Intermolecular Alkylboration of gem-Disubstituted Allenes for Stereoselective Synthesis of Multi-alkylated Allylic Boronates

Abstract

Allylic boronates have been recognized as essential building blocks in organic synthesis, as the carbon–boron bond can be stereospecifically transformed into carbon–carbon and carbon–heteroatom bonds at the γ- or α-position. Among the possible substitution patterns of multi-substituted allylic boronates, 2,3,3-all-carbon-trisubstituted allylic boronates that possess a stereodefined tetrasubstituted alkene moiety are attractive precursors of contiguous and densely substituted sp2- and sp3-carbon skeletons. However, the stereoselective synthesis of sterically congested tetrasubstituted alkene moiety in the trisubstituted allylic boronates, especially having four different substituents, is highly challenging. Here, I developed the copper(I)-catalyzed the alkylboration reaction of gem-dialkylallenes for the synthesis of differentially 2,3,3-trialkylsubstituted allylic boronates. The products were obtained in high yield with high regio- and stereoselectivity. Furthermore, a density functional theory (DFT)-based computational mechanistic study revealed that the π-coordination of the allenes to boryl copper(I) species and the following borylcupration is the key step for determining the regio- and stereoselectivity.