Synthesis of 2-Aryl-10b,10c-dimethyl-10b,10c-dihydropyrenes and a Study of Their Conjugation Behavior: Linear Relationship between the Degree of Conjugation and the Electronic Nature of Substituents.

Abstract

A new Gomberg-Bachmann reaction between 10b,10c-dimethyl-10b,10c-dihydropyrene (3) and an arenediazonium salt under acidic conditions afforded a mixture of mono- and diarylated derivatives of 3. The major product was 2-aryl-10b,10c-dimethyl-10b,10c-dihydropyrene 6. The desired products were obtained only from diazonium salts prepared from anilines with at least one strongly electron-withdrawing substituent. The combined conjugation and electronic effects of the arene substituent on the diatropicity of the dihydropyrene moiety in the monoarylated product is insignificant. The conjugation effect in 6 is, however, readily observed by the color appearance of 6 in the solid state and the significant red shift in the absorption bands between 300 and 520 nm in its electronic spectrum. A linear relationship was in fact obtained when the degree of conjugation (rho band lambda(max) values) is correlated with the electronic nature (sigma(+) values) of the corresponding substituents.