Unprecedented endo-oxidative cyclometallation and [4 + 3] cycloaddition of diene-vinylcyclopropanes

The traditional exo-oxidative cyclometallation (exo-OCM) is a key step in transition-metal-catalyzed reactions. Here, for the first time, we propose an endo-oxidative cyclometallation (endo-OCM) reaction as a new reaction mode. Realization of endo-OCM will add a new concept to chemistry and help the future design of endo-[m + n], [m + n + o] cycloadditions and other reactions. We report here that endo-OCM exists in a newly developed Rh(I)-catalyzed intramolecular [4 + 3] cycloaddition of Z-diene-vinylcyclopropanes (Z-diene-VCPs), which provide an efficient way to access challenging 5/7-fused bicyclic skeletons with a bridgehead ethyl substituent. In this [4 + 3] reaction, the Z-diene serves as a 4-carbon synthon, while the VCP acts as an unprecedented 3-carbon synthon (two carbon from the vinyl group and one carbon from the cyclopropyl group). Quantum chemical calculations have been carried out to analyze factors affecting the competition of exo- and endo-OCMs, and to understand why [3 + 2] and expected [4 + 3] reactions of Z-diene-VCPs do not happen.