Synthesis, Thermolysis, and Photolysis of the Azoalkanes Spiro[4,5‐diazatricyclo[4.3.0.0 3,7 ]non‐4‐ene‐8,2′‐[1.3]dioxolane] and 4,5‐Diazatricyclo[4.3.0.0 3,7 ]non‐4‐en‐8‐one: On the Mechanism of the Oxadi‐π‐methane Rearrangement of 5‐Norbornen‐2‐one

4,5‐Diazatricyclo[4.3.0.0 3,7 ]non‐4‐en‐8‐one ( 4 ) and spiro[4,5‐diazatricyclo[4.3.0.0 3,7 ]non‐4‐ene‐8,2′‐[1,3]dioxolane] ( 5 ) were prepared starting from bicyclo[2.2.1]hept‐5‐en‐2‐one ( 1 ) by cycloaddition with 4‐phenyl‐4 H ‐1,2,4‐triazole‐3,5‐dione (PTAD) to the corresponding urazole 7 , followed with acetalization to the urazole 9 and subsequent hydrolysis and oxidation. The regioselectivity of the PTAD‐cycloaddition leading to urazole 7 was confirmed by an X ‐ray analysis. Direct and benzophenone‐sensitized photolyses of azoalkane 5 gave the tricycloalkane 13 , while in the thermolysis also the pyrazole 12 was formed. Direct and benzophenone‐sensitized photolyses of azoalkane 4 yielded bicyclo[2.2.1]hept‐5‐en‐2‐one ( 1 ), tricyclo[3.2.0.0 2,7 ]heptan‐3‐one ( 2 ), and bicyclo[3.2.0]hept‐3‐en‐6‐one ( 3 ). Thermolysis gave only the bicyclic ketones 1 and 3 . It is postulated that the photolysis and thermolysis of azoalkane 4 first lead to diazenyl diradicals 19a , b , which are differentiated in their chemical behavior on account of spin state multiplicities (singlet versus triplet) and electronic configurations (D σ , σ versus D σ , π ). Formation of the oxadi‐π‐methane‐type 1,3‐diradical 20 by denitrogenation involving double C – N cleavage represents a minor product channel.