No abstract

Up to 600 K, chlorinated benzenes and naphthalenes are inert in a hydrogen atom donating solvent such as 9,10-dihydroanthracene. However, when a hydroxyl or amine group is attached to the 2 or 4 position relative to chlorine, a surprisingly facile and selective hydrodehalogenation occurs at temperatures between 530 and 630 K. These features are the result of the onset of tautomeric equilibria for the chlorophenols or -anilines, creating the corresponding enones or imines, respectively, as reactive intermediates. The mechanism of dehalogenation involves the rate-determining transfer of a hydrogen atom from the solvent to the reactive intermediate according to a reverse radical disproportionation (RRD), followed by elimination of chlorine. The reactivity of mono- and dichlorophenols, chloroanilines, 4-chloro-1-naphthol, and 4-chloro-1-naphthylamine dissolved in 9,10-dihydroanthracene has been investigated. By means of density functional theory (DFT) calculations, the energies for the tautomers and their hydrogen affinities have been established. The experimental selectivities could be adequately reproduced by the computed data.

No abstract

The generally accepted ∆ f HЊ 298 values for cyclohexa-2,4-and -2,5-dienone 1 and 2 respectively) were critically revised, based on three different approaches: (1) evaluation of available kinetic data; (2) group additivity; (3) density functional theory calculations at the B3LYP/6-31G(d,p) level with isodesmic reactions. The kinetics of the gas-phase Claisen rearrangement of allyl phenyl ether (3) sets ∆ f HЊ 298 for 1 at ≥ Ϫ9 kcal mol Ϫ1 , at least 8 kcal mol Ϫ1 above the current literature value of Ϫ17 ± 3 kcal mol Ϫ1 . Recent results on the gas-phase recombination of phenoxyl radicals also indicate a value for 1 15 kcal mol Ϫ1 above ∆ f HЊ 298 of phenol (40, Ϫ23 kcal mol Ϫ1 ). Our DFT calculations led to Ϫ7.3 and Ϫ8.0 kcal mol Ϫ1 for 1 and 2, respectively. Simple group additivity approaches yield values higher by ca. 6 kcal mol Ϫ1 -likely to be due to underestimating conjugation and specific structural effects. The hitherto accepted difference of 4 kcal mol Ϫ1 between the two isomers also appears to be too large. The literature values for the analogous hydrocarbons, 5-methylenecyclohexa-1,3-diene (32) and 3-methylenecyclohexa-1,4-diene (33) are critically reviewed as well on the basis of similar approaches; we prefer ∆ f HЊ 298 (32) = ϩ43 kcal mol Ϫ1 rather than the literature value of ϩ35 kcal mol Ϫ1 , and a value for 33 close to 43 kcal mol Ϫ1 .