Alcohols, catalytic oxidation preparation

Catalytic oxidant.1 In combination with N-methylmorpholine N-oxide (7,244) as the stoichiometric oxidant, this ruthenium compound can be used as a catalytic oxidant for oxidation of alcohols to aldehydes or ketones in high yield in CH2C12 at 25°. Addition of 4A molecular sieves is generally beneficial. Racemization is not a problem in oxidation of alcohols with an adjacent chiral center. Tetrabutylammonium perruthenate can also be used as a catalytic oxidant, but the preparation is less convenient. 

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). 

Perhaps the most important recent discovery in catalytic oxidation of alcohols is the use of a catalyst prepared from [Pd(OAc)2] and sulfonated batophenanthroline (see Scheme 8.1 above). This catalyst was found to oxidize primary and secondary, as well as benzylic and allylic alcohols with close to quantitative yields and 90-100 % select vities to the corresponding aldehydes or ketones (Scheme 8.4) [18]. The easy oxidation of non-activated secondary alcohols is particularly noteworthy since in general these are rather unreactive towards O2. 

The first chapter concerns the chemistry of the oxidation catalysts, some 250 of these, arranged in decreasing order of the metal oxidation state (VIII) to (0). Preparations, structural and spectroscopic characteristics are briefly described, followed by a summary of their catalytic oxidation properties for organic substrates, with a brief appendix on practical matters with four important oxidants. The subsequent four chapters concentrate on oxidations of specific organic groups, first for alcohols, then alkenes, arenes, alkynes, alkanes, amines and other substrates with hetero atoms. Frequent cross-references between the five chapters are provided. 

Numerous methods for the synthesis of salicyl alcohol exist. These involve the reduction of salicylaldehyde or of salicylic acid and its derivatives. The alcohol can be prepared in almost theoretical yield by the reduction of salicylaldehyde with sodium amalgam, sodium borohydride, or lithium aluminum hydride by catalytic hydrogenation over platinum black or Raney nickel or by hydrogenation over platinum and ferrous chloride in alcohol. The electrolytic reduction of salicylaldehyde in sodium bicarbonate solution at a mercury cathode with carbon dioxide passed into the mixture also yields saligenin. It is formed by the electrolytic reduction at lead electrodes of salicylic acids in aqueous alcoholic solution or sodium salicylate in the presence of boric acid and sodium sulfate. Salicylamide in aqueous alcohol solution acidified with acetic acid is reduced to salicyl alcohol by sodium amalgam in 63% yield. Salicyl alcohol forms along with -hydroxybenzyl alcohol by the action of formaldehyde on phenol in the presence of sodium hydroxide or calcium oxide. High yields of salicyl alcohol from phenol and formaldehyde in the presence of a molar equivalent of ether additives have been reported (60). Phenyl metaborate prepared from phenol and boric acid yields salicyl alcohol after treatment with formaldehyde and hydrolysis (61). 

The industrial preparation of formaldehyde has occurred since the late 1800s and involves the catalytic oxidation of methanol 2CH,OH,. + 0 ,. —> 2CH 0,.. The oxidation takes place at temperatures between 400°C and 700°C in the presence of metal catalysts. Metals include silver, copper, molybdenum, platinum, and alloys of these metals. Formaldehyde is commonly used as an aqueous solution called formalin. Commercial formalin solutions vary between 37% and 50% formaldehyde. When formalin is prepared, it must be heated and a methanol must be added to prevent polymerization the final formalin solution contains between 5% and 15% alcohol. 

Griffith, W., S. Ley, G. Whitcombe, and A. White (1987) Preparation and use of tetra-n-butylammonium per-ruthenate (TBAP reagent) and tetra-n-propylammonium per-ruthenate (TpAP reagent) as new catalytic oxidants for alcohols. Chemical Communications, 1625-1627. 

RuCl. (terpy)] can be prepared by reaction of RuCl3 with terpy in refluxing EtOH for 3 h.259,1249,1252 The corresponding complex of l,3-bis(4-methyl-2-pyridylimino)isoindoline (L) [RuC13L] can be prepared by the same method1240,1260 and has been used for the catalytic oxidation of alcohols.1260... 

It is possible to use PDC as a catalytic oxidant (10 mol %), with bis(trimethylsilyl) peroxide as the cooxidmt, for the preparation of carbcmyl compounds. It is necessary to add the cooxidant slowly via syringe pump since the actual oxidizing agent (15) is unstable in solution. A range of primary and secondary alcohols were oxidized in good yields by this method (Table 14). Fortunately, isolated double bonds are inert under these conditions. 

Chromium(VI) oxide can be used as a catalytic oxidant for alcohols with r-butyl hydroperoxide as the cooxidant. This reagent appears to be selective for allylic and benzylic over saturated alcohols, though ( )/(Z)-isomerization has been observed during the preparation of a,3-unsaturated aldehydes. This reagent is also a good oxidant for allylic and ben lic C—bonds these may be competing pathways in more sophisticated substrates. ... 

A copper catalysed click (azide-alkyne cycloaddition) reaction has been used to prepare a fluorous-tagged TEMPO catalyst (Figure 7.20). TEMPO is a stable organic free radical that can be used in a range of processes. In this case, its use in metal-free catalytic oxidation of primary alcohols to aldehydes using bleach as the terminal oxidant was demonstrated. The modified TEMPO can be sequestered at the end of the reaction on silica gel 60 and then released using ethyl acetate for reuse in further reactions in this way the TEMPO was used four times with no loss in activity. 

Catalytic systems available for the synthesis of pentafluoroethane (CF3CHF2 HFC-125) are essentially similar to those reported for the synthesis of HCFC-123 and HCFC-124. If one starts with PCE, the end point after the initial HF addition is HFC-125. Again, as with HCFC-124, a suitable pentahaloethane such as HCFC-123 or even HCFC-124 can also be employed. Active catalysts seem to contain chrome in some form. Pretreatment of such catalysts have been reported to modify activity. For example. Firth and Foil (15) have claimed that if a chromium hydroxide precursor is treated with steam prior to calcination to chromium(III) oxide, the activity is superior to the one prepared without such a pretreatment. Swamer (66) has disclosed that chromium oxide gel prepared by special techniques involving the addition of alcohol during the preparative procedure is an excellent catalyst for both HF addition as well as halogen exchange. 

The vapor-phase dehydrogenations just mentioned are applicable only to the preparation of aldehydes that tolerate such high temperatures. A catalytic oxidation of alcohols carried out by passing a current of air or oxygen through solutions of alcohols in solvents such as heptane [56] or ethyl acetate [55] in the presence of platinum [55, 56], or, better still, platinum dioxide [56], or active cobalt oxide [1136 is applicable even to alcohols that cannot be vaporized and that contain double bonds (equation 205) [56]. 

Industrial preparation is generally patterned after these laboratory methods, but with use of cheaper reagents alcohols are oxidized catalytically with air, or by dehydrogenation over hot copper. 

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