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The Strained Sesquiterpene β-Caryophyllene as a Probe for the Solvent-Assisted Epoxidation Mechanism



In our attempt to synthesize β-caryophyllene oxide in food-compatible conditions, we observed the uncatalyzed and highly selective epoxidation of β-caryophyllene, a strained bicyclic sesquiterpene, in ethanol with aqueous H2O2 under radical-suppressing conditions without the addition of a catalyst. The unusual reactivity of β-caryophyllene allowed us to use it as a probe for the mechanism of the solvent-assisted epoxidation in a wide range of organic solvents. A kinetic study was performed to investigate the epoxidation mechanism; an excellent correlation was found between the observed epoxidation rates in different solvents and the Abraham’s hydrogen bond formation parameters of these solvents. By means of computational analysis, it was found that the main role of the solvent consists of the stabilization of the elongated O[BOND]O bond of H2O2 in the transition state through hydrogen-bond donation to the leaving OH moiety of H2O2. α-Humulene was found to possess similar reactivity as β-caryophyllene whereas isocaryophyllene—the unstrained isomer of β-caryophyllene—was unreactive.