Hydroboration. 82. Asymmetric Hydroboration of Representative Cis Disubstituted and Heterocyclic Olefins with Dicaranylboranes of High Enantiomeric Purity

Article abstract of DOI:10.1021/jo00248a004

Dicaranylboranes (2-^dIcr2BH and 4-^dIcr2BH) are prepared by the reaction of borane-methyl sulfide with (+)-2- and (+)-3-carene, respectively.The reagents readily hydroborate prochiral cis disubstituted olefins to yield the corresponding trialkylboranes, which, upon oxidation, give chiral alcohols of 77-93percent ee (2-^dIcr2BH) and 36-75percent ee (4-^dIcr2BH). 2-^dIcr2BH and 4-^dIcr2BH convert cis-alkenes into alcohols of the opposite absolute configurations.The trialkylborane hydroboration products, upon treatment with benzaldehyde, eliminate (+)-2- or (+)-3-carene, providing the corresponding benzyl boronates.Thus, the chiral auxiliary is recovered, and the alcohols can be obtained by oxidation of the benzyl boronates or alternatively by their hydrolysis and oxidation of the boronic acids.Trans disubstituted and trisubstituted olefins are hydroborated by both reagents with partial elimination of (+)-2- or (+)-3-carene and low asymmetric induction.On the other hand, the hydroboration-oxidation of five-membered ring heterocyclic olefins proceeds rapidly to give the corresponding chiral alcohols of 11-85percent ee.Convenient procedures for the preparation of high purity (+)-2- and (+)-3-carene are described.