How borane reagents based on alpha-pinene control stereoselectivity in the reductions of carbonyl groups with different stereogenic elements. 3. The effect of the relative size and conformation of the carbonyl substituents on the stereoselectivity of the Ipc(2)BCl, Eap(2)BCl, B-t-Bu-IpcBCl, and B-t-Bu-EapBCl reagents. A semiempirical study

The reductions of a more elaborate prostereogenic nonchiral and chiral ketones with the representative borane reagents derived from (+)-alpha-pinene confirm that the overall stereoselectivity, similarly to the previously studied reductions of benzaldehyde and acetophenone, is also controlled, early along the reaction coordinate, by the structure of the borane reagent. The stereoselectivity reflects the energy differences among syn-1, 3-interactions of the smallest of the three boron substituents in the reacting borane conformation with the two carbonyl substituents in the substrate. For the B-alkyl-IpcBCl reagents and the R(S)-CO-R(L) substrates, these are the interactions of the B-Cl with the C-R(S) and C-R(L) bonds. To minimize developing intermolecular syn-1,3-interactions with the reagent's B-Cl bond during the reaction, in the conformationally mobile R(S)-CO-R(L) systems, the R(L) and R(S) groups seek to adopt a more reactive conformation even at the expense of the unfavorable intramolecular interactions.