Aromatization of 1,4-Cyclohexadienes with Tetracyanoethylene: A Case of Varying Mechanisms

Article abstract of DOI:10.1021/jo00249a020

The aromatization of 1,4-cyclohexadiene and four alkyl-substituted 1,4-cyclohexadienes with tetracyanoethylene was examined and found in four of five cases to involve two competing mechanisms.Most of each reaction proceeded by concerted ene addition followed by breakdown of the ene product, probably heterolytically.Rate constans for diene reaction were determined in acetonitrile-d3 and p-dioxane-d8.Adducts were isolated in three cases and rate constants for adduct breakdown determined for isolated compounds.Where the adduct could be observed but not isolated, a constant was calculated through computer simulation of the rate data.The minor mechanism competing with the ene addition displayed not detectable intermediates and seemed most consistent with electron-proton-electron-proton or electron-proton-hydrogen-atom transfer.Total reaction rate varied by a factor of over 4 * 10⁵, yet with one exception, the ratio of the two pathways varied very little.One possible explaonation for this, the presence of a common rate determining step preceding any hydrogen transfer (such as SET) was ruled out by the finding of a large primary isotope effect for hexadeuterio-1,4-cyclohexadiene disappearance (k_H/k_D = 5.2).With one diene, 3,3-dimethyl-1,4-cyclohexadiene, the otherwise minor mechanism became the sole one, as the adduct formed was clearly not a concerted ene adduct.However, in this case aromatization also required a 1,2 methyl shift, and the fact that quantitative collapse to an adduct, without rearrangement, occured instead ruled out a simple cation intermediate from hydride transfer.A reversible electron transfer therefore seems the likeliest first step for the minor mechanism.