Electron Transfer‐Initiated Epoxidation and Isomerization Chain Reactions of β‐Caryophyllene

The abundant sesquiterpene β‐caryophyllene can be epoxidized by molecular oxygen in the absence of any catalyst. In polar aprotic solvents, the reaction proceeds smoothly with epoxide selectivities exceeding 70 %. A mechanistic study has been performed and the possible involvement of free radical, spin inversion, and electron transfer mechanisms is evaluated using experimental and computational methods. The experimental data—including a detailed reaction product analysis, studies on reaction parameters, solvent effects, additives and an electrochemical investigation—all support that the spontaneous epoxidation of β‐caryophyllene constitutes a rare case of unsensitized electron transfer from an olefin to triplet oxygen under mild conditions (80 °C, 1 bar O₂). As initiation of the oxygenation reaction, the formation of a caryophyllene‐derived radical cation via electron transfer is proposed. This radical cation reacts with triplet oxygen to a dioxetane via a chain mechanism with chain lengths exceeding 100 under optimized conditions. The dioxetane then acts as an in situ‐formed epoxidizing agent. Under nitrogen atmosphere, the presence of a one‐electron acceptor leads to the selective isomerization of β‐caryophyllene to isocaryophyllene. Observations indicate that this isomerization reaction is a novel and elegant synthetic pathway to isocaryophyllene.