Platinum aromatic aldehydes

In contrast to phenolic hydroxyl, benzylic hydroxyl is replaced by hydrogen very easily. In catalytic hydrogenation of aromatic aldehydes, ketones, acids and esters it is sometimes difficult to prevent the easy hydrogenolysis of the benzylic alcohols which result from the reduction of the above functions. A catalyst suitable for preventing hydrogenolysis of benzylic hydroxyl is platinized charcoal [28], Other catalysts, especially palladium on charcoal [619], palladium hydride [619], nickel [43], Raney nickel [619] and copper chromite [620], promote hydrogenolysis. In the case of chiral alcohols such as 2-phenyl-2-butanol hydrogenolysis took place with inversion over platinum and palladium, and with retention over Raney nickel (optical purities 59-66%) [619]. 

Hydrogen and a catalyst.2 0 The most common catalysts are platinum and ruthenium, but homogeneous catalysts have also been used.281 Before the discovery of the metal hydrides this was one of the most common ways of effecting this reduction, but it suffers from the fact that C=C, CssC, C=N and C=N bonds are more susceptible to attack than C=0 bonds.282 For aromatic aldehydes and ketones, reduction to the hydrocarbon (9-37) is a side reaction, stemming from hydrogenolysis of the alcohol initially produced (0-78). 

Unsymmetrical secondary amines are readily prepared in good yields by the catalytic reduction of Schiff bases at moderate temperatures in high-or low-pressure equipment. Many examples have been cited. The intermediate imines are prepared from primary amines and aldehydes—very seldom from ketones—and may be used without isolation (cf. method 431). For the preparation of aliphatic amines, e.g., ethyl-w-propylamine and n-butylisoamylamine, a prereduced platinum oxide catalyst is preferred with alcohol as the solvent. Schiff bases from the condensation of aromatic aldehydes with either aromatic or aliphatic amines are more readily prepared and are reduced over a nickel catalyst. In this manner, a large number of N-alkylbenzylamines having halo, hydroxyl, or methoxyl groups on the nucleus have been made. Reductions by means of sodium and alcohol and lithium aluminum hydride have also been described,... 

On the other hand, use of acetic acid as the solvent, or addition of a few drops of concentrated HCl or HCIO4, facilitates formation of the hydrocarbon. Platinum oxide is rapidly deactivated by aromatic aldehydes through reduction of the catalyst to a lower oxidation state. This difficulty is circumvented by reactivation after shaking the reaction mixture with air or by various additives such as Fe ... 

Platinum oxide-Fe or Cu-containing catalysts allow hydrogenation of furfural to furfurylalcohol". Ruthenium catalysts (Ru—C, RuOj) are successful in this specific case they have an activity well preserved through reuses. Otherwise Ru exhibits little activity in the heterogeneous hydrogenation of aromatic aldehydes. Other heterogeneous catalysis include platinized (PtC ) Raney Ni and copper chromite. 

Palladium is usually the prefeired metal of choice for aromatic aldehyde hydrogenation in neutral non-polar solvents such as hexane, DMF, or ethyl acetate (5-100 °C and 1-10 bar) although ruthenium, which is less active, can be considered and run in aqueous alcohol at similar temperatures and pressures. If higher pressures are accessible ruthenium may be preferable because of its lower (historical) cost. Its use has recently been reviewed [4]. Although platinum and rhodium could... 

Catalysts that are active for hydrogenation can be used equally for hydro-genolysis of an N-N bond Raney nickel is the catalyst of choice for this purpose 162 use of platinum or palladium catalysts generally leads only to the substituted hydrazine,144 145 N-N fission being rare in these cases.146 Experience to date does not permit prediction with certainty as to which compounds will suffer N-N fission on noble-metal catalysts it has been observed mainly with hydrazones and azines of aromatic aldehydes, but such catalysts have often been used in acid solution for reduction of hydrazones and azines to the corresponding hydrazines. 

For aliphatic aldehydes and ketones, reduction to the alcohol can be carried out under mild conditions over platinum or the more-active forms of Raney nickel. Ruthenium is also an excellent catalyst for reduction of aliphatic aldehydes and can be used to advantage with aqueous solutions. Palladium is not very active for hydrogenation of aliphatic carbonyl compounds, but is effective for the reduction of aromatic aldehydes and ketones excellent yields of the alcohols can be obtained if the reaction is interrupted after absorption of one mole of hydrogen. Prolonged reaction, particularly at elevated temperatures or in the presence of acid, leads to hydrogenolysis and can therefore be used as a method for the reduction of aromatic ketones to methylene compounds. 

Methyl esters are produced in the electrolytic oxidation of methanolic solutions of aldehydes in the presence of sodium cyanide at a platinum anode the eight examples that have been studied show yields of 38 to 80%. Carefully purified butyraldehyde reacts with RuHgfPPha) to give butyl butyrate aliphatic and aromatic aldehydes undergo this dimerization process, which may have considerable synthetic potential." The electrolysis of furan-2-carboxylic acid (66) provides an efficient synthesis of 4,4-dimethoxy-esters (67), typically in 77 % yield (Scheme 35). "... 

In the presence of a catalytic amount of a palladium, platinum, or copper catalyst, disilanes also react with various 9-unsaturated carbonyls to effect conjugate addition of silyl groups. The process can be performed in an enantioselective manner using a chiral phosphorus ligand. The presence of silyl inflates as a co-catalyst allows palladium-catalyzed silylation of aromatic aldehydes (Scheme 3-15). ... 

V-Acylsaccharins prepared by treatment of the sodium salt of saccharin with acyl chlorides were reduced by 0.5 molar amounts of sodium bis(2-methoxyethoxy)aluminum hydride in benzene at 0-5° to give 63-80% yields of aliphatic, aromatic and unsaturated aldehydes [1108 Fair yields (45-58%) of some aliphatic aldehydes were obtained by electrolytic reduction of tertiary and even secondary amides in undivided cells fitted with platinum electrodes and filled with solutions of lithium chloride in methylamine. However, many secondary and especially primary amides gave 51-97% yields of alcohols under the same conditions [130]. 

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. ... 

Emerson and Waters alkylated primary aromatic amines with C2-C7 aliphatic aldehydes and benzaldehyde over Raney Ni in the presence of sodium acetate as a condensing agent and obtained /V-alkylanilincs in 47-65% yields.33 With C2-C5 aldehydes, up to 10% of the tertiary amines were produced, but no tertiary amines were found in the case of heptanal and benzaldehyde. With acetaldehyde in the absence of sodium acetate, aniline was recovered unchanged over platinum oxide and a mixture of amines resulted over Raney Ni, compared to 41 and 58% yields of N-ethylaniline over platinum oxide and Raney Ni (eq. 6.12), respectively, in the presence of sodium acetate. 

Figure 111. Emission of aldehydes, acrolein and various polynuclear aromatic hydrocarbons of two passenger cars equipped with an IDI/NA and with a DI/NA diesel engine, once without and once with a diesel oxidation catalyst, in the US-FTP 75 vehicle test cycle (monolith catalyst with 62 cells cm dedicated diesel washcoat formulation with a platinum loading of 1.76 g 1 in the fresh state vehicle dynamometer tests according to the US-FTP 75 vehicle test procedure, with passenger cars equipped with a DI/NA and with an IDI/NA diesel engine of displacement 2.0 1). Reprinted with permission from ref [70], 1990 Society of Automotive Engineers, Inc.

Primary aromatic amines can be reductively alkylated in satisfactory yields by aliphatic aldehydes in alcoholic solution containing a platinum or nickel catalyst at 3 atm hydrogen pressure if sodium acetate is also present.1002... 

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