Alkenes as Alkyne Equivalents in Radical Cascades Terminated by Fragmentations: Overcoming Stereoelectronic Restrictions on Ring Expansions for the Preparation of Expanded Polyaromatics

Chemoselective
interaction of aromatic enynes with Bu3Sn radicals can
be harnessed for selective cascade transformations,
yielding either Sn-substituted naphthalenes or Sn-indenes. Depending
on the substitution at the alkene terminus, the initial regioselective
5-exo-trig cyclizations can be intercepted at the 5-exo stage via
either hydrogen atom abstraction or C–S bond scission or allowed
to proceed further to the formal 6-endo products via homoallylic ring
expansion. Aromatization of the latter occurs via β-C–C
bond scission, which is facilitated by 2c,3e through-bond interactions,
a new stereoelectronic effect in radical chemistry. The combination
of formal 6-endo-trig cyclization with stereoelectronically optimized
fragmentation allows the use of alkenes as synthetic equivalents of
alkynes and opens a convenient route to α-Sn-substituted naphthalenes,
a unique launching platform for the preparation of extended polyaromatics.

芳香炔烯与三丁基锡（Bu3Sn）自由基的化学选择性相互作用，可应用于选择性级联反应，得到锡取代萘与锡取代茚两类产物。依据烯烃端基的取代基差异，初始发生的区域选择性5-exo-trig环化反应可在5-exo阶段通过两种路径被截停：一是通过氢原子抽提或C-S键断裂终止反应，二是继续通过高烯丙基扩环，得到形式上的6-endo产物。后者（即形式上的6-endo产物）的芳构化过程通过β-C-C键断裂完成，该断裂过程可借助2c,3e键间相互作用得到强化——这是自由基化学领域中一种全新的立体电子效应。将形式上的6-endo-trig环化与经立体电子效应优化的裂解过程相结合，可将烯烃用作炔烃的合成等价物，并为α-锡取代萘的制备开辟了一条便捷路径；而α-锡取代萘是构建稠合多芳烃的独特起始平台。