From α-pinene feedstock to value-added products: scalable and recyclable copper(ii) catalysts for allylic oxidation

Abstract

α-Pinene is one of the most abundant and low-cost terpenes that can be used as an attractive and renewable feedstock for generating added-value products via allylic oxidation. Aiming to develop more sustainable catalytic materials for the functionalization of terpenes, in this work two new 2D coordination polymers (CPs), {[Cu₂(μ-pdc)(μ₃-pdc)(H₂mdea)(H₂O)₂]·2H₂O}_n (Cu-mdea) and {[Cu₂(μ-pdc)(μ₃-pdc)(H₃tipa)(H₂O)₂]·4H₂O}_n (Cu-tipa) {H₂pdc = 3,4-pyridinedicarboxylic acid, H₂mdea = N-methyldiethanolamine and H₃tipa = triisopropanolamine} were synthesized via a self-assembly method under green conditions. These compounds were obtained from low-cost and environmentally tolerable reagents, and their scale-up process was also optimized in a ‘crystalline reactor’ from the milligram to gram scale. The obtained CPs were applied as efficient and recyclable heterogeneous catalysts for the mild oxidation of α-pinene. The effects of various reaction parameters, temperature-dependent and mechanistic features, and catalyst stability were investigated in detail, leading to up to 93% of α-pinene conversion with good yields of 4-tert-butylperoxy-2-pinene (42%) and verbenone (25%) as main oxidation products. This study extends the application of α-pinene as a renewable feedstock for the synthesis of value-added oxidation products using recyclable heterogeneous catalysts and mild reaction conditions.