Strain-Release Controlled [4+2+1] Reaction of Cyclopropyl-Capped Diene-Ynes/Diene-enes and CO Catalyzed by Rhodium

ABSTRACT: Achieving transition metal-catalyzed reactions of diene-enes/diene-ynes and carbon monoxide (CO) to deliver [4+2+1] cycloadducts, rather than the kinetically favored [2+2+1] products, is challenging. Here, we report that this can be solved by adding a cyclopropyl (CP) cap to the diene moiety of the original substrates. The resulting cyclopropyl-capped diene-ynes/diene-enes can react with CO under Rh catalysis to give [4+2+1] cycloadducts exclusively without forming [2+2+1] products. This reaction has a broad scope and can be used to synthesize useful 5/7 bicycles with CP moiety. Of the same importance, the CP moiety in the [4+2+1] cycloadducts can act as the intermediate group for further transformations so that other challenging bicyclic 5/7 and tricyclic 5/7/5 and 5/7/6, 5/7/7 skeletons, which are widely found in natural products, can be obtained. The mechanism of this [4+2+1] reaction has been investigated by quantum chemistry calculations, and the role of the CP group in avoiding the possible side reaction of [2+2+1] reaction have been identified, showing that the [4+2+1] is controlled by releasing the ring strain in the methlenecyclopropyl (MCP) group (about 7 kcal/mol) in the CP-capped dienes.