Theoretical Study of Thermal Isomerization of Silacyclobutene to Cyclopropene

Abstract

The reaction pathway and energetics for the conversion of silacyclobutene to cyclopropene, which plays a role as an intermediate in complicated thermal reactions of silacyclobutene, are discussed using B3LYP/6-31+G(d,p) level calculations. There are two reaction pathways in its thermal isomerization: (1) cyclopropene is formed via silabutadiene that is formed by the ring opening of silacyclobutene in a conrotatory process; (2) cyclopropene is directly formed by a 1,2-siloxyl shift in silacyclobutene. The first mechanism involves two-step processes. Two kinds of ring-opening reactions based on the Woodward−Hoffmann rule result in the formation of cis- and trans-silabutadienes that involve an SiC double bond. Calculated activation barriers for the formation of cis- and trans-silabutadienes are 36.5 and 21.8 kcal/mol, respectively. Thus, the trans form is energetically more favorable than the cis form in this ring-opening step. This result is in good agreement with experimental observations that the methanol adduct derived from the trans form is detected in the presence of methanol. However, the next transition state with respect to a 1,4-siloxyl shift takes place only in the cis form, due to the orientation of the siloxyl group. A calculated activation barrier for the 1,4-siloxyl shift in cis-silabutadiene is 17.2 kcal/mol. Thus, the first step is the rate-determining step in this two-step mechanism. The second mechanism involves a 1,2-siloxyl shift that leads to the direct formation of cyclopropene. The activation barrier of this reaction is calculated to be 40.0 kcal/mol. IRC calculations are performed to trace this 1,2-siloxyl shift in the second mechanism. Since the activation barrier of the direct formation of cyclopropene is energetically comparable to that of the ring-opening reaction toward the cis-silabutadiene, the B3LYP DFT calculations suggest that both reaction pathways are likely to occur under thermal conditions.