Thermal Decomposition of Chroman. Reactivity of o-Quinone Methide

The gas phase thermal decomposition of 3,4-dihydro-2 H -1-benzopyran (chroman, 1 ) has been studied between 760 and 1110 K in different bath gases and hydrogen donors. In nitrogen, the unimolecular rate parameters are k 1 (s - 1 ) = 10 15.3 exp(−263 (kJ mol - 1 )/ RT ). The activation energy is slightly higher than the bond dissociation energy (BDE) of the phenoxylic C−O bond. The decomposition starts with elimination of ethene and formation of 6-methylene-2,4-cyclohexadien-1-one ( o -quinone methide, 2 ). Quinone methides are important intermediates in the chemistry of lignin. In the high temperature range (860−980 K) 2 decomposes cleanly into CO, benzene, and small amounts of fulvene, obeying k 2 (s - 1 ) = 10 14.8 exp(−281 (kJ mol - 1 )/ RT ). Reverse radical disproportionation of 2 with toluene is mainly responsible for o -cresol formation. In cis -2-butene at 770 K, exclusively cis -2,3-dimethylchroman is formed. This stereospecificity suggests a concerted retro-Diels−Alder mechanism and is not compatible with the high Arrhenius parameters, indicative of a stepwise, biradical mechanism.