Catalytic condensation of 3-carene with formaldehyde

This work is a first comprehensive study of the catalytic condensation of renewable 3-carene with formaldehyde (FA) for the one-step preparation of terpenoid trans-4-hydroxymethyl-2-carene, a commercially significant fragrance and a chiral platform chemical. Commonly available Brønsted and Lewis acids, as well as aluminosilicates (halloysite nanotubes, K-10 montmorillonite, H-Beta-25 zeolite) are able to catalyze the reaction in acetic acid, however, selectivity to the desired product is limited due to further transformations by (i) acetylation and (ii) addition of a second FA molecule. Compounds of successive condensation with FA are mainly formed in the presence of ZnCl ₂ and LiClO ₄, giving up to 50 % yield, while weak to moderate acidic halloysite and K-10 (45–104 µmol/g) result in a low conversion of 3-carene with a prevalence of acetylation products (up to 46 %). The largest selectivity towards trans-4-hydroxymethyl-2-carene at substrate conversion of 50 % was observed with phosphoric acid (50–66 %). The yield of the desired terpenoid increased significantly with an excess of FA or the catalyst loading, because the amount of the active form of formaldehyde is the key factor in the reaction. Presence of water in the system also led to a certain selectivity increase explained by inhibition of the side reaction. Under optimized conditions using H ₃PO ₄-AcOH mixture as a catalyst at 15 °C, selectivity to the target product was ca. 67 %. A detailed mechanism for the 3-carene condensation with FA has been proposed. The DFT calculations show that further transformations of trans-4-hydroxymethyl-2-carene are energetically favorable in agreement with the experimental data The reaction pathways were also confirmed by kinetic modeling. Synthesis of the desired product was performed on a scale up to 25 g and a possibility of 3-carene recycling has been demonstrated.