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

✍ Scribed by Philip Clements; George E. Gream; Paul K. Kirkbride; Simon M. Pyke

Publisher
John Wiley and Sons
Year
2005
Tongue
German
Weight
328 KB
Volume
88
Category
Article
ISSN
0018-019X

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✦ Synopsis

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 __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).

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