The Benzo[3,4]cyclobuta[1,2]cycloheptenyl cation. A cycloheptatrienyl (tropylium) ion analogue of biphenylene

Abstract

The title cation (5), which is an analogue of biphenylene in which one of the benzenoid rings has been replaced by a cycloheptatrienyl ion (tropylium) ring, has been prepared by hydride ion abstraction from 6H- benzo[3,4]cyclobuta[1,2]cycloheptene (19) ('homobiphenylene'). The compound (19) was obtained by a sequence of brominations and dehydrobrominations from 5,6,7,8,9,9a-hexahydro-4bH- benzo[3,4]cyclobuta[1,2]cycloheptene (13). N.m.r. spectroscopy indicates that the cation (5) is best considered as possessing two essentially independent 6π electron systems. Nucleophilic capture of cation (5) by methoxide ion yields 9-methoxyfluorene, instead of the predicted product, 4b-methoxy-4bH-benzo[3,4]cyclobuta[1,2]cycloheptene. This rearrangement of derivatives of 4bH- benzo[3,4]cyclobuta[1,2]cycloheptene to derivatives of fluorene appears to be a general reaction, and mechanistic aspects of this interesting transformation are discussed.