TEMPERATURE-DEPENDENCE OF THE ALPHA-BOND VERSUS BETA-BOND CLEAVAGE IN THE DIRECT AND TRIPLET-SENSITIZED PHOTOLYSIS OF AZOALKANES OF THE 2,3-DIAZABICYCLO[2.2.1]HEPT-2-ENE TYPE

On direct irradiation, the azoalkanes anti-hexahydro-1,4:5,8-dimethanophthalazines (5a, 1,4-H; 5b, 1,4-Me; 5c, 1,4-Ph) do not only give the expected housanes 6 through a C-N bond cleavage and subsequent denitrogenation but also yield, in the order 5a > 5b > 5c, the rearranged aziranes 7 through beta C-C bond cleavage. The increased formation of the heterocycles 7 on tripler sensitization with benzophenone and the suppression of these rearrangement products by the triplet quenchers trans-piperylene and 1,3-cyclohexadiene show that the aziranes 7 are derived from the triplet state of the azoalkanes (3)5* by beta C-C bond cleavage. The higher amounts of housanes 6 at higher temperatures suggest that intersystem crossing (ISC) to (3)5* competes with denitrogenation in the singlet-excited azoalkanes (1)5*. Since ISC should be relatively temperature independent, the temperature effect is rationalized in terms of an activation barrier for alpha C-N bond cleavage in the singlet-excited azoalkane (1)5*, which is more readily overcome at higher temperatures to form increased amounts of housane 6. From the temperature dependence of the direct and sensitized photoreactions of azoalkane 5a, the relative activation energies were estimated to be greater than or equal to 3.3 and greater than or equal to 10.5 kcal/mol for alpha C-N scission from the singlet and triplet states and greater than or equal to 7.9 kcal/mol for the beta C-C bond cleavage from the triplet.