Hydrogenation Homogeneous catalytic

A wide range of soluble metal complexes has proved to exert catalytic activity in hydrogenation of unsaturated molecules. Most attention, however, has focused on Group Vm elements since they give rise to the most active catalytic systems. 

An early and popular complex, [IrCl(CO)(PPh3)2], called the Vaska complex,113 and its derivatives can be used for the hydrogenation of unsaturated hydrocarbons.26,31 

Very high rates can be achieved for hydrogenations with dichlorotris(triphenyl-phosphine)ruthenium(II) [RuCl2(PPh3)3], which is complementary to the two important rhodium complexes mentioned before.114,115 In catalytic hydrogenations it is transformed to [RuHCl(PPh3)3], which is often the true catalyst. 

Cyanide-containing cobalt catalysts, particularly potassium pentacyanocobalta-te(II) K3[Co(CN)5], are used in the reduction of activated alkenes (conjugated dienes).26,31 [Co(CO)4]2 is best known as a hydroformylation catalyst, but hydrogenation is also possible under specific conditions. Phosphine-substituted analogs are more successful. 

A number of Ziegler-type catalysts based on early transition metals and tri-alkylaluminum or similar organometallic compounds are soluble in hydrocarbon solvents and function as homogeneous hydrogenation catalysts.24,26 Many of the 

Certain types of functionality can be removed and replaced by hydrogen under catalytic hydrogenation conditions. This is called hydrogenolysis. For example, aromatic halogen substituents are usually removed by reduction over metal catalysts. Aliphatic halides are less reactive but hydrogenolysis is promoted by base. Carbon oxygen bonds at benzyl and allyl positions are cleaved by catalytic hydrogenation.  

In agreement with this mechanism is the fact that the addition has been demonstrated to be syn for several typical alkenes. The rate of diimide reductions has been shown to be affected by torsional and angle strain in the alkene. More strained double bonds react at accelerated rates. For example, the more strained trans double bond is selectively reduced in cis, rrcm5-l,5-cyclodecadiene. Diimide 

Most reductions of carbonyl and other functional groups are now done with reagents that transfer a hydride ion from boron or aluminum. The numerous 

CHAPTER 5 REDUCTION OF CARBONYL AND OTHER FUNCTIONAL GROUPS 

O 11 RCOH RCH2OH Pd, Ni, Ru Very strenuous conditions required 

In benzene or similar solvents, tris(triphenylphosphine)halogenorhodium(I) complexes, RhX[P(C6H5)3]3, are extremely efficient catalysts for the homogeneous hydrogenation of nonconjugated olefins and acetylenes at ambient temperature and pressures of 1 atmosphere (6). Functional groups (keto-, nitro-, ester, and so on) are not reduced under these conditions. 

Some generalizations that pertain are (1) Terminal olefins are more rapidly reduced than internal olefins (2) conjugated olefins are not reduced at 1 atmosphere (3) ethylene is not hydrogenated. Rates of reduction compare favorably with those obtained by heterogeneous catalysts such as Raney nickel or platinim oxide. In fact, the hydrogenation of some olefins may be so rapid that the temperature of the solution (benzene) is raised to the boiling point. 

One useful feature of this reducing system is its apparent ability to allow deuteration of double bonds without scrambling. Although the precise stereochemistry of the addition remains to be established, the incorporation of only two deuterium atoms per double bond has been clearly demonstrated (7). 

The preparation of one of the complexes is given, as well as some examples of its use. 

A 500-ml rcund-bottom flask is equipped with a reflux condenser, a gas inlet tube, and a gas outlet leading to a bubbler. The flask is charged with a solution of rhodium (III) chloride trihydrate (2 g) in 70 ml of 95 % ethanol. A solution of triphenylphosphine (12 g, freshly recrystallized from ethanol to remove the oxide) in 350 ml of hot ethanol is added to the flask, and the system is flushed with nitrogen. The mixture is refluxed for 2 hours, following which the hot solution is filtered by suction to obtain the product. The crystalline residue is washed with several small portions of anhydrous ether (50 ml total) affording the deep red crystalline product in about 85% yield. 

We have seen numerous examples of the hydrogenation of alkenes catalyzed by various finely divided metals such as Ni, Pt, Pd, and Rh. In all those cases, the metal acted as a heterogeneous catalyst, present as a solid while the alkene was in solution. The idea of carrying out hydrogenations in homogeneous solution seems far-fetched inasmuch as no solvent is capable of simultaneously dissolving both metals and hydrocarbons. Nevertheless, there is a way to do it. 

Rhodium is a good catalyst for alkene hydrogenation (Section 6.1), as are many of its complexes such as tris(triphenylphosphine)rhodium chloride (Wilkinson s catalyst). 

Like rhodium itself, Wilkinson s catalyst is an effective catalyst for alkene hydrogenation. It is selective, reducing less-substituted double bonds faster than more-substituted ones and C=C in preference to C=0. Unlike rhodium metal, however, Wilkinson s catalyst is soluble in many organic solvents. 

The mechanism of the hydrogenation of propene in the presence of Wilkinson s catalyst is shown in Mechanism 14.3. 

Humulene is a naturally occurring hydrocarbon present in the seed cone of hops and has been synthesized several times. In one of these, the retrosynthetic strategy was based on the disconnection shown. Deduce the structure, including stereochemistry, of an allylic bromide capable of yielding humulene by an intramolecular Suzuki coupling in the last step in the synthesis. Represent the boron containing unit as—B(OH)2. 

The scope of palladium-catalyzed cross-coupling has expanded beyond C—C bond formation to include C—O and C—N bond-forming methods. 

---

The only weakness of the homogeneous catalytic hydrogenation of elastomer is the removal of the catalyst from... 

With the recent development of zeolite catalysts that can efficiently transform methanol into synfuels, homogeneous catalysis of reaction (2) has suddenly grown in importance. Unfortunately, aside from the reports of Bradley (6), Bathke and Feder (]), and the work of Pruett (8) at Union Carbide (largely unpublished), very little is known about the homogeneous catalytic hydrogenation of CO to methanol. Two possible mechanisms for methanol formation are suggested by literature discussions of Fischer-Tropsch catalysis (9-10). These are shown in Schemes 1 and 2. 

This chapter aims to provide an overview of the current state of the art in homogeneous catalytic hydrogenation of C=0 and C=N bonds. Diastereoselec-tive or enantioselective processes are discussed elsewhere. The chapter is divided into sections detailing the hydrogenation of aldehydes, the hydrogenation of ketones, domino-hydroformylation-reduction, reductive amination, domino hydroformylation-reductive amination, and ester, acid and anhydride hydrogenation. 

This chapter provides a review of the progress in reaction art, reactor techniques and process technology with respect to homogeneous catalytic hydrogenation of diene-based polymers, in accordance with the homogeneous hydrogenation theme of this handbook. 

P. G. Jessop, T. Ikariya, R. Noyori, Homogeneous Catalytic Hydrogenation of Supercritical Carbon Dioxide , Nature 1994,368,231-233. 

---