The Photochemical Synthesis and Denitrogenation of 8,9‐diazadibenzo[ c , e ]isobullvalene

Starting from dibenzo[ a , c ]cyclooctene ( 4 ) and 4‐methyl‐3 H ‐1,2,4‐triazol‐3,5(4 H )‐dione (MTAD), the strained skeleton of the title azo compound 1 is assembled in a tandem photo‐ Diels ‐ Alder addition/di‐π‐methane rearrangement sequence. The synthesis is completed by a stepwise hydrolytic oxidation of the ensuing triazolidine‐dione 2 with nickel peroxide. Thermolysis of 1 in benzene solution is shown to be governed by an initial 1,3‐dipolar cycloreversion which leads, via an intermediate diazo compound 11 , to cyclobuta[ 1 ]phenanthrene 8 and two further carbene‐derived C 16 H 12 products. Photolysis of 1 at 350 nm leads in modest yield (12%), via a diazenyl diradical, to an unstable bridged bicyclobutane 10 (dibenzooctavalene). MNDO calculations suggest the latter to have a rapidly inverting, twisted structure of C 2 symmetry.