title: Electron transfer-initiated epoxidation and isomerization chain reactions of β-caryophyllene.
abstract: 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.
authors.affiliation: Centre for Surface Chemistry and Catalysis, KU Leuven, Kasteelpark Arenberg 23, 3001 Leuven (Belgium), Fax: (+32) 6321998.
authors.forename: Bart
authors.initials: B
authors.lastname: Steenackers
authors.forename: Nicolò
authors.initials: N
authors.lastname: Campagnol
authors.forename: Jan
authors.initials: J
authors.lastname: Fransaer
authors.forename: Ive
authors.initials: I
authors.lastname: Hermans
authors.forename: Dirk
authors.initials: D
authors.lastname: De Vos
mesh_terms: D002384:Catalysis; D004583:Electrons; D007536:Isomerism; D015394:Molecular Structure; D000081223:Polycyclic Sesquiterpenes; D012717:Sesquiterpenes / Q000737:chemistry*
chemical_list: D000081223:Polycyclic Sesquiterpenes; D012717:Sesquiterpenes; C024714:caryophyllene
keywords: epoxidation; isomerization; radical ions; reaction mechanisms; sesquiterpenes