URAZOLE-BRIDGED BICYCLIC AZOALKANES - GENERATION, NITROXYL TRAPPING, AND ESR-SPECTRA OF THE HETEROSUBSTITUTED 4,5-DIAZA-1,3-CYCLOPENTANEDIYL DIRADICALS THROUGH DIRECT AND BENZOPHENONE-SENSITIZED PHOTOCHEMICAL DEAZETATION

The Diels-Alder reaction of isopyrazoles 3 with 4-methyl-1,2,4-triazoline-3,5-dione afforded the azoalkanes 4 in high yields. The stereochemistry of the syn- and anti-diastereomeric derivatives 4b was established on the basis of spectral data and an X-ray structure determination for syn-4b. Photochemical loss of dinitrogen through direct as well as benzophenone-sensitized photolysis led to the corresponding housanes 5. For the stereolabeled syn- and anti-4b azoalkanes, the respective housanes anti- and syn-5b were obtained as mixtures, the latter in preference (syn/anti > 90:10). Above -10 degrees C, the housanes 5b isomerized thermally to a 96:4 (at -30 degrees C) thermodynamic mixture of syn/anti, for which a van't Hoff treatment gave Delta G(243 K) = -4.4 kJ/mol in favor of syn-5b, Delta H = 4.2 kJ/mol, and Delta S = 36 J/mol K. AM1 calculations confirm that the syn-fib diastereomer is the preferred one. The triplet-sensitized photolysis at -30 degrees C of the separate syn- and anti-4b azoalkanes gave the same thermodynamically controlled mixture of housanes 5b (syn/anti = 96:4), which speaks for a planar triplet 1,3 diradical T-IIb as intermediate. The intervention of triplet diradicals T-IIa,d was confirmed by ESR spectroscopy under matrix isolation (T less than or equal to 77 K), and the triplet ground state was established by a Curie plot for the dimethyl derivative T-IIa. Since in the direct photolysis of the separate syn- and anti-4b azoalkanes also predominantly the syn-5b housane was obtained, fast intersystem crossing of the singlet 1,3 diradical S-IIb to the triplet species T-IIb is proposed. The small memory effect (retention) in the direct photolysis in solution implies a puckered conformation for the singlet 1,3 diradical S-IIb. Indeed, in the direct deazetation under matrix conditions at -78 degrees C, the memory effect (retention) is unequivocally established, i.e. syn/anti-5b = 24:76 for azoalkane syn-4b and syn/anti > 99:1 for anti-4b. In the presence of the nitroxyl radical scavenger 1,1,3,3-tetramethylisoindolin-2-yloxyl, the 1,3 diradicals were efficiently trapped in form of the isomeric bis-alkoxyamines 7 in the triplet-sensitized as well as direct photolyses of azoalkanes 4a,d. These unprecedented results imply that heteroatom substitution does not generally reduce the lifetime of triplet diradicals.