The thermal isomerization of a tetrahydro-2,3-disilanaphthalene into a 1,2-disilacyclobutane - The conversion of a formal silene [2+4] cyclodimer into the [2+2] cycloadduct

1,2,3,8a-Tetrahydro-1-mesityl-5,7,8a-trimethyl-2,2,3,3- tetrakis(trimethylsilyl)-2,3-disilanaphthalene (3), the formal [2+4] cyclodimer of the transient 2-mesityl-1,1-bis(trimethylsilyl)-silene (2), on thermal treatment gradually isomerizes to give a mixture of the [2+2] products (E)- and - predominantly - (Z)-3,4-dimesityl-1,1,2,2-tetrakis(trimethylsilyl)-1,2-disilacyclobutane [(E/Z)-4]. By prolonged heating or at higher temperatures both (E/Z)-3 and (Z)-4 are converted into (E)-4, the thermodynamically most stable head-to-head dimer of 2. Possible pathways of the isomerization processes are discussed.