The hydrogenation of olefins

As will become apparent during the ensuing discussion, an understanding of the above processes, in terms of the elementary steps occurring on the catalyst surface, is essential to the specification of precise mechanisms for olefin hydrogenation. 

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As a result of rather extensive work on the hydrogenation of olefins 139,151 mechanism originally proposed by Horiuti and Polanyi is currently accepted ... 

The hydrogenation of -olefins requires isomerization of the double bond to the 14 position prior to hydrogenation. 

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) ... 

Rennard and Kokes (39) in their paper stated directly that their purpose was just to study the catalytic activity of palladium hydride in the hydrogenation of olefins, in this case ethylene and propylene. Kokes (39a) in his article recently published in Catalysis Reviews summarizes the results of studies on such catalytic systems. 

A MOF constructed from rhodium paddlewheel clusters linked to porphyrinic ligands already discussed in Section 4.3.1.1 shows an interesting synergetic behavior when the porphyrinic rings are loaded with metals like Cu , Ni , or Pd . In the hydrogenation of olefins, the hydride species at the rhodium center is transferred to the coordinated olefin adsorbed on a metal ion in the center of the porphyrin ring to form an alkyl species, and next this alkyl species reacts with a hydride species activated at the rhodium center to form the alkane [81]. 

The high-surface-area TUD-1 can serve as an anchor for many catalysts. Si- or Al-Si-TUD-1 (24,25) can be used as a support for various noble metals (Pt, PtPd, Ir, etc.). This will provide catalysts suitable for the hydrogenation of olefins and aromatics. In the refining industry, one use is the hydrogenation of polynuclear aromatics ( PNAs ) in diesel fuel, which can lower the fuel s toxic properties. Also, jet fuel has an aromatics constraint, designed to lessen smoke formation. Cracked stocks (e.g., coker or visbreaker liquids) generally have undesirable olefins (especially a-olefins) that also need to be saturated prior to final processing. 

According to the proposed mechanism, the hydrogenation of olefin by iridium catalyst should conform to the following rate expression,... 

The related dihydride-dichloro complex OsH2Cl2(P Pr3)2 is also an active catalyst for the hydrogenation of olefins, diolefms, and a-(3-unsaturated ketones,14 but attempts to hydrogenate phenylacetylene show a rapid deactivation of the catalyst due to formation of a hydride-carbyne complex.54... 

The range of reactions which have been examined is wide (248) and includes hydrogenations (256), ammonia synthesis (257), polymerizations (257), and oxidations (258). Little activity has occurred in this area during the past few years. Recent reports of the effects of sonication on heterogeneous catalysis include the liquefaction of coal by hydrogenation with Cu/Zn (259), the hydrogenation of olefins by formic acid with Pd on carbon (260), and the hydrosilation of 1-alkenes by Pt on carbon (261). 

Scheme 6.1 Elementary steps for the hydrogenation of olefins with d° transition-metal complexes.

Asymmetric catalytic reduction reactions represent one of the most efficient and convenient methods to prepare a wide range of enantiomerically pure compounds (i.e. a-amino acids can be prepared from a-enamides, alcohols from ketones and amines from oximes or imines). The chirality transfer can be accomplished by different types of chiral catalysts metallic catalysts are very efficient for the hydrogenation of olefins, some ketones and oximes, while nonmetallic catalysts provide a complementary method for ketone and oxime hydrogenation. 

The Hartree-Fock method was in any case the method of choice for the first quantitative calculations related to homogeneous catalysis. It was the method, for instance, on a study of the bonding between manganese and hydride in Mn-H, published in 1973 [28]. The first studies on single steps of catalytic cycles in the early 1980 s used the HF method [29]. And it was also the method applied in the first calculation of a full catalytic cycle, which was the hydrogenation of olefins with the Wilkinson catalyst in 1987 [30]. The limitations of the method were nevertheless soon noticed, and already in the late 1980 s, the importance of electron correlation was being recognized [31]. These approaches will be discussed in detail in the next section. 

An interesting effect of pH was found by Ogo et al. when studying the hydrogenation of olefins and carbonyl compounds with [Cp Ir(H20)3] (Cp = ri -CsMej) [89]. This complex is active only in strongly acidic solutions. From the pH-dependence ofthe HNMR spectra it was concluded that at pH 2.8 the initial mononuclear compound was reversibly converted to the known dinuclear complex [(Cp Ir)2(p-OH)3] which is inactive for hydrogenation. In the strongly acidic solutions (e.g. 1 M HCIO4) protonation of the substrate olefins and carbonyl compounds is also likely to influence the rate ofthe reactions. 

Unmodified poly(ethyleneimine) and poly(vinylpyrrolidinone) have also been used as polymeric ligands for complex formation with Rh(in), Pd(II), Ni(II), Pt(II) etc. aqueous solutions of these complexes catalyzed the hydrogenation of olefins, carbonyls, nitriles, aromatics etc. [94]. The products were separated by ultrafiltration while the water-soluble macromolecular catalysts were retained in the hydrogenation reactor. However, it is very likely, that during the preactivation with H2, nanosize metal particles were formed and the polymer-stabilized metal colloids [64,96] acted as catalysts in the hydrogenation of unsaturated substrates. 

W(CH3CN)(C0)3(TPPMS)2] was obtained in the reaction of TPPMS and [W(CH3CN)3(C0)3], and was used as catalyst in hydrogenation of benzene in water/heptane biphasic systems [164]. At 100 °C and 70 bar H2 the catalytic activity was found rather low (average TOP 1 h ). The same complex is also active in the hydrogenation of olefins (e.g. 1-hexene, 2,3-dimethyl- 1 -butene). 

Indeed, in many cases the creation of soluble nanoparhcles has provided singular catalyhc activities/selectivities that differ from those expected for both molecular (single-site) and heterogeneous (mulh-site) catalysts [40, 41]. As a result, iridium nanoparticles have attracted much interest in terms of their catalyhc performance in the hydrogenation of olefins, ketones and aromahc compounds. 

The coordinatively unsaturated hydride complex is proposed to be active site in the hydrogenation of olefins under mild conditions [24]. 

M/AI2O3 (M = Cr, Mo, W) catalysts were ultra-active for the hydrogenation of ethylene when compared with others prepared by traditional methods from appropriate salts-a low oxidation state of M is of crucial importance for these catalysts to be active in the hydrogenation of olefins. 

An excellent review of the problems of the enantioselective heterocatalytic hydrogenation of prochiral double bonds, covering the literature up to 1970, has been compiled by Izumi57). Raney nickel catalysts modified with chiral amino acids or dipeptides gave only very moderate enantiomeric excesses of between 0 and 10% in the hydrogenation of olefins, carbonyl compounds or oximes 57). Only Raney nickel modified with (S)-tyrosine furnished a higher enantiomeric excess in the products58). 

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. 

Rhodium catalysts generated from the sulfonated phosphine 23 (Table 2) were effective in the hydrogenation of olefins in an aqueous/organic or in a homogeneous methanol system, substantially higher rates being observed in the latter system.131 For example, the TOF observed in the hydrogenation of 1-hexene in the biphasic system was 220 h 1 compared to 7860 h 1 in methanol.131... 

Rhodium catalysts modified with carboxylated phosphines 45 (Table 3 n=5, n=7)229 and phosphonium phosphines 103 (Table 5 n=2,3,6,10)255 form very active catalytic systems for the hydrogenation of olefins in aqueous/organic two phase systems. 

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