Preparation of some substituted bicyclo[4,2,0]octa-1,3,5-trienes (benzocyclobutenes).

The reaction of benzyne (generated by the thermolysis of o- diazoniobenzoate) with the following alkenes has been examined: 1,1- dichloroethene (5) (vinylidene chloride), 1,1-dichloro-propene (7), 1,1-dichloro-2-methylpropene (17), 2-methylprop-1-en-1-yl acetate (18) and 2-chloro-prop-2-enenitrile (α-chloroacrylonitrile) (22). Substituted bicyclo[4,2,0]octa-1,3,5-trienes resulting from 2+2 cycloaddition were obtained from (5), (7), and (22), while the reaction involving the en01 acetate (18) gave only ?ene? product, and that involving (17) gave an intractable mixture. (Z)- and (E)-1-Chloro-2-(o- chlorophenyl)propene were obtained as minor products in the reaction of benzyne with (7): these products result from a chlorine atom transfer to the aromatic ring, and a novel mechanism involving a diradical → carbene rearrangement is postulated.