Reactions of a stable (phosphanyl)(silyl)carbene with aliphatic aldehydes:: [2+1] versus [2+2] addition to a carbonyl group

The reactions between the stable (phosphanyl)(silyl)carbene 1 and different aliphatic aldehydes have been investigated for the first time. Results reveal that two competitive processes take place. The kinetically most favorable one is a concerted [2+1] cycloaddition to carbonyl leading to oxiranes as single diastereomers, with the trans arrangement of the alkyl and phosphoranyl groups, as the major products. Simultaneously, a [2+2]-like addition provides short-lived oxaphosphetene intermediates that rapidly evolve to the corresponding olefins with E stereochemistry, which are the thermodynamically most stable compounds. DFT calculations establish the mechanism for both processes and a rationale for the observed diastereoselectivity. (C) Wiley-VCH Verlag GmbH & Co. KGaA, 69451 Weinheim, Germany, 2003.