Olefinic bonds, catalytic hydrogenations

A very significant recent development in the field of catalytic hydrogenation has been the discovery that certain transition metal coordination complexes catalyze the hydrogenation of olefinic and acetylenic bonds in homogeneous solution.Of these catalysts tris-(triphenylphosphine)-chloror-hodium (131) has been studied most extensively.The mechanism of the deuteration of olefins with this catalyst is indicated by the following scheme (131 -> 135) ... 

If R and R1 are not a methyl group, the process generates a chiral carbon center (C ). The overall catalytic addition of hydrogen to olefinic bonds generally is nearly always cis (7, p. 407) and to the olefinic face coordinated to the metal. This cis-endo-addition produces a chiral center(s) when one olefinic face is preferentially coordinated. 

First, solvent molecules, referred to as S in the catalyst precursor, are displaced by the olefinic substrate to form a chelated Rh complex in which the olefinic bond and the amide carbonyl oxygen interact with the Rh(I) center (rate constant k ). Hydrogen then oxidatively adds to the metal, forming the Rh(III) dihydride intermediate (rate constant kj). This is the rate-limiting step under normal conditions. One hydride on the metal is then transferred to the coordinated olefinic bond to form a five-membered chelated alkyl-Rh(III) intermediate (rate constant k3). Finally, reductive elimination of the product from the complex (rate constant k4) completes the catalytic cycle. 

Acetylenic aromatic acids having the triple bond Hanked by carboxyl and an aromatic ring were partially reduced to olefinic aromatic acids by chromous sulfate in aqueous dimethylformamide at room temperature in high yields. Phenylpropiolic acid afforded irani -cinnamic acid in 91% yield [195]. Its sodium salt in aqueous solution gave on catalytic hydrogenation over colloidal platinum at room temperature and atmospheric pressure 80% yield of cis-cinnamic acid if the reaction was stopped after absorption of 1 mol of hydrogen. Otherwise phenylpropanoic acid was obtained in 75-80% yield [992]. 

As a third example for an organocatalytic reaction, based on multiple hydrogen bonding and mechanistically investigated by DFT, we selected olefin epoxidation with hydrogen peroxide in fluorinated alcohol solvents, such as 1,1,1,3,3,3-hex-afluoro-2-propanol (HFIP) (Scheme 8). Here we encounter a new type of catalytic hydrogen bond the cooperative hydrogen bond. 

Hydrogenation of ditermamine in acetic acid over Pt gave tetrahydroditerm-amine (149). There were absorption bands in the IR spectrum of 149 at 1630 cm (>N—CO—) and at 1680 cm (double bond). There were no signals for olefinic protons in the H-NMR spectrum. Catalytic hydrogenation of diter-... 

Hirai et al.129 studied the hydrogenation of olefins catalyzed by poly(acrylic acid)-Rh(II) complexes in homogeneous solutions. The catalytic activity of the polymer-Rh complex was about 103 times that of the acetato-Rh complex. When olefins having another functional group, such as diallylether, allylaldehyde, and cyclohexene-1 -one, were used as the substrates, the olefinic bond was preferentially hydrogenized by the polymer-Rh complex. The polymer ligand was presumed to exercise a steric effect. 

Catalytic hydrogenation of triple bonds and the reaction with DIBALH usually give the cis olefin. Most of the other methods of triple-bond reduction lead to the more thermodynamically stable trans olefin. However, this is not the case with the method involving hydrolysis of boranes or with the reductions with activated zinc, hydrazine, or NHjOSOyH, which also give the cis products. 

When an oxidation or a reduction could be considered in a previous chapter, this was done. For example, the catalytic hydrogenation of olefins is a reduction, but it is also an addition to the C==C bond and was treated in Chapter IS. In this chapter are discussed only those reactions that do not fit into the nine categories of Chapters 10 to 18. An exception to this rule was made for reactions that involve elimination of hydrogen (9-1 to 9-6) which were not treated in Chapter 17 because the mechanisms generally differ from those in that chapter. 

Asymmetric synthesis (i) has gained new momentum with the potential k use of homogeneous catalysts. The use of a transition metal complex with chiral ligands to catalyze a synthesis asymmetrically from a prochiral substrate is beneficial in that resolution of a normally obtained racemate product may be avoided. In certain catalytic hydrogenations of olefinic bonds, optical purities approaching 100% have been attained (2,3,4,5) hydrogenations of ketones (6,... 

In contrast with these results, catalytic cracking yields a much higher percentage of branched hydrocarbons. For example, the catalytic cracking of cetane yields 50-60 mol of isobutane and isobutylene per 100 mol of paraffin cracked. Alkenes crack more easily in catalytic cracking than do saturated hydrocarbons. Saturated hydrocarbons tend to crack near the center of the chain. Rapid carbon-carbon double-bond migration, hydrogen transfer to trisubstituted olefinic bonds, and extensive isomerization are characteristic.52 These features are in accord with a carbo-cationic mechanism initiated by hydride abstraction.43,55-62 Hydride is abstracted by the acidic centers of the silica-alumina catalysts or by already formed carbocations ... 

In the ensuing discussion, it will be assumed that structure L is the relevant species in alkyne hydrogenation, and that the catalytically active adsorbed state of an alkadiene can be represented as a jr-olefin complex in which either one or both olefinic bonds interact with the surface. 

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