Oxidation of Alkanes and Alkenes

Gold-catalyzed oxidation of styrene was firstly reported by Choudhary and coworkers for Au NPs supported on metal oxides in the presence of an excess amount of radical initiator, t-butyl hydroperoxide (TBHP), to afford styrene oxide, while benzaldehyde and benzoic acid were formed in the presence of supports without Au NPs [199]. Subsequently, Hutchings and coworkers demonstrated the selective oxidation of cyclohexene over Au/C with a catalytic amount of TBHP to yield cyclohexene oxide with a selectivity of 50% and cyclohexenone (26%) as a by-product [2]. Product selectivity was significantly changed by solvents. Cyclohexene oxide was obtained as a major product with a selectivity of 50% in 1,2,3,5-tetramethylbenzene while cyclohexenone and cyclohexenol were formed with selectivities of 35 and 25%, respectively, in toluene. A promoting effect of Bi addition to Au was also reported for the epoxidation of cyclooctene under solvent-free conditions. 

Caps and coworkers studied the solvent effect in the epoxidation of stilbene by varying solvents and the supports [200], In methylcyclohexane (MCH), the activated radical species proposed were MCH peroxy radicals, which were formed by the radical transfer from TBHP and reaction with molecular oxygen. Except for MCH, the solvent effect is not fully understood however, the choice of solvent and supports that can trap or stabilize the radical species affected the catalytic performance of Au. 

Aerobic oxidation of cyclohexane to cyclohexanol or cyclohexanone over Au catalysts has also been studied under solvent-free conditions and without radical initiators [201-203]. Gold on A1203 was relatively cyclohexanol selective while Au supported on more acidic supports such as silica exhibited cyclohexanone selective [201, 202]. Gold on MCM-41 showed a good selectivity to cyclohexanone of 76% accompanied by the formation of cyclohexanol (21%) at 19% conversion [202]. 

Although bulk gold is poorly active as a catalyst, Au nanoparticles (NPs) attached to various support materials exhibit unique catalytic properties in oxidation. General features can be summarized as follows  

Highly active Au catalysts can be prepared by an appropriate selection of preparation methods such as co-precipitation (CP), deposition-precipitation (DP), deposition-reduction (DR) and solid grinding (SG) with dimethyl Au(III) acetylacetonate, depending on the kind of support materials and reactions targeted. 

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The equilibrium (1) at the electrode surface will lie to the right, i.e. the reduction of O will occur if the electrode potential is set at a value more cathodic than E. Conversely, the oxidation of R would require the potential to be more anodic than F/ . Since the potential range in certain solvents can extend from — 3-0 V to + 3-5 V, the driving force for an oxidation or a reduction is of the order of 3 eV or 260 kJ moR and experience shows that this is sufficient for the oxidation and reduction of most organic compounds, including many which are resistant to chemical redox reagents. For example, the electrochemical oxidation of alkanes and alkenes to carbonium ions is possible in several systems... 

The supported Co2+-substituted Wells-Dawson POM, Cs6H2[P2W17061Co(OH2)], on silica was stable up to 773 K and catalyzed the heterogeneous oxidation of various aldehydes to the corresponding carboxylic acids with 02 as a sole oxidant [116], The H5PV2Mo10O40 POM, impregnated onto meso-porous MCM-41, catalyzed the aerobic oxidation of alkanes and alkenes using isobutyraldehyde as a... 

Meta11ophtha11ocyan1nes, porphyrines and salen complexes encaged In mainly Y zeolites have been reported to be active and shape selective in the oxidation of alkanes and alkenes. 

Mn TG, PS-macro glycine amides oxidation of alkanes and alkenes [70]... 

In a subsequent study Devynck and co-workers81,82 studied the electrochemical oxidation of alkanes and alkenes in triflic acid monohydrate. The acidity of CF3SO3H H20 was found to be intermediate between that of aqueous acid media and superacidity. Alkanes undergo two-electron oxidation, whereas alkenes are protonated to yield carbenium ions in this medium. In addition to various transformations characteristic of carbenium ions [Eqs. (5.36)—(5.38)], they undergo a reversible disproportionation to give an alkane and an aldehyde [Eqs. (5.40)]. 

The rate constants for oxidation of alkanes and alkenes with peroxynitrous acid in aqueous-gas phase are bell-shaped functions of the volume ratio between the liquid and gas phases. The kinetics of the generation of OH radicals and its importance in understanding the mechanism of lipid membrane oxidation has been stressed.236... 

A variety of alkylbenzenes undergo anodic acetoxylation, in which the loss of an a proton and solvation of the radical cation intermediate form the basis of side-chain and nuclear acetoxylation, respectively.30Sa b The nucleophilicity of the solvent can be diminished by replacing acetic acid with TFA. The attendant increase in the lifetimes of aromatic radical cations has been illustrated in anodic oxidations.308 Radical cations also appear to be intermediates in the electrochemical oxidation of alkanes and alkenes.309a-c... 

F. G. Doro, J. R. L. Smith, A. G. Ferreira, M. D. Assis, Oxidation of alkanes and alkenes by iodosylbenzene and hydrogen peroxide catalysed by halogenated manganese porphyrins in homogeneous solution and covalently bound to silica, J. Mol. Catal. A Chem. 164 (2000) 97. 

Vanadium. Vanadyl complexes of the three ligands 14, 24, and 28 were prepared from vanadyl sulfate and the metal-free ligands. They have the expected spectroscopic properties, and this is confirmed by the X-ray crystal structure of the vanadyl complex of 24, which is shown in Fig. 23(a) and has metal-donor bond distances in the expected range. In terms of reactivity, these complexes are rather unspectacular. No haloperoxidase reactivity was observed, and oxidation of alkanes and alkenes is rather sluggish (138). This is not unexpected since with the pentadentate ligands there is no free site at the vanadium(IV) center for metal-substrate interaction. The same is true for the vanadium(V) oxo-peroxo complexes with tetradentate coordination of the pentadentate ligand. 

Aldehydes may be formed as intermediates in the oxidation of alkanes and alkenes, initiated by a hydroxyl radical, and by direct attack of ozone upon alkenes. Attack by OH is the dominant sink as shown in reaction (23)... 

The major objective of this chapter is to provide a critical review of the kinetics and mechanisms of free radical oxidation of alkanes and alkenes and the techniques for their measurement and determination under mild conditions in the liquid phase. A brief discussion of photooxygenation (singlet oxygen) reactions is included for completeness. Literature has been reviewed carefully through 1975 and updated with references to mid-1978. 

Oxidation of hydrocarbons. Several variants of the aldehyde-metal salt combination effect the oxidation of alkanes and alkenes, among them salen-cobalt, CuCl2/18-crown-6, and vanadium-substituted zeolites. " A change of ligand for the cobalt ion has dramatic effects on the course of oxidation [allylic oxidation vs. epoxidation of cyclohexene by (1) and (2), respectively]. 

These Pd-Ti systems were active in the oxidation of other substrates such as alkanes, alkenes and alcohols. Hexane was hydroxylated into 2- and 3-hexanols, which were further oxidized in part to the corresponding ketones. In this case the product turnover was sensitive to the concentration of HCl. The addition of MeOH was effective as in the case of oxidation by H, , over TS-1. Finally we note that shape selectivity was found in the oxidation of alkanes and alkenes similarly to what was observed for the oxidation where H2O2 was used as oxidant the rates for oxidation of cyclic alkanes and alkenes were much lower than those of linear alkanes and alkenes. 

Indirect electrochemical oxidative carbonylation with a palladium catalyst converts alkynes, carbon monoxide and methanol to substituted dimethyl maleate esters (81). Indirect electrochemical oxidation of dienes can be accomplished with the palladium-hydroquinone system (82). Olefins, ketones and alkylaromatics have been oxidized electrochemically using a Ru(IV) oxidant (83, 84). Indirect electrooxidation of alkylbenzenes can be carried out with cobalt, iron, cerium or manganese ions as the mediator (85). Metalloporphyrins and metal salen complexes have been used as mediators for the oxidation of alkanes and alkenes by oxygen (86-90). Reduction of oxygen and the metalloporphyrin generates an oxoporphyrin that converts an alkene into an epoxide. 

Novel and innovative approaches for the selective catalytic homogeneous oxidation of alkanes and alkenes to produce industrially important alcohols, aldehydes, ketones, and epoxides aie still urgently needed. Homogeneous catalytic oxidation is a process that is more selective to products and, as well, the reactions aie carried out at lower temperatures in comparison to heterogeneous catalytic oxidation reactions. However, the separation of the homogeneous catalyst from the oxidation products is energy intensive for example, distillation, as well as possible thermal decomposition of the catalyst during distillation. We will discuss... 

Oxidations of alkane and alkene were atten ted by a system involving Sn porphyrins as a sensitizer and Fe porphyrins as an oxygen transfer agent, where... 

Suslick and his coworkers examined the shape selective oxidation of alkanes and alkenes catalyzed by the iron and manganese complexes of the bis-pocket porphyrin (Mn(TTPPP) and Fe(TTPPP) where TTPPP is tetrakis(2,4,6-triphenylphenyl)porphyrin) with a variety of oxidants such as mCPBA, PhIO, F5-PhIO [290]. The selectivities for terminal hydroxylation of n-alkanes are found to be very similar. While the details of the reactive intermediates are not clear, a peracid-Fe TTPPP adduct was obtained as a stable complex at -78°C to measure IR spectrum (Vc=o- 1744 cm l) [139]. However, an expected structure of an oxidant-metalloporphyrin complex could be too crowded to react with olefins. Uponwarming the solution temperature up to room temperature, the... 

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