Umpolung of tropone: the reaction of 2-halogenocycloheptadienone enolates with tropylium cations and several other cationic electrophiles. Preparation of novel 2-substituted and 2,7-disubstituted tropones

Abstract

The reactions of 2-halogenotropones (3a–c) with hydride reagents giving 2-halogenocycloheptadienone enolates (4a–c) have been studied to provide a reverse polarity (umpolung) of tropone. The structures of 2-chlorocycloheptadienone enolate (4a) and 2-chloro-7-deuteriocycloheptadienone enolate (4D) were confirmed by 1H NMR spectral studies. The enolates were easily reacted with tropylium and substituted tropylium cations to give 2-(2,4,6-cycloheptatrienyl)tropone (8) and its derivatives, (13b–d) and (14b–d) in good yields. The other cationic electrophiles, such as benzo- and dibenzotropylium cations, di- and triphenylcyclopropenylium cations as well as tricarbonyl(cyclopentadienylium)iron and tricarbonyl(cyclohexadienylium)iron, were also reacted with the enolate to give the corresponding 2-substituted tropones (21–27) in good to modest yields. In a similar fashion, 2-halogeno-7-substituted cycloheptadienone enolates (28a–e) were generated through the reaction of 2-chlorotropone (3a) with Grignard and organolithium reagents as well as the reaction of 2-bromo-7-methoxytropone (3d) and 2,7-dibromotropone (3e) with hydride reagent. They reacted also with tropylium cation to give 2,7-disubstituted tropones (29a–e).