Hydrogenation homogeneous catalysts

Phosphines avoid becoming tram to hydride groups whenever possible. Oxidative-addition of hydrogen is cis, and the monophosphine complex can form an intermediate in which the two phosphines are trans to each other. This then adds olefin to form intermediate 4. The diphosphine complex cannot form an intermediate where phosphine is not tram to hydride and this changes the equilibrium for oxidation-addition of hydrogen. The mechanism proceeds via an olefin rather than a hydride path, so that complex 5 is an intermediate. Hydrogenation of 5 produces the intermediate complex 6  

The hydride tram to the phosphine migrates to the olefin and this tram destabilisation probably contributes to the rapid rate of reaction. High enantioselectivity ( 90%) has been restricted to hydrogenation of prochiral o-acetylaminocinnaniic derivatives and a variety of chiral phosphines have been used. The highest enantioselectivity was obtained with the dppe-analogues (-)DIPAMP (7) and (S,S)-Chiraphos (8) 

It is evident that by substituting bidentate tertiary phosphine ligands for monophosphines it is possible to cause major changes in the reactivity of a metal-phosphine complex. We discuss in other sections the contrasting profile of biological activity of chelated diphosphine complexes compared to related monophosphine complexes. 

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, trans-l,5-cyclodecadiene.  

Hydrogen chloride and hydrogen bromide add to olefins to give addition products. Many years ago, it was noted that additions usually take place to give the product in which the halogen atom is attached to the more substituted end of the olefin. This type of behavior was sufficiently general that the name Markownikoff s 

In nucleophilic solvents, products that arise from reaction of the solvent with the intermediate may be encountered. For example, reaction of cyclohexene with hydrogen bromide in acetic acid gives cyclohexyl acetate as well as cyclohexyl 

Because of the involvement of carbonium ion intermediates, rearrangement is a possibility. Reaction of t-butylethylene with hydrogen chloride in acetic acid gives both rearranged and unrearranged product. The rearranged acetate may also be 

The stereochemistry of the addition of hydrogen halides to a variety of alkenes has been investigated. The addition of hydrogen chloride to 1-methylcyclopentene is 

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An even more effective homogeneous hydrogenation catalyst is the complex [RhClfPPhsfs] which permits rapid reduction of alkenes, alkynes and other unsaturated compounds in benzene solution at 25°C and 1 atm pressure (p. 1134). The Haber process, which uses iron metal catalysts for the direct synthesis of ammonia from nitrogen and hydrogen at high temperatures and pressures, is a further example (p. 421). 

Good reviews of asymmetric homogeneous hydrogenation catalysts and their syntheses are those of Caplar et al. 28) and Vineyard et al. (106). 

RhCl(PPh3)3 is a very active homogenous hydrogenation catalyst, because of its readiness to engage in oxidative addition reactions with molecules like H2, forming Rh—H bonds of moderate strength that can subsequently be broken to allow hydride transfer to the alkene substrate. A further factor is the lability of the bulky triphenylphosphines that creates coordinative unsaturation necessary to bind the substrate molecules [44]. 

RhCl(PPhi)i as a homogenous hydrogenation catalyst [44, 45, 52]. The mechanism of this reaction has been the source of controversy for many years. One interpretation of the catalytic cycle is shown in Figure 2.15 this concentrates on a route where hydride coordination occurs first, rather than alkene coordination, and in which dimeric species are unimportant. (Recent NMR study indicates the presence of binuclear dihydrides in low amount in the catalyst system [47].)... 

A. J. Bircii u. D. H. Williamson, Homogeneous Hydrogenation Catalysts in Organic Synthesis, Organic Reactions, Vol. XXIV, S. 4-186, 1976. 

The efficiency of Crabtree s catalyst as a catalyst for small molecule hydrogenation has been known for many years. Unlike many homogeneous hydrogenation catalysts, Crabtree s catalyst is able to reduce hindered olefins at favourable rates.7 It has never been reported as a catalyst for the hydrogenation of rubber except for its use in the hydrogenation of bulk PBD.8 This paper describes the first use of Crabtree s catalyst in the hydrogenation NBR. Kinetic data are presented and analyzed to understand the underlying chemistry. 

More recently homogeneous hydrogenation catalysts, such as RhCl(Ph3P)3, have been developed which are soluble in the reaction medium. These are believed to transfer H to an alkene via a metal hydride intermediate they, too, lead to a considerable degree of SYN stereoselectivity in hydrogen addition. 

Other advances over the past few years have been the development of (a) homogeneous hydrogenation catalysts for substrates normally not readily reduced, e.g., aromatics, isonitriles, and nitro compounds, and (b) a number of catalyst systems with unusual selectivity properties, e.g., with the capability of reducing a,/3-unsaturated aldehydes to the corresponding a,/3-unsaturated alcohols (see Sections II,B,2 and VII). 

The synthesis of cationic rhodium complexes constitutes another important contribution of the late 1960s. The preparation of cationic complexes of formula [Rh(diene)(PR3)2]+ was reported by several laboratories in the period 1968-1970 [17, 18]. Osborn and coworkers made the important discovery that these complexes, when treated with molecular hydrogen, yield [RhH2(PR3)2(S)2]+ (S = sol-vent). These rhodium(III) complexes function as homogeneous hydrogenation catalysts under mild conditions for the reduction of alkenes, dienes, alkynes, and ketones [17, 19]. Related complexes with chiral diphosphines have been very important in modern enantioselective catalytic hydrogenations (see Section 1.1.6). 

Following Wilkinson s discovery of [RhCl(PPh3)3] as an homogeneous hydrogenation catalyst for unhindered alkenes [14b, 35], and the development of methods to prepare chiral phosphines by Mislow [36] and Horner [37], Knowles [38] and Horner [15, 39] each showed that, with the use of optically active tertiary phosphines as ligands in complexes of rhodium, the enantioselective asymmetric hydrogenation of prochiral C=C double bonds is possible (Scheme 1.8). 

In the context of 1H chemical shifts and determination of the reaction mechanism of homogeneous hydrogenation catalysts, one usually tries to observe hydride-intermediates that typically resonate at high field (-5 to -30 ppm). Agostic bonds (see Fig. 11.1) also tend to have a hydride-like proton chemical shift. 

Both temperature and pressure are important parameters/variables in NMR measurements of homogeneous hydrogenation catalysts. Usually, a certain hydrogen pressure is needed to form the active catalyst. The temperature controls the rate of reactions. Sometimes, temperatures above room temperature are needed for example, the reaction shown in Figure 11.3 occurs at a hydrogen pressure of 3 atmos and temperatures above 318 K. In other cases, intermediates can only be observed at temperatures below room temperature. Modern NMR instruments routinely allow measurements to be made in the range of, for example 170 to 410 K, but this range can easily be extended by the use of special NMR probes. 

The binuclear precursor (di-,u-chloro-bis-[ /4-2,5-norbomadiene]-rhodium(I)) = [(Rh(NBD)Cl]2 is well suited for the in-situ preparation of a variety of homogeneous hydrogenation catalysts, if tertiary phosphines (here PMe3, PMe2Ph,... 

In recent years, considerable effort has been made to immobilize homogeneous hydrogenation catalysts because of the obvious potential advantages, such as improved separation and catalytic performance [4 b, 40]. Although beyond the... 

In 1968, Knowles et al. [1] and Horner et al. [2] independently reported the use of a chiral, enantiomerically enriched, monodentate phosphine ligand in the rhodium-catalyzed homogeneous hydrogenation of a prochiral alkene (Scheme 28.1). Although enantioselectivities were low, this demonstrated the transformation of Wilkinson s catalyst, Rh(PPh3)3Cl [3] into an enantioselective homogeneous hydrogenation catalyst [4]. 

As already mentioned, the most important industrial application of homogeneous hydrogenation catalysts is for the enantioselective synthesis of chiral compounds. Today, not only pharmaceuticals and vitamins [3], agrochemicals [4], flavors and fragrances [5] but also functional materials [6, 7] are increasingly produced as enantiomerically pure compounds. The reason for this development is the often superior performance of the pure enantiomers and/or that regulations demand the evaluation of both enantiomers of a biologically active compound before its approval. This trend has made the economical enantioselective synthesis of chiral performance chemicals a very important topic. 

Crabtree described the use of dibenzo[a,e]cyclooctatetraene, a potent selective poison of homogeneous hydrogenation catalysts, as a tool to distinguish between homogeneous and heterogeneous catalysis in the hydrogenation of hexene with a range of catalysts [24]. 

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