Abstract
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 k1 (s-1) = 1015.3 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 k2 (s-1) = 1014.8 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.
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