Photolysis of 2,3-diazabicyclo[2.2.2]oct-2-ene: Electronic spin determines the distribution of products

Article abstract of DOI:10.1021/ja00123a005

The distribution of products from the photolysis of 2,3-diazabicyclo[2.2.2]oct-2-ene (DBO) is determined primarily by the electronic spin state of the cyclohexane-1,4-diyl intermediate. DBO undergoes photolysis by direct irradiation of the N=N chromophore and singlet-sensitization to produce 1,5-hexadiene and bicyclo[2.2.0]hexane with a product ratio of 51:49 in n-octane. Triplet-sensitized photolysis in n-octane produces a product ratio of 70:30 1,5-hexadiene - bicyclo[2.2.0]hexane. The ¹⁴N/¹⁵N kinetic isotope effect on deazatization is 1.025 for DBO and 1.004 for the more-strained analogue, 2,3-diazabicyclo[2.2.1]hept-2-ene (DBH). The large kinetic isotope effect on DBO photolysis is consistent with rate-limiting C - N bond scission in excited DBO. The amount of hexadiene can be increased by heavy atom solvents that contain bromine and iodine, or by an increase in photolysis intensity, to a maximum of 70:30 hexadiene:bicyclohexane. The product distribution and ¹⁵N kinetic isotope effect is insensitive to solvent viscosity, temperature, and magnetic field (up to 7.05 T). The ratio of products can be quantitatively accounted for by fast intersystem crossing (ISC) due to spin - orbit coupling as the boat conformer of the cyclohexane-1,4-diyl is forming. A statistical distribution of 25:75 singlet-triplet cyclohexane-1,4-diyl in the boat geometry is produced by direct and singlet-sensitized photolysis. The singlet cyclohexane-1,4-diyl boat collapses to form bicyclohexane with unit efficiency, whereas the triplet boat is long-lived such that relaxation to the twist-boat occurs. The triplet cyclohexane-1,4-diyl twist-boat is unreactive toward ring closure or bond scission and can only undergo further reaction after ISC to the singlet spin state. In the twist-boat, the singlet cyclohexane-1,4-diyl undergoes ring-opening to 1,5-hexadiene at a rate that is 7/3 faster than it undergoes ring closure to [2.2.0] bicyclohexane. Triplet-sensitized photolysis also produces cyclohexane-1,4-diyl in the boat geometry, but the singlet spin state is not populated because it is 22 kcal/mol higher in energy.