35934-83-9Relevant articles and documents
The regioselectivity of the Birch reduction
Zimmerman, Howard E.,Wang, Patricia A.
, p. 2205 - 2216 (2007/10/02)
The reaction mechanism of the Birch reduction was investigated with a view of determinig how the regioselectivity is controlled. Regioselectivity is determined in the first step of radical anion protonation and in the second step of cyclohexadienyl carbanion protonation. It was ascertained that the rate-determining step of the Birch reduction of anisole was radical anion protonation, consistent with the observation of Krapcho and Bothner-By in the case of benzene reduction. A new approach to determining the regioselectivity of the two steps of the Birch reduction was devised. This was predicated on an enhanced primary deuterium isotope effect anticipated for radical anion protonation relative to that expected for cyclohexadienyl carbanion protonation. The approach utilized a partially deuterated medium. The method was applied to the reductions of anisole, 1,3-dimethoxybenzene, 3-methoxytoluane, and 2-methoxynaphthalene. The basic assumption of greater selectivity of the radical anion of the first step relative to the carbanion of the second step was explored in the cases of benzene and anisole and confirmed. In the examples studied, ortho protonation of the radical anion was found to predominate. With a view of understanding the regioselectivity of the two steps, quantum mechanical computations were carried out on several facets of the reaction. Electron density distributions of the radical anions were determined as well as the energies of radical products of some radical anion protonations. Similarly, the energies were obtained for the partially protonated radical anion species at several points along the reaction coordinate. In addition, electron densities were obtained for cyclohexadienyl anion. Theory was then correlated with experiment.
Aromatization of 1,4-Cyclohexadienes with Tetracyanoethylene: A Case of Varying Mechanisms
Jacobson, Barry M.,Soteropoulos, Patricia,Bahadori, Sheila
, p. 3247 - 3255 (2007/10/02)
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 (kH/kD = 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.
