Novel [2 + 1] Concerted Reaction Path for Disilacyclobutenes with Acetylene

Abstract

Thermal reactions of benzodisilacyclobutene (1) and disilacyclobutene (2) with acetylene were investigated theoretically. The reactions are thought to proceed via the conventional Diels–Alder reaction of disilabutadiene, the conrotatory ring-opening product of disilacyclobutene, with acetylene. However, this mechanism is incompatible with the observed similar reactivities of 1 and 2 with acetylene and the retention of stereochemistry during the reaction. In our previous paper, we proposed an alternative [2 + 1] cycloaddition pathway that involved the direct addition of acetylene to the Si–Si σ–bond of 1 without ring opening. In this study, we extensively investigated the reaction pathways for both 1 and 2 on a theoretical basis. We found that charge transfer (CT) played a key role in the [2 + 1] cycloaddition pathway. On the basis of natural bond orbital (NBO) analysis, the interaction of the Si–Si σ-bond orbital (donor) with the π* orbital (acceptor) of the acetylene was attributed mainly to the CT proce...