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

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 process. Finally, an experiment to
verify the [2 + 1] cycloaddition mechanism was proposed, in which
the use of triacetylene as a terminal alkyne would allow the key intermediate
in the pathway to be trapped.