Copper-Catalyzed Defluorinative Allylboration of Allenes with Trifluoromethyl Alkenes

Abstract

An N-heterocyclic carbene/Cu-catalyzed coupling of allenes, bis(pinacolato)diboron, and trifluoromethyl alkenes is reported. The method allows access to stereodefined borylated 1,1-difluoro-1,5-dienes with high levels of selectivity. The integration of a 1,5-diene scaffold with boron and fluorine functionalities makes these products versatile building blocks for the synthesis of structurally diverse and valuable organofluorine compounds such as gem-difluoroalkenes, difluoromethylene units, and alkenyl fluorides. Mechanistic studies and density functional theory calculations shed light on key mechanistic aspects of the catalytic process and suggest that the in situ generated LiCl is essential for the reaction by assisting the oxidative addition of the trifluoromethyl alkene to the catalytically generated allylcopper species. It is found that the polarization of the key C─F bond induced upon binding to LiCl results in a significant decrease of destabilizing Pauli repulsion, which is translated into a remarkable reduction of the activation barrier of the oxidative addition step.