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 perox ide in the presence of catalytic amounts of several manganese(III) porphyri n complexes with electron withdrawing and electron donating groups was exam ined. The reactions were carried out at room temperature in acetonitrile, u sing aqueous hydrogen peroxide as oxidant and ammonium acetate as co-cataly st. The oxidation reactions of 3-carene (1) showed high conversion of the subst rate with all metalloporphyrins tested and four major products were identif ied and characterised, namely alpha -3,4-epoxycarane (7), beta -3,4-epoxyca rane (8), 3-caren-5-one (9) and 3-carene-2,5-dione (10). Nerol (2) oxidatio n reactions gave rise to 2,3-epoxynerol(ll), 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 oxidat ion 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. (C) 2001 Elsevier Science B.V. All rights reserved.