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. H 2 SO 4 , 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% Me 2 S) 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 Me 2 S. 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.