Oxidations of Alkenes

Alkenes are readily oxidized, products depending strongly upon the solvent and also upon nucleophiles present. 

Oxidation of alkenes to methyl ketones using oxygen has been developed in both synthetic organic chemistry and industrial processes. A well-known example is the Wacker process using PdCl2 and CUCI2 as catalysts in acidic water or an organic solvent [Eq. (7)]. 

Wacker oxidation of 1-hexene has been carried out in scC02-[BMIM][PFg] at 40 °C and 12.5 MPa total pressure (2.1 MPa O2) [47]. The main products for this reaction were 2-hexanone and 3-hexanone, with 2-hexanone being the desired product. The conversion approached 100% after 17 h in aU the solvent systems studied (SCCO2 only, [BMIM][PFg] only, scC02-[BMlM][PFg]) as well as in a solventless reaction. Very similar rates were observed in scC02and in scC02-[BMlM][PFgj. 

Partial oxidation of n-hexadecane has been achieved under the more extreme conditions of scH20, but this gave a complex mixture of small (Cl to C4) hydrocarbons, as well as C02, CO and H2 [6], 

The epoxidation of alkenes vdth metaHoporphyrins has been studied as model reactions of cytochrome P-450 [37]. Ruthenium porphyrins such as Ru(OEP) (PPh3)Br (OEP = octaethylporphyrinato) have been examined for the catalytic oxidation of styrene with PhIO [38]. Hirobe and coworkers [39] and Groves and 

Non-porphyrin ruthenium complexes such as [RuCl(DPPP)2] (DPPP = l,3-bis (diphenylphosphino)propane), [Ru(6,6-Cl2bpy)2(H20)2], binaphthyl-mthenium complex, and RUCI3 catalyze oxidations of alkenes with PhIO [45], t-BuOOH [46], PhI(OAc)2 [47], or H2O2 [48] to give the corresponding epoxides in moderate yields. 

The ruthenium-catalyzed aerobic oxidation of alkenes has been explored by several groups. Groves and coworkers reported that Ru(TMP)(0)2 (3)-catalyzed aerobic epoxidation of alkenes proceeds under 1 atm of molecular oxygen without any 

If one could trap the intermediate 5 with external nucleophiles, such as water, a new type of catalytic oxidation of alkenes could be performed. Indeed, a transformation of alkenes into a-ketols was discovered to proceed highly effldently. Thus, the low-valent ruthenium-catalyzed oxidation of alkenes with peracetic acid in an aqueous solution under mild conditions gives the corresponding a-ketols, which are important key structures of various biologically active compounds (Eq. (7.21)) [60]. 

Typically, the RuCl3-catalyzed oxidation of 3-acetoxy-l-cyclohexene (6) with peracetic acid in H2O-CH3CN-CH2CI2 (1 1 1) gave (2R, 3S )-3-acetoxy-2-hydroxycy-clohexanone (7) chemo- and stereoselectively in 78% yield (Eq. (7.22)). 

When alkenes react with a hot, basic solution of KMn04 they are oxidized to carboxylic acids. If straight chain alkenes are used, monocarboxylic acids are produced. 

Preparation of carboxylic acids by the reaction of Grignard reagents with carbon dioxide is the most commonly used method. 

Carbonation of Grignard reagents is similar to the addition of Grignard reagents to aldehydes and ketones. However, in the former reaction, carboxylate is produced, not alcohol. The magnesium carboxylate produced does not dissolve in ethers but when dissolved in acid, carboxylic acids are formed. 

HzO or the OH- ion may react with the carbonyl group of an ester to form carboxylic acids. For example esters can be hydrolyzed by heating with water in the presence of an acid catalyst. 

Oxidefj pj of alkenes may produce glycols (hydroxyl groups on adjacent carbons) or oxidation may cleave the alkene at the double bond as in ozonohjsis. 

Alkynes produce carboxylic acids when undergoing ozonolysis, 

In general, alkenes are more easily oxidized than alkanes by chemical oxidizing agents. These reagents attack the pi electrons of the double bond. The reactions may be useful as chemical tests for the presence of a double bond or for synthesis. 

When compared to Cr-Si02/sol-gel material (prepared using no surfactant), both catalysts were able to oxidise the biomass-based alkene substrates with similar distributions of the types of products obtained from each substrate. However, the selectivity of the Cr-MCM-41 catalyst was much higher than the Cr-SiOa/sol-gel material. The differences in the combined product selectivities between the two catalyst materials for p-pinene, a-pinene, and limonene decreased from 92 to 74%, 83 to 59%, and 75 to 48%, respectively. Although the sol-gel material has a larger pore size and higher number of pores, the overall selectivity decreased these results are 

A year later, MCM-41 was used to prepare a Cr(salpr)MCM-41 (salpr=Ar,Ai -bis(3-salicylidenaminopropyl)amine) catalyst for the benzylic oxidation of allqrl aromatics. Masteri-Farahani et al. used a chloropropyl-modified MCM-41 material that was mixed with a [Cr(salpr)(H20)Cl] complex to produce their chromium catalyst. Using tert-bulylhydroperoxide (TBHP) as an oxidant, a mixture of the catalyst and substrate in solvent (chloroform or the substrate itself) was reacted for 7 h to afford the oxidised product. The substrates tested were toluene and ethyl benzene, which showed high selectivity towards the oxidised product benzaldehyde or acetophenone, respectively. 

When chloroform was used as the solvent, significantly lower conversions were observed compared to using the substrate as the solvent. This is due to the higher temperatures used under solvent-free conditions, which results in increased activation of the allq lperoxo chromium intermediate at these temperatures and faster transfer of the o)ygen atom. Thus, the conversion of toluene to benzaldehyde increased from 19 to 98% in the absence of chloroform. Similarly, the conversion of ethylbenzene to acetophenone increased from 25 to 98%. The selectivities under either set of conditions were similar for the oxidation of toluene. However, when ethylbenzene was used as the solvent in its oxidation, the selectivity to acetophenone increased from 67 to 89%, showing the beneficial effect of the higher boiling point solvent. 

Several advantageous links to green chemistry are seen in this study. First, the use of the substrate as the solvent in the reaction limits the use of auxiliary substances. Also, the catalyst choice of a low-valent chromium complex helps to reduce the overall toxicity of the catalyst in the reaction. The catalyst material is also heterogeneous, which allows for easy separation of the catalyst and the product. Finally, the desired product was obtained in high selectivity, which reduced the amount of byproduct waste produced. 

The major product from the former is cyclohexen-3-one, along with minor amounts of cyclohexene oxide (17). Epoxide formation has also been identified as a minor product from cyclopentene autoxidation. The intermediacy of 3-cyclohexene hydroperoxide was proposed in this report but not verified/ Subsequent work on the autoxidation of cyclohexene using RhCl(PPh3)3 verified this premise/ and a number of review articles have emphasized this conclusion.The involvement of preformed hydroperoxide has been verified by comparing the rate of cyclohexene oxidation both with hydroperoxide present, and also when the cyclohexene is purified free from peroxide. In the former case the reaction is rapid and there is no induction period. Under conditions where the cyclohexene is peroxide free the reaction proceeds more slowly, and there is an induction period of close to three hours (using IrCl(CO)(PPh3)2 as catalyst) as the hydroperoxide intermediate is being performed (18)  

The mechanistic details of the subsequent chemistry are unclear, but it is apparent that the catalytic decomposition of hydroperoxide follows a chain process that resembles a Haber-Weiss pathway. It has also been found that complexes show a greater activity for cyclohexene autoxidation that do complexes, but no rationale has been provided to explain this observation. 

As a final generalization, it should be mentioned that these radical chain processes are common when transition metal compounds other than phosphine complexes are used. Catalytic oxidations have been carried out with a wide range of metal complexes, and a comparison has been observed between reactions catalyzed by phosphine complexes and those with acetyl-acetonate or other ligands. 

A ruthenium porphyrin complex immobilized in a polymer can be used for catalytic epoxidation with 2,6-dichloropyridine N-oxide [112], Nitrous oxide (N2O) can be also used as oxidant for the epoxidation of trisubstituted olefins in the presence of ruthenium porphyrin catalyst [113], Asymmetric epoxidations have been reported using chiral ruthenium porphyrin complexes 35 [114], 36 [115], and 37 [116] (Eq. 3.62). 

A Ru-containing polyoxometalate, [WZnRu2(0H)(H20)](ZnW9034)2 (Eq. 3.64) [119] and a sterically hindered ruthenium complex, [Ru(dmp)2(CH3CN)2](PF6) (dmp = 2,9-dimethyl-l,10-phenanthroline) [120] are effective for the epoxidation with molecular oxygen. 

The three most common alkene oxidation reactions are epoxidation, dihydrox-ylation, and ozonolysis. Epoxides are formed when an alkene is treated with a peroxyacid, such as mCPBA. Since both C-O bonds are formed in the same step (described as a concerted mechanism), the stereochemistry of the starting alkene is preserved in the product. 

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.  

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The attack of OH obeys the Markovnikov rule. Higher alkenes are oxidized to ketones and this unique oxidation of alkenes has extensive synthetic appli-cations[23]. The oxidation of propylene affords acetone. Propionaldehyde is... 

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. 

Three membered rings that contain oxygen are called epoxides At one time epox ides were named as oxides of alkenes Ethylene oxide and propylene oxide for exam pie are the common names of two industrially important epoxides... 

Hydroboration-oxidation of alkenes (Section 6 11) H and OF add to the double bond with a regioselectivity opposite to that of Markovnikov s rule This is a very good synthetic method addition is syn and no rearrangements are observed... 

Overall the stereospecificity of this method is the same as that observed m per oxy acid oxidation of alkenes Substituents that are cis to each other m the alkene remain CIS m the epoxide This is because formation of the bromohydrm involves anti addition and the ensuing intramolecular nucleophilic substitution reaction takes place with mver Sion of configuration at the carbon that bears the halide leaving group... 

The intermediate formed in the oxidation of alkenes by permanganate ion is considered a cycHc manganate(V) ester (92). Investigations have suggested that manganate(V) intermediates play a significant role in virtually all permanganate oxidation reactions. It is therefore the further reactions of the... 

Most frequent are oxidations of alkenes that can be converted to a series of compounds such as epoxides, halohydnns and their esters, ozonides (1,2,4 tri-oxolanes), a-hydroxyketones, a-hydroxyketone fluorosulfonates, ot-diketones, and carboxylic acids and their denvatives... 

Chlorohydnns and 1,2-dichloro denvatives are obtamed by oxidation of alkenes with fert-butyl hypochlorite when the reaction is performed in acetic acid instead of water, chlorohydrm acetate is formed [Ji] (equation 25)... 

Peroxy acid oxidation of alkenes (Sections 6.18 and 16.9) Peroxy acids transfer oxygen to alkenes to yield epoxides. Stereospecific syn addition is observed. 

The oxidation of alkenes and allylic alcohols with the urea-EL202 adduct (UELP) as oxidant and methyltrioxorhenium (MTO) dissolved in [EMIM][BF4] as catalyst was described by Abu-Omar et al. [61]. Both MTO and UHP dissolved completely in the ionic liquid. Conversions were found to depend on the reactivity of the olefin and the solubility of the olefinic substrate in the reactive layer. In general, the reaction rates of the epoxidation reaction were found to be comparable to those obtained in classical solvents. 

Thomson v Click Organic Interactive to use a web-based palette to predict products of the hydroboration/oxidation of alkenes. 

Alkenes can be aminated in the allylic position by treatment with solutions of imido selenium compounds R—N—Se=N—R. The reaction, which is similar to the allylic oxidation of alkenes with Se02 (see 14-4), has been performed with R = t-Bu and R=Ts. The imido sulfur compound TsN=S=NTs has also been used... 

Oxidation of alkenes with noble metal salts... 

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