Syntheses of Unsymmetrical 1,4-Bifunctional Allylboron Reagents via Cu-catalyzed Highly Regio- and Stereoselective 1,4-Protoboration of Dienylboronates and Analysis of the Origin of Chemoselective Aldehyde syn-(Hydroxymethyl)allylation.

The syntheses of unsymmetrical 1,4-bifunctional allylboron reagents via Cu-catalyzed highly regio- and stereoselective 1,4-protoboration of dienylboronates were developed. The resulting allylboronates underwent chemoselective allylboration with aldehydes followed by oxidative workup to give diol products with high diastereoselectivity. Transition state analysis revealed that the disfavored transition states suffer from either a severe A1,3 allylic strain or 1,3- syn-pentane interactions. Minimization of such nonbonding 1,3- syn-pentane interactions is proposed to be the origin of observed chemoselectivity of the reaction.