Thermal decomposition of chroman. Reactivity of o-quinone methide

The gas phase thermal decomposition of 3,4-dihydro-2H-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 band. 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.