Gold-Catalyzed Functionalization of Unactivated C(sp~3)-H Bonds by Hydride Transfer Facilitated by Alkynylspirocyclopropanes

摘要

Transition-metal-catalyzed coupling reactions involving unactivated C(sp~3)-H bonds constitute one of the most active research areas in both industry and academia. In particular, the development of methods that enable C-C bond formations through the selective cleavage of ubiquitous C(sp~3)-H bonds is highly desirable as it allows for new retrosynthetic disconnections, thus minimizing the number of steps and the generation of waste.A major drawback of this transformation lies in the control of the selectivity. In general, selective functionalizations are achieved by the assistance of directing groups that bring the catalyst closer to the desired C—H bond. Alternatively, good control of the selectivity has been reported by the intramolecular thermal or Lewis acid catalyzed 1,5-hydride transfer reaction (HT). In contrast to hydride transfer to electron-poor alkenes, the use of electronically unbiased alkynes as acceptor partners requires the assistance of platinum or gold catalysts. Thus, a variety of gold- or platinum-catalyzed cleavages of C(sp~3)—H bonds through intramolecular HT to alkynes have been described recently by the groups of Sames, Gagosz, arid others. As a carbocation is initially generated, this strategy has been limited to C(sp~3)—H bonds bearing a stabilizing functionality at the a position (OR, NR2, aryl; Scheme la). Considering this limitation, hydride transfer to alkynes from aliphatic, nonbenzylic, positions constitutes a challenging transformation.