Hydrogenation of alkenes and alkynes

Catalytic hydrogenation is commonly used for the reduction of alkenes, alkynes, aromatic hydrocarbons, and aromatic heterocycles, carbonyl derivatives, nitriles, and nitro compounds. The reaction with alkenes proceeds on the surface of a heterogeneous metal catalyst, via cleavage of diatomic hydrogen and adsorption 

As mentioned, the hydrogen adsorption process is better understood for the reduction of alkenes with homogeneous catalysts, and Wilkinson s catalyst (340) has been studied extensively. Wilkinson s catalyst 

There are many other homogeneous catalysts, involving several different transition metals. Vaska s catalyst (345) is one of the more important and it also reacts with hydrogen, converting the square planar catalyst to the octahedral 346.356 Reduction appears to involve replacement of the triphenylphosphine ligand with hydrogen or an alkene in a reversible process.3l2 

With proper choice of catalyst, high chemoselectivity can be achieved. In the Hagiwara et al. synthesis of (I )-(-i-)-muscopyridine, for example, hydrogenation of 347 was a Pd-C catalyst gave a 98% yield of diketone 

with no reduction of the ketone units. 57 Alkenes can be reduced in the presence of benzylic substituents without hydrogenation. In the Cossy et al. synthesis of (+)-sedamine, hydrogenation of 349 was a Pt02 catalyst gave a 94% yield of 350 without hydrogenolysis of the benzylic ester group.3 8 

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Hydrogenation of benzene and other arenes is more difficult than hydrogenation of alkenes and alkynes Two of the more active catalysts are rhodium and platinum and it IS possible to hydrogenate arenes m the presence of these catalysts at room temperature and modest pressure Benzene consumes three molar equivalents of hydrogen to give cyclohexane... 

The hydrogenation of alkenes and alkynes in water can also use silanes as hydrogen sources. Tour reported that by using palladium acetate as catalyst, triethoxysilane reduced C-C unsaturated bonds to saturation in a mixture of THF and water.18 The reaction showed excellent chemo- and stereoselectivity. Water was essential to the reaction. In the absence of water, 95% of the starting alkene remained unchanged (Eq. 3.3). 

Synthesis of Alkanes and Cycloalkanes 4.18A Hydrogenation of Alkenes and Alkynes... 

Duckett381 reported on the use of parahydrogen-induced polarization (PHIP) to delineate the pathways involved in the catalytic hydrogenation of alkenes and alkynes by [Ru3(CO)12 x(PPh3)x] (x = 1 or 2) and showed that the mechanism is highly dependent on the solvent. Bassett and... 

The high activity of the polymeric compound [Pd5(PPh2)]n (n 4) [22] in the hydrogenation of alkenes and alkynes was ascribed to the initial DMF-assisted dissociation of the polymer (Eq. (7)) the resulting [Pd5(PPh)2] species forms an adduct with the substrate molecule (Sub) (Eq. (8)) which, after reaction with H2, leads to the hydrogenation product and gives back [Pd5(PPh)2] (Eq. (9)), which re-enters the catalytic cycle. 

The LiAlH4 may be replaced by four moles of LiAl(0- -Bu)3H, but when only two moles of the latter are used, the hydride precipitates as (Cp2ZrH)20(Cp2ZrH2) (88). The (Cp2ZrH2)x acts as a catalyst for the hydrogenation of alkenes and alkynes (106). 

Furthermore, in the presence of Pd or Rh catalysts, the Cp2TiCl/H20 couple can also be used for the hydrogenation of alkenes and alkynes by hydrogen transfer from water (Scheme 33) [83]. 

Impure samples of 1 containing a green impurity were originally believed to catalyze hydrogenation, but this activity is really due to the green complex, formulated as 2. This complex is more effective than Pd/C for hydrogenation of alkenes and alkynes, and can reduce nitroarenes to anilines directly. Reduction of internal alkynes affords cis-alkenes with slight contamination with an alkane. 

Reaction of Ceo with Pd2(dba)3 yields the organometallic polymer CeoPd . Material of varying composition may be prepared, but those with n > 3 are active for the catalytic hydrogenation of alkenes and alkynes. This material is proposed to consist of catalytically active palladium atoms distributed on an inert 3D polymer CeoPds. 

As we have already shown, the creation of novel carbon-carbon bonds usually results in the formation of functionalized derivatives (the Wurtz coupling and Friedel-Crafts alkylation are probably the only exceptions). That is why a set of special reductive methods was devised to remove residual functionality that is unwanted in the final structure. The well-known hydrogenation of alkenes and alkynes belongs to this group of non-isohypsic transformations. Several other pathways available for the reductive removal of various functions will briefly be considered below. 

Table 19. Hydrogenation of alkenes and alkynes by Rh(III) and Pd(II) chelates of 120). Reaaion ...

One of the first metal-catalyzed reactions learned by organic chemistry students is the Pd-catalyzed hydrogenation of alkenes and alkynes. The reaction is stereo-... 

Table 2 Catalytic hydrogenation of alkenes and alkynes mediated by 52 ...

Hydrogenation of alkenes and alkynes is accomplished in the presence of a metal catalyst which attracts both the hydrogen and hydrocarbon to its surface. As a result of the reactants being adsorbed onto the same surface, the reaction occurs with cis addition. 

Polymer-supported reagent. Palladium(II) chloride supported on polymeric diphenylbenzylphosphine is a very active catalyst for hydrogenation of alkenes and alkynes and for isomerization of double bonds. 

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