Hydrogenation over Palladium Catalysts

Supported palladium catalysts have often been used with an organic base such as pyridine and quinoline as additive or solvent to depress hydrogenation of resulting olefins and thus improve selectivity. For example, the selective hydrogenation of steroidal 17- 

TABLE 4.1 Percent Trans Isomer in Olefinic Product with Uptake of 1 mol of Hydrogen by 4-Undecyne in the Presence of Palladium Catalysts 1  

A new Pd-Pb alloy catalyst has been reported to be more selective than commercial Lindlar catalyst in the hydrogenation of triple bonds, that is, in the hydrogenation of 2-butyne to (Z)-2-butene and phenylacetylene to styrene.34 The high selectivity of the alloy catalyst was confirmed in the syntheses of (Z)-l 1-hexade-cenyl acetate and (Z)-l 1-tetradecenyl acetate where particularly high stereoselectivity was required. 

Robins and Walker obtained 1-vinylcyclohexanol in high yield simply by hydrogenating 1-ethynylcyclohexanol over 2% Pd-SrC03 in methanol until the calculated amount of hydrogen had been absorbed (eq. 4.12).42 

In a patent dealing with the selective hydrogenation of alkynols, use of palladium catalysts in combination with lower aliphatic amines such as butylamine, ethanolamine, and ethylenediamine, or in liquid ammonia was claimed to be more effective than use in the presence of higher amines, and superior to Lindlar catalyst in both activity and selectivity.43 Thus, linalool was obtained almost quantitatively by hydrogenation of 3,7-dimethyl-6-octen-l-yn-3-ol over Pd-CaC03 in the presence of butylamine (eq. 4.13). 

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Sato et al. carried out detailed studies on the possibilities of transformation of tetrazolo[l,5-tf]pyrazines 54 to 2-aminopyrazines 56 < 1994S931 >. These authors found that the generally used methods for this conversion fail because the starting compound exists in the stable bicyclic form 54, whereas partial formation of the azide valence bond isomer 55 would be necessary for the success of the transformation. Application of special reaction conditions succeeded, however hydrogenation over palladium catalyst in the presence of ammonium hydroxide or treatment with stannous chloride in a mixture of methanol and hydrochloric acid solved this problem. Thus, a great number of derivatives of 54 was reduced to the corresponding 2-aminopyrazine 56 in medium to high yields (45-100%). 

The carbon-nitrogen double bond of phenanthridine can be reduced selectively by hydrogenation over Raney nickel, and attempted reductive dechlorination of 6-chloro derivatives in the presence of this catalyst normally results in the formation of the corresponding 5,6-dihydro compounds.106 Hydrogenations over palladium catalysts are more successful.203 325 Desulfurization of phenanthridinthione... 

When succinic anhydride was hydrogenated over palladium catalysts, y-butyrolactone was produced in high yield in ethyl acetate at 30-95°C and 7.3-10.8 MPa H2 (eq. 10.34).61... 

Hydrogenation is effected over Raney nickel at 100-130 atm. Low-pressure hydrogenation over palladium catalysts has also been used for the succinates. ... 

A much more frequently used reaction is the cleavage of unsaturated compounds to aldehydes (equations 98 and 99). Alkenes and cycloalkenes that possess one or two hydrogens at the double bonds are oxidized by ozone to ozonides, which have to be reduced to prevent a subsequent oxidation to acids by the excess oxygen atom. Reductions are carried out, usually without isolation of the ozonides, by catalytic hydrogenation over palladium catalyst [80, 81,1106] or Raney nickel [55] or by treatment with... 

In contrast to benzene rings, tropones, 18, and tropolones, 19, are easily hydrogenated over palladium catalysts at low temperatures and pressures in neutral solvents (Eqns. 17.21-22). 2,83 acetic acid the ketone group of the saturated tropolone is also reduced. ... 

Arrhenius behaviour in ethyne hydrogenation over palladium catalysts. Appl Catal 55 L5... 

Since benzyl groups can be removed from N,N-dibenzylcyclopropylamines by catalytic hydrogenation over palladium catalysts, primary cyclopropylamines are accessible by this methodology. Thus, the theoretically interesting tricyclopropylamine [106,107] could be prepared from benzylcyclopropylformamide by a sequence of reductive cydopropanation of the formyl group, hydrogenolytic debenzylation, N-formylation, and repeated reductive cydopropanation [106,107]. 

Coke formation during acetylene hydrogenation over palladium catalyst could promote hydrogen transfer to acetylene and ethylene adsorbed molecules leading to a decrease in ethane selectivity and an increase in ethylene loss. 

This section is divided into three different parts Sect.B, catalytic hydrogenation over palladium catalysts Sect.C, Pd-catalyzed reductions with group 14 metal hydrides and Sect.D, Pd-catalyzed reductions with other hydrogen donors. 

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