Hydrogenation stereochemistry, catalyst effect

Product stereochemistry is a function of the specific catalyst used for hydro-genation. For example, palladium generally gives more of the thermodynamically stable product than other catalysts. This effect has been attributed to an increased rate of equilibration of the steps in the hydrogenation process. Consequently, palladium should not be used to hydrogenate readily isomerizable olefins such as A - and A -steroids. ... 

Some olefinic molecules have a second function that anchors itself on the catalyst surface in such a way so as to enforce addition of hydrogen to its own side of the molecule. This anchoring effect, dubbed haptophilicity 112,113), has been observed by many investigators 0,19,74,78,90,120). An example of Gula and Spencer 47) illustrates how the anchoring tendencies of a function remote from the point of saturation may influence the stereochemistry. 

Solvents and pH may have a marked effect on stereochemistry as was illustrated in Chapter 1, and the generality given there is useful, A further example of the stereochemical influence that may be exerted by proper choice of catalyst and solvent is shown in the hydrogenation of a complex enamine, By proper choice of conditions high yields of either the cis or trans product could be obtained. Selected results are shown below (52) (data used with permission). 

Solvents regularly used in organic reactions are used in heterogeneous catalysis of organic reactions. When solvent information is known, it accompanies other reaction information in each chapter. It must be remembered, however, that the solvent may interact with the catalyst surface and be converted into something undesirable or may combine with or modify one or more of the reactants. The example in Table 1.351 shows the rather minor effect of solvents on the stereochemistry of hydrogenation of the exo double bond in a spatane precursor. 

This stereochemical model explains the stereochemistry of the hydrogenation of MAA over TA-Ni system. It also predicts that the TA-Ni catalyst can be effective for the enantioselective hydrogenation of some prochiral ketones with excellent e.e. values (70%). 

Previous work has shown that the electronic characteristics of the benzene substituent in triarylphosphine chlororhodium complexes have a marked influence on the rate of olefin hydrogenation catalyzed by these compounds. Thus, in the hydrogenation of cyclohexene using L3RhCl the rate decreased as L = tri-p-methoxyphenylphosphine > triphenylphosphine > tri-p-fluorophenylphosphine (14). In the hydrogenation of 1-hexene with catalysts prepared by treating dicyclooctene rhodium chloride with 2.2-2.5 equivalents of substituted triarylphosphines, the substituent effect on the rate was p-methoxy > p-methyl >> p-chloro (15). No mention could be found of any product stereochemistry studies using this type of catalyst. 

Next, the mechanism of the Type II reactions is discussed. To discriminate one of the enantiofaces of the acceptor it is desirable to place and to activate the electrophiles in a chiral environment. At the same time, effective activation of the Michael donor is required. In Shibasaki s ALB-catalyzed reaction (Scheme 3), it was proposed that the aluminum cation functioned as a Lewis acid to activate enones at the center of the catalyst, and that the Li-naphthoxide moiety deproton-ated the a-hydrogen of malonate to form the Li enolate (Scheme 9). Such simultaneous activation of both reactants at precisely defined positions became feasible by using multifunctional heterobimetallic complexes the mechanism is reminiscent of that which is operative in the active sites of enzymes. The observed absolute stereochemistry can be understood in terms of the proposed transition state model 19. Importantly, addition of a catalytic amount of KOt-Bu (0.9equiv. to ALB) was effective in acceleration of the reaction rate with no deterioration of the... 

None of these mechanistic proposals is sufficiently general to use to rationalize all of the stereochemical data observed on the hydrogenation of a,[3-unsaturated ketones. By a judicious combination of segments of each of these proposals along with the Horiuti-Polanyi mechanism (2), it is possible, however, to develop a uniform mechanistic rationale that can be useful in determining the effect of solvent on product stereochemistry. In addition, the influence of hydrogen availability, the type and quantity of catalyst, and the nature of other substituents on the reacting molecule on the product isomer distribution can also be more readily understood. 

XII), there is only a slight increase in the amount of cis product formed (13). The effect of varying base concentrations on product stereochemistry in the hydrogenation of octalone is shown in Fig. 7. As in the acid-promoted reaction (Fig. 6), the product distribution in base is composed of three straight line segments one in very dilute base, a second at intermediate concentrations, and a third in more concentrated basic media. The position of the first breakpoint is dependent on the quantity of catalyst used, whereas the position of the second breakpoint depends on the amount of substrate present (20,24). 

Rules 1 and 2 may be accepted as a generalization based primarily on the results obtained over platinum catalysts. However, there have been known many examples of the exception to this rule,153 since the stereochemistry of hydrogenation may be influenced by many factors, such as the solvent, the temperature, the hydrogen pressure, and the basic or acidic impurity associated with catalyst preparation, as well as the activity of the catalyst, and since the effects of these factors may differ sensitively with the catalyst employed and by the structure of the ketone hydrogenated. 

Rylander et al. studied the effect of carriers and water on the stereochemistry of hydrogenation of o-, m-, and / -xylenes over rhodium and ruthenium catalysts at room temperature and an initial hydrogen pressure of 0.44 MPa.66 As seen from the results shown in Table 11.6, carbon-supported catalysts give less trans isomers than do the other supported catalysts. With a few exceptions, rhodium catalysts tend to produce the trans isomers more than do ruthenium catalysts. It is noted that the presence of water greatly reduced the proportion of trans isomer in the hydrogenations of o- and m-xylenes with Ru-C and of p-xylene with Rh-C. 

TABLE 11.6 Effects of Carriers and Water on the Stereochemistry of Hydrogenation of o-, in-, andp-Xylenes with Rhodium and Ruthenium Catalysts 2 ... 

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