Cascade Vinylation/8π-Electrocyclization and Cu(II)-Catalyzed Dehydrogenation toward Highly Stable Formally Antiaromatic Cycloheptatrienyl Anions

An approach to the synthesis of seven-membered systems via the chain elongation of nucleophilic propenes and subsequent 8 pi-electrocyclization is proposed. The cascade reaction yields either cycloheptadienes or bicycloheptenes, and the latter are formed via a 6 pi-electrocyclization of intermediate cycloheptadienyl anion which was proved to be reversible in a basic medium. The electrocyclic nature of the ring-closing reactions was supported by density functional theory and DLPNO/CCSD(T) calculations. Highly electron-deficient cycloheptatrienes can be obtained from cycloheptadienes or bicycloheptenes via oxidation either intro-duced into the cascade reaction or performed as a separate reaction, with the overall yield of up to 81%. The oxidation step was performed by means of a rarely encountered Cu(II)-catalyzed dehydrogenation of cycloheptadienes or bicycloheptenes, and so the reaction mechanism was proposed. Stable formally 8 pi-antiaromatic cycloheptatrienyl-anion containing compounds were obtained, and some correlations between their UV-vis spectra and the structure of the distorted cycloheptatrienyl-anion moiety were clarified. Additionally, a base-induced retro-[2 + 2]-cycloaddition in a bicycloheptene derivative gave cyanotetra-(methoxycarbonyl)cyclopentadienyl cesium.

Automated summary

This study proposes a method for the synthesis of seven-membered systems through the chain elongation of nucleophilic propenes and subsequent electrocyclization reactions. The cascade reaction can yield either cycloheptadienes or bicycloheptenes, with the latter being formed via a reversible 6π-electrocyclization of an intermediate cycloheptadienyl anion. The electrocyclic nature of the ring-closing reactions is supported by computational calculations. Highly electron-deficient cycloheptatrienes can be obtained through oxidation reactions. Additionally, a retro-[2+2]-cycloaddition reaction was observed in a bicycloheptene derivative.