Chiral N-Hydroxyalkyl Pyrid-2-Ylidenes: A New Class of Ligands for Copper-Catalyzed Asymmetric Allylic Alkylation

Abstract

A class of chiral N-heterocyclic carbenes derived from pyridine, namely N-hydroxyalkyl pyrid-2-ylidenes, was developed. Capitalizing the remarkable steric and electronic features of the pyrid-2-ylidene core with the presence of a chiral hydroxyalkyl-chelating arm on the nitrogen atom, these ligands demonstrated high performance in copper-catalyzed asymmetric allylic alkylation of dialkylzincs to various allylic or dienic phosphates with high γ-selectivity (>98%) and enantioselectivity (up to 95% ee). Importantly, the catalyst loading can be decreased to below 0.5 mol% without any loss of catalyst efficiency, thus outperforming N-hydroxyalkyl imidazoline-2-ylidene congeners. Moreover, thanks to the versatile post-transformation of the resulting enantioenriched skipped 1,4-dienes, various relevant building blocks were synthesized, notably a key intermediate in the total synthesis of (+)-Phorbasin C. Furthermore, by involving a transient oxazolidine, which acts as a masked carbene before the insertion of the metal center, a well-defined but air-sensitive N-hydroxyalkyl pyrid-2-ylidene copper(I) chloride complex was isolated. Deuteration experiments and computational studies provided valuable insights into the formation of the oxazolidine and the corresponding copper complex.