Synthesis and rearrangements of 1,1′-bi(benzocyclobutylidene) and its derivatives

Convenient new preparations of 1-halogeno-1,2-dihydrobenzocyclobutenes (5) and (6) and 1,1′-bi(benzocyclobutylidene)(4) are described. The hydrocarbon (4) is susceptible to cationic rearrangements: it forms 5,10-dihydroindeno[2,1-a]indene (16) on protonation in acetic anhydride, and is oxidised by selenium(IV) oxide to a mixture of the spiroketone (17) and 5H-indeno[1,2-c]-[2]benzopyran-2-one (18). The mechanisms of these and other rearrangements are discussed.