Hydrogenation to Alcohols

TABLE 10.1 Hydrogenation of Fatty Acids to Alcohols over Copper Catalysts  

TABLE 10.2 Hydrogenation of Dicarboxylic Acids to Glycols over Copper Catalysts  

Carnahan et al. obtained good yields of alcohols and glycols by hydrogenation of lower mono- and dicarboxylic acids over ruthenium dioxide or Ru-C at 135-225°C and 34-69 MPa H2 (eqs. 10.1 and 10.2).8 In general, the optimum temperature was about 150°C. The chief side reaction was hydrogenolysis of the alcohols, as exemplified in the formation of ethanol from oxalic acid and of butanol and propanol from succinic acid (see eq. 10.2). Platinum and palladium catalysts were ineffective under similar or even more severe conditions. 

Broadbent et al. have found that the rhenium blacks prepared by reducing rhenium heptoxide are highly effective catalysts for the hydrogenation of carboxylic acids to alcohols at 150-170°C for monocarboxylic acids (eq. 10.3) and at 200-250°C for dicarboxylic acids (eq. 10.4) under 13.5-27 MPa H2.10,11 Rhenium heptoxide can be reduced to the active blacks in appropriate solvent (ethanol, 1,4-dioxane, acetic acid, or water) at 120-220°C and 15-21 MPa H2 for 1-2 h, or more conveniently, in situ in 

Novotny found that neat trifluoroacetic acid could be hydrogenated to 2,2,2-tri-fluoroethanol in the presence of rhodium or iridium catalyst under much milder con- 

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Amination. Isopropyl alcohol can be aminated by either ammonolysis ia the presence of dehydration catalysts or reductive ammonolysis usiag hydrogeaatioa catalysts. Either method produces two amines isopropylamine [75-31-0] and diisopropylamine [108-18-9]. Virtually no trisubstituted amine, ie, triisopropyl amine [122-20-3], is produced. The ratio of mono- to diisopropylamine produced depends on the molar ratio of isopropyl alcohol and ammonia [7664-41-7] employed. Molar ratios of ammonia and hydrogen to alcohol range from 2 1—5 1 (35,36). 

Fig. 6. Campholenic aldehyde (81) reacts with 2-butanone to produce ketones that are hydrogenated to alcohols having the odors indicated.

Rosenthal and co-workers (91, 92) studied the cobalt hydroformylation of various unsaturated carbohydrates. As with other a,j8-unsaturated ethers, addition of the formyl group occurred almost exclusively at the double-bond carbon a to the oxygen. High yields of product were obtained, but hydrogenation to alcohol was facile, even under mild conditions, as noted in Eq. (43) ... 

The isomer aldehydes are completely hydrogenated to alcohols which are partially dehydrated to ethers and carbonylated to esters. 

A reaction related to CO hydrogenation to alcohols has recently been reported (46,46a, 46b, 46c)-, the products are apparently siloxanes. Reactions were carried out in the presence of high concentrations of hydrosilanes, as follows ... 

Aldehydes have been catalytically hydrogenated to alcohol products in a range of supercritical solvents under otherwise mild conditions.314... 

Since the aldehydes are easily hydrogenated to alcohols, which are the most frequently required end-products, the reactions are often run to make the alcohol. Much later it was discovered that a rhodium triphenylphosphine complex, developed by Wilkinson, was a much better catalyst for many hydroformylation reactions as it required milder conditions (lower temperature and pressure) and gave a higher selectivity. 

Aldehydes are easily hydrogenated to alcohols but ketones are more difficult to reduce because of steric hindrance. Hydrogenolysis is a problem with the catalytic reduction of carbonyls, particularly when linked to aromatic systems. Pd and H2 reduce alkenes faster than carbonyls. Metal catalyst Pt is commonly used for the reduction of carbonyls. For example, the Adams catalyst (Pt02) reduces 2-naphthaldehyde (6.31) to 6.32 in 80% when used with FeCls as a promoter. When excess of the promoter is used the product is 2-methylnaphthalene (6.33), which is also obtained by the reduction of 6.31 with Pd on BaS04 and H2. 

Cobalt catalysts completely dominated industrial hydroformylation rmtil the early 1970s, when rhodium catalysts were commercialized. Most aldehydes produced are hydrogenated to alcohols or oxidized to carboxylic acids. Esterification of the alcohols with phthalic anhydride produces dialkyl phtha-late plasticizers that are primarily used for polyvinyl chloride plastics - the largest single end-use. Detergents and surfactants make up the next largest category, followed by solvents, lubricants, and chemical intermediates. 

In 1952, it was discovered by Schiller that rhodium salts generated highly active hydroformylation catalysts. It was from these early studies that rhodium was estimated to be 1000 to 10 000 times more active than cobalt. Rhodium was also found to be very selective to aldehydes, with httle hydrogenation to alcohols observed under normal catalysis conditions. It was suggested early on that HRh(CO)4 was the active catalyst species, analogous to HCo(CO)4, and the same monometallic mechanism was proposed (Scheme 6). 

One possible explanation of the formation of the primary alcohol is that the oxirane isomerizes to aldehyde in the first step (Eq. 142) the aldehyde then being hydrogenated to alcohol. This is not probable, for oxiranes participate in hydrogenolysis under conditions where oxo compounds do not react at all with hydrogen or only very slowly moreover, the reduction of the oxo compound does not occur on a surface covered with the oxirane. Thus, formation of alcohols and oxo compounds is the result of simultaneous processes. 

The stereochemistry of the cathodic reaction of carbonyl compounds has been extensively studied from synthetic and mechanistic aspects. In this section the reaction is discussed in three parts hydrogenation to alcohols, hydrodimerization to pinacols, and cross-dimerization with unsaturated compounds. 

Only activated monoenes are hydrogenated . These include carvene, mesityl oxide, 2-cyclohexenone, and benzalacetone . Some styrenes are hydrogenated a-functionalized styrenes react, but )S-functionalized styrenes do not - " . Similarly, only activated ketones such as benzil, diacetyl and p-benzoquinone are hydrogenated to alcohols " . Often catalytic reduction of a ketone is observed only in the presence of added OH . The base is believed to react with an intermediate to give [Co(CN)j(OH)] and the reduced substrate . Aryl ketones such as acetophenone and benzophenone are not reduced . Several examples of nitro and nitroso compound reductions have been reported - . ... 

Since aldehydes are reactive, subsequent reactions occur to varying extent under the conditions necessary for cobalt-catalyzed hydroformylations, e.g. 10% of the aldehyde is further hydrogenated to alcohol . This product forms acetals with the parent aldehyde which makes separation and purification difficult. 

Non-activated carbon-carbon double bonds react catalytically and rapidly with H2 and CO at 80°C and a few atmospheres pressure (<1 mPa). The milder conditions are attractive for the synthesis of aldehydes without high pressure equipment and for minimizing competitive but undesirable reactions. Among the latter are partial hydrogenation to alcohols which form acetals, aldol condensation products, ketone formation and hydrogenation to alkane. 

Peroxide compounds should readily hydrogenate to alcohols... 

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