Oxidation of unsaturated monoterpenes with hydrogen peroxide catalysed by manganese(III) porphyrin complexes

Oxidation of (+)-3-carene (1), nerol (2) and geraniol (3) by hydrogen peroxide in the presence of catalytic amounts of several manganese(III) porphyrin complexes with electron withdrawing and electron donating groups was examined. The reactions were carried out at room temperature in acetonitrile, using aqueous hydrogen peroxide as oxidant and ammonium acetate as co-catalyst. The oxidation reactions of 3-carene (1) showed high conversion of the substrate with all metalloporphyrins tested and four major products were identified and characterised, namely -3,4-epoxycarane (7), -3,4-epoxycarane (8), 3-caren-5-one (9) and 3-carene-2,5-dione (10). Nerol (2) oxidation reactions gave rise to 2,3-epoxynerol (11), 6,7-epoxynerol (12) and 2,3,6,7-diepoxynerol (13). In the case of geraniol (3), besides 2,3-epoxygeraniol (14), 6,7-epoxygeraniol (15) and 2,3,6, 7-diepoxygeraniol (16), the oxidation reactions afforded 6,7-epoxygeranial (17). The terminal 6,7 double bond of nerol and geraniol was preferentially epoxidised. The regioselectivity induced by different porphyrins was investigated.