On the enthalpy of formation of cyclohexa-2,4- and -2,5-dienone

Abstract

The generally accepted Δ_fH^°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 Δ_fH^°₂₉₈ for 1 at ≥−9 kcal mol⁻¹, at least 8 kcal mol⁻¹ above the current literature value of −17 ± 3 kcal mol⁻¹. Recent results on the gas-phase recombination of phenoxyl radicals also indicate a value for 1 15 kcal mol⁻¹ above Δ_fH^°₂₉₈ of phenol (40, −23 kcal mol⁻¹). Our DFT calculations led to −7.3 and −8.0 kcal mol⁻¹ for 1 and 2, respectively. Simple group additivity approaches yield values higher by ca. 6 kcal mol⁻¹—likely to be due to underestimating conjugation and specific structural effects. The hitherto accepted difference of 4 kcal mol⁻¹ 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 Δ_fH^°₂₉₈ (32) = +43 kcal mol⁻¹ rather than the literature value of +35 kcal mol⁻¹, and a value for 33 close to 43 kcal mol⁻¹.