Síntese de produtos fragrantes através da oxidação catalítica de olefinas de origem natural

Abstract

Linalool, a monoterpenic allylic alcohol, has been efficiently and selectively oxidized by dioxygen in methanol and ethanol solutions using a chloride-free Pd(OAc)2/Cu(OAc)2 catalytic system under non-acidic conditions. Corresponding allylic ether arising from the exclusive oxidation of a trisubstituted internal double bond has been characterized as a major product in each alcohol. In methanol, this product, 7-methoxyhotrienol, was obtained in a virtually quantitative yield. It is remarkable that a monosubstituted terminal double bond of the substrate remains intact. In acetic acid, catalytic palladium turnover was achieved only by using benzoquinone as a terminal oxidant, with the regioselectivity of allylic oxidation being completely switched from that in alcohols. A major product is 8-linalyl acetate formed with up to 85% selectivity. All linalool derivatives obtained have a pleasant scent with a flower or fruit tinge and are natural products found either in exotic plants or in grape wines. To the best of our knowledge, the present work reports a first synthesis of these functionalized monoterpenes potentially useful as components of synthetic perfumes. Based on the results obtained, the mechanism of this unusual oxidation reaction involving the formation of various organometallic intermediates is discussed, with bimetallic Pd-Cu catalysis being suggested. Reactivity of other monoterpenes, such as nerolidol, β-citronelene, citronelol, nerol, citral, citronelal, mircene and limonene, has been tested under similar conditions. The results obtained support the mechanism proposed for the oxidation of linalool and confirm the importance of the deactivated by allylic hydroxyl group terminal double bond for the palladium promoted selective oxidation of the internal double bond of the substrate. Some simple terminal olefins have also been converted into corresponding carbonilic compounds, which shows the efficiency the developed Pd-Cu system for this class of substrates as well. This work also describes a novel process for the oxidation of monoterpenic allylic alcohols with dioxygen catalyzed by doped magnetites (formula, Fe3−xMxO4 (M = Co; Mn; x = 0–0.75) in a solvent-free medium. Linalool was converted to the correspondent oxide with reasonable selectivity at ca. 25 % conversion. Nerol and geraniol were converted into citral, albeit the selectivity for this product did not exceed ca. 45 % due to its high reactivity under the reaction conditions. The catalysts undergo no metal leaching and can be easily recovered either magnetically or by filtration.