Comparative chemistry of eta(3)-oxaallyl and eta(3)-allyl rhodium(I) complexes in the hydrosilylation of cyclopropyl ketones: observation of an unprecedented rearrangement

Addition of eta(3)-oxaallyl bis(triphenylphosphine)rhodium(I) (1) or eta(3) -allyl bis(triphenylphosphine)rhodium(I) (2) to a mixture of cyclopropyl ph enyl ketone and triethylsilane promotes two competitive catalytic reactions . Complex 1 gives high yields of the silyl ether 13, while complex 2 favors an unprecedented cyclopropyl carbinyl ring-opening reaction providing good yields of enol silane (Z)- and (E)-14 (4.1 Z/E ratio). Studies of catalyst turnover provide evidence that two monomeric rhodium complexes account for the competing reactions. The cyclopropyl carbinyl catalysis occurs under a narrow reaction regime with a specific set of ketone substrates suitable f or the reaction. Chemicals such as triethylsilane activate the catalysis le ading to 13, while triphenylphosphine strongly inhibits the formation of 14 . A mechanism involving a polar transition state for the cyclopropyl carbin yl rearrangement is proposed. (C) 1999 Elsevier Science S.A. All rights res erved.