Cycloaddition Reactions of 7‐Benzylidenecycloocta‐1,3,5‐triene with Ethenetetracarbonitrile and 4‐Phenyl‐3 H ‐1,2,4‐triazole‐3,5(4 H )‐dione

An ( E )/( Z) mixture (3 : 2) of 7‐benzylidenecycloocta‐1,3,5‐triene ( 5 ) is obtained when 1‐benzylcycloocta‐1,3,5,7‐tetraene ( 7 ), prepared by an improved procedure, is treated with t ‐BuOK in THF. Alternatively, a ca. 9 : 1 mixture ( E )/( Z )‐ 5 can be prepared in a Wittig reaction involving benzaldehyde and cycloocta‐2,4,6‐trien‐1‐ylidenetriphenylphoshorane ( 9 ). Treatment of ( E )/( Z )‐ 5 88 : 12 with ethenetetracarbonitrile (TCNE) gave a complex mixture of products, from which seven mono‐adducts and two bis‐adducts were isolated ( Sect. 2.2.1 ). Of the mono‐adducts, four are π 4+ π 2 adducts: two (( E )‐ and ( Z )‐isomers) are derived from valence tautomers of the two isomers of ( E )/( Z )‐ 5 , while it is tentatively suggested that the other two (again ( E )‐ and ( Z )‐isomers) are formed from the intermediacy of a pentadienyl zwitterion ( Sect. 2.3 ). The remaining three mono‐adducts, two of which are epimers, are π 8+ π 2 adducts. It is suggested that they are derived from the intermediacy of homotropylium zwitterions ( Sect. 2.3 ). For the two bis‐adducts, it is postulated that they are derived from an initial π 2+ π 2 cycloaddition involving the homotropylium zwitterions followed by π 4+ π 2 cycloaddition to the valence tautomer of each of the π 2+ π 2 cycloadducts. With 4‐phenyl‐3 H ‐1,2,4‐triazole‐3,5(4 H )‐dione ( 6 ), ( E )/( Z )‐ 5 91 : 9 yielded two π 4+ π 2 cycloadducts (( E )‐ and ( Z )‐isomers) as well as two epimeric π 8+ π 2 cycloadducts ( Sect. 2.2.2 ). The intermediacy of pentadienyl (tentative suggestion) and homotropylium zwitterions accounts for the formation of the products ( Sect. 2.3 ).