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 <b>11</b>, which has two inequivalent diazo groups, have been investigated in solutions at room temperature and in matrixes at 12 K. Irradiation of <b>11</b> in benzene containing methanol gave a mixture of the spironorcaradiene <b>13</b> and the diazo ketone <b>17</b> as primary isolable photoproducts. The former <b>13</b> originated from the diazo ketene <b>20</b>, which was formed from the initial extrusion of N₂ from the 2-position of <b>11</b>, while the latter product <b>17</b> was derived from the diazo ketene <b>24</b> which was generated by the initial decomposition of the diazo group at the 4-position of <b>11</b>. The product distribution was remarkably dependent upon the excitation wavelength:  <b>13</b> was predominantly obtained in the photolysis with light of >350 nm, while the irradiation with long-wavelength light (>420 nm) exclusively gave <b>17</b>. The consistent wavelength effects were observed in photoreactions in an Ar matrix at 12 K. The irradiation of <b>11</b> matrix-isolated in Ar with light of >350 nm afforded <b>20</b> in preference to the isomer <b>24</b> as the first-formed intermediates, while <b>24</b> 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 <b>11</b> causes a preferential cleavage of the diazo group at the 2-position and that the selective extrusion of N₂ 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 <b>11</b> is discussed on the basis of theoretical calculations with the PM3 CI method.