Preparation and chemical behaviour of exo-methylene compounds: isoelectronic compounds of 5-methylenecyclohexa-1,3-diene

Abstract

Some exo-methylene compounds such as 9-methylene-9,10-dihydrophenanthrene (2) were photolytically prepared and their chemical behaviour was investigated. Treatment of (2) with acid (CF₃CO₂H) or base (^–OBu^t) led to the formation of 9-methylphenanthrene, with bromine to 9-bromomethylphenanthrene. Reaction with dimethyl acetylenedicarboxylate did not give Diels–Alder adduct but gave an ene adduct. Treatment with ethenetetracarbonitrile also gave an ene adduct. Reaction with carbene or carbenoids gave 9,10-dihydrophenanthrene-9-spirocyclopropanes. Reaction with ethenetetracarbonitrile was examined for other exo-methylene compounds, 1-methylene-1,2-dihydronaphthalene and 2-methylene-2,3-dihydrobenzo[b]thiophene. The aromatisation may be responsible for the reactions of these exo-methylene compounds except for the reaction with carbenes.