Pt-Catalyzed Oxidations

Much less information has been gained on the oxidation of the Pt(II) alkyl to Pt(IV). In one elegant study, oxidation of the chloroplatinum-mettiyl complex by Pt-labeled Pt(IV) chloride formed a Pt(IV) methyl complex without Tt enrichment (Equation 18.14). This result shows that oxidation of the Pt(II) alkyl complexes by Pt(IV) occurs by transfer of chlorides from the Pt(IV) to the platinum alkyl complex, not by transfer of the alkyl group to the Pt(IV) center.  

Directed Oxidations, Animations, and Halogenations of Alkanes and Arenes  

Related strategies have been successfully applied to the halogenation of aliphatic and aromatic C-H bonds. Yu reported iodination of alkyl chains attached to an oxazoline that binds the palladium catalysts (Equation 18.19a). In this iodination, the combination of PhI(OAc)j and are used as the reagents to effect iodination and oxidation. The use of directing groups on arenes has also made possible the regioselective halogenation of arenes. As illustrated by the catalyzed and uncatalyzed processes in Equation 18.19b, the palladium-catalyzed process can occur faster than the uncatalyzed process to form a product directed by the catalyst, rather than the electronic properties of the arene. Under microwave conditions, directed fluorination of arene C-H bonds can also occur (Equation 18.19c).  

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A catalyst may play an active role in a different sense. There are interesting temporal oscillations in the rate of the Pt-catalyzed oxidation of CO. Ertl and coworkers have related the effect to back-and-forth transitions between Pt surface structures [220] (note Fig. XVI-8). See also Ref. 221 and citations therein. More recently Ertl and co-workers have produced spiral as well as plane waves of surface reconstruction in this system [222] as well as reconstruction waves on the Pt tip of a field emission microscope as the reaction of H2 with O2 to form water occurred [223]. Theoretical simulations of these types of effects have been reviewed [224]. 

The maximum 0os value is thus computed for the transient of Fig. 5.6b to be 0os 0.5. This, in view of the definition of the promotion index, PI, (Chapter 4) and the observed p value (p 16.5) gives a PIQ5- value of the order of 30, in good qualitative agreement with PI0s- values for other Pt catalyzed oxidations.5... 

A striking feature of the effect of current on the CO oxidation oscillations is shown in Fig. 8.33. It can be seen that the frequency of oscillations is a linear function of the applied current. This holds not only for intrinsically oscillatory states but also for those which do not exhibit oscillations under open-circuit conditions, such as the ones shown on Fig. 8.31. This behaviour is consistent with earlier models developed to describe the oscillatory behaviour of Pt-catalyzed oxidations under atmospheric pressure conditions which are due to surface Pt02 formation35 as analyzed in detail elsewhere.33... 

Since S03/H2S04 is clearly not the most desirable system for industrial applications, a formidable challenge is to find an oxidant that oxidizes Pt(II) much faster than S03 does, operates in an environmentally friendly solvent, and can be (like SVI/SIV) reoxidized by oxygen from air. Ideally, the reduced oxidant would get reoxidized in a continuous process, such that the oxidant acts as a redox mediator. In addition, the redox behavior has to be tuned such that the platinum(II) alkyl intermediate would be oxidized but the platinum(II) catalyst would not be completely oxidized. Such a system that efficiently transfers oxidation equivalents from oxygen to Pt(II) would be highly desirable. A redox mediator system based on heteropolyacids has been reported for the Pt-catalyzed oxidation of C-H bonds by 02, using Na8HPMo6V6O40... 

Furan-2,5-dicarboxylic add also has tremendous industrial potential, because it could replace oil-derived diadds such as adipic or terephthalic acid as monomers for polyesters and polyamides [98, 99]. This diadd can be synthesized by Pt-catalyzed oxidation with 02 of 5-hydroxymethylfurfural the latter is obtained by acid-catalyzed dehydration of D-frudose or frudosans (inulin) the latter, however, are too expensive as starting materials, and yields from glucose-based waste raw materials are no higher than 40%. Therefore, the potential attractive option of furan-2,5-dicarboxylic acid will develop only after an effident generation of 5-hydroxymethylfurfural from forestry waste materials has been developed. The same compound is also the starting material for the synthesis of other interesting chemicals obtained by oxidative processes, such as 5-hydroxymethylfuroic add, 5-formylfuran-2-carboxylic add and the 1,6-dialdehyde. 

J. M. Smith (J. M. Smith, Chemical Engineering Kinetics, 2nd ed., McGraw-Flill, New York, 1970, p. 395) presents the following observed data for Pt-catalyzed oxidation of SO2 at 480°C obtained in a differential fixed-bed reactor at atmospheric pressure and bulk density of 0.8 g/cm. ... 

It is important to notice that Ej decreases linearly with eO with a slope of -1, in excellent agreement with the observed decrease in activation energy E with eO in the Pt-catalyzed oxidation of C2H4 and CH4, where cleavage of the metal-oxygen bond is rate-limiting (Figure 21) and with recent theoretical ab initio calculations. ... 

Scheme V,4. The catalytic cycle proposed for the Pt -catalyzed oxidation of [nethane in the gas phase.

PftCe02. This result provides a further evidence that carbonaceous materials must have been deposited on the catalyst during wet oxidation of phenol, and the catalyst deactivation during wet oxidation of phenol must be due to the deposits of carbonaceous materieds. Carbon deposits on alumina support were reported to be oxidized at temperatures higher than 440°C[14]. On the other hand participation of Pt metal could catalyze oxidation of carbon deposits, and Pt-catalyzed oxidation of carbon is known to occur generally at temperatures around 280°C [15,16]. A separate TPO analysis was carried out with Pt powder alone after reaction for 3hr with initial phenol concentration 5000 mg/L, and the result is shown in Figure 9. The carbonaceous deposit on Pt powder was burned off at narrow temperature range around 250 C and was completely removed below 300"C. 

The Pt-catalyzed oxidation of NO to NOa is a well-known reaction [10], FTIR-studies have shown that with NO surface nitrate species can be formed on alumina supports. The presence of platinum promoted the surface nitrate formation [11]. Furthermore, Burch and Watling concluded from steady-state kinetic studies that Pt-NOa species take part in the mechanism for the selective reduction of NO with propane, being a highly abundant species on the platinum surface [6]. Thus we ascribe the gap in the i N-balance to the formation of Pt-i NOa species (9) and to an eventual spill-over of i NOa to the alumina surface. 

A mild Pt-catalyzed oxidation of tertiary amines to their N-oxides by aqueous H 2O2 was recently reported by Strukul ] 118]. Platinum(I I) complexes of the general formula 34 were used, and the reaction is proposed to involve tertiary amine-Pt-peroxide complexes in which an oxygen transfer to the amine nitrogen occurs. Various tertiary amines were oxidized in good yields and high selectivity with 34c as the catalyst. 

Zerella, M. and Bell, A.T. (2006) Pt-catalyzed oxidative carbonylation of methane to acetic acid in sulfuric acid. 

Figure 8.7 The catalytic cycle for Pt-catalyzed oxidation of methane. BIPYM coordination not shown for clarity.

This efficient conversion is a good example of the potential of gold catalysis to produce useful chemicals from biomass-derived starting materials. Methylfuroate formed from furfural is useful for flavor and fragrance applications and has potential as an industrial solvent. FDMC derived from HMF is a monomer that could replace the terephthalic acid used for making polyester plastics. The course of the reaction is quite different from when a platinum catalyst is used because furan dicar-baldehyde is the product from the Pt-catalyzed oxidation of HMF in water. 

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