Stereochemistry of C7-allyl yohimbine explored by X-ray crystallography

X-ray crystallographic analysis revealed that the palladium-catalyzed β-allylation of yohimbine proceeded in a (7S)-selective manner. The crystal structure had an indolenine unit that was generally unstable in air. A stereoselective outcome was obtained when the palladium π-allyl complex approached yohimbine from the less-hindered pro-S side. However, during reserpine allylation—because the structure of reserpine is that of a transoid-3, 15-ring junction—the palladium π-allyl complex approached from both sides: pro-S and pro-R. A computational method was developed to discuss this selectivity. Experimental details and considerations of the reaction are provided.

► An indole in yohimbine was directly allylated by cat. Pd(0) into a 3H-indole.
► X-ray crystallographic analysis revealed the solid structure of (7R)-allyl yohimbine.
► Steric effects dominate stereoselective approach of the π-allyl Pd complex.