Selective oxidation with aqueous

Whilst the majority of the discussion thus far has been concerned with metallo-substituted redox molecular sieves, it is important to note that proto-nated zeolite forms can also be employed for selective oxidation with aqueous hydrogen peroxide. An excellent example of this is the study conducted by the Mobil Oil Corporation.52 Their work has shown that a number of protonated zeolites such as H-ZSM-5 or zeolite-/ can be used with hydrogen peroxide to catalyse the oxidation of cyclic ketones to lactones or the co-hydroxycarboxylic acids (Figure 4.12). 

The discovery of titanium silicalite-1 (TS1) by Enichem scientists (20-22) and its commercial use as a catalyst for a variety of selective oxidations with aqueous hydrogen peroxide under mild conditions (Figure 1.3) constituted a major breakthrough in oxidation catalysis. 

The discovery, in the mid-eighties, of the remarkable activity of TS-1 as a catalyst for selective oxidations with aqueous H2O2 fostered the expectation that this is merely the progenitor of a whole family of redox molecular sieve catalysts with unique activities. However, the initial euphoria has slowly been tempered by the realization that framework substitution/attachment of redox metal ions in molecular sieves does not, in many cases, lead to a stable heterogeneous catalyst. Nevertheless, we expect that the considerable research effort in this area, and the related zeolite-encapsulated complexes, will lead to the development of synthetically usefril systems. In this context the development of chiral ship-in-a-bottle type catalysts for intrazeolitic asymmetric oxidation is an important goal. Such an achievement would certainly justify the appellation mineral enzyme . 

Commercial production of ethanolamines (EOA) is by reaction of ethylene oxide with aqueous ammonia. The ethylene oxide reacts exothermically with 20% to 30% aqueous ammonia at 60 to 150°C and 30 to 150 bar in a tubular reactor to form the three possible ethanolamines (mono-ethanolamine - MEA, di-ethanolamine - DEA and tri-ethanolamine - TEA) with high selectivity. The product stream is then cooled before entering the first distillation column where any excess ammonia is removed overhead and recycled. In the second column, ammonia and water are removed and the EOA s are separated in a series of vacuum distillation columns. 

A similar two-phase procedure involves addition over 15 min. of the theoretical quantity of chromic acid (from Na2Cr207-2 HjO, H2SO4, and water) to a solution of the alcohol in ether. After 2 hrs. at 25-30° the ether layer is separated and the product isolated. Oxidation of (-menthol gave (-menthone in 97% yield with only a trace of d-isomenthone, whereas 3-4% of this isomer was present on oxidation with aqueous chromic acid (50-55°), H2Cr04 in 90% acetic acid (25°), or HjCrO,-acetone (5-10°), and the yields in these oxidations were 90, 71, and 86%. This procedure has been used for Ihc selective oxidation of steroidal l6/l,2()o-diols to l6-keto-20 -ols. ... 

In the basic solution (run 5), the reaction rate was greatly reduced and the amount of acetic acid still increased. These results indicate that the oxidation may occur preferentially on the undissociated forms of the acids (pK, of succinic acid = 4.16, pKj = 5.61), rather than on the carboxylate ions, in agreement with previous results on selective oxidation of aqueous solutions of alcohols over noble metal catalysts. Slightly basic conditions favor the desorption of the acid salt from the surface and prevents C-C bond rupture and over-oxidation, whereas acidic pH favor the adsorption of the carboxylic acid and its further oxidation [14-15]. Similar results were observed by Imamura, et al. [11] in the oxidation of formic acid or acetic acid over 5 % Ru/CeOj. 

Alcohols are oxidized slowly with PdCh. Oxidation of secondary alcohols to ketones is carried out with a catalytic amount of PdCh under an oxygen atmo-sphere[73.74]. Also, selective oxidation of the allylic alcohol 571 without attacking saturated alcohols is possible with a stoichiometric amount of PdfOAc) in aqueous DMF (1% H OifSll],... 

Secondary alcohols are oxidized at room temperature to ketones in high yields by HOCl generated in situ from aqueous NaOCl and acetic acid (109,110). Selective oxidation in the presence of a primary alcohol is possible. In methanol, aldehydes are oxidized to methyl esters (110). Under the proper conditions, alcohols can be esterified with HOCl forming isolable alkyl hypochlorites. 

Nickel peroxide is a solid, insoluble oxidant prepared by reaction of nickel (II) salts with hypochlorite or ozone in aqueous alkaline solution. This reagent when used in nonpolar medium is similar to, but more reactive than, activated manganese dioxide in selectively oxidizing allylic or acetylenic alcohols. It also reacts rapidly with amines, phenols, hydrazones and sulfides so that selective oxidation of allylic alcohols in the presence of these functionalities may not be possible. In basic media the oxidizing power of nickel peroxide is increased and saturated primary alcohols can be oxidized directly to carboxylic acids. In the presence of ammonia at —20°, primary allylic alcohols give amides while at elevated temperatures nitriles are formed. At elevated temperatures efficient cleavage of a-glycols, a-ketols... 

Chlorins, e.g. 14, form adducts with osmium(VIII) oxide, which can be hydrolyzed in aqueous sodium sulfide to bacteriochlorindiols, e g. 2, or isobacteriochlorindiols, e.g. 3. Thus, similar to diimide reductions of chlorins, metal-free tetraphenylchlorin 14 (M = 2H) is selectively oxidized to a corresponding bacteriochlorin 2 whereas the zinc chlorin gives an isobac-teriochlorin 3 on oxidation with osmium(VIII) oxide.40 With less symmetrical chlorins, very complex mixtures of constitutional isomers and stereoisomers are formed by /i-bishydroxyla-tion.17... 

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). 

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