Aerobic Palladium(II)/Copper(II)-Catalyzed Oxidation of Olefins under Chloride-Free Nonacidic Conditions

Abstract

A chloride-free Pd(OAc)2/Cu(OAc)2 system catalyzes the oxidation of organic substrates by molecular oxygen in nonacidic methanol solutions. Terminal olefins, i.e., 1-hexene, 1-octene, styrene, and 2-vinylnaphthalene, give the corresponding methyl ketones without a significant double-bond isomerization of long-chain substrates. The catalyst also promotes an unusual allylic oxidation of sterically encumbered trisubstituted internal double bonds in acyclic polyenes able to form η2−η2-chelates with palladium. Linalool and nerolidol, mono- and sesquiterpenic allylic alcohols, give corresponding allylic ethers arising from the exclusive oxidation of the internal double bonds. Monosubstituted terminal double bonds in both substrates remain intact due to the deactivating effect of the allylic hydroxyl group. Dihydromyrcene, a nonfunctionalized monoterpenic diene, gives allylic ether and methyl ketone arising from the oxidation of both trisubstituted and terminal double bonds, respectively. On the other hand, internal double bonds in the substrates without a terminal olefinic bond, such as citronellol, citronellal, nerol, and citral, do not undergo oxidation under similar conditions. A steric possibility for chelation of the terminal and internal double bonds on the same palladium atom seems to be a crucial factor to determine the reactivity of the internal double bonds in these substrates toward oxidation. The presence of the second η2-coordinated double bond in these chelates favors the activation of the allylic C−H bond of the olefin, which results in η3-allyl intermediates and, then, in allylic ethers. Nerol, a primary allylic alcohol, is oxidized to (Z)-citral, although the process is complicated by further nonselective transformations of citral. All oxygenated terpenic derivatives obtained in the present work have a pleasant scent with a flower or fruit tinge and are potentially useful as components of synthetic perfumes.

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

“…[12,14,28] Studying the reactivity of eugenol and safrole we observed that their acetic acid solutions with catalytic amounts of PdCl 2 , CuCl 2 , and LiCl readily consumed dioxygen; however, only small amounts of the corresponding methyl ketones were detected. Most of the reacted substrates were converted into high boiling products which were not detectable by GC.…”

“…[7] However, the Wacker process requires large amounts of chloride ions and acid to maintain a catalytic cycle; therefore, the system is highly corrosive and often causes the formation of chlorinated side products. Although systems with various alternative halide-free co-catalysts, such as CuA C H T U N G T R E N N U N G (OAc) 2 , heteropoly acids, nitrates, and benzoquinone, have been intensively studied in attempts to reduce the environmental impact of these processes, [8][9][10][11][12][13][14] the reoxidation of palladium(0) during the catalytic cycle under more friendly conditions remains a critical challenge.…”

“…[14][15][16][17][18][19][20] Nevertheless, only a few examples of ligand-modulated palladiumcatalyzed oxidations of terminal alkenes into methyl FULL PAPERS ketones using molecular oxygen as the sole oxidant have been published. [15,16,20] Recently, Kaneda and coworkers have discovered that the use of dimethylacet-A C H T U N G T R E N N U N G amide as a solvent allows one to perform an aerobic oxidation of a number of terminal alkenes without the need for additional co-catalysts or special ligands.…”

Selective oxidation of rhodinol to citral using H2O2-platinum black system under microwave irradiation

“…However, synthetic work has become more challenging where greener concept of chemical synthesis are required to overcome problems such as long reaction times [5], expensive oxidants [6][7], toxic solvents [8][9] and the simplicity of the work up procedures.…”

Palladium‐Catalyzed Oxidation of Phenyl‐Substituted Alkenes using Molecular Oxygen as the Sole Oxidant

“…[1] However, the Wacker process requires large amounts of CuCl 2 , chloride ions, and acid to maintain the catalytic cycle; therefore, the system is highly corrosive and often causes the formation of chlorinated side products. To overcome these problems, much effort has been devoted to the development of alternative halide-free co-catalysts, such as CuA C H T U N G T R E N N U N G (OAc) 2 , heteropoly acids, nitrates, and benzoquinone; [2][3][4][5][6][7][8] as well as, more recently, palladium catalysts modulated by special ligands. [8][9][10][11][12][13][14] In the latter systems, oxidatively robust (usually, nitrogen-containing) ligands are used to stabilize reduced palladium and promote its regeneration directly by molecular oxygen avoiding the use of corrosive additives.…”