Platinum aldehydes

The most obvious way to reduce an aldehyde or a ketone to an alcohol is by hydro genation of the carbon-oxygen double bond Like the hydrogenation of alkenes the reac tion IS exothermic but exceedingly slow m the absence of a catalyst Finely divided metals such as platinum palladium nickel and ruthenium are effective catalysts for the hydrogenation of aldehydes and ketones Aldehydes yield primary alcohols... 

Common catalyst compositions contain oxides or ionic forms of platinum, nickel, copper, cobalt, or palladium which are often present as mixtures of more than one metal. Metal hydrides, such as lithium aluminum hydride [16853-85-3] or sodium borohydride [16940-66-2] can also be used to reduce aldehydes. Depending on additional functionahties that may be present in the aldehyde molecule, specialized reducing reagents such as trimethoxyalurninum hydride or alkylboranes (less reactive and more selective) may be used. Other less industrially significant reduction procedures such as the Clemmensen reduction or the modified Wolff-Kishner reduction exist as well. 

Pla.tinum. Platinum catalysts that utilize both phosphine and tin(Il) haUde ligands give good rates and selectivities, in contrast to platinum alone, which has extremely low or nonexistent hydroformylation activity. High specificity to the linear aldehyde from 1-pentene or 1-heptene is obtained using HPtSnClgCO(1 1P) (26), active at 100°C and 20 MPa (290 psi) producing 95% -hexanal from 1-pentene. 

A further improvement in platinum catalysis is claimed from use of tin(Il) haUde and phosphine ligands which are rigid bidentates, eg, l,2-bis(diphenylphosphinomethyl)cyclobutane (27). High rates for a product containing 99% linear aldehyde have been obtained. However, a pressure of 10 MPa (1450 psi) H2 CO is requited. 

Styrene oxide [96-09-3] M 120.2, b 84-86 /16.5mm, d 1.053, n 1.535. Fractional distn at reduced pressure does not remove phenylacetaldehyde. If this material is present, the styrene oxide is treated with hydrogen under 3 atmospheres pressure in the presence of platinum oxide. The aldehyde, but not the oxide, is reduced to 6-phenylethanol) and separation is now readily achieved by fractional distn. [Schenck and Kaizermen J Am Chem Soc 75 1636 1953.]... 

Platinum, especially platinum oxide, has been used by many investigators (5), Platinum oxide, when used with aldehydes is apt to be deactivated before reduction is completed. Deactivation is inhibited by small amounts of ferrous or stannous chlorides (59,82). This type of promoter can also sharply curtail hydrogenolysis if it is a troublesome reaction (Rylander and Starrick, 1966). Deactivated systems can often be regenerated by shaking the reaction mixture with air (2,8,21 J3,96). The usefulness of this regenerative technique transcends aldehyde reductions it frequently is worth resorting to. 

Reduction of unsaturated aldehydes seems more influenced by the catalyst than is that of unsaturated ketones, probably because of the less hindered nature of the aldehydic function. A variety of special catalysts, such as unsupported (96), or supported (SJ) platinum-iron-zinc, plalinum-nickel-iron (47), platinum-cobalt (90), nickel-cobalt-iron (42-44), osmium (<55), rhenium heptoxide (74), or iridium-on-carbon (49), have been developed for selective hydrogenation of the carbonyl group in unsaturated aldehydes. None of these catalysts appears to reduce an a,/3-unsaturated ketonic carbonyl selectively. 

In certain reductions it is an advantage to reduce the platinum oxide to platinum black by shaking with hydrogen in the presence of solvent only, before the substance to be reduced is added to the mixture. More often the catalyst is reduced in the presence of the substance to be reduced with aldehydes for example the platinum black is usually more finely divided and generally more active if prepared in presence of the aldehyde. 

The most successful class of active ingredient for both oxidation and reduction is that of the noble metals silver, gold, ruthenium, rhodium, palladium, osmium, iridium, and platinum. Platinum and palladium readily oxidize carbon monoxide, all the hydrocarbons except methane, and the partially oxygenated organic compounds such as aldehydes and alcohols. Under reducing conditions, platinum can convert NO to N2 and to NH3. Platinum and palladium are used in small quantities as promoters for less active base metal oxide catalysts. Platinum is also a candidate for simultaneous oxidation and reduction when the oxidant/re-ductant ratio is within 1% of stoichiometry. The other four elements of the platinum family are in short supply. Ruthenium produces the least NH3 concentration in NO reduction in comparison with other catalysts, but it forms volatile toxic oxides. 

Aldehydes and ketones can be converted to ethers by treatment with an alcohol and triethylsilane in the presence of a strong acid or by hydrogenation in alcoholic acid in the presence of platinum oxide. The process can formally be regarded as addition of ROH to give a hemiacetal RR C(OH)OR", followed by reduction of the OH. In this respect, it is similar to 16-14. In a similar reaction, ketones can be converted to carboxylic esters (reductive acylation of ketones) by treatment with an acyl chloride and triphenyltin hydride. " ... 

Hydroformylation represents the most industrially important homogeneous catalysed reaction by volume [2, 3]. The petrochemical, agrochemical and pharmaceutical industries are particularly interested in this transformation. The reaction uses syngas (COiH mix) and a catalyst, commonly rhodium or platinum, to transform an olefin into an aldehyde (Scheme 9.1) [4]. 

As mentioned in Section 3.2, hydrogenation is by far the most investigated catalytic reaction and palladium the most commonly employed metal, followed by platinum. The most common substrates for catalytic hydrogenation tests are simple alkenes, cyclic alkenes and unsaturated carbonylic compounds. In the latter case, conjugated substrates (a,P-unsaturated aldehydes, acrylic acid) have received particular attention. 

Before showing how direct peroxidation of active group works, it should be mentioned that platinum catalyses oxidation of alcohols at ambient temperature in aldehydes (provided they are primary). The exothermicity of the reaction is sufficient to cause the alcohol to combust by heating a thread of platinum at a high temperature. Note that 2-butanol (but not isopropanol) is classified in list B, producing peroxides that become expiosive at a certain concentration in the lists of peroxidable compounds set up by Du Pont de Nemours company (see p.261). 

The discussion of the activation of bonds containing a group 15 element is continued in chapter five. D.K. Wicht and D.S. Glueck discuss the addition of phosphines, R2P-H, phosphites, (R0)2P(=0)H, and phosphine oxides R2P(=0)H to unsaturated substrates. Although the addition of P-H bonds can be sometimes achieved directly, the transition metal-catalyzed reaction is usually faster and may proceed with a different stereochemistry. As in hydrosilylations, palladium and platinum complexes are frequently employed as catalyst precursors for P-H additions to unsaturated hydrocarbons, but (chiral) lanthanide complexes were used with great success for the (enantioselective) addition to heteropolar double bond systems, such as aldehydes and imines whereby pharmaceutically valuable a-hydroxy or a-amino phosphonates were obtained efficiently. 

Podlovchenko Bl, Petrii OA, Frumkin AN, Lai H. 1966. The behaviour of a platinized-platinum electrode in solutions of alcohols containing more than one carbon atom, aldehydes and formic acid. J Electroanal Chem 11 12-25. 

Besides Wacker oxidation, other transition-metal catalyzed oxidations have also been carried out in aqueous medium. For example, methyl groups can be selectively hydroxylated by platinum salts in water.88 In this way, p-toluenesulfonic acid was oxidized to benzy-lic alcohol, which was subsequently oxidized into the aldehyde (Eq. 3.19).89... 

During hydrogenation of aldehydes, especially over platinum oxide, catalyst deactivation occurs. The reasons for this deactivation are not well understood and several theories exist.6... 

The influence of steric and electronic effects of diphosphites (111) and (112) have been studied with regard to their catalytic performance on the hydroformylation of styrene catalyzed by platinum complexes. The highest chemoselectivity to aldehyde (71%) and regioselectivity to branched aldehyde (85%), with an enantiomeric excess of 86%, was obtained with the plat-inum(II)-SnCl2 catalytic system associated with ligand (25, 45)-bis(5)-(lll).340... 

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