Bridgehead substitution, azoalkanes

Contrary to the rotationally symmetric bridgehead substitution (azoalkanes 5a,b,c), which leads to the unselective triplet reaction channel and selective formation of anti-configured (retained) housanes (Scheme 7), the direct photolysis of the rotationally unsymmetric bridgehead-substituted azoalkanes 5d,e is moderately diastereoselective for both the singlet and triplet modes of photolysis (Scheme 9), as evidenced by the synjanti (inversion/retention) ratio. Thus, the photolysis of 5d,e affords under singlet conditions (high-temperature direct photolysis) predominantly the retained housanes anti-14d,e syn anti 21 79 for housane 14d and 33 67 for 14e) while under triplet conditions (low-temperature direct or benzophenone-sensitized photolysis), the inverted diastereomer syn-14d,e is favored syn anti 61 39 for 14d and 70 30 14e). 

Dehnitive for triplet-state photochemistry, the a-CN-bond cleavage has been recently disclosed in the direct irradiation of azoalkanes 5. A prominent feature of the photolysis of azoalkanes 5 (Scheme 4) is the fact that the syn/anti product diastereoselectivity depends on temperature and bridgehead substitution. The temperature dependence of the direct photolysis of azoalkanes 5 stems from the competition between the singlet and triplet reaction channels (Scheme 4), the latter dominating at low temperature. Mechanistic details of this diastereoselective process is considered below, subsequent to a general discussion on the stereochemical inversion phenomenon in the photochemical nitrogen extrusion. 

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