Cycloaddition reactions of 7-benzylidenecycloocta-1,3,5-triene with ethenetetracarbonitrile and 4-phenyl-3H-1,2,4-triazole-3,5(4H)-dione

Abstract

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 4+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 8+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 2+2 cycloaddition involving the homotropylium zwitterions followed by 4+2 cycloaddition to the valence tautomer of each of the 2+2 cycloadducts. With 4-phenyl-3H-1,2,4-triazole-3,5(4H)-dione (6), (E)/(Z)-5 91 : 9 yielded two 4+2 cycloadducts ((E)- and (Z)-isomers) as well as two epimeric 8+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).