Phase Oxidation of Alkenes

The first extensive attempt to use nitrous oxide as a selective oxidant in the liquid phase was made in the early 1950s by ICI researchers Bridson-Jones et al. [167]. The gas-phase 

The authors [167] tested a significant number of organic substrates. The oxidation of alkenes leading to aldehydes and ketones was the most interesting discovery. However, the reaction selectivity was quite low, not exceeding 65% in best cases. Because of very the harsh reaction conditions, which are difficult to provide in laboratory practice, these modest results did not stimulate further studies and then virtually dropped out of the researchers sight. 

In 2002, this type of N20 oxidation was re-discovered by Panov et al. [168]. Being guided by quite a different idea, the authors [168] used milder conditions and obtained much better selectivity, which in many cases exceeded 90%. Such a high selectivity was shown to relate to a non-radical type reaction mechanism as well as to a remarkable feature of the oxidant. N20 reacts solely with alkene C=C bonds and is inert towards all other bonds. Therefore, reaction products having no double bonds are not subjected to overoxidation. Only non-oxidation side processes maybe a reason for decreasing selectivity. 

The ability of nitrous oxide to forma 1,3-dipole (Section 7.2) seems to be of critical importance for the reaction with alkenes. The oxygen transfer proceeds via the 1,3-dipolar cycloaddition mechanism, assuming intermediate formation of a 1,2,3-oxadiazoline complex, the decomposition of which leads to a carbonyl compound  

This mechanism, first suggested by Bridson-Jones et al. [167], explains all experimental results and recently was strongly supported by quantum chemical calculations [169-171]. 

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Good yields of carbonyl compounds have also been obtained from the vapor-phase oxidation of alkenes by steam and air over palladium catalysts supported on charcoal.413 In this case, no copper cocatalyst is needed, presumably because palladium(II) is not reduced to palladium(O), but remains in the form of a stabilized palladium(Il) hydride which can react with 02 to give the hydroperoxidic species. 

H.F.W.J. van Breukelen, M.E. Gerritsen, V.M. Ummels, J.S. Broens J.H.C. van Hooff (1997). Stud. Surf Sci. Catal., 105, Part A-C, 1029-1035. Application of CoAlPO-5 molecular sieves as heterogeneous catalysts in liquid phase oxidation of alkenes with dioxygen. 

Halohydrins are easily prepared and dehydrohalogenation occurs readily at low temperatures. Another way epoxides can be formed is by catalytic vapor-phase oxidation of alkenes to form oxiranes. 

With higher alkenes, three kinds of products, namely alkenyl acetates, allylic acetates and dioxygenated products are obtained[142]. The reaction of propylene gives two propenyl acetates (119 and 120) and allyl acetate (121) by the nucleophilic substitution and allylic oxidation. The chemoselective formation of allyl acetate takes place by the gas-phase reaction with the supported Pd(II) and Cu(II) catalyst. Allyl acetate (121) is produced commercially by this method[143]. Methallyl acetate (122) and 2-methylene-1,3-diacetoxypropane (123) are obtained in good yields by the gas-phase oxidation of isobutylene with the supported Pd catalyst[144]. 

The Phase-Transfer-Assisted Permanganate Oxidation of Alkenes and Alkynes... 

The versatility of permanganate as an oxidant has been greatly enhanced in the past decade by the observation that it can be solubilized in nonaqueous solvents with the aid of phase transfer agents (1). The literature contains descriptions for the use of this procedure for the oxidation of alkenes (2-13), alkynes (13-18), aldehydes (19), alcohols (20), phenols (21,22), ethers (23), sulfides (24,25), and amines (20,26). The dehydrogenation of triazolines has also been achieved by the use of permanganate and a phase transfer agent (27). ... 

Solvent Effects. Information on the effect of solvent polarity of the phase transfer assisted permanganate oxidation of alkenes has been obtained by studying the oxidation of methyl cinnamate by tetrabutylammonium permanganate in tv/o different solvents, acetone and methylene chloride (37). 

Liquid-Phase Oxidation of High Molecular Weight 1-Alkenes... 

Gutbrod, R., R. N. Schindler, E. Kraka, and D. Cremer, "Formation of OH Radicals in the Gas Phase Ozonolysis of Alkenes The Unexpected Role of Carbonyl Oxides, Chem. Phys. Lett., 252, 221-229 (1996). 

In short, the Criegee intermediate from alkene-ozone reactions can contribute, in principle, to the gas-phase oxidation of S02. In practice, it is likely less important than reaction with OH. In addition, as we shall see, even the OH-SOz gas-phase reaction is, under many conditions, swamped out by reactions occurring in the liquid phase found in clouds and fogs. As a result, the CI-S02 reaction may contribute in some circumstances but is unlikely to be a major contributor to S02 oxidation as a whole. 

It has recently been found that NEt3 is a gas-phase promoter for propene epoxidation by supported gold catalysts [245]. In more recent studies, Hughes et al. reported that catalytic amounts of peroxides could initiate the oxidation of alkenes with 02, without the need for sacrificial H2 [243]. The process worked for a range of substrates (cyclohexene, ds-stilbene, styrene and so on) and even in the absence of solvent hence, we may refer to this as green technology. 

The gas-phase oxidation of ethylene to ethylene oxide over a supported silver catalyst was discovered in 1933 and is a commercially important industrial process. Using either air or oxygen, the ethylene oxide is produced with 75% selectivity at elevated temperatures (ca. 250 °C). Low yields of epoxides are obtained with propylene and higher alkenes so that other metal-based catalysts are used. A silver-dioxygen complex of ethylene has been implicated as the active reagent.222... 

Oxidation of alkenes to ketones.3 Both internal and terminal alkenes are oxidized by PdCl2 (with CuCl2 as reoxidant) in water-polyethylene glycol (PEG), serving as the phase-transfer catalyst as well as the solvent (9, 360, 376). This oxidation is more facile than that catalyzed by quaternary ammonium salts, which is applicable only to terminal alkenes. 

Some synthetically useful oxidations of alkenes by permanganate can be performed under controlled conditions. For example, 1-decene could be oxidized to nonanoic acid in 91% yield by permanganate in the presence of the phase-transfer agent Aliquat 336.319 In a benzene solution with crown ether and permanganate, a-pinene is oxidized to cis-pinonic acid in 90% yield (equation 110).314... 

The synthetic applications of the palladium-catalyzed oxidation of alkenes to ketones have recently been reviewed.639 Improvements in the Wacker palladium-catalyzed ketonization of terminal alkenes have been obtained using phase-transfer catalysis,641 polyethylene glycol642 or phosphomolybdovanadic acids.643... 

Phase transfer oxidation of alkenes Ru04/NaI04, 0s04... 

Van Sickle DE, Mayo F, Arluck RM. Liquid-phase oxidation of cyclic alkenes. J Am Chem Soc 1965 87 4824 4832. 

The selective oxidation of organic molecules is one of the most important processes used in the chemical industry. Its reactions fall into two broad categories (i) gas-phase oxidation of hydrocarbons (alkanes, alkenes) to oxygenated products and (ii) further oxidation of molecules containing one or more oxygen atoms (mainly in the liquid phase). 

A detailed study of the oxidation of alkenes by O on MgO at 300 K indicated a stoichiometry of one alkene reacted for each O ion (114). With all three alkenes, the initial reaction appears to be the abstraction of a hydrogen atom by the O ion in line with the gas-phase data (100). The reaction of ethylene and propylene with O" gave no gaseous products at 25°C, but heating the sample above 450°C gave mainly methane. Reaction of 1-butene with O gives butadiene as the main product on thermal desorption, and the formation of alkoxide ions was proposed as the intermediate step. The reaction of ethylene is assumed to go through the intermediate H2C=C HO which reacts further with surface oxide ions to form carboxylate ions in Eq. (23),... 

Molybdenum trioxide constitutes an active model catalyst for the oxidation of propene in the presence of gas-phase 02 at temperatures above approximately 600 K (Grzybowska-Swierkosz, 2000). Reduction of M0O3 in propene and oxidation of Mo02 in 02 were investigated by time-resolved XAFS spectroscopy combined with mass spectrometry (Ressler et al., 2002). Reduction and reoxidation of M0O3 x are of particular interest because they constitute the two fundamental transformations of the so-called redox mechanism for partial oxidation of alkenes on molybdenum oxide catalysts. 

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