Syntheses and structure of 8-, 7-, and 6-membered silacycloallenes

Encouraged by their recent success in the synthesis and structure determination of a tetrasilacyclohexyne, the authors have pursued the syntheses of strained silacycloallenes and report herein the first examples of isolable 6- and 7-membered rings containing 1,2-diene units. A key factor in their synthetic success was the finding that 1,3-bis(trimethylsilyl)-1-propyne is quantitatively converted to an allenyl dianion upon treatment with 2 equiv of n-BuLi in ether. Quenching of the allenyl dianion with [alpha],[omega]-dichloropolysilanes leads in each case to good yields of the corresponding cyclic allenes. 15 refs., 3 figs., 1 tab.