Hydrogenolysis, of esters

Selective Hydrogenolysis of Esters and Acids to Aldehydes and Alcohols... 

Hydrogenolysis of esters to aldehydes or alcohols needs high temperatures and high pressures. Moreover, it leads to the formation of acids, alcohols, and hydrocarbons. In contrast, bimetallic M-Sn alloys (M = Rh, Ru, Ni) supported on sihca are very selective for the hydrogenolysis of ethyl acetate into ethanol [181]. For example while the selectivity to ethanol is 12% with Ru/Si02, it increases up to 90% for a Ru-Sn/Si02 catalyst with a Sn/Ru ratio of 2.5 [182]. In addition, the reaction proceeds at lower temperatures than with the classical catalysts (550 K instead of temperatures higher than 700 K). The first step is the coordination of the ester to the alloy (Scheme 46), and most probably onto the tin atoms. After insertion into the M - H bond, the acetal intermediate decomposes into acetaldehyde and an ethoxide intermediate, which are both transformed into ethanol under H2. 

Only scare data is available in the literature on the application of rhenium containing mono- or bimetallic catalysts in the hydrogenolysis of esters to alcohols. Decades ago Broadbent and co-workers studied the hydrogenation of organic carbonyl compounds (aldehydes, ketones, esters, anhydrides, acids,... 

Hydrogenolysis of esters to aldehydes or alcohols is difficult to attain either by homogeneous or heterogeneous catalysis. In fact, high temperatures and high pressures are required to achieve the reaction, leading to a non-selective hydrogenolysis with formation of acids, alcohols, CO2, CO and hydrocarbons. 

Hydrogenolysis of esters to acids can occur provided the R —O bond is weakened, for instance when R is a vinyl, allyl, or aryl group [equation (d)]. Thus, hydrogenolysis of an enol ester is a synthetic method for facile removal of carbonyl oxygens that applies to enol trifluoromethanesulfonates [equation (e)] ... 

When an Ru/Si02 catalyst is modified by reaction with SnBu4 (Sn/Ru = 2.4) to give RuSn2,4/Si02, the catalytic activity for hydrogenolysis of esters to aldehydes or alcohols (Scheme 73) is about the same as with unmodified catalyst, but the selectivity for the cleavage of the C-OR bond increases up to 90%. ... 

Scheme 73 Hydrogenolysis of esters to aldehydes or alcohols on RuSn2,4/Si02 catalyst.

Despite the extended studies on supported Sn-Ru catalysts used in different selective hydrogenation reactions and hydrogenolysis of esters there is a need to further investigate the effect of catalyst preparation and pretreatment parameters on the performance of this bimetallic system. 

Nevertheless only scare data is available in the recent literature on the application of Group VIII noble metal (M) or rhenium-based mono- and Re-M bimetallic catalysts, in the hydrogenolysis of esters or hydrogenation of acids to alcohols. Recently a few publications, - and patents. have been reported on the transformation of different carbonyl compounds (saturated and unsaturated esters, acids and carboxamides) over rhenium-containing catalysts. In the bimetallic catalysts used for the hydrogenation of carbonyl compounds the rhenium was combined with Pd, or Rh. In the case of catalysts used for the hydrogenation of unsaturated carbonyl compounds the rhenium is usually modified with tin. ... 

Reaction rates listed in Table 23 indicate that, due to the absence of ionic promoter species the activity of Pt/AbOs catalyst is very low. The addition of rhenium to the Pt/AbOs catalyst increased the hydrogenolysis activity at least by a factor of 20. The activity of the two different types of Re-Pt catalysts is in the same order of magnitude. Ru is often used as a component of catalysts applied for the hydrogenolysis of esters.22 Therefore, the addition of Ru to Re-Pt catalysts further improved the hydrogenolysis activity and the selectivity to butanol. It is noteworthy that the selectivity of RePt and RuRePt catalysts is significantly higher than that of monometallic Pt catalyst. 

Besides hydrocarbons and heavy ester, small amounts of ethers and unidentified products were also detected in the reaction mixtures. It is also noteworthy that in the hydrogenolysis of esters no water is formed in the main reactions, only minor amount of water is formed during in situ reduction of catalysts and in the side reactions. Therefore, in the hydrogenolysis of ethyl dodecanoate no information can be obtained about the water tolerance of the CuO-ZnO-AbOs catalysts. 

Selective hydrogenolysis of esters and acids to aldehydes and alcohols... 

This mechanism is comparable to that proposed for the hydrogenolysis of esters. Note also that during the reaction esters are formed which can also undergo the hydrogenolysis reaction, by the mechanism depicted above. 

This section lists examples of the hydrogenolysis of esters, R COOR — RH. For the conversion R COOR — R R" or RR" (R"=alkyl) see section 68 (Alkyls and Aryls from Esters). The reduction of esters of sulfonic acids (e.g. ROTs — RH) is included in section 160 (Hydrides from Halides and Sulfonates)... 

Turek, T., Trimm, D.L., Cant, N.W., 1994. The catalytic hydrogenolysis of esters to alcohols. Catalysis Reviews Science and Engineering 36, 645-683. 

Yan, T.Y., Albright, L.F., Case, L.C., 1965. Hydrogenolysis of esters, particularly perfluorinated esters. Industrial Engineering Chemistry Research 4,101-107. 

Fig. 8.24 Tentative reaction pathways for hydrogenolysis of esters in a cascade system. (From Z U, R. S. Assary, A C. Atesin, L A. Curtiss, T. J. Marks, Rapid ether and alcohol C-0 bond hydrogenolysis catalyzed by tandem high-valent metal triflate+supported Pd catalysts. J. Am. Chem. Soc. 136 (2014) 104—107. Copyright 2014 American Chemical Society).

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