Adding value to terpenes: copper-catalyzed oxidation of α-pinene in water under micellar conditions

Abstract

The development of sustainable protocols for the conversion of renewables such as terpenes into value-added products is currently in high demand, which motivates the search for inexpensive and environmentally tolerable catalytic systems and reaction conditions. In the present study, three new prospective catalysts featuring mono-, di-, and tricopper(II) cores were easily assembled in aqueous ethanol medium from copper(II) ions, amino alcohol and carboxylic acid ligands. Their catalytic performance was evaluated in water under micellar conditions with 1% of PS-750-M surfactant, while studying the mild oxidation of α-pinene as an abundant, low-cost, and renewable feedstock. A water-soluble monocopper(II) complex proved to be the most promising catalyst for the oxidation of α-pinene with tert-butyl hydroperoxide under micellar conditions, leading to high substrate conversion (87%) and good yields of the main products (tert-butylperoxy-2-pinene, verbenone, and pinene oxide). A partially water-soluble 1D coordination polymer based on dicopper(II) units also showed a notable catalytic behavior. The effects of different reaction parameters and mechanistic features were investigated. This work opens up the use of micellar catalysis systems and aqueous-medium conditions for the oxidative functionalization of α-pinene and other terpene feedstocks into value-added products.