Trapping of 1,2-benzo-1,3,4-cycloheptatriene as evidence for a strained cyclic allene structure

8-brom-6,7-dihydrobenzocycloheptene 16 and 9-bromo-6,7-benzocycloheptadiene 17 have been synthesized via a new and simple method, and their allene reactions were studied. Treatment of 16 with a base in the presence of DPIBF (diphenylisobenzofuran) gave the strained bicyclic allene 9, which underwent a cycloaddition reaction to yield 25 and 26. On the other hand, the reaction of the vinyl bromide 17 with a base, either in the presence or absence of DPIBF, resulted in the formation of 7H-benzocycloheptadiene 21, rather than the alkyne 22. (C) 1999 Elsevier Science Ltd. All rights reserved.