First measurements of absolute rate constants for oxacarbene intermediates produced in the photochemistry of benzocyclobutenedione

Article abstract of DOI:10.1021/ja00180a030

The photochemistry of benzocyclobutenedione in solution has been examined by use of laser flash photolysis techniques. Two rapid processes appear to compete. One leads to the formation of the long-lived bisketene IV, which can be readily observed (λ_max 380 nm). The other process leads to oxacarbene III. This carbene could not be monitored directly, but probing techniques, with pyridine and adamantanethione as probes, lead to readily detectable ylides that can be employed to characterize the kinetic behavior of III. Thus, III lives about 20 μs in toluene and can be trapped by typical carbene scavengers such as alcohols. Carbene trapping by alcohols is in fact a complex reaction, since it also leads to equilibration with a short-lived intermediate (studied in detail in methanol/acetonitrile) that in turn can be trapped by alcohols, azides, and halides. This intermediate is tentatively assigned to carbocation VIII formed by solvent protonation of III.