Stereoselective rhodium-catalyzed reaction of allenes with organoboronic reagents for diversified branched 1,3-alkadienes.

Abstract

The terminal isoprene unit, as the simplest branched 1,3-diene unit, exists in a wide range of natural products and bioactive molecules. Herein, we report a stereoselective rhodium-catalyzed reaction of allenes with readily available methyl pinacol boronic ester, providing a straightforward approach to isoprene derivatives with a very high E-stereoselectivity. Its synthetic potential has been illustrated by a concise synthesis of natural product schinitrienin. Such a protocol can be easily extended to aryl and alkenyl boronic reagents affording 2-aryl or -alkenyl substituted 1,3-dienes, which are also of high importance in organic synthesis but remain challenging for their selective synthesis, with a remarkable stereoselectivity. A series of deuterium-labeling experiments indicate a unique mechanism, which involves reversible β-H elimination as well as hydrometalation and isomerization of the allylic rhodium species.