Substrate-controlled diastereoselectivities in catalyzed and uncatalyzed hydroborations of acyclic allylic alcohol derivatives: secondary orbital effects involving d.pi.-p.pi. interactions

Abstract

Rhodium-mediated hydroborations of allylic alcohol derivatives followed by oxidation give predominantly syn-2-methyl 1,3-diols whereas conventional hydroborations of the same substrates afford mostly anti products. It is proposed that mixing of σ*-orbitals involved in bonding at the asymmetric center with π*-orbitals of the alkene lowers the LUMO involved in complexation of rhodium, and this could control diastereofacial selectivities in catalyzed hydroborations. Steric effects in catalyzed hydroborations are also discussed. The resulting hypotheses are tested with respect to catalyzed and uncatalyzed hydroborations of phenyl-substituted allylic alcohol derivatives and Ph(C 6 F 5 )CHCMe=CH 2