Enantioselective Synthesis of 2,3-Disubstituted Azetidines via Copper-Catalyzed Boryl Allylation of Azetines

Abstract

Disclosed here is a highly enantioselective difunctionalization of azetines for convenient access to chiral 2,3-disubstituted azetidines, a family of important scaffolds previously lacking general access. With Cu/bisphosphine as a catalyst, two versatile functionalities (boryl and allyl) were installed on azetine with concomitant construction of two new stereogenic centers. This represents a rare demonstration of Cu-catalyzed asymmetric boryl alkylation of electron-rich olefins and C═C bonds in strained heterocycles. The use of allyl phosphates proved critical not only to overcome the low reactivity of the borylated alkylcuprate intermediate toward alkylation but also to avoid competing side reactions. Remarkably, in almost all cases, single isomers were obtained with complete regio-, enantio-, and diastereoselectivies on the azetidine motif as well as excellent control on the double bond configuration. The mild conditions exhibited outstanding functional group compatibility and chemoselectivity. The versatile boryl and allyl functionalities allowed for easy transformations of the products to other useful chiral azetidines previously lacking straightforward access. Control experiments and kinetic studies indicated that the reaction proceeds by a fast boryl cupration of azetine followed by rate-determining allylation via an intrinsically controlled S_N2′ pathway.