric ligands work as well as, or in some cases better than symmetric ligands (Trost B M et al, Angew Chem Int Ed, 2002, 41, 1929).

A group from Merck Process Research (Kiska S W et al, J Amer Chem Soc, 2002, 124, 12656) has found, in an elegant mechanistic study, that carbon monoxide is vital for catalyst turnover, which occurs via the mechanism shown below. Intermediate (X) was not active unless CO was added or Mo(CO)₆ was present giving the species (Y), which is the active catalyst.