Laser flash photolysis study of the photochemistry of ortho-benzoylbenzaldehyde

Ketene-enols 4 and 5 have been generated by laser flash photolysis of ortho-benzoylbenzaldehyde (3) and kinetically and spectroscopically characterized. In benzene or acetonitrile, the E ketene-enol, 4, shows absorption at 340 and 400 nm and a lifetime in excess of 1 ms, whereas the Z ketene-enol, 5, shows maxima at 360 and 430 nm and a lifetime of only 1.5 μs. At shorter time scales we observed a weak absorption (λmax = 580 nm) tentatively assigned to biradical 6 with a lifetime of 140 ns. The E ketene-enol is readily quenched by oxygen, dienophiles, methanol, and water, with quenching rate constants ranging from 3.6 × 103 M−1 s−1 (for methanol as a quencher) to 2.2 × 108 M−1 s−1 (for diethylketomalonate). At high water concentrations (typically > 10 M) a new species, 7, was detected showing maximum absoiption at 510 nm and a growth lifetime of 7 μs. In deuterated water and using the same concentration as before we observed a formation lifetime for 7 of 10 μs, which results in an isotope effect of ~ 1.5. It is proposed that 5 is the main precursor for 7. Steady-state irradiation of 3 in deaerated methanol leads to the formation of dihydroanthraquinone (9), a strongly colored and fluorescent (λmax = 475 nm, τn = 29 ns) species, whereas 3-phenylphthalide (2, R = Ph) is the main product when the irradiation is performed in benzene. Steady-state quenching of product formation by diethyl ketomalonate gives a Stern–Volmer constant of 380 M−1 from which we conclude that 5 is the ketene-enol responsible for product formation, in agreement with the laser flash photolysis results.