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 nonacidic 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 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. On the basis of 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.

Notes:

192HUTimes Cited:19Cited References Count:40