Thermal and photochemical [2+2] cycloreversion of a 1,2-disilacyclobutane and a 1,2-digermacyclobutane

Upon heating of the highly sterically congested 1,1,2,2-tetrakis(trimethylsilyl)dispiro[3,3',4,4'-biadamantane-1,2-disilacyclobutane] (2) in solution in the presence of trapping reagents, such as 1,3-butadienes, styrene, phenylacetylene, and methanol, the trapping products of the silene bis(trimethylsilyl)adamantylidenesilene (1) are formed with high regioselectivity and good yields. Photolysis of 2 at -196 degrees C in methylcyclohexane in the absence of trapping agents produces tetrakis(trimethylsilyl)disilene (3). Photolysis of 2 in solution in the presence of 1,3-butadiene leads to a 2:1 mixture of 1,1,2,2-tetrakis(trimethylsilyl)-1,2-disilacyclohex-4-ene (10)-the trapping product of the disilene 3-and of 1,1-bis(trimethylsilyl)-1-silocyclopent-3-ene (11)-the trapping product of bis(trimethylsilyl)silylene (4)-together with 2,2'-biadamantylidene. The results of laser flash photolysis and of additional trapping experiments suggest that the sole primary product in the photolysis of 2 is the disilene 3, which dissociates under further irradiation to produce the silylene 4. Heating of 1,1,2,2-tetrakis(trimethylsilyl)dispiro[3,3',4,4'-biadamantane-1,2-digermacyclobutane] (14) in solution results in the quantitative formation of 2,2'-biadamantylidene and of germanium-containing oligomers, while neither bis(trimethylsilyl)adamantylidenegermene (15) nor tetrakis(trimethylsilyl)digermene (16) or bis(trimethylsilyl)germylene (17) could be trapped. Upon photolysis 14 behaves similarly to 2, leading to 16 and 17, which could be trapped by 1,3-butadiene.