Over-oxidation

Oxidation if 1° and 2° alcohols to aldehydes and ketones- No over oxidation... 

Oxidation of primary alcohols to aide hydes (Section 15 10) Pyridinium di chromate (PDC) or pyridinium chloro chromate (PCC) in anhydrous media such as dichloromethane oxidizes primary al cohols to aldehydes while avoiding over oxidation to carboxylic acids... 

Reactions with Ammonia and Amines. Acetaldehyde readily adds ammonia to form acetaldehyde—ammonia. Diethyl amine [109-87-7] is obtained when acetaldehyde is added to a saturated aqueous or alcohoHc solution of ammonia and the mixture is heated to 50—75°C in the presence of a nickel catalyst and hydrogen at 1.2 MPa (12 atm). Pyridine [110-86-1] and pyridine derivatives are made from paraldehyde and aqueous ammonia in the presence of a catalyst at elevated temperatures (62) acetaldehyde may also be used but the yields of pyridine are generally lower than when paraldehyde is the starting material. The vapor-phase reaction of formaldehyde, acetaldehyde, and ammonia at 360°C over oxide catalyst was studied a 49% yield of pyridine and picolines was obtained using an activated siHca—alumina catalyst (63). Brown polymers result when acetaldehyde reacts with ammonia or amines at a pH of 6—7 and temperature of 3—25°C (64). Primary amines and acetaldehyde condense to give Schiff bases CH2CH=NR. The Schiff base reverts to the starting materials in the presence of acids. 

Oxydehydrogenation of /i-Butenes. Normal butenes can be oxidatively dehydrogenated to butadiene in the presence of high concentration of steam with fairly high selectivity (234). The conversion is no longer limited by thermodynamics because of the oxidation of hydrogen to water. Reaction temperature is below about 600°C to minimise over oxidation. Pressure is about 34—103 kPa (5—15 psi). 

Two different pathways have been proposed to explain the over-oxidation reaction of malonaldehyde. Huebner and his collaborators (32) based their conclusion on the observed behavior of digitoxose and suggested that malonaldehyde (7) was oxidized by three molar equivalents of periodate with the concomitant formation of three molar equivalents of formic acid ... 

The decomposition of NO is a very slow catalytic reaction. Amirazmi, Benson, and Boudart recently studied the kinetics over platinum and over oxides of copper, cobalt, nickel, iron and zirconium from 450 to 900°C. They found that the kinetics is first order in NO with concentrations from 1.5 to 15%, and that oxygen has a strong inhibiting effect. Even at these temperatures, the kinetics is about a factor of 1000 too low for automotive usage (97). 

The kinetics of NO reduction by hydrogen and CO was studied by Ayen and Peters. Hydrogen reduction of NO over oxides of copper, zinc, and chromium was studied at 375-425°C. The products formed include... 

The major difficulty encountered in the preparation of sulphoxides by this method is a facile over-oxidation to the corresponding sulphones. 

The enzymatic oxygenation process is of particular value as there is a significant difference in the formation rates of sulfoxides and sulfones. The initial conversion of sulfide to the optically active sulfoxide by an MO is usually very fast compared to the subsequent oxidation step to sulfone, upon which chirality is lost (Scheme 9.26). In many cases, over-oxidation to sulfone is not observed at all when employing MOs. 

In the presence of anunonium bromide cobalt (ref. 22) and manganese (ref. 23) have been shown to catalyze the ammoxidation of methylaromatics to the corresponding aromatic nitriles (Fig. 20). It is interesting to compare this homogeneous, liquid phase system with the more well-known vapour phase ammoxidation of alkylaromatics over oxidic catalysts (ref. 4). 

The proposed catalytic cycle, which is based on experimental data, is shown in Scheme 6. Loss of 2 equiv. of N2 from 5 (or alternatively 1 equiv. of N2 or 1 equiv. of H2 from complexes shown in Scheme 3) affords the active species a. Olefin coordination giving b is considered to be preferred over oxidative addition of H2. Then, oxidative addition of H2 to b provides the olefin dihydride intermediate c. Olefin insertion giving d and subsequent alkane reductive elimination yields the saturated product and regenerates the catalytically active species a. 

Calcium tartronate was precipitated and hence samples required acidification prior to the filtration step necessary to remove the catalyst. The chief product of over-oxidation was oxalic acid. However, conversion to oxalic acid proceeds at a relatively low rate and yields of the former are consequently high. This is probably partly due to the tartronate being precipitated, effectively hindering further oxidation. 

Tartronic acid was oxidised to mesoxalic acid on 6%Pt2%Bi/C, prepared by exchange/redox, under acidic conditions (reaction f, Scheme 1) (29% yield at 53% conversion, pH=1.5). Figure 10 shows that the conversion rate of tartronic acid is high at first but decreases as the reaction proceeds, probably because the formed mesoxalic acid is more strongly adsorbed on the surface than tartronic acid. The initial high selectivity tapers off due to over-oxidation. 

The purpose of this work was to increase the A3 selectivity at low conversion through a catalyst modification. Previous studies of phenol alkylation with methanol (the analogue reaction) over oxides and zeolites showed that the reaction is sensitive to acidic and basic properties of the catalysts [3-5]. It is the aim of this study to understand the dependence of catalyst structure and acidity on activity and selectivity in gas phase methylation of catechol. Different cations such as Li, K, Mg, Ca, B, incorporated into y-Al203 can markedly modify the polarisation of the lattice and consequently influence the acidic and basic properties of the surface [5-8] which control the mechanism of this reaction. 

The role of oxygen on the allyhc oxidation of cyclohexene over the FePcCli6-S/TBHP catalytic system was determined by using 2 labelled oxygen. Since more than 70% of the main cyclohexene oxidation products, 4,11, and 12, had labelled oxygen, we can assure that molecular oxygen acts as co-oxidant. However, under the reaction conditions the over-oxidation of 4 seems to be unavoidable. Labelled 2, 3- epoxy-l-cyclohexanone (13), 2-cyclohexen-l, 4-dione (14), and 4-hydroxy-2-cyclohexen-l-one (15) were detected as reaction products. 

Furthermore, there is no proof for over-oxidation of the primary alcohol of 5 in a carboxylate or for cleavage of the glycosidic bond and release of methanol under the conditions applied. However, additional signals in the NMR spectrum of the reaction mixture are observed between 80 and 85 ppm, which are ascribed to side products formed from 6 by aldol condensations in alkaline solution. 

It was clearly important to drive adduct formation to completion any unreacted starting material 32 was not rejected in isolation of the product 33 and the result was an impurity in the finasteride product. Excess DDQ was required to ensure complete conversion of 32, but any DDQ remaining after adduct formation produced over-oxidation products in the thermolysis. The three over-oxidation products shown in Figure 3.4 were identified. Sequential dehydrogenation in the B-ring... 

If unreacted DDQ was quenched prior to thermolysis, over-oxidation products could be limited to less than 1%. 1,3-Cyclohexanedione was used to quench unreacted reagent prior to thermolysis. Purity following thermolysis was significantly improved and no problems were found on the laboratory scale. The fully optimized conditions are provided in Scheme 3.16. The reaction was run on pilot scale and performed as expected. 

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