Remarkable wavelength-dependent photoreactions of the bis(diazo) ketone having inequivalent diazo groups: studies in fluid solutions and in low-temperature matrixes

The photoreactions of the bis(diazo) ketone 11, which has two inequivalent diazo groups, have been investigated in solutions at room temperature and in matrixes at 12 K. Irradiation of 11 in benzene containing methanol gave a mixture of the spironorcaradiene 13 and the diazo ketone 17 as primary isolable photoproducts. The former 13 originated from the diazo ketene 20, which was formed from the initial extrusion of N(2) from the 2-position of 11, while the latter product 17 was derived from the diazo ketene 24 which was generated by the initial decomposition of the diazo group at the 4-position of 11. The product distribution was remarkably dependent upon the excitation wavelength: 13 was predominantly obtained in the photolysis with light of >350 nm, while the irradiation with long-wavelength light (>420 nm) exclusively gave 17. The consistent wavelength effects were observed in photoreactions in an Ar matrix at 12 K. The irradiation of 11 matrix-isolated in Ar with light of >350 nm afforded 20 in preference to the isomer 24 as the first-formed intermediates, while 24 was mainly obtained in the long-wavelength irradiation (>420 nm). On the basis of these experimental data, we conclude that the short-wavelength irradiation of 11 causes a preferential cleavage of the diazo group at the 2-position and that the selective extrusion of N(2) from the 4-position is practically achieved by the irradiation with long-wavelength light. The reason for the selective cleavage of the two inequivalent diazo groups of 11 is discussed on the basis of theoretical calculations with the PM3 CI method.