Studies in the cyclopropa-arene series: cyclopropa[b]naphthalenes
1,1,1a,7a-Tetrachloro-1a,2,7,7a-tetrahydro-2,7-diphenylcyclopropa[b]naphthalene (5) affords an isolable gem-dihalogenocyclopropa[b]naphthalene (7) on dehydrohalogenation, whereas the tetrabromo-analogue (6) does not. The reaction of the gem-dihalide (7) with a small excess of phenyl- or ethyl-magnesium bromide surprisingly results in halogen exchange and formation of gem-dibromocyclopropa[b]naphthalene (8) in high yield. Only with a large excess of Grignard reagent are the halogeno-substituents of (7) replaced by alkyl and aryl groups. Whereas the gem-diethyl derivative (13) is isolable, the gem-diphenyl compound (12) suffers ready ring cleavage, rearranging to a benzo[b]fluorene (18). With the gem-dichloride (7), methylmagnesium iodide effects the equivalent of monohalogen exchange and Grignard reduction to give a benzo[c]fluorene (20).