Enhancement of electron affinity of benzocycloheptene-1,4,7-trione by substitution

Abstract

To obtain new efficient electron acceptors, three bromine-substituted benzocycloheptene-1,4,7-triones, 14, 15, 16, and 7-dicyanomethylene-7H-benzocycloheptene-1,4-dione 36 have been synthesized. The electron affinities of the bromine-substituted quinones were found to be raised considerably as the number of bromine atoms increased compared with the parent trione 3. A linear correlation between E₁^redox and E_LUMO(MNDO) has been shown. The dicyanomethylene substitution (36) of the tropone carbonyl of 3 caused an increase in the electron affinity by E₁+ 0.2 V. To obtain an acceptor carrying electron-withdrawing substituents on C₆ and C₈ of 3, the 6,8-dimethoxycarbonyl derivative 18 was oxidized and gave a ring-contracted product 22 in fair yield. The formation is discussed in terms of electron distribution in the molecule of the expected product 19.