Solvent-controlled stereodivergent synthesis of E- and Z-enamines via metal-free formal C(sp2)-H amination of α-substituted styrenes

Abstract

C(sp²)-H amination represents an attractive approach for the synthesis of enamines, which is intrinsically associated with the challenge of controlling of stereochemistry and primarily relying on transition-metal catalysis. Herein, a metal-free C(sp²)-H amination of α-substituted styrenes has been achieved, leading to stereodivergent formation of both E- and Z-enamines in 50 ​%–97 ​% yield under mild conditions by using PhI(OAc)₂ as a green oxidant and ortho-phenylenediamines as nitrogen source. Interestingly, the Z/E selectivity can be controlled readily by switching the reaction medium. E-isomers were formed preferentially in THF, whereas n-hexane favored the formation of Z-isomers. Preliminary mechanistic studies suggested that in situ formed ortho-benzoquinone diimides are the key intermediates, and there is a correlation between solvent polarity and stereoselectivity. This study enriches the chemical repertoire of ortho-benzoquinone diimides particularly with respect to sustainable amination.