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.延续我们此前针对带有环外硅碳双键的第14族环所开展的研究工作，本文报道了一类此前尚未被合成的酰基取代1,4-二硅杂环己-2,5-二烯(1,4-disilacyclohexa-2,5-diene)的合成方法与反应性能。1,1,4,4-四(三甲基硅基)-1,4-二硅杂环己-2,5-二烯(1)在加入1当量叔丁醇钾(KOtBu)后，可干净地转化为硅负离子1-K。随后，硅负离子1-K可与多种亲电试剂发生反应，得到预期的取代产物，包括化合物4与5。当用波长λ>300 nm的辐射进行光解时，化合物4和5会发生布鲁克型(Brook-type)1,3-硅→氧迁移反应，主要生成对应的带有环外硅碳双键的1,4-二硅杂环己二烯类产物。这类亚稳态硅烯仅能通过合适的淬灭产物进行表征。将化合物4与叔丁醇钾反应后，再加入1当量二甲基苯基氯硅烷(PhMe2SiCl)，出乎意料地得到了硅基化的1,4-二硅杂降冰片二烯笼状化合物8与9，而非预期的环外硅烯产物。通过采用以四氢呋喃(THF)为溶剂的类导体极化连续介质模型(conductor-like polarizable continuum model)下的B3LYP-GD3/6-31+G(d)级别密度泛函理论(density functional theory)计算，以及对经适当衍生化后得到的不稳定中间体产物的分离与表征，我们阐明了该反应所涉及的硅代彼得森型(sila-Peterson-type)反应机理。