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

摘要

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 retionalized 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 ~3DR after ISC. In contrast, the denitrogenation of the intermediary diazenyl diradical ~1DZ along the S_H2 pathway (inversion) for the singlet process is quite insensitive to these remote steric effects.