Silicon-carbon unsaturated compounds. 76. Photochemical and thermal behavior of 1-silacyclobut-3-enes generated from the reaction of pivaloyltris(trimethylsilyl)silane with tert-butylacetylene

Irradiation of trans-1-silacyclobut-3-ene 2, obtained by thermal isomerization of [2 + 2] cycloadduct 1 arising from the co-thermoysis of pivaloyltris(trimethylsilyl)silane with řerř-butylacetylene, with a low-pressure mercury lamp afforded an equilibrium mixture consisting of 2 and its isomer, cisl-silacyclobut-3-ene 3, in a ratio of 1:1. Similar irradiation of the cis isomer 3 gave the mixture involving equal amounts of 2 and 3. The photolysis of 2 in the presence of tert-butylalcohol gave two isomers of the cyclopropane derivative, arising from the reaction of sila bicyclobutane, A and A', with alcohol. The thermolysis of 3 at 250 °C produced 2 in quantitative yield. Similar thermolysis of 3 in the presence of tert-butylalcohol, however, afforded no alcohol adducts, but 2 was obtained as the sole product. The DFT calculations with the use of cis-2,4-dimethyl-1-siloxy-1,2-bis(silyl)-1- silacyclobut-3-ene as a starting compound and řraní-2,4-dimethyl- l-siloxy-l,2-bis(silyl)-l-silacyclobut-3ene as a product indicated that a pentacoordinate silicon intermediate plays an important role in the thermal isomerization of 3 to 2.