Mechanistic insights on the bicycloaddition of conjugated dienes with internal alkynes catalyzed by the [CoBr2(dppp)]/Zn/ZnI2 system

Abstract A catalytic system of [CoBr2(diphosphine)]/Zn/ZnX2 is widely used in cobalt-catalyzed cycloaddition and hydrovinylation reactions, but the in situ-generated catalytic species largely remain unclear. This report provides a detailed mechanistic investigation of the [CoBr2(dppp)]/Zn/ZnI2 system [dppp: 1,3-bis(diphenylphosphino)propane] regarding the bicycloaddition of conjugated dienes with internal alkynes. During efforts to prepare new Co(I) species, we found an alternative synthesis of a cationic Co(I) species [Co(dppp)2]+[BArF]− by a single-electron reduction of [CoBr2(dppp)] with sodium naphthalene followed by the treatment of NaBArF. In quest of the active species generated by [CoBr2(diphosphine)]/Zn/ZnX2, UV-vis absorption spectroscopy, ³¹P{1H} NMR spectroscopy, and controlled experiments on the bicycloaddition revealed that Zn itself cannot work as a reductant, but that the presence of ZnI2 encourages the reduction of neutral Co(II) and Co(I) species. For the reduction of cationic Co(I) species, Zn/ZnI2 and the addition of a conjugated diene are indispensable. X-band EPR experiments of the reduced species from [CoBr2(diphosphine)]/Zn/ZnI2 show an air-sensitive and intense signal at g = 2.07 at 77 K, which is consistent with a Co(0) d9 (S = 1/2) complex. The overall study suggests that [CoBr2(diphosphine)]/Zn/ZnI2 generates a Co(0) species as an active catalyst for the bicycloaddition.