Alkenes oxidation with molecular oxygen

In contrast to oxidation in water, it has been found that 1-alkenes are directly oxidized with molecular oxygen in anhydrous, aprotic solvents, when a catalyst system of PdCl2(MeCN)2 and CuCl is used together with HMPA. In the absence of HMPA, no reaction takes place(100]. In the oxidation of 1-decene, the Oj uptake correlates with the amount of 2-decanone formed, and up to 0.5 mol of O2 is consumed for the production of 1 mol of the ketone. This result shows that both O atoms of molecular oxygen are incorporated into the product, and a bimetallic Pd(II) hydroperoxide coupled with a Cu salt is involved in oxidation of this type, and that the well known redox catalysis of PdXi and CuX is not always operalive[10 ]. The oxidation under anhydrous conditions is unique in terms of the regioselective formation of aldehyde 59 from X-allyl-A -methylbenzamide (58), whereas the use of aqueous DME results in the predominant formation of the methyl ketone 60. Similar results are obtained with allylic acetates and allylic carbonates[102]. The complete reversal of the regioselectivity in PdCli-catalyzed oxidation of alkenes is remarkable. 

Free-radical reactions of tervalent phosphorus acids have been covered to some extent in Section II (addition of PX3 to alkenes, equation 15-17) and in Section IV (oxidations with molecular oxygen). Several other reactions occur via radicals, e.g. reactions with peroxides, certain disulphides and certain halogen compounds. However, these reactions are the subject of chapters by Bentrude and Danko wski in Vol. 1 of this series and will not be covered here. 

With the supply of large amounts of propane in the 1950s the search began to find a system for its direct oxidation with molecular oxygen to yield acrolein. Attempts with cuprous oxide marked the beginning of the technical development of alkene oxidation in the gas phase by metal oxide catalysts [22]. But this system showed weak points in the conversion (20%) [23,24] and in the selectivity, with the consequence that most of the propane added had to be recycled and many side products had to be removed. The development and introduction of the bismuth molybdate/bismuth phosphomolybdate system (Sohio, 1957) as a catalyst [25-27] and the following application for propane... 

Dioxygen is a cheap and ideal source of oxygen but it is very difficult to activate and there are relatively few examples of 02 oxidations catalyzed by zeolite-encapsulated complexes. Encapsulated CoPc is active for the oxidation of propene to aldehyde, whereas the free complex is inactive.104 A triple catalytic system, Pd(OAc)2, benzoquinone, and a metal macrocycle, was used to oxidize alkenes with molecular oxygen at room temperature.105 Zeolite-encapsulated FePc106-108 and CoSalophen (Scheme 7.5)107109 complexes were used as oxygen-activating catalysts. With the use of a Ru complex instead of Pd(OAc)2 in the triple catalytic system, primary alcohols can be oxidized selectively to aldehydes.110... 

A new stereoselective epoxidation catalyst based on a novel chiral sulfonato-salen manganese(III) complex intercalated in Zn/Al LDH was used successfully by Bhattacharjee et al. [125]. The catalyst gave high conversion, selectivity, and enantiomeric excess in the oxidation of (i )-limonene using elevated pressures of molecular oxygen. Details of the catalytic activities with other alkenes using both molecular oxygen and other oxidants have also been reported [126]. 

Using this scheme, we can track the original alkyl radical through the most likely mechanisms for oxidation at low temperatures that lead to chain-branching. Once formed from the parent molecule (R—H), an alkyl radical (R ) can react with molecular oxygen to form an alkene and hydroperoxyl (HO2 ) radical [Equation (2)], via... 

Alkenes and 1,3-dicarbonyl compounds together with molecular oxygen can be cyclized in a one-step synthesis to 1,2-dioxan-3-ols 159 by a thermodynamically-controlled Mn(lll)-based oxidation (Scheme 43). 

Another aspect of the gas-phase homogeneous oxidation reaction at high temperatures is that the major reaction of the alkyl radicals (ethyl and propyl) is their reaction with molecular oxygen to form alkene and 02H radical ... 

Catalytic oxidations by copper compounds are mainly homolytic in nature. Copper salts have been extensively used in conjunction with molecular oxygen, peroxides and persulfate for the oxidation of a variety of alkenic and aromatic hydrocarbons.56,584... 

In practice in the literature of the past 20 years the important results with ruthenium in epoxidation are those where ruthenium was demonstrated to afford epoxides with molecular oxygen as the terminal oxidant. Some examples are presented (see later). Also ruthenium complexes, because of their rich chemistry, are promising candidates for the asymmetric epoxidation of alkenes. The state of the art in the epoxidation of nonfunctionalized alkenes is namely still governed by the Jacobsen-Katsuki Mn-based system, which requires oxidants such as NaOCl and PhIO [43,44]. Most examples in ruthenium-catalysed asymmetric epoxidation known until now still require the use of expensive oxidants, such as bulky amine oxides (see later). 

The reaction with molecular oxygen gives an jj -peroxy complex, [Pt(jj -02)(PPh3)2], in which the oxygen is reactive, and the complex can oxidize PPhs, RNC, and alkenes to OPPh3, RNCO, and the epoxide. SO2, NO2, NO, and CO2 react to give the sulphate, dinitrato, dinitro, and carbonate complexes. The dioxygen complex is planar and seems to have substantial Pt character. ... 

Aqua(phosphine)ruthenium(II) complexes [121] are useful for activation of molecular oxygen, and catalytic oxidation of cyclohexene can be carried out with 1 atm of O2 [121a,bj. The ruthenium catalyst bearing perfluorinated 1,3-diketone ligands catalyzes the aerobic epoxidation of alkenes in a perfluorinated solvent in the presence of i-PrCHO [122]. Asymmetric epoxidations of styrene and stilbene proceed with 56-80% e.e. with ruthenium complexes 38-40 (Figure 3.2) and oxidants such as PhI(OAc)2, PhIO, 2,6-dichloropyridine N-oxide, and molecular oxygen [123-125]. 

Punniyamurthy T, Velusamy S, Iqbal J. Recent advances in transition metal catalyzed oxidation of organic substrates with molecular oxygen. Chem. Rev. 2005 105 2329-2363. McGarrigle EM, GUheany DG. Chromium- and manganese-salen promoted epoxidation of alkenes. Chem. Rev. 2005 105 1563-1602. 

Gasoline hydrocarbons volatilized to the atmosphere quickly undergo photochemical oxidation. The hydrocarbons are oxidized by reaction with molecular oxygen (which attacks the ring structure of aromatics), ozone (which reacts rapidly with alkenes but slowly with aromatics), and hydroxyl and nitrate radicals (which initiate side-chain oxidation reactions) (Stephens 1973). Alkanes, isoalkanes, and cycloalkanes have half-lives on the order of 1-10 days, whereas alkenes, cycloalkenes, and substituted benzenes have half- lives of less than 1 day (EPA 1979a). Photochemical oxidation products include aldehydes, hydroxy compounds, nitro compounds, and peroxyacyl nitrates (Cupitt 1980 EPA 1979a Stephens 1973). 

Reddy, M. M. Punniyamurthy, T. Iqbal, J., Cobalt Catalyzed Oxidation of Cyclic Alkenes with Molecular Oxygen Allylic Oxidation Versus Double BondAttack. Tetrahedron Lett. 1995, 36,159. 

C. Dobler, G. M. Mehltretter, M. Beller, Atom-efficient oxidation of alkenes with molecular oxygen Synthesis of diols, Angew. Chem. Int. Ed. Engl. 38 (1999) 3026. 

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