Palladium catalyst for reductions

Hydrogenation of 3-pyridinecarboxylic acids is apt to be accompanied by extensive decarboxylation (2S), but this unwanted reaction can be prevented by carrying out the reaction in the presence of one equivalent of base (33,79). Ruthenium (33), rhodium (29), platinum oxide (2S,59), and palladium (30) have all proved effective catalysts for reduction of pyridinecarboxylic acids to the saturated acid. 

Palladium catalyst foe partial ee DUCTION OF ACETYLENES, 46, 89 Palladium on charcoal, catalyst for reductive methylation of ethyl p-mtrophenylacetate, 47, 69 in reduction of l butyl azidoacetate to glycine J-butyl ester 4B, 47 Palladium oxide as catalyst for reduction of sodium 2 nitrobenzene sulfinate, 47, S... 

Indolines are produced in good yield from 1-benzenesulfonylindoles by reduction with sodium cyanoborohydride in TFA at 0°C (Equation 5) (89TL6833). If acyl groups are present at C-2 or C-3 in the substrate, they are reduced to alkyl groups. Indole is also reduced to 2,3-dihydroindole by sodium cyanoborohydride and acetic acid or triethylamineborane and hydrochloric acid. An alternative method for preparing indolines involves treatment of indoles with formic acid (or a mixture of formic acid and ammonium formate) and a palladium catalyst (82S785). Reduction of the heterocyclic ring under acidic conditions probably involves initial 3-protonation followed by reaction with hydride ion. 

However, the use of hydrazine hydrate and a palladium catalyst for the reduction gave high yields of the amines. The crystalline amines all decomposed on exposure to air or hot solvents. The rates of decomposition varied thus, compounds 122 were stable for up to 1 hr, whereas 123 was much more stable, only decomposing after several months. 

Stalder, C. J., Chao, S., Summers, D. P., and Wrighton, M. S. (1983). Supported Palladium Catalysts for the Reduction of Sodium Bicarbonate to Sodium Formate in Aqueous Solution at Room Temperature and One Atmosphere of Hydrogen. Journal of the American Chemical Society, 105, 6318-6320. 

Polymerization of triethoxysilane in water catalyzed by Pd(OAc)2 results in a palladium-deposited siloxane that can be used as a heterogeneous catalyst for reductions with hydrogen. 

In spite of the successful use of NHCs in a number of palladium-catalyzed reactions, no system for hydrogenation was reported until 2005. This can be easily explained as it had been observed that hydridopalladium-carbene species decompose due to attack of the hydride on the carbene, which results in its reductive elimination to yield the corresponding imidazolium salt [ 190]. However, Cavell and co-workers recently showed that the oxidative addition of imidazolium salts to bis-carbenic palladium complexes leads to isolable NHC-hydridopalladium complexes [191]. This elegant work evidenced the remarkable stabilizing effect of NHC ligands in otherwise reactive species and led to the development of the first NHC-palladium catalyst for hydrogenation. 

Platinum Catalyst for Reductions (CoU. Vol. i, 452) Directions are given for the preparation of a colloidal platinum (or palladium) catalyst in an anhydrous, alkaline medium. This catalyst is said to be particularly valuable for the reduction of nitriles, oximes, and nitrostyrenes to pure prinaary amines. Skiia and Keil, Ber. 63, 424 (r932). 

The dicyano-palladium complex (03P)2Pd(CN)2 is a remarkably effective catalyst for reduction to monoene, even without adding tin(II) chloride. (It does not combine with tin(II) chloride.) At 90°C., in 12 hrs., it gave 82.8% monoene. At the same time, the percentage of stearate in the product rose to 5.4%, indicating that the dicyano-palladium compound is not quite as selective as the tin-containing catalysts. An attempt... 

In Figure 2 are compared the activities of various palladium catalysts for the reductions carried out with carbon monoxide (9) (in these experiments, the hydrogen necessary for the reduction is obtained by the palladium catalyzing the reaction CO -f- H2O — CO2 -f- H2). It can be... 

Bailar and co-workers have used several complexes as catalysts for reduction of linolenic ester to linoleic or oleic ester without any reduction to the saturated stearic ester. A considerable portion of this work was carried out using a mixed catalyst consisting of tin(II) chloride and a platinum(II) complex. However, catalytic work has also been carried out using palladium and nickel complexes, the former again being used along with tin(II) chloride J91). The experimental details have been recently reviewed (f90) so that this article is concerned with the conclusions and mechanistic aspects rather than with the direct results. 

This paper describes the use of polydentate ligands to optimize the performance of palladium catalysts for CO2 reduction and to probe mechanistic aspects of catalytic reactions. Polydentate ligands can be used to precisely control coordination environments, electronic properties, and specific steric interactions that can lead to new insights into the relationship between catalyst structure and activity. 

Lindlar, H. Dubuis, R. Palladium catalyst for partial reduction of acetylenes. Org. Synth. 1973, J, 880-882. 

---