Liquid-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 N2O 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. N2O 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 may be 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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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. 

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

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

Differences between copper and silver have been seen for the heterogeneously catalysed liquid-phase oxidation of cumene. Copper appears to initiate the reaction by decomposition of the hydroperoxide. The subsequent free-radical reaction occurs in the liquid phase. There is some evidence that silver initiates, this process via an adsorbed molecular oxygen species. If this is correct, this species is reacting in a different manner when compared to its reaction with gas-phase alkenes. In the latter, O—O bond cleavage occurs while the cumene oxidation mechanism would necessarily involve Ag—O bond cleavage. 

The catalytic activity of various complex compounds of technetium was tested in the metathesis of olefins [17], epoxide ring opening reactions [18], epoxide ring formation by reaction of cyclohexene with fe/7-butylhydroperoxide [19,20], and the preferred production of tram-epoxides in the liquid phase oxidation of cw/tram-n-alkenes [21 j. 

Pb(C2H5)4 is employed as a chain-starter for the alkylation of alkanes or cycloalkanes by alkenes or cycloalkenes [697], and in the liquid-phase oxidation of alkylaromatic hydrocarbons to give hydroperoxides [892]. Pb(C2Hs)4 catalyzes the photochemical addition of HBr [661, 681] or of H2S or mercaptans to alkenes, such as propene [661, 679, 681], and it catalyzes the intermolecular condensation of arylalkanes in sunlight or on heating [684]. 

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. 

Alkylation. Friedel-Crafts alkylation (qv) of benzene with ethylene or propylene to produce ethylbenzene [100-41 -4], CgH10, or isopropylbenzene [98-82-8], C9H12 (cumene) is readily accomplished in the liquid or vapor phase with various catalysts such as BF3 (22), aluminum chloride, or supported polyphosphoric acid. The oldest method of alkylation employs the liquid-phase reaction of benzene with anhydrous aluminum chloride and ethylene (23). Ethylbenzene is produced commercially almost entirely for styrene manufacture. Cumene [98-82-8] is catalytically oxidized to cumene hydroperoxide, which is used to manufacture phenol and acetone. Benzene is also alkylated with C1Q—C20 linear alkenes to produce linear alkyl aromatics. Sulfonation of these compounds produces linear alkane sulfonates (LAS) which are used as biodegradable deteigents. 

Homometathesis of cyclic alkenes affords larger rings. For example, cyclohex-adecadiene (57) was prepared by liquid-phase metathesis of cyclooctene (56) using WCl5/Et3Al [21], or in the gas phase by short-time contact of cyclooctene to a supported catalyst of Re207 activated with Me4Sn [22], Cyclohexadecenone (58), a muscone-type perfume, is produced commercially from 57 by selective oxidation. 

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). 

In both the synthetic organic laboratory and industry, the first and foremost procedure for the preparation of oxiranes is the direct oxidation of alkenes. Significant new results have been achieved in the development of methods of oxidizing alkenes in the liquid phase. The major aim is the attainment of an oxidation reaction under the mildest possible experimental conditions, which allows an increase in the selectivity of oxirane formation and permits the selective oxidation of more sensitive compounds. Since the various methods of preparing oxiranes were reviewed quite recently, the individual oxidation procedures will be mainly illustrated here with some more recent examples. Surveys concentrating on stereo-controlled epoxidations and assymmetric synthetic methods have been published. "... 

Palladium catalysts are widely used in liquid phase aerobic oxidations, and numerous examples have been employed for large-scale chemical production (Scheme 8.1). Several industrially important examples are the focus ofdedicated chapters in this book Wacker and Wacker-type oxidation of alkenes into aldehydes, ketones, and acetals (Scheme 8.1a Chapters 9 and 11), 1,4-diacetoxylation of 1,3-butadiene (Scheme 8.1b Chapter 10), and oxidative esterification of methacrolein to methyl methacrylate (Scheme 8.1c Chapter 13). In this introductory chapter, we survey a number of other Pd-catalyzed oxidation reactions that have industrial significance, including acetoxylation of ethylene to vinyl acetate (Scheme 8. Id), oxidative carbonylation of alcohols to dialkyl oxalates and carbonates (Scheme 8.1e), and oxidative coupling of dimethyl phthalate to 3,3, 4,4 -tetramethyl biphenylcarboxy-late (Scheme 8.1f). 

Dioxepanes have been manufactured by treatment of alkenes in the liquid phase with oxygen and carbon monoxide with PdCyCuCl as a catalyst, for example synthesis of (12 X = CN) from acrylonitrile <94JAP0608778i>, or by oxidation of alkenes with Th(III)-acetate in 1,4-diols <76BCJ3285>. Treatment of perfluoro(2-methyl-2-pentene) with 1,4-butanediol and triethylamine leads to (27)... 

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