SUBSTRATE-CONTROLLED DIASTEREOSELECTIVITIES IN CATALYZED AND UNCATALYZED HYDROBORATIONS OF ACYCLIC ALLYLIC ALCOHOL DERIVATIVES - SECONDARY ORBITAL EFFECTS INVOLVING D-PI-P-PI INTERACTIONS

Rhodium-mediated hydroborations of allylic alcohol derivatives 1-9 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 delta*-orbitals involved in bonding at the asymmetric center with pi*-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 17-19 and Ph(C6F5)CHCM(e) = CH2 (21) (a model substrate with aromatic groups of a similar size but different electronic properties attached to the chiral center). All experimental observations described here for catalyzed hydroborations of chiral alkenes are consistent with the proposals outlined above.