GENERATION AND TRAPPING OF 7-NORBORNYLIDENECARBENE AND A HEPTACYCLIC ANALOG OF THIS ALKYLIDENECARBENE

Reaction of 7-(dibromomethylene) norbornane (2) wi th n-BuLi resulted in alpha-elimination of Br-2 with concomitant formation of 7-norbornyl idenecarbene (4a). When this reaction was performed in the absence of an alkene trapping agent, the col-responding cumulated triene, 5, was produced (47% yield). When 4a was generated in the presence of norboma diene, the corresponding [2 + 2] cycloadduct, 7, was formed in 82% yie ld. Alkylidenecarbene 4a thus generated in the presence of cis-2-buten e resulted in stereospecific formation of the corresponding cis cycloa dduct, 8 (74% yield), thereby suggesting that 4a functions as a single t unsaturated cal bene in this reaction. Similar reactions of 7-(dibro momethylene)-HCTD (11) with n-BuLi performed (i) in the absence of add ed alkene and (ii) in the presence of cyclohexene resulted in the form ation of cumulated triene 14 (69% yield) and [2 + 2] alkylidenecalbene -alkene cycloadduct 15 (51%, yield), respectively. We find no evidence for the formation of cycloalkynes (i.e., 4b or 13b) in any of the rea ctions reported herein. (C) 1998 Elsevier Science Ltd. All rights rese rved.