Palladium alkyne hydrogenation

The use of dispersed or immobilized transition metals as catalysts for partial hydrogenation reactions of alkynes has been widely studied. Traditionally, alkyne hydrogenations for the preparation of fine chemicals and biologically active compounds were only performed with heterogeneous catalysts [80-82]. Palladium is the most selective metal catalyst for the semihydrogenation of mono-substituted acetylenes and for the transformation of alkynes to ds-alkenes. Commonly, such selectivity is due to stronger chemisorption of the triple bond on the active center. 

It was concluded that the high selectivity observed in the hydrogenation experiments using 26 b is explained by the relatively strong coordination of the alkyne to the palladium center, which only allows for the presence of small amounts of alkene complexes. Only the latter are responsible for the observed minor amounts of ( )-alkene, which was shown to be a secondary reaction product formed by a subsequent palladium-catalyzed, hydrogen-assisted isomerization reaction. Since no n-octane was detected in the reaction mixture, only a tiny... 

Scheme 17. DFT-derived mechanism for the alkyne hydrogenation by a neutral palladium(0)-bisphospine complex. ...

The selectivity values observed in the first stage of the reaction, summarised in Table 23, show similar trends from metal to metal to those observed in alkyne hydrogenation. Again, palladium is almost completely... 

Teschner D, Borsodi J, Wootsch A, Revay Z, Havecker M, Knop-Gericke A, Jackson SD, Schlogl R. The roles of subsurface carbon and hydrogen in palladium-catalyzed alkyne hydrogenation. Science. 2008 320(5872) 86-89. 

Grigg and co-workers [203] have very recently described an ingenious cascade process. Tliis is initiated by the addition of Bu3SnH to alkyne cyclization and anion capture follow and afford polycyclic products (Scheme 4-52). A palladium-catalyzed hydrogenation of the double bond in ajS-unsaturated A -acyloxy azolidinones was described by Wu et al. [204] the diastereoselectivity was low. 

Kinetic measurements are only available for the reactions catalyzed by pumice-supported nickel, palladium, and platinum (100). These reactions were zero-order in hydrocarbon and first-order in hydrogen, indicating in a superficial way the similarity of this reaction to that of alkyne hydrogenation and confirming the disparity in the strengths of adsorption of the reactants. 

On balance, palladium offers the best combination of activity and selectivity at reasonable cost, and for these reasons has become the basis of the most successful commercial alkyne hydrogenation catalysts to date. Because of their inherently high activity, these catalysts contain typically less than 0.5 % (by weight) of active metal-to preserve selectivity at high alkyne conversion. Despite the prominence of these catalysts, other active metals are used in fine chemicals applications. Of particular utility is the nickel boride formulation formed by the action of sodium borohydride on nickel(II) acetate (or chloride). Reaction in 95 % aqueous ethanol solution yields the P2-Ni(B) catalyst and selectivity in alkyne semi-hydrogenation has been demonstrated in the reaction of 3-hexyne to form cw-3-hexene in 98 % yield [15,16] ... 

Zeolites have been successfully employed as catalyst supports for palladium. In one example of a disubstituted alkyne hydrogenation, additional treatment with diphenyldiethoxysilane increased the yield of 3-nonene from 3-nonyne by 57 % (40 to 97 %) [60]. The limited accessibility of zeolite A assumes only the extra-framework Pd sites are catalyzing the reaction ... 

The outstanding quality of palladium in both these respects is well known but not well understood. Foremost among the complexities that have to be addressed are (i) the tendency to form hydride phases, and (ii) the frequent formation, especially with ethyne, of a carbonaceous overlayer, derived perhaps from oligomers, some of which escape into the fluid phase. Both these factors are invoked to account for palladium s remarkable properties, but both are responsive to reaction conditions and especially to particle size support effects may also operate. So many factors have to be kept in play when discussing mechanisms that one may safely conclude that none so far suggested is wholly satisfactory. The rich literature on alkyne hydrogenation deserves careful attention, and should be a fine source of inspiration for further research. 

Palladium-Catalyzed Hydrogenation of Alkynes and Conjugated Dienes... 

A similar Nicolas-Pauson-Khand combination was used in a synthesis of the ketone analogue of biotin 7.98, required for biochemical studies (Scheme 7.25). In this case, the Nicholas reaction was intermolecular, between allyl thiol as the nucleophile and carbocation 7.94 generated from alcohol 7.93. The Pauson-Khand reaction was then between the dicobalt complexed alkyne 7.95 and the double bond from the thiol moiety. The Pauson-Khand reaction proceeded with no stereoselectivity, and the diastereoisomers had to be chromatographically separated at a later stage. The synthesis was completed by reduction of the alkene of cyclopentenone 7.96, without using palladium-catalysed hydrogenation due to the sulfide moiety, and ester hydrolysis. 

BOnnemann, H., Brijoux, W., Siepen, K., Hormes,)., Franke, R., Pollmann, J. and Rothe, J. (1997) Surfactant stabilized palladium colloids as precursors for ds-selective alkyne-hydrogenation catalysts. Applied Organometallic Chemistry, 11, 783-96. 

The conditions for hydrogenation of alkynes are similar to those employed for alkenes In the presence of finely divided platinum palladium nickel or rhodium two molar equivalents of hydrogen add to the triple bond of an alkyne to yield an alkane... 

Hydrogenation of alkynes may be halted at the alkene stage by using special catalysts Lindlar palladium is the metal catalyst employed most often Hydrogenation occurs with syn stereochemistry and yields a cis alkene... 

Lindlar catalyst (Section 9 9) A catalyst for the hydrogenation of alkynes to as alkenes It is composed of palladium which has been poisoned with lead(II) acetate and quino line supported on calcium carbonate... 

Alkynes can be reduced to yield alkenes and alkanes. Complete reduction of the triple bond over a palladium hydrogenation catalyst yields an alkane partial reduction by catalytic hydrogenation over a Lindlar catalyst yields a cis alkene. Reduction of (he alkyne with lithium in ammonia yields a trans alkene. 

A synthetically useful virtue of enol triflates is that they are amenable to palladium-catalyzed carbon-carbon bond-forming reactions under mild conditions. When a solution of enol triflate 21 and tetrakis(triphenylphosphine)palladium(o) in benzene is treated with a mixture of terminal alkyne 17, n-propylamine, and cuprous iodide,17 intermediate 22 is formed in 76-84% yield. Although a partial hydrogenation of the alkyne in 22 could conceivably secure the formation of the cis C1-C2 olefin, a chemoselective hydrobora-tion/protonation sequence was found to be a much more reliable and suitable alternative. Thus, sequential hydroboration of the alkyne 22 with dicyclohexylborane, protonolysis, oxidative workup, and hydrolysis of the oxabicyclo[2.2.2]octyl ester protecting group gives dienic carboxylic acid 15 in a yield of 86% from 22. 

The increase of selectivity in consecutive reactions in favor of the intermediate product may be sometimes extraordinarily high. Thus, for example, in the already cited hydrogenation of acetylene on a platinum and a palladium catalyst (45, 46) or in the hydrogenation or deuteration of 2-butynes on a palladium catalyst (57, 58), high selectivities in favor of reaction intermediates (alkenes) are obtained, even though their hydrogenation is in itself faster than the hydrogenation of alkynes. 

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