Nickel alkyne hydrogenation

This postulate has several implications regarding the mechanism of alkyne hydrogenation these will be discussed in Sect. 4.3. It should be noted, however, that there is as yet little or no direct evidence for structure L, although analogous structures are known to exist with organometallic complexes [161], Such a structure is also consistent with the positive surface potentials observed for acetylene adsorption on evaporated nickel films [88]. 

Various complexes of Cu, Ni, and Pd, especially nickel phosphite complexes, such as Ni[P(0-o-C6H4Me)3]4, are active for the addition of HCN to alkenes and alkynes. Hydrogen cyanide will oxidatively add to low-valent metal complexes,... 

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] ... 

Alkyne hydrogenation on NiS catalysts NisS2 on die nickel surface is active in selective hydrogenation, but NiS is not... 

In the presence of catalysts such as palladium, platinum, or nickel, alkynes can be hydrogenated to the corresponding alkanes. 

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... 

A variety of alternate methods for the reductive coupling of aldehydes and alkynes have been developed. A number of important hydrometallative strategies have been developed, although most of these methods require the stoichiometric formation of a vinyl metal species or metallacycle. A very attractive hydrogenative coupling method has recently been developed, and its scope is largely complementary to the nickel-catalyzed methods. A very brief overview of these methods is provided below. 

Three new methods for the conversion of alkynes to (Z)-alkenes were reported, although Lindlar semi-hydrogenation still remains as the most convenient method. Copper (I) hydride reagent could reduce alkynes to (Z)-alkenes as shown in Scheme 3 [12]. Yoon employed nickel boride prepared on borohy-dride exchange resin for selective hydrogenation of alkynes to (Z)-alkenes (Scheme 4) [13]. 

A catalyst that permits hydrogenation of an alkyne to an alkene is the nickel boride compound called P-2 catalyst. 

Some of these coupling reactions can be made catalytic if hydrogen is eliminated and combines with the anion, thus leaving the nickel complex in the zero-valent state. Allylation of alkynes or of strained olefins with allylic acetates and nickel complexes with phosphites has been achieved (example 38, Table III). 

The use of the nickel cluster [Ni4(CNCMe3)7] in hydrogenation of alkynes was recently described (222, 223). 

The mechanism was proposed to involve the formation of a nickel metallacycle by the oxidative cyclization of Ni(0) with the aldehyde and alkyne, followed by conversion of the metallacycle to product by a transmetalation/reductive elimination sequence. If R possesses a P-hydrogen, then P-hydride elimination after the transmetalation step generates the product with R = H in some instances. The mechanism was shown to be ligand dependent, and the mechanism depicted below is undoubtedly oversimplified. ... 

Hydrogenation using Raney nickel is carried out under mild conditions and gives cis alkenes from internal alkynes in yields ranging from 50 to 100% [356, 357, 358, 359, 360]. Half hydrogenation of alkynes was also achieved over nickel prepared by reduction of nickel acetate with sodium borohydride (P-2 nickel, nickel boride) [349,361,362] or by reduction with sodium hydride [49], or by reduction of nickel bromide with potassium-graphite [363]. Other catalysts are palladium on charcoal [364], on barium sulfate [365, 366], on... 

Complete reduction of alkynes to alkanes is easily accomplished by catalytic hydrogenation, especially using palladium [386, 387], platinum oxide and active nickel catalysts [559]. 

Hydrogen undergoes catalytic hydrogenation adding to unsaturated hydrocarbons, such as alkenes and alkynes forming alkanes. The reaction is catalyzed by nickel, platinum or palladium catalysts at ambient temperature. Hydrogenation of benzene over platinum catalyst yields cyclohexane, C6H12. 

Addition of hydrogen atoms in the presence of a metal catalyst to double or triple bonds is known as hydrogenation or catalytic hydrogenation. Alkenes and alkynes are reduced to alkanes by the treatment with H2 over a finely divided metal catalyst such as platinum (Pt—C), palladium (Pd—C) or Raney nickel (Ni). The platinum catalyst is also frequently used in the form of Pt02, which is known as Adams s catalyst. The catalytic hydrogenation reaction is a reduction reaction. 

The observation was a significant finding since at the time, when the only synthetic method to reduce alkynes selectively was their conversion by heterogeneous catalytic hydrogenation (Raney nickel) to cis alkenes. The dissolving-metal reduction provided easy access to high-purity trans alkenes since the latter do not readily react further under the conditions used. The efficient reduction of 1-alkynes in this system requires the presence of ammonium ion.196... 

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