Platinum ketone hydrogenation

Further evidence for surface effects upon the stereochemistry of electrochemical reduction of ketones comes from the discovery that the nature of the cathode material may effect stereochemistry. Reduction of 2-methylcyclo-hexanone affords pure trans-2-methylcyclohexanone at mercury or lead cathodes, a mixture of cis and trans alcohols (mostly trans) at nickel, and pure cis alcohol at copper 81 >. Reduction could not be effected at platinum presumably hydrogen evolution takes place before the potential necessary for reduction of the ketone can be reached. 

Trifluoroacetic acid is the best solvent for several platinum-catalysed hydrogenation processes. For example the rate of reduction of ketones to secondary alcohols is about three times faster than in acetic acid. The ketones are taken in concentrated solutions to keep the reactions faster. 

Rules 1 and 2 may be accepted as a generalization based primarily on the results obtained over platinum catalysts. However, there have been known many examples of the exception to this rule,153 since the stereochemistry of hydrogenation may be influenced by many factors, such as the solvent, the temperature, the hydrogen pressure, and the basic or acidic impurity associated with catalyst preparation, as well as the activity of the catalyst, and since the effects of these factors may differ sensitively with the catalyst employed and by the structure of the ketone hydrogenated. 

Aldehydes and ketones are usually hydrogenated to the alcohol over platinum, rhodium or ruthenium catalysts at 25 -60°C and 1-5 atmospheres pressure. Platinum catalyzed hydrogenations are generally best run in acidic media while with rhodium or ruthenium neutral or basic solvents are preferred. Hydrogenations run over ruthenium catalysts are facilitated by the presence of water which makes ruthenium a particularly effective catalyst for the hydrogenation of sugars and other water soluble aldehydes and ketones.2... 

Hydrogenation solvent. Trifluoroacetic acid appears to be the best solvent for the platinum-catalyzed hydrogenation of ketones. The rate in this solvent is approximately three times the rate in acetic acid. A relatively high concentration of the ketone should be used, since the reactions are slow in dilute solution. The alcohol is frequently obtained as the ether. 

The lower selectivity of PtHFAU catalysts is due to the very rapid formation of Ce cyclic hydrocarbons (family 1). The same trend has been found in the case of acetone transformation [3]. This can be explained by the lower activity of the palladium relatively to the platinum to hydrogenate the C=0 bond. This lower activity which has been found in the case of cyclohexanone hydrogenation on platinum group metals was explained by a weaker adsorption of the ketone on Pd in comparison with Pt and Ru [9]. 

Platinum oxide hydrogen chloride Ethers from ketones s. 19, 240... 

Platinum oxide hydrogen chloride a-Amino- from a-isonitroso-ketones s. 17,29... 

Reductive amination of aldehydes and ketones gives primary, secondary, and tertiary amines. Sodium triacetoxyborohydride (STAB-H) [93] or platinum-catalyzed hydrogenations are used as shown in Equations 6.59 [94] and 6.60 [95]. 

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... 

Reduction. Most ketones are readily reduced to the corresponding secondary alcohol by a variety of hydrogenation processes. The most commonly used catalysts are palladium, platinum, and nickel For example, 4-methyl-2-pentanol (methyl isobutyl carbinol) is commercially produced by the catalytic reduction of 4-methyl-2-pentanone (methyl isobutyl ketone) over nickel. 

Because of the presence of alkali in Raney nickel, ketones are hydrogenated over this catalyst to yield the more stable, equatorial alcohol e.g. 59) as the predominant product, Similar results can be expected with platinum in basic media or with platinum oxide in an alcoholic solvent since this catalyst also contains basic impurities. 

The hydrogenation of 5a-cholestanone (58) in methanolic hydrobromic acid over platinum gives 3j5-methoxycholestane ° (61). This compound is also obtained from the palladium oxide reduction of (58) in methanol in the absence of acid. Hydrogenation of 5 -cholestanone also gives the 3j5-methoxy product under these conditions. Reduced palladium oxides are quite effective for the conversion of ketones to ethers. The use of aqueous ethanol as the solvent reduces the yield of ether. Ketals are formed on attempted homogeneous hydrogenation of a 3-keto group in methanol. ... 

Stability toward reduction makes hydrogen fluoride a good medium for different hydrogenation processes [1, 2] It is a useful solvent for the hydrogenation of benzene in the presence of Lewis acids [f ] Anhydrous hydrofluonc acid has pronounced catalytic effect on the hydrogenations of various aromatic compounds, aliphatic ketones, acids, esters, and anhydrides in the presence of platinum dioxide [2] (equations 1-3)... 

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. 

TDicvclohexvl-2-(2 -pvridvl)ethylene hydrochloride (15 grams) in 150 ml of ethanol was hydrogenated in the presence of platinum oxide at about 60 pounds per square inch of hydrogen pressure. The product, 1,1-dicvclohexvl-2-(2 -piperidyl)ethane hydrochloride, crystal-Ii2ed from a mixture of methanol and methyl ethyl ketone as a white solid melting at 243 to 245.5 C. 

A mixture of 0.5 gram of platinum oxide and a solution of 2.0 grams (0.0067 mol) of 3,4-dihydroxyphenyl-2-pyridyl ketone hydrobromide in 20 ml of water and 80 ml of ethanol Is hydrogenated on the Parr apparatus using an initial hydrogen pressure of 50 psi at room temperature. The reaction mixture is filtered, the filtrate concentrated in vacuo and the residue triturated with acetone to give erythro-3,4-dihydroxyphenyl-2-piperidinylcarbinol hydrobromide, MP 210° to 211°C (decomposition). 

The benzylic position of an alkylbcnzene can be brominated by reaction with jV-bromosuccinimide, and the entire side chain can be degraded to a carboxyl group by oxidation with aqueous KMnCfy Although aromatic rings are less reactive than isolated alkene double bonds, they can be reduced to cyclohexanes by hydrogenation over a platinum or rhodium catalyst. In addition, aryl alkyl ketones are reduced to alkylbenzenes by hydrogenation over a platinum catalyst. 

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. " ... 

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