Z  E photoisomerization of benzylidenebenzocyclobutenones via ketene-allene intermediates. A laser flash photolysis study

Abstract Benzylidenebenzocyclobutenones I -E and I -Z undergo photoinduced isomerization via the intermediacy of ketene-allen II which in acetonitrile has a lifetime of ∼26 μs and can be trapped with water and methanol. Trapping by the latter occurs with a rate constant of 1.2 × 10 4 M −1 s −1 and leads to the expected ester. In addition to Z E isomerization, product studies reveal the formation of dimeric products in very low quantum yields.