p-Menthadienes as Biorenewable Feedstocks for a Monoterpene-Based Biorefinery

A terpene-based biorefinery is described that uses crude sulfate turpentine (CST) and gum turpentine (GT) to produce mixtures of p-menthadienes (p-MeDs) as biorenewable terpene feedstocks. An acid catalyzed ring opening reaction (6 m aq. H₂SO₄, 90 °C) is first used to convert the major bicyclic monoterpenes (α-pinene, β-pinene, and 3-carene) in untreated CST (or GT with 5 mol% Me₂S) into mixtures of monocyclic p-MeDs. These unpurified sulfurous p-MeD mixtures (α-terpinene, γ-terpinene, and isoterpinolene) are then used as feedstocks for oxidative aromatization (OA), ozonolysis, Diels–Alder, and hydrogenation reactions to produce p-cymene, fragrances, anti-oxidants, drugs, biopolymers, and biofuels. Mechanistic studies of the OA reaction used to convert the p-MeDs into p-cymene reveal that p-cymene hydroperoxide acts as an initiator to produce polar radical intermediates that are stabilized by DMSO generated in situ through aerobic oxidation of Me₂S. This enables CST and GT to be converted into biorenewable p-cymene in 50–60% yields (two steps) using a process that only requires aqueous acid, oxygen, heat, and a final distillation step.