Aldehydes, from catalytic oxidation

Two key intermediates in the production of vitamin A are citral and the so-called C5 aldehyde. In the modem routes to these intermediates, developed by BASF and Hoffmann-La Roche, catalytic technologies are used (see Fig. 2.29 and 2.30). Thus, in the synthesis of citral, the key intermediate is 2-methyl-l-butene-4-ol, formed by acid-catalyzed condensation of isobutene with formaldehyde. Air oxidation of this alcohol over a silver catalyst at 500°C (the same catalyst as is used for the oxidation of methanol to formaldehyde) affords the corresponding aldehyde. Isomerization of 2-methyl-l-butene-4-ol over a palladium-on-charcoal catalyst affords 2-methyl-2-butene-4-ol. The latter is then reacted with the aldehyde from the oxidation step to form an enol ether. Thermal Claisen rearrangement of the enol ether gives citral (see Fig. 2.29). 

Ca.ta.lysis, The mechanism of hydrogen abstraction from alcohols to form aldehydes (qv) over silver has been elucidated (11). Silver is the principal catalyst for the production of formaldehyde (qv), the U.S. production of which was 4 x 10 metric tons in 1993. The catalytic oxidation of... 

Concern for the conservation of energy and materials maintains high interest in catalytic and electrochemistry. Oxygen in the presence of metal catalysts is used in CUPROUS ION-CATALYZED OXIDATIVE CLEAVAGE OF AROMATIC o-DIAMINES BY OXYGEN (E,Z)-2,4-HEXADIENEDINITRILE and OXIDATION WITH BIS(SALI-CYLIDENE)ETHYLENEDIIMINOCOBALT(II) (SALCOMINE) 2,6-DI-important industrial method, is accomplished in a convenient lab-scale process in ALDEHYDES FROM OLEFINS CYCLOHEXANE-CARBOXALDEHYDE. An effective and useful electrochemical synthesis is illustrated in the procedure 3,3,6,6-TETRAMETHOXY-1,4-CYCLOHEX ADIENE. ... 

Synthesis of aldehydes from alcohols is an important transformation in several applications. In small scale oxidations still chromic acid is being used as a stoichiometric oxidant of alcohols, which leads to a large amount of toxic waste and it is also expensive. Catalytic routes have been reported using palladium catalyst [18], or TEMPO (see also Figure 15.13) as a radical catalyst for the oxidation of alcohols [19], or combinations of TEMPO and copper [20] related work is mentioned in the references of these articles. The mechanism of... 

More recently, using the cyclometallated iridium C,(7-benzoate derived from allyl acetate, 4-methoxy-3-nitrobenzoic acid and BIPHEP, catalytic carbonyl crotylation employing 1,3-butadiene from the aldehyde, or alcohol oxidation was achieved under transfer hydrogenation conditions [274]. Carbonyl addition occurs with roughly equal facility from the alcohol or aldehyde oxidation level. However, products are obtained as diastereomeric mixtures. Stereoselective variants of these processes are under development. It should be noted that under the conditions of ruthenium-catalyzed transfer hydrogenation, conjugated dienes, including butadiene, couple to alcohols or aldehydes to provide either products of carbonyl crotylation or p,y-enones (Scheme 16) [275, 276]. 

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. 

Catalytic oxidation of ozonides over platinum appears to be accompanied by the same ester by-product disadvantage found in the thermal process. Chain degradation by other reactions is less serious, however, and transesterification does not occur. The method can therefore be used to prepare a half-ester of a dicarboxylic acid from an ester of a suitable unsaturated acid. If ozonide autoxidation occurs by the route, ozonide — aldehyde — peracid, with the latter acting as precursor of both acid and ester products (20-24), it is interesting to compare reaction rates observed in the present study with the rate of uptake of oxygen by... 

These results indicate that the same crystalline face does not necessarily exhibit the same catalytic properties with different molecules. Thus, the (010) face of a-Mo03 is selective for the formation of aldehydes from alcohols while it promotes essentially the deep oxidation of olefins. It is expected that the studies on structure-sensitive reactions will be made more quantitative using recent methods to determine the number of surface M=0 species (425 —7). It should be noted that the earlier observation on the specificity of Mo03 crystalline faces in propylene oxidation has been obtained on oriented Mo03-graphite catalysts (425k). Non-structure-sensitive reactions have also been reported (425k). 

In experiments (i) and (iv), no oxidation was observed after 50 h, in toluene / oxygen at 80 °C implying that a perruthenate derived species was responsible for the catalysis and not Ru02. In experiments (i) and (iii), approximately 10% and 70% oxidation respectively, to the aldehyde was observed after 3 days under the same conditions and complete leaching of potassium perruthenate was observed in both cases. The used solid material from experiments (ii) and (iii) showed no catalytic oxidative activity when reused.. 

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.

TEMPO has a double role - it generates the oxoamonium salt, responsible for the oxidation (Scheme 1), and inhibits the further oxidation of aldehydes and ketones, which occurs instead via free-radical chain processes under the same conditions, but in the absence of TEMPO [5a], Thus, this catalytic system is highly effective for one of the most demanding transformations, the selective synthesis of aldehydes from benzylic and nonbenzylic alcohols. 

Despite their obvious economical and ecological importance, few catalytic systems are available for the transformation of alcohols into aldehydes and ketones, using molecular oxygen or air as the ultimate, stoichiometric oxidant (5). Moreover, most of the currently available catalytic oxidation processes suffer from severe limitations, being usually only effective with reactive alcohols, such as benzylic and allylic ones, or requiring high pressures, temperatures, and catalyst loading. 

The catalytic oxidation in the presence of various heteropoly compounds of lower olefins to unsaturated aldehydes and subsequent conversion into unsaturated nitriles are described in Ref.225-231. Copper phthalocyanine is produced in 92% yield from phthalic anhydride in the presence of 12-molybdophosphoric acid232. ... 

The C5 aldehyde intermediate is produced from butadiene via catalytic oxidative acetoxylation followed by rhodium-catalyzed hydroformylation (see Fig. 2.30). Two variations on this theme have been described. In the Hoffmann-La-Roche process a mixture of butadiene, acetic acid and air is passed over a palladium/tellurium catalyst. The product is a mixture of cis- and frans-l,4-diacetoxy-2-butene. The latter is then subjected to hydroformylation with a conventional catalyst, RhH(CO)(Ph3P)3, that has been pretreated with sodium borohydride. When the aldehyde product is heated with a catalytic amount of p-toluenesulphonic acid, acetic acid is eliminated to form an unsaturated aldehyde. Treatment with a palladium-on-charcoal catalyst causes the double bond to isomerize, forming the desired Cs-aldehyde intermediate. 

This chapter highlights the ruthenium-catalyzed dehydrogenative oxidation and oxygenation reactions. Dehydrogenative oxidation is especially useful for the oxidation of alcohols, and a variety of products such as ketones, aldehydes, and esters can be obtained. Oxygenation with oxo-ruthenium species derived from ruthenium and peroxides or molecular oxygen has resulted in the discovery of new types of biomi-metic catalytic oxidation reactions of amines, amides, y3-lactams, alcohols, phenols, and even nonactivated hydrocarbons tmder extremely mild conditions. These catalytic oxidations are both practical and useful, and ruthenium-catalyzed oxidations will clearly provide a variety of futrue processes. 

Due to the wide availability of aliphatic aldehydes from hydroformylation (cf. Section 2.1.1), the principal method for the production of C3-C]o carboxylic acids is the catalytic oxidation of the corresponding aldehyde (eq. 1). 

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