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

The generally accepted Delta (f)H(298)degrees 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 r earrangement of allyl phenyl ether (3) sets Delta (f)H(298)degrees for 1 at greater than or equal to -9 kcal mol(-1), at least 8 kcal mol(-1) above th e current literature value of - 17 +/- 3 kcal mol(-1). Recent results on th e gas-phase recombination of phenoxyl radicals also indicate a value for 1 15 kcal mol(-1) above Delta (f)H(298)degrees of phenol (40, -23 kcal mol(-1 )). Our DFT calculations led to -7.3 and -8.0 kcal mol(-1) for 1 and 2, res pectively. Simple group additivity approaches yield values higher by ca. 6 kcal mol(-1)-likely to be due to underestimating conjugation and specific s tructural effects. The hitherto accepted difference of 4 kcal mol(-1) betwe en the two isomers also appears to be too large. The literature values for the analogous hydrocarbons, 5-methylenecyclohexa-1,3-diene (32) and 3-methy lenecyclohexa-1,4-diene (33) are critically reviewed as well on the basis o f similar approaches; we prefer Delta (f)H(298)degrees (32) = +43 kcal mol- L rather than the literature value of +35 kcal mol(-1), and a value for 33 close to 43 kcal mol(-1).