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

Abstract

An approach to the synthesis of seven-membered systems via the chain elongation of nucleophilic propenes and subsequent 8π-electrocyclization is proposed. The cascade reaction yields either cycloheptadienes or bicycloheptenes, and the latter are formed via a 6π-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 introduced 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 8π-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.

Cyсlopentadienone and Pyrone Derivatives as Precursors of Electron-Deficient Cycloheptatrienes: Quantum Chemical Investigation and Synthesis

“…Thus, two bulky phenyl groups in 9a obstruct isomerization and the initially formed symmetrical isomer is prevalent. Previously it was shown that bulky aryl groups make the transition into axial isomer of heptaarylcycloheptatrienes irreversible and [1,5]-hydride shift takes place only when the initially formed equatorial conformer is long-living. 45,46 In our cases ester groups are less bulky and, moreover, alkyl groups in 9b,c afford simplified conformational flip and, thus, [1,5]-hydride shift 4 to make the more stable unsymmetrical isomer prevalent (in 9b) or equal (in 9c) under the same conditions.…”

“…Previously it was shown that bulky aryl groups make the transition into axial isomer of heptaarylcycloheptatrienes irreversible and [1,5]-hydride shift takes place only when the initially formed equatorial conformer is long-living. 45,46 In our cases ester groups are less bulky and, moreover, alkyl groups in 9b,c afford simplified conformational flip and, thus, [1,5]-hydride shift 4 to make the more stable unsymmetrical isomer prevalent (in 9b) or equal (in 9c) under the same conditions. Milder heating in acetic anhydride again afforded the initial symmetrical isomer of 9b, which rearranges into a mixture of isomers upon heating at 120 °C (see SI).…”

“…The introduction of multiple acceptor substituents onto the cycloheptatriene ring significantly alters its reactivity. In particular, unlike parent cycloheptatriene that forms electrophilic tropylium cation 1,2 electron-deficient cycloheptatrienes [3][4][5] form stable cycloheptatrienides, 5 reacting with both nucleophilic and electrophilic species to form various carbo-and heterocyclic compounds. 1,[6][7][8][9][10] There are only several general methods for the synthesis of such cycloheptatrienes.…”