Hydrogenation of esters

The catalyst, which may be regarded as complementary to Raney nickel (Section VI,5) is largely used for the hydrogenation of esters (esters of monobasic and of dibasic acids to alcohols and glycols respectively) ... 

Sodium borohydride reduces esters but the reaction is too slow to be useful Hydrogenation of esters requires a special catalyst and extremely high pressures and tern peratures it is used m industrial settings but rarely m the laboratory... 

The catalytic hydrogenation of esters is of great commercial importance. It is one of the industrial methods used to produce long-chain fatty alcohols (eg, dodecyl and decyl alcohols) from fatty acid methyl esters (33). The method is also suitable for the conversion of dimethyl 1,4-cyclohexanedicarboxylate [94-60-0] into 1,4-cydohexanedimethanol [105-08-8] an important intermediate in the manufacturing of polyesters. 

Hydrogenation of esters may give alcohols, acids, ethers, or hydrocarbons the product composition seems to depend largely on the structure of the substrate. 

Alcohols are the most frequently formed products of ester hydrogenolysis. The hydrogenation of esters to alcohols is a reversible reaction with alcohol formation favored at high pressure, ester at low pressure (/). Copper chromite is usually the catalyst of choice. Details for the preparation of this catalyst (/7) and a detailed procedure for hydrogenation of ethyl adipate to hexamethylene glycol (/[Pg.80]

Platinum catalyzes the hydrogenation of most functional groups. It will not catalyze the hydrogenation of esters, acids, and amides. 

The hydrogenation of esters remains a challenge. Some recent progress has been reported by Teunissen and Elsevier [71, 72] where a mixture of Ru(acac)3 and MeC(CH2PPh2)3 was used to hydrogenate aromatic and aliphatic esters to the alcohols in MeOH at 100-120 °C with 85 bar H2. 

Table 15.14 Hydrogenation of esters using ruthenium catalysts.3 ...

Scheme 15.20 Two-stage hydrogenation of esters giving ethylene glycol (EG), without decomposition products. MG = methyl glycolate.

Fatty Acid Esters and Fatty Alcohols Fatty acid esters are obtained by transesterification of triglycerides (vegetable oils) or by esterification of fatty acid with alcohol or polyols. Fatty alcohols are obtained by hydrogenation of esters on metal catalysts. Fatty acid esters and fatty alcohols are useful platform molecules to prepare surfactants, emulsifier, lubricants and polymers. 

Temperature affects the rate of hydrogenation but not as much as is usual with other chemical reactions. The rise in temperature from 50° to 100° and from 25° to 100° caused, respectively, fourfold and 12-fold increases in the rates of hydrogenation of esters over Raney nickel W6 [61]. The increased rate of hydrogenation may reduce the selectivity of the particular catalyst. 

With the same excess of catalysts hydrogenations of the esters over Raney nickel could be carried out at temperatures as low as 25-125° at 350atm with comparable results (80% yields). However, benzene rings were saturated under these conditions [55]. In addition to nickel and copper, zinc and chromium oxides, rhenium obtained by reduction of rhenium heptoxide also catalyzes hydrogenation of esters to alcohols at 150-250° and 167-340 atm in 35-100% yields [42]. 

Hydrogenation of esters, with copper chromite and Raney nickel, 8, 1 Hydrohalogenation, 13, 4 Hydroxyaldehydes, aromatic, 28, 1 a-Hydroxyalkylation of activated olefins, 51, 2... 

The rate law for the hydrogenation of esters to alcohols shows a dependence on the square of the hydrogen pressure. 

H. Adkins, Catalytic Hydrogenation of Esters to Alcohols, Org. React. 1954, 8, 1-27. 

The a hydrogens of esters are weakly acidic, and they can be deprotonated to give eno-... 

H2. The equilibrium concentrations of esters may become higher with increasing temperatures. Usually for the hydrogen pressures above 21 MPa and temperatures below 260°C, the concentration of esters at equilibrium is very small and practically not important. As the most favorable conditions for the reaction at 250°C, Adkins recommends an initial hydrogen pressure of 24 MPa, the maximum pressure of 41 MPa at the temperature of reaction, and 21 MPa or higher at completion.32 Typical hydrogenations of esters of mono- and dicarboxylic acids are given in eqs. 10.735 and 10.8,36 respectively. 

Adkins H (1954) Catalytic hydrogenation of esters to alcohols In Adams R (ed) Organic reactions, Vol VIII, Wiley, New York 9... 

The copper-chromium oxide ("copper chromite") catalyst, which was developed by Adkins, has been considered to be a complement of Raney nickel for hydrogenation reactions.26 This catalyst, which is, essentially, copper supported on chromium oxide or copper-chromium oxide,22-29 is useful for the hydrogenation of esters and amides but does not affect the saturation of... 

An alternate procedure involves the precipitation of mixed copper and chromium hydroxides from a solution of chromium nitrate and copper nitrate by the addition of an sodium bicarbonate and then heating the precipitate to 300°-500°C.30 In this procedure, the copperchromium ratio can be varied over a wide range. A ratio between four and eight was optimum for use in the hydrogenation of esters to alcohols (Eqn. 13.6).30 A related Zn-CrO catalyst prepared by the decomposition of precipitated zinc-copper hydroxides was effective in the hydrogenation of unsaturated esters to unsaturated alcohols (Eqn. 13.7). 0 The presence of a small amount of alumina increased catalyst activity and selectivity. Some of these catalysts, however, tend to become colloidal on use so they can present separation problems. O... 

Reduction. Hydrogenation of esters in the presence of Ru(acac>3 and MeCfCH PPhj) provides the alcohols. Double bonds are also saturated. 

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