Hydrogenation of Olefinic Substrates

Most of the studies dealing with the effect of the composition of reaction mixture on the catalytic hydrogenation in the liquid state have been devoted to olefinic substrates. The reactions proceed at readily measurable rates already under normal conditions, so that their experimental investigation is not technically demanding. The system enables the structure of compounds undergoing hydrogenation to be varied within broad limits it also makes possible the use of a broad variety of solvents, and is very suitable for the given purpose. 

In studies of structure effects it is necessary that the catalyst should appear as a constant parameter. Generally, such studies may be performed using a great variety of catalysts, but the possibility of generalization of the data is very restricted. This phenomenon is especially marked in the cases of hydrogenation of olefinic substrates, which on some catalysts proceed selectively, while on others they are accompanied by side reactions, mainly by isomerization and sometimes also by hydrogenolysis. 

The causes of the selectivity of hydrogenation catalysts, similarly to the mechanism of hydrogenation and isomerization reactions of olefins are complicated problems, which so far have resisted any satisfactory solution 46). 

Papers (4, 47, 48) demonstrate that, while the character of the carrier (silica gel, active carbon) of the active component has no pronounced influence on the process of hydrogenation, there are distinct differences in the effect of the active components themselves. Side reactions occurred on rhodium and palladium catalysts, while on platinum catalysts they could not be observed in most cases (migration of the double bond, cis-trans isomerization). These reactions occurred only if a sufficient amount of hydrogen was present in the reaction mixture (part of hydrogen is irreversibly consumed by hydrogenation). Neither the carrier alone nor the catalyst in an inert atmosphere provoked any side reactions, which shows that hydrogen in one of its forms participates directly in the isomerization process. 

The observed phenomena may be adequately explained on grounds of the mechanism of gradual addition of hydrogen species to the double bond, the basic form of which is called the Horiuti-Polanyi mechanism (49)  

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EDITOR S NOTE In 1982, J Halpem (University of Chicago) reported that rhodium complexes containing chiral phosphine ligands catalyze the hydrogenation of olefinic substrates such as alpha-aminoacrylic acid derivatives, producing chiral products with very high optical yields. 

Polymer-supported chiral catalysts have likewise been prepared in order to obtain access to reusable systems (cf. Section 3.1.1, especially Section 3.1.1.3). For example, copolymerized functionalized BINAP [160, 161] could be applied in the enantioselective hydrogenation of olefinic substrates (up to 94 % ee). Similarly, the copolymerization of vinyl-BINAPHOS with styrene derivatives led to a heterogenized auxiliary which made it possible to hydroformylate styrene and vinyl acetate (Rh catalysis) with selectivities and enantioselectivities close to those provided by the parent homogeneous catalytic system [162]. 

The results described above lead to a conclusion that the parameter t can characterize the solvent with respect to its influence on the rate of hydrogenation of olefinic substrates, but that the applicability of this parameter is far from universal. 

The results obtained so far in the study of the effect of solvents on the kinetics of hydrogenation of olefinic substrates indicate the applicability of LEER. On the other hand, however, similarly to the results of the investigation of the effect of structure of substrates, they suggest the necessity of a complex approach to reaction systems, because the investigation of any particular effect regardless of the other effects leads to an exaggerated simplification, and the individual results cannot be applied to other systems. 

Tandem or domino reactions using hydroformylation as the first step allow the immediate transformation of the formed aldehydes into other valuable chemical compounds (see Chapter 5) [41]. As discussed previously, the hydrogenation of olefinic substrates or product aldehydes is a commonly observed side reaction in the hydroformylation with Ru complexes. On the other hand, the reduction of the aldehydes can be desired. 

Double-bond migrations during hydrogenation of olefins are common and have a number of consequences (93). The extent of migration may be the key to success or failure. It is influenced importantly by the catalyst, substrate, and reaction environment. A consideration of mechanisms of olefin hydrogenation will provide a rationale for the influence of these variables. 

In conclusion, the above summary of oxidation methods shows that there is still room for further improvements in the field of selective olefin epoxidation. The development of active and selective catalysts capable of oxidizing a broad range of olefin substrates with aqueous hydrogen peroxide as terminal oxidant in inexpensive and environmentally benign solvents remains a continuing challenge. 

Polymerization of a BINAP derivative (Figure 14) followed by complexation with [ RuC12 (benzene) 2] afforded a catalyst showing high enantioselectivities for the hydrogenation of various substrates such as dehydroaminoacids, ketoesters, olefins, and ketones.135 The catalyst may be re-used four times with negligible loss of enantioselectivity and activity. 

Kaneda et al. reported substrate-specific hydrogenation of olefins using the tri-ethoxybenzamide-terminated polypropylene imine) dendrimers (PPI) as nanoreactors encapsulating Pd nanoparticles (mean diameter 2-3 nm) [59]. The catalytic tests were performed in toluene at 30 °C under dihydrogen at atmospheric pressure (Table 9.3). The hydrogenation rates were seen to decrease with increasing generation of dendrimers, from G3 to G5. 

M. Ooe, M. Murata, T. Mizugaki, K. Ebitani, and K. Kaneda, Dendritic nanoreactors encapsulating Pd particles for substrate-specific hydrogenation of olefins. Nano Lett. 2,999-1002 (2002). 

Even in an excess of ligands capable of stabilizing low oxidation state transition metal ions in aqueous systems, one may often observe the reduction of the central ion of a catalyst complex to the metallic state. In many cases this leads to a loss of catalytic activity, however, in certain systems an active and selective catalyst mixture is formed. Such is the case when a solution of RhCU in water methanol = 1 1 is refluxed in the presence of three equivalents of TPPTS. Evaporation to dryness gives a brown solid which is an active catalyst for the hydrogenation of a wide range of olefins in aqueous solution or in two-phase reaction systems. This solid contains a mixture of Rh(I)-phosphine complexes, TPPTS oxide and colloidal rhodium. Patin and co-workers developed a preparative scale method for biphasic hydrogenation of olefins [61], some of the substrates and products are shown on Scheme 3.3. The reaction is strongly influenced by steric effects. 

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. 

In conclusion, the peculiarities of hydrogenation of olefins in aqueous solutions show that by shifting acid-base equilibria the aqueous environment may have important effects on catalysis through changing the molecular state ofthe substrate or the catalyst or both. 

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. 

In asymmetric hydrogenation of olefins, the overwhelming majority of the papers and patents deal with hydrogenation of enamides or other appropriately substituted prochiral olefins. The reason is very simple hydrogenation of olefins with no coordination ability other than provided by the C=C double bond, usually gives racemic products. This is a common observation both in non-aqueous and aqueous systems. The most frequently used substrates are shown in Scheme 3.6. These are the same compounds which are used for similar studies in organic solvents salts and esters of Z-a-acetamido-cinnamic, a-acetamidoacrylic and itaconic (methylenesuccinic) acids, and related prochiral substrates. The free acids and the methyl esters usually show appreciable solubility in water only at higher temperatures, while in most cases the alkali metal salts are well soluble. 

The BINAP-Rh catalyzed hydrogenation of functionalized olefins has a mechanistic drawback as described in Section 1.2.1. This problem was solved by the exploitation of BINAP-Ru(ll) complexes.Ru(OCOCH3)2(binap) catalyzes highly enantioselective hydrogenation of a variety of olefinic substrates such as enamides, a, (3- and (3,y-unsaturated carboxylic acids, and allylic and homoallylic alcohols (Figure 1.9). " " Chiral citronellol is produced in 300 ton quantity in year by this reaction. ... 

A number of reactions, principally of olefinic substrates, that can be catalyzed by supported complexes have been studied. These include hydrogenation, hydrosilylation, hydroformylation, polymerization, oxidative hydrolysis, acetoxylation, and carbonylation. Each of these will be considered in turn together with the possibility of carrying out several reactions consecutively using a catalyst containing more than one kind of metal complex. 

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