Preferred formation of anti-housane (retention) in the sensitized photodenitrogenation of cyclopentane-annellated DBH-type azoalkanes through long-range steric effects in the cyclization of the triplet cyclopentane-1,3-diyl diradicals

The photolysis of the cyclopentene- and cyclopentane-annellated DBH-type azoalkanes 1a and 1b affords under singlet conditions (high-temperature direct photolysis) predominantly the inverted housanes syn-2 in similar amounts for both derivatives 2a and 2b. Under triplet conditions (low-temperature direct or benzophenone-sensitized photolysis), the photolysis leads to the retained housane anti-2 as the major diastereomer, but with a substantial difference in the syn/ anti-housane ratio for 2a (38:62) and for 2b (6:94). This significant difference in the anti stereoselectivity of the triplet pathway is mechanistically rationalized in terms of long-range steric interactions between the annellated ring and the gem-dimethyl-substituted methylene bridge during the cyclization of the cyclopentane-1,3-diyl triplet diradical (DR)-D-3 after ISC. In contrast, the denitrogenation of the intermediary diazenyl diradical (1)DZ along the S(H)2 pathway (inversion) for the singlet process is quite insensitive to these remote steric effects. ((C) Wiley-VCH Verlag GmbH & Co. KGaA, 69451 Weinheim, Germany, 2003).