Rearrangement reactions of bicyclic systems: Part VI. 1 A [ 14C]-tracer study of the acid-catalysed skeletal rearrangement of 1,4-dihydro-1-methoxy-1,4-etheno- naphthalene (1-methoxybenzobarrelene) and its 5,6,7,8-tetrahalogeno-derivatives

Abstract

A degenerate skeletal rearrangement occurs during the acid catalysed conversion of 1-methoxybenzobarrelene and its tetrachloro- and tetrafluoro-derivatives into the corresponding 3,4-dihydro-1,4-ethenonaphthalen-2(1H)-ones (benzobarrelenones). A [ 14C]-tracer study has confirmed that two pathways operate. In both pathways the methoxylated bridgehead C-1 of the benzobarrelene becomes the carbonyl C-2 of the benzobarrelenone. The pathways are distinguished by the fate of the benzobarrelene bridgehead C-4: in one pathway aryl migration leaves this formerly bridgehead C-4 as the olefinic C-5 of the newly formed benzobarrelenone; in the other pathway the bridgehead C-4 remains bonded to the aryl ring while a double migration of other skeletal bonds leaves it as the bridgehead C-4 of the benzobarrelenone. The pathway involving aryl migration is favoured in each case examined. The preference for aryl migration is greater in concentrated (> 98%) sulphuric acid than in aqueous (80%) sulphuric acid and in both cases the preference increases as the aryl substitution varies from F 4 to Cl 4 to H 4.