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-d, and p-dioxane-d8. Adducts were isolated in three cases and rate constants for adduct 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 consistent with electron-proton-electron-proton or electron-proton-hydrogen-atom transfer. Total reaction rate varied by a factor of over 4 X lo5, 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 (kH/kD = 5.2). With one diene, 3,3-dimethyl-l,4-~yclohexadiene, 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, 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.Tetracyanoethylene (TCNE) reacts with 1,4-cyclohexadienes (1,4-CHD's) to give tetracyanoethane and the corresponding arene.'I2 Although this reaction has not been applied with anything approaching the frequency of similar aromatization reactions using quinones (e.g. ochloranil or 2,3-dichloro-5,6-dicyanobenz~quinone~) yet, it is often very mild, has synthetic utility,* and perhaps might see more use if it were better understood.Nishiguchi et al. proposed a stepwise mechanism involving hydride abstraction or electron-proton-electron transfer to give a cyclohexadienyl cation for the TCNE r e a~t i o n .~ A little earlier, Haselbach and Rossi, during an investigation directed largely a t another matter, had isolated a reactive intermediate, 1, from the reaction of c?
