Cyclohexene, catalytic oxidation

The catalytic oxidation of isophorone (259—261) or P-isophorone (262,263) to ketoisophorone [1125-21 -9] (2,6,6-trimethyl-2-cyclohexen-l,4-dione) has been reported. Ketoisophorone is a building block for synthesis in terpene chemistry and for producing compounds of the vitamin A and E series. 

The epoxidation method developed by Noyori was subsequently applied to the direct formation of dicarboxylic acids from olefins [55], Cyclohexene was oxidized to adipic acid in 93% yield with the tungstate/ammonium bisulfate system and 4 equivalents of hydrogen peroxide. The selectivity problem associated with the Noyori method was circumvented to a certain degree by the improvements introduced by Jacobs and coworkers [56]. Additional amounts of (aminomethyl)phos-phonic acid and Na2W04 were introduced into the standard catalytic mixture, and the pH of the reaction media was adjusted to 4.2-5 with aqueous NaOH. These changes allowed for the formation of epoxides from ot-pinene, 1 -phenyl- 1-cyclohex-ene, and indene, with high levels of conversion and good selectivity (Scheme 6.3). 

Oxalate 111 is formed when the reaction is carried out in the presence of air. In that case, catalytic oxidation of cyclohexene to cyclohexen-3-ol takes place. The alcohol reacts with... 

A study of the catalytic oxidation of six stereoisomers of 5-cyclohexene-l,2,3,4-tetrol revealed448 that, in all cases, an allylic hydroxyl group is selectively attacked, and a consideration of their favored conformations suggested that quasi-axial groups are selectively dehydrogenated before those that are quasi-equatorial. 

The titanosilicate version of UTD-1 has been shown to be an effective catalyst for the oxidation of alkanes, alkenes, and alcohols (77-79) by using peroxides as the oxidant. The large pores of Ti-UTD-1 readily accommodate large molecules such as 2,6-di-ferf-butylphenol (2,6-DTBP). The bulky 2,6-DTBP substrate can be converted to the corresponding quinone with activity and selectivity comparable to the mesoporous catalysts Ti-MCM-41 and Ti-HMS (80), where HMS = hexagonal mesoporous silica. Both Ti-UTD-1 and UTD-1 have also been prepared as oriented thin films via a laser ablation technique (81-85). Continuous UTD-1 membranes with the channels oriented normal to the substrate surface have been employed in a catalytic oxidation-separation process (82). At room temperature, a cyclohexene-ferf-butylhydroperoxide was passed through the membrane and epoxidation products were trapped on the down stream side. The UTD-1 membranes supported on metal frits have also been evaluated for the separation of linear paraffins and aromatics (83). In a model separation of n-hexane and toluene, enhanced permeation of the linear alkane was observed. Oriented UTD-1 films have also been evenly coated on small 3D objects such as glass and metal beads (84, 85). 

Effect of Platinum Oxidation State in the Catalytic Oxidation of Cyclohexene... 

Fig. 59. Catalytic oxidative dehydrogenation of cyclohexene (O, surface catalysis) and oxidation of acetaldehyde ( , bulk-type II) the catalyst was HjPMonO supported on Si02. Masses catalyst 0.2 g for cyclohexene and 0.1 g for acetaldehyde. (From Ref. 327.)...

Fig. 60. Correlations between catalytic activity and oxidizing ability for (a) oxidation of acetaldehyde (surface reaction) and (b) oxidative dehydrogenation of cyclohexene (bulk-type 11 reaction). (From Ref. 327.) r(aldehyde) and r(hexene) show the rates of catalytic oxidation of acetaldehyde and oxidative dehydrogenation of cyclohexene, respectively. (From Ref. 337.) r( CO) is the rate of reduction of catalysts by CO r(H2) is the rate of reduction of catalysts by H2. M, denotes M,H3-,PMO 2O40. Na2-1, 2, 3, and 4 are Na2HPMoi2O40 of different lots, of which the surface areas are 2.8, 2.2, 1.7, and 1.2 m2 g, respectively.

This subject has recently been reviewed.647 Several additional papers have appeared on the catalytic oxidation of alkenes by 02 in the presence of PdCl(MeCN)2N02(148).64S Terminal alkenes and trans- cyclooctene yield the corresponding ketones, cyclopentene and cyclohexene the corresponding allyl alcohol, and bicyclic alkenes the corresponding epoxide. Heterometallacy-clopentanes such as (152) have been isolated from the reaction of (148) with norbornene (dicy-clopentadiene), and characterized by X-ray crystallography.6486 Glycol monoacetates were obtained from the reaction of (148) with terminal alkenes in acetic acid.649... 

The prevalence for epoxidation seems to be independent of the electronic structure of the porphyrin or the type of oxidant. For example, the catalytic oxidation of cyclohexene la with molecular oxygen and isobutyraldehyde in the presence of Fe3+(TPP)C1 (TPP = tetraphenylporphyrin) in 1,2-dichloroethane yielded exclusively the epoxide 4a and only trace amounts of the allylic alcohol 2a or ketone 3a, as demonstrated by Nam et al. [111]. 

All the polymers of Table III have been applied for the epoxidation of olefins with alkyl hydroperoxides. For example, the polymers with iminodiacetic acid or diethylene triamine groups were used for the regioselective epoxidation of (E)-geraniol with t-BuOOH to the 2,3-epoxide (225), whereas the Mo anchored to the diphenylphosphinopolystyrene catalyst is used in the epoxidation of cyclohexene with t-BuOOH (228). The polymer-supported molybdenyl thioglycolate has also been used for the catalytic oxidation of thiols and phosphines with air or pyridine N-oxide as the oxidant (234). 

As mentioned above, catalytic oxidation of olefins via coordination catalysis with an intermediate such as LnM (olefin) 02 seemed an attractive possibility, and Collman s group (45) tentatively invoked such catalysis in the 02-oxidation of cyclohexene to mainly 2-cyclo-hexene-1-one promoted by IrI(CO)(PPh3)2, a complex known to form a dioxygen adduct. Soon afterwards (4, 46, 47) such oxidations involving d8 systems generally were shown to exhibit the characteristics of a radical chain process, initiated by decomposition of hydroperoxides via a Haber-Weiss mechanism, for example Reactions 10 and 11. Such oxidations catalyzed by transition-metal salts such as... 

Figure 6. Correlations between catalytic activity and oxidizing ability (a) oxidation of acetaldehyde (surface type) and surface oxidizing ability (b) oxidative dehydrogenation of cyclohexene (bulk type) and bulk oxidizing ability [4, 38] (/(aldehyde) and /(hexene) are the rates of catalytic oxidation of acetaldehyde and cyclohexene, respectively).

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]. 

In the catalytic oxidative dehydrogenation of cyclohexene (OXD), both the flow of molecular oxygen in the system [33] and the presence of peroxides [34] in cyclohexene feed stock were found to be of prime importance as operational conditions. 

M. Guo, Catalytic oxidation of cyclohexene to adipic acid with a reaction-controlled phase transfer catalyst. Chin. J. Catal. 24 (2003) 483. 

The catalytic oxidation experiments were carried out in a round bottom flask equipped with condenser and stirrer. Typically, 6 or 12 mmol of substrate, 2 mmol bromobenzene (internal standard), 9 ml solvent (1,2-dichloroethane) and 100 mg zeolite (which contains typically around 2.9 pmol V (0.15wt%), TBHP/V ratio = 2070) were heated to 70°C, after which 6 mmol of TBHP in a 5.3 mmol chlorobenzene solution (which at the same time can function as an internal standard) were added to start the reaction. A sample was taken immediately afterwards. Before and during the reaction the mixture was purged with nitrogen for oxidations with cyclohexene and cyclooctene. Samples were filtrated over cotton wool and/or alumina, and triphenylphosphine was added to remove TBHP. In case of acetone as the solvent at 70°C, reactions were performed in a 50 ml autoclave and the reaction mixture was only purged with nitrogen before heating. After reaction TBHP was determined by iodometric titration. 

Our results with the CoNaY catalysts is quite comparable to the results of Lunsford and Dai [4]. They have reported the catalytic oxidation of cyclohexene with Co2,5NaY as catalyst and t-BuOOH as initator. They found a conversion of cyclohexene of 49.7 9 with a product distribution of 1-3 of 53.0, 39.4 and 6.0, respectively. 

Catalytic Activity and Product Distribution of the Catalytic Oxidation of Cyclohexene by... 

In order to unveil some of the details concerning the mechanism of the catalytic-oxidation of cyclohexene with CrY and NMP, some further experiments were performed. 

Influence of Different Parameters on the Catalytic Activity and Product Distribution of the Catalytic Oxidation of Cyclohexene by Metal-Exchanged Zeolite Y and l-Methyl-2-... 

Catalytic oxidation of cyclohexene and styrene were carried out with the following complexes... 

Catalytic oxidation of cyclohexene with cumene hydroperoxide gave 2 cyclohexen-l-ol and 2-cyclohexenone. Cydohexene oxide was not formed under the reaction conditions. The effect of various solvents on the oxidation reactions was smdied. The reaction was performed in polar, nonpolar, protic and aprotic solvents. Si ificant variation in product yield was observed. Table-1 shows the distribution of product yield with different solvents. Maximum conversion was observed in chloroform. The eflSciency of the catalyst for 2-cyclohexen-l-ol formation in chloroform is of the order, catalyst (2) > (3) > (1). When methanol was used as solvent the selectivity fijr epoxide was highest with catalyst (3), In this complex the selectivity fiar the formation of the allylic oxidation products are comparatively low. With catalyst (l),m methanol as solvent, 2-cyclohexen-l-ol was obtained in higher yield than eporade. With catalyst (1) the yield of the product was maximum when benzene was used as the solvent. Percentage conversion was the highest (39%). 

Catalytic oxidation of cyclohexene with NMO gave mainly 2-cyclohexenone.With complexes (2) and (3) minor amounts of 2-cyclohexen-l-ol was also formed. Table-3 shows the product yield with NMO as the oxidising agent. 

Catalytic oxidation of cyclohexene with CHP gave 2-cyclohexen-l-ol and 2-cyclohexenone respectively. In polar protic solvents epoxidation was significant. Oxidation of cyclohexene with NMO gave low conversion. The major product is 2-cyclohexenone. Styrene does not react with NMO under the reaction conditions. Oxidation of styrene with CHP gave benzaldehyde as the predominant product. 

However, catalytic oxidations of cyclohexene or ethylbenzene in the presence of RhCl(PPh3)3 give the products characteristic of free-radical reactions 36 thus cyclohexene gives cyclohexenone, cyclohexenol, cyclohexyl hydroperoxide and unidentified products. 

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