Silicon-carbon unsaturated compounds.: 74.: Thermal behavior of 1-silacyclobut-3-enes generated from the reaction of acylpolysilanes with tert-butylacetylene

The thermolysis of 2-adamantyl-3-tert-butyl-2-(trimethylsiloxy)-1,1-bis(trimethylsilyl)-1-silacyclobut-3-ene(2a), obtained by the reaction of adamantoyltris(trimethylsilyl)silane with tert-butylacetylene, at 250 degrees C for 5 h gave two regioisomers, trans-4-adamantyl-2-tert-butyl- and trans-2-adamantyl-4-tert-butyl-1-(trimethylsiloxy)-1,2-bis(trimethylsilyl)-1-silacyclobut-3-ene (3a and 4a) in a ratio of 1.5:1, in 93% combined yield. The structure of 4a was confirmed by X-ray crystallographic analysis. Similar thermolysis of 2,3-di-tert-butyl-2-(trimethylsiloxy)-1,1-bis(trimethylsilyl)-1-silacyclobut-3-ene (2b), formed by the reaction of pivaloyltris(trimethylsilyl)silane with tert-butylacetylene, however, afforded a single isomer, trans-2,4-di-tert-butyl-1-(trimethylsiloxy)-1,2-bis(trimethylsilyl)-1-silacyclobut-3-ene (3b), in 93% yield. When 2a was heated in a sealed tube at 190 degrees C for 5 h, 1-adamantyl-2-(tert-butyl)-3[(trimethylsiloxy)bis(trimethylsilyl)silyl]cycloprop-1-ene (5a), which cleanly isomerized at 250 degrees C to give a mixture of 3a and 4a, was obtained in quantitative yield. Similarly, the thermolysis of 2b at 190 degrees C for 5 h afforded 1,2-di-tert-butyl-3-[(trimethylsiloxy)bis(trimethylsilyl)silyl]cycloprop-1-ene (5b) in 90% yield, as the sole product. Treatment of 2a with methanol in a sealed glass tube at 160 degrees C gave an alkene derivative (7a) with an E configuration, arising from addition of methanol to a silicon-carbon double bond in the 1-sila-1,3-butadiene generated from the ring opening of 2a, in 84% yield. Similar treatment of 2b with methanol afforded (E)-1,2-di-tert-butyl-2-[(methoxybis(trimethylsilyl)silyl)methyl]-1-(trimethylsiloxy)ethene (7b) in 83% yield. The co-thermolysis of 2a,b with 2,3-dimethylbutadiene at 180 degrees C produced the respective silacyclohexene derivatives (8a,b), arising from [2 + 4] cycloaddition of the 1-sila-1,3-butadienes with the butadiene.