PHOTOLYSIS OF 2,3-DIAZABICYCLO[2.2.2]OCT-2-ENE - ELECTRONIC SPIN DETERMINES THE DISTRIBUTION OF PRODUCTS

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 st ate of the cyclohexane-1,4-diyl intermediate. DBO undergoes photolysis by direct irradiation of the N=N chromophore and singlet-sensitizatio n to produce 1,5-hexadiene and bicyclo[2.2.0]hexane with a product rat io of 51:49 in n-octane. Triplet-sensitized photolysis in n-octane pro duces a product ratio of 70:30 1,5-hexadiene-bicyclo[2.2.0]hexane. The N-14/N-15 kinetic isotope effect on deazatization is 1.025 for DBO an d 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 consi stent with rate-limiting C-N bond scission in excited DBO. The amount of hexadiene can be increased by heavy atom solvents that contain brom ine and iodine, or by an increase in photolysis intensity, to a maximu m of 70:30 hexadiene:bicyclohexane. The product distribution and N-15 kinetic isotope effect is insensitive to solvent viscosity, temperatur e, and magnetic field (up to 7.05 T). The ratio of products can be qua ntitatively accounted for by fast intersystem crossing (ISC) due to sp in-orbit coupling as the boat conformer of the cyclohexane-1,4-diyl is forming. A statistical distribution of 25:75 singlet-triplet cyclohex ane-1,4-diyl in the boat geometry is produced by direct and singlet-se nsitized photolysis. The singlet cyclohexane-1,4-diyl boat collapses t o 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 s inglet 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-sensi tized photolysis also produces cyclohexane-1,4-diyl in the boat geomet ry, but the singlet spin state is not populated because it is 22 kcal/ mol higher in energy.