Theoretical Study on the Formation of Silacyclopropene from Acylsilane and Acetylene via Silene-to-Silylene Rearrangement

Density functional theory calculations have been performed for a proposal of possible mechanisms of the thermal reaction of an acylsilane, pivaloyltris-(trimethylsilyl)silane, and bis(trimethylsilyl)acetylene yielding a silacyclopropene, 1-[(tert-butyl)bis(trimethylsilyl)ethyl]-1-trimethylsiloxy-2,3-bis(trimethylsilyl)-1-silacycloprop-2-ene. Two reaction pathways in which two different silyl species play a role were considered based on analogous reactions of acylsilane and alkyne: (i) A silene (Si=C) intermediate derived from the acylsilane reacts with the acetylene to yield a silacyclobutene intermediate as a result of a stepwise [2 + 2] cycloaddition, and a ring-opening reaction of the silacyclobutene triggers the formation of the silacydopropene. (ii) The silene intermediate is rearranged to a silylene intermediate, and then [2 + 1] cycloaddition of the acetylene and the silylene gives the silacyclopropene. The high activation energy calculated for the [2 + 2] cycloaddition indicates that the silene would not react with the acetylene, which is consistent with the experimental result that no silacyclobutene intermediate was observed. On the other hand, the second reaction pathway involving the silene-to-silylene rearrangement and the [2 + 1] cycloaddition is more realistic from thermodynamic and kinetic points of view. All the calculated results strongly suggest that the silyl species reacting with bis(trimethylsilyl)acetylene is not silene but silylene.