Catalytic hydrogenation selectivity

T. Mathe, Platinum Met. Rev., 1998, 42, 108-115. Palladium-Mediated Heterogeneous Catalytic Hydrogenations. Selectivity of Liquid-Phase Reactions for the Fine Chemicals Industry. 

Metal hydrides reduce preferably polar double bonds, whereas catalytic hydrogenation is somewhat selective for non-polar double bonds. Selective protection of amino groups in amino acids. 

Hydrogenation. Acetylene can be hydrogenated to ethylene and ethane. The reduction of acetylene occurs in an ammoniacal solution of chromous chloride (20) or in a solution of chromous salts in H2SO4 (20). The selective catalytic hydrogenation of acetylene to ethylene, which proceeds... 

The Corey process is also useful for the synthesis of PGs of the 1 and 3 series. Catalytic hydrogenation of (34) (see Fig. 5) with 5% Pd/C at — 15-20°C results in selective reduction of the 5,6-double bond. Subsequent transformations analogous to those in Figure 5 lead to PGE (9) and PGF (10). The key step for synthesis of the PG series is the Wittig reaction of (29) with the appropriate unsaturated CO-chain yUde (170). 

For a particular phenol, the authors required a protective group that would be stable to reduction (by complex metals, catalytic hydrogenation, and Birch conditions) and that could be easily and selectively removed. 

Significant quantities of Cj and C, acetylenes are produced in cracking. They can be converted to olefins and paraffins. For the production of high purity ethylene and propylene, the contained Cj and C3 acetylenes and dienes are catalytically hydrogenated leaving only parts per million of acetylenes in the products. Careful operation is required to selectively hydrogenate the small concentrations of acetylenes only, and not downgrade too much of the wanted olefin products to saturates. 

Catalytic hydrogenation has been utilized extensively in steroid research, and the method has been found to be of great value for the selective and stereospecific reduction of various functional groups. A number of empirical correlations concerning selectivity and product stereochemistry compiled for steroid hydrogenations has been listed in a previous review. ... 

Since the stereochemical course of a catalytic hydrogenation is dependent on several factors, " an understanding of the mechanism of the reaction can help in the selection of optimal reaction conditions more reliably than mere copying of a published recipe . In the first section the factors which can influence the product stereochemistry will be discussed from a mechanistic viewpoint. In subsequent sections the hydrogenation of various functional groups in the steroid ring system will be considered. In these sections both mechanistic and empirical correlations will be utilized with the primary emphasis being placed on selective and stereospecific reactions. 

Another route to 5a compounds (57) proceeds from the dienol ether (58) by selective catalytic hydrogenation of the A -double bond with concomitant shift of the 3,4-double bond to the 2,3-position. If the hydrogenation is carried out in the presence of traces of base, double bond migration is suppressed and the difficultly accessible A -enol ethers of 5a-series (59) are thus obtained. 

The technique of catalytic hydrogenation can be applied almost universally to unsaturated systems, and therein lies its chief advantage (7). By appropriate selection of catalyst, pressure, and temperature, a remarkable variety of substrates can be made to undergo hydrogenation, many of them under hydrogen pressure not exceeding 50 psi (see Appendix 3 for description and use of low-pressure hydrogenation apparatus). 

The literature on catalytic hydrogenation is very extensive, and it is tempting to think that after all this effort there must now exist some sort of cosmic concept that would allow one to select an appropriate catalyst from fundamentals or from detailed knowledge of catalyst functioning. For the synthetic chemist, this approach to catalyst selection bears little fruit. A more reliable, quick, and useful approach to catalyst selection is to treat the catalyst simply as if it were an organic reagent showing characteristic properties in its catalytic behavior toward each functionality. For this purpose, the catalyst is considered to be only the primary catalytic metal present. Support and... 

Solvents are often used in catalytic hydrogenation (< 7). Solvents may be one of the best means available for markedly altering the selectivity, a fact not sufficiently appreciated. Solvents also help to moderate the heat of hydroge nation, to aid in catalyst handling and recovery, and to permit the use of solid substrates. A convenient solvent may be the product itself or the solvent used in a prior or subsequent step. 

It can be summarized from the available data in Table 3 that supported palladium catalysts selectively hydrogenated carbon-carbon double bonds in the presence of the nitrile group in NBR. However, there is no detailed fundamental study on heterogeneous catalytic hydrogenation of nitrile rubber in the literature that can provide an insight into the reaction. The available information is limited since most of the literature is patented. 

In view of the unexpected effects of the C-2 and C-3 substituents on the reaction of C-4 sulfonates, it is worthwhile to point out the observations made with some 2,3-dideoxy derivatives. Treatment of ethyl 2,3-dideoxy-4,6-di-0-methylsulfonyl-D-ert/ hro-hexopyranoside (40) with sodium iodide and acetone at 115°C. caused the displacement of the C-6 mesylate group selectively to give 41. Catalytic hydrogenation then gave the corresponding ethyl 4-0-methylsulfonyl-2,3,6-trideoxy- -D-en/ /iro-hexoside in good overall yield (83%) (72). 

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