Synthesis and Unusual Reactivity of Acyl-Substituted 1,4-Disilacyclohexa-2,5-dienes

In continuation of our recent studies on group 14 rings
with exocyclic
silicon–carbon double bonds, we report here on the synthesis
and reactivity of previously unknown acyl-substituted 1,4-disilacyclohexa-2,5-dienes.
1,1,4,4-Tetrakistrimethylsilyl-1,4-disilacyclohexa-2,5-diene 1 cleanly afforded the silyl anion 1-K after
addition of 1 equiv of KOtBu. 1-K subsequently could be reacted with various electrophiles to the
expected substitution products including compounds 4 and 5. When photolyzed with λ > 300 nm radiation, 4 and 5 undergo Brook-type 1,3-Si → O
migration reactions to generate the corresponding 1,4-disilacyclohexadienes
with exocyclic SiC bonds as the primary products. These metastable
silenes only could be characterized in form of appropriate quenching
products. The reaction of compound 4 with KOtBu followed by the addition of 1 equiv of PhMe2SiCl surprisingly gave the silylated 1,4-disilanorbornadiene
cages 8 and 9 instead of the expected exocyclic
silene. The responsible sila-Peterson-type mechanism could be elucidated
by density functional theory calculations at the conductor-like polarizable
continuum model (THF) B3LYP-GD3/6-31 + G­(d) level and by the isolation
and characterization of unstable intermediate products after proper
derivatization.