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

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 adduct, probably heterolytically. Rate constants for diene reaction were determined in acetonitrile-d₃ and p-dioxane-d₈. Adducts were isolated in three cases and rate constants foradduct breakdown determined for the 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 no detectable intermediates and seemed most consistentwith electron-proton-electron-proton or electron-proton-hydrogen-atom transfer. Total reaction ratevaried by a factor of over 4 × 10⁵, yet with one exception, the ratio of the two pathways varied very little. One possible explanation 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-l,4-cyclohexadiene, the otherwise minor mechanism became the sole one, asthe adduct formed was clearly not a concerted ene adduct. However, in this case aromatizationalso required a 1,2 methyl shift, and the fact that quantitative collapse to an adduct, without rearrangement, occurred instead ruled out a simple cation intermediate from hydride transfer. A reversible electron transfer therefore seems the likeliest first step for the minor mechanism.