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

Abstract

The generally accepted ΔfH°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 ΔfH°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 ΔfH°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 ΔfH°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.