A ligand-enabled, copper(ii)-catalyzed, highly selective and efficient synthesis of 2E-alkenylfurans from ynenones

Abstract

We describe a ligand-enabled Cu(II)-catalyzed approach for highly selective synthesis of 2E-alkenylfurans 2 from ynenones under an air atmosphere. In this reaction, the amide group on our designed 1,10-phenanthroline framework had a key role in promoting the α-H internal 1,2-migration to Cu(II) carbene species and stabilizing the in situ-generated carbocation. This approach prevented the external oxygen participating to oxidize α-furyl copper carbene rapidly to 2-acylfuran. With the bifunctional ligand L1, the Cu(II) catalyst became highly efficient at catalyzing ynenones selective to 2E-alkenylfurans 2 under an air atmosphere at room temperature in excellent yields. Moreover, the ligand could lower the Cu(II) catalyst loading dramatically to ppm-level at a higher temperature and concentration, with turnover numbers of the Cu(II) catalyst up to 9300. This economical and practical approach with good application was mild and environmentally friendly.