Platinum-catalyzed oxidations

Perhaps the most familiar example of heterogeneous catalysis is the series of reactions that occur in the catalytic converter of an automobile (Figure 11.12). Typically this device contains 1 to 3 g of platinum metal mixed with rhodium. The platinum catalyzes the oxidation of carbon monoxide and unburned hydrocarbons such as benzene, C6H6 ... 

C.G. Vayenas, and J. Michaels, On the Stability Limit of Surface Platinum Oxide and its role in the oscillatory behavior of Platinum Catalyzed Oxidations, Surf. Sci. 120, L405-L408 (1982). 

Rebrov, E. V., Duinkerke, S.A., de Croon, M. H. J. M., Schouten, J. C., Optimization of heat transfer characteristics, flow distribution, and reaction processing for a microstructured reactor/ heat-exchanger for optimal peformance in platinum catalyzed ammonia oxidation, Chem. Eng. 93 (2003) 201-216. 

Ertl G, Norton PR, Riistig J. 1982. Kinetic oscillations in the platinum-catalyzed oxidation of CO. Phys Rev Lett 49 177. 

A mixture of Pt(ll) and metallic Pt in an aqueous medium was shown to oxidize ethane to yield acetic and glycolic acids. A series of deuterium-exchange processes enabled a complex mechanism to be elucidated metallic platinum catalyzes the oxidation of intermediate alcohols to acid products, whereas the Pt(ll) salt activates the initial alkene (Scheme 7X29... 

The palladium-catalyzed asymmetric hydrosilylation of styrenes has been applied to the catalytic asymmetric synthesis of l-aryl-l,2-diols from arylacetylenes (Scheme 6).46 Thus, ( )-l-aryl-2-(trichlorosilyl)ethenes, which are readily generated by platinum-catalyzed hydrosilylation of arylacetylenes, were treated with trichlorosilane and the palladium catalyst coordinated with MOP ligand 12f to give 1 -aryl-1,2-bis(silyl)ethanes, oxidation of which produced the enantiomerically enriched (95-98% ee) 1,2-diols. 

The latest developments in the mechanistic understanding of platinum catalyzed alkane oxidation have involved Pt complexes with chelating ligands having mainly - but not exclusively - nitrogen donors. This review will focus on recent studies of these chelated Pt systems. The selection of the literature is somewhat subjective and will not be comprehensive. However, it should provide a true flavor of the current state of progress in the field. 

Secondary phosphine oxides are known to be excellent ligands in palladium-catalyzed coupling reactions and platinum-catalyzed nitrile hydrolysis. A series of chiral enantiopure secondary phosphine oxides 49 and 50 has been prepared and studied in the iridium-catalyzed enantioselective hydrogenation of imines [48] and in the rhodium- and iridium-catalyzed hydrogenation functionalized olefins [86]. Especially in benzyl substituted imine-hydrogenation, 49a ranks among the best ligands available in terms of ex. 

In 1985 Jakobs et al. studied polypyrrole (PPy) covered platinum and gold electrodes for the ORR,167,168 One interesting result of the work was that, compared to a bare gold electrode, the PPy covered gold reduced oxygen at a lower overpotential.168 Further, the PPy covered electrodes, when in the oxidized state, catalyzed peroxide decomposition and thus improved selectivity to water.168... 

Anhydro-L-gulono-1,4-lactone (62) was prepared100 in 62% yield by the platinum-catalyzed oxidation of 1,4-anhydro-D-glucitol. Methyl 3,4,5-tri-0-acetyl-2,6-anhydro-L-gulonate (63, R = H) was obtained from D-glucuronic acid by way of the tetraacetate (63, R = OAc) and the thioglycoside101,102 (63, R = SPh) (76%), followed by reduction in the presence of Raney nickel to afford 63 (R= H) (68%). 

Miyaura and co-workers reported the platinum-catalyzed diboration of allenes with bis(pinacolato)diboron (Scheme 16.52) [57]. The catalytic cycle involves a sequence of oxidative addition of bis(pinacolato)diboron to Pt(0), insertion of an allene into the B-Pt bond and reductive elimination of an allylic boronate, re-producing the Pt(0) species. (Z)-Allylic boronates are formed stereoselectively in the reaction with monosubstituted allenes, which strongly suggests a pathway via a vinylplatinum species rather than a Jt-allylplatinum species. 

This approach was coupled to a system of three NAD+-dependent enzymes comprised of alcohol dehydrogenase (EC 1.1.1.1), aldehyde dehydrogenase (EC 1.2.1.3), and formate dehydrogenase (EC 1.2.1.2) to create an electrode theoretically capable of complete oxidation of methanol to carbon dioxide, as shown in Eigure 5. The anode was, in turn, coupled to a platinum-catalyzed oxygen cathode to produce a complete fuel cell operating at pH 7.5. With no externally applied convection, the cell produced power densities of 0.67 mW/cm at 0.49 V for periods of less than 1 min, before the onset of concentration polarization. 

A mild one-pot procedure based on a platinum-catalyzed diborylation of 1,3-butadienes (see Eq. 30) gives doubly allylic boronate 144, which adds to an aldehyde to form a quaternary carbon center in the intermediate 145 (Eq. 105). The use of a tartrate auxiliary in this process leads to good levels of enantiose-lectivity in the final diol product, which is obtained after oxidation of the primary alkylboronate intermediate. Although examples of aliphatic, aromatic, and unsaturated aldehydes have been described, enantioselectivities vary widely (33 to 74% ee), and are good only for aliphatic aldehydes. An intramolecular variant of this interesting tandem reaction is also known. ... 

SCHEME 158. Platinum-catalyzed Baeyer-ViUiger oxidation using H2O2... 

Immeasurable ions are formed when hydrocarbons in an air stream are oxidized in the presence of a heated platinum filament (3,10). The experimental evidence suggests that these ions are produced by the chemical oxidation reactions catalyzed by the platinum surface (10). That this process is one of chemionization is supported by the fact that the extent of ionization observed depended greatly on the molecular structure of the particular hydrocarbon being oxidized. This is in contrast to the ionization measured in flames, where the extent of ionization for saturated hydrocarbons, for example, does not appear to depend in any way on the molecular structure but only on the number of carbon atoms (9). 

A more detailed investigation of the major reaction parameters influencing the platinum-catalyzed oxidation of isosorbide (3) was undertaken212 in... 

Much less is known concerning the platinum-catalyzed hydroformylations. However, a reasonable catalytic cycle can be constructed (Scheme 3) from the available information on the generation and reactions of many of the intermediate complexes shown.6,8,9,15 The ability of platinum to catalyze hydroformylation reactions while palladium is not a good catalyst could be due to the ability of platinum to achieve the +4 oxidation state more readily. 

Platinum-catalyzed oxidation of methyl a-D-fructofuranoside (43) with oxygen is selective for C-6, and fructuronic acid (44) was obtained in 80% yield.151... 

Platinum-catalyzed oxidation of D-glucitol affords L-gulose and D-glucose. The specificity of this reaction is probably due to both statistical and steric factors. A related example is the conversion of L-sorbose into L-vy/o-2-hexulosonic acid in 62% yield (see Ref. 1, p. 1129). Similar oxidation of 1,2-acetals of a-D-glucofuranose and a-D-xylofuranose affords the respective glycuronic acids whereas, under more drastic conditions, the former acetal is converted into 1,2-O-isopropylidene-o -D-vy/o-hexofuranos-5-ulosono-6,3-lactone, a synthetic precursor of ascorbic acids. 

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