Pentadienyl zwitterions

A DFT study of the 2 + 2/4 + 2-cycloaddition of benzyne with thiophene to produce 1-naphthyl- and 2-naphthyl phenyl sulfides led to a modification of the originally proposed reaction pathway.1 The reaction of an E-Z mixture of 7-benzylidenecycloocta-1,3,5-triene (1) with TCNE yields a mixture of 2 + 2- [(2) and (3)], 4 + 2- [(4) and (5)], and 8 + 2- (6) cycloadducts. The presence of pentadienyl and homotropylium zwitterions accounts for the products isolated (Scheme l).2... 

Pentadienyl anion-like [61] photocyclizations occur from molecules having a heteroatom connected to two C=C bonds (olefmic and/or aromatic). This results in a reactive zwitterionic intermediate 128 which, upon oxidative dehydrogenation, produces derivatives of pyrroles, furans, thiophenes, and selenophenes 129 (Scheme 8.37) [62],... 

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