Palladium-Catalyzed Oxidation of Monoterpenes:  Novel Aerobic Pd(II)/Cu(II)-Catalyzed Oxidation of Linalool under Chloride-Free Nonacidic Conditions

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.

Highly efficient bimetallic iron‐palladium catalyzed Michael‐type Friedel–Crafts reactions of indoles with chalcones

“…In addition, we found that the electron effect of enones' substituents had a profound impact on the yields of the reaction. For the same indole, enones with electron-donating substituents would give higher yields than those with electron-withdrawing substituents (entries [8][9][10][11], and the trend also applied to indoles. Interestingly, the enones with ortho-substituents afforded higher yields than those with para-substituents (entries 10 and 11).…”

“…The bifunctional, bimetallic catalysts have been applied to various organic transformations in recent decades, [1 -3] such as cyanation, [4] Friedel-Crafts acylation, [5] Friedel-Crafts alkylation, [6 -8] oxidation, [9] hydroformylation, [10] allylic etherification, [11] nitro-Mannich reaction [12] and other rections. [13 -15] Heterobimetallic catalysts offer superior results in terms of efficiency and selectivity relative to the individual ones.…”

Palladium‐Catalyzed Aerobic Oxidation of Naturally Occurring Allylbenzenes as a Route to Valuable Fragrance and Pharmaceutical Compounds

“…After carrying out the reaction and cooling to room temperature, the excess of oxygen was slowly vented from the autoclave. The products were isolated by a column chromatography (silica gel 60) using mixtures of hexane and CH 2 Cl 2 as eluents and identified by 1 H, and 13 C NMR and/or GC-MS.…”

Palladium-Catalysed Oxidation of Bicycle Monoterpenes by Hydrogen Peroxide in Acetonitrile Solutions: A Metal Reoxidant-Free and Environmentally Benign Oxidative Process

“…The oxidation of natural olefins by dioxygen in systems type Wacker (PdCl 2 /CuCl 2 ) is limited because the CuCl 2 reoxidant acting as a Lewis acid, promote undesirable carbon skeletal rearrangements of those substrates, which are accompanied by nucleophilic addition of chloride ions and or protic solvents [23]. The selectivity reported for the oxidation product in those reactions does not exceed 25% [24].…”