Hydrogenation with homogeneous catalysts

Exercise 11-6 a. Would you expect a carbon-nitrogen triple bond to be hydrogenated more, or less, easily than a carbon-carbon triple bond b. Why is it difficult to hydrogenate a tetrasubstituted alkene such as 2,3-dimethyl-2-butene  

Exercise 11-7 Accurate AH° and AG° values in kcal mole 1 for hydrogen addition to 1-butene, c/s- and frans-2-butene in the gas phase at 25° follow  

Hydrogen addition to multiple bonds is catalyzed by certain complex metal salts in solution. This may be described as homogeneous catalysis and, compared to heterogeneous catalysis, is a relatively new development in the area of hydrogenation reactions. Rhodium and ruthenium salts appear to be generally useful catalysts  

The most thoroughly investigated homogeneous hydrogenation catalyst is the four-coordinate rhodium complex Rhf (C6H5)3P]3C1. This catalyst is called 

11 Alkenes and Alkynes II. Oxidation and Reduction Reactions. Acidity of Alkynes 

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The mechanisms of hydrogenation of alkenes over finely divided metals such as nickel, platinum, and so on (Section 11-2) now are understood in a general way. However, these reactions are extremely difficult to study because they occur on a metallic surface whose structure is hard to define. In contrast, the mechanisms of hydrogenation with homogeneous catalysts are known in considerable detail and provide insight into their heterogeneous counterparts. 

The rate of hydrogen (or deuterium) uptake with homogeneous catalysts is usually faster in benzene-alcohol (methanol or ethanol) solvent systems or in acetone than in tetrahydrofuran or in benzene alone. Whereas... 

The potential importance of homogeneous catalytic reactions in synthesis gas transformations (i.e., hydrogenation of carbon monoxide) has been widely recognized in recent years. In the first place, such systems could provide structural and mechanistic models for the currently more important, but more difficult to study, heterogeneous catalysts. Secondly, product selectivity is generally more readily achievable with homogeneous catalysts, and this would be an obviously desirable feature in an efficient process converting synthesis gas to useful chemicals and fuels. 

In broad terms there are three types of catalyst for transfer hydrogenation dehydrogenases heterogeneous and homogenous metal catalysts. Here, the first two are mentioned for completeness, and the main focus of this chapter will be asymmetric transfer hydrogenation with homogenous metal catalysts. 

Hydrogen donors that function poorly with homogenous catalysts include hydrazine hydrate, alkenes (e.g., cyclohexene), and ascorbic acid. This is somewhat surprising as they can be very effective in heterogeneous transfer hydrogenation. 

Fig. 37.2 Chemo- and diastereoselective hydrogenations of complex molecules with homogeneous catalysts.

Fig. 45.2 Possible qualitative concentration profiles in the liquid boundary layer for homogeneous hydrogenations with molecular catalysts.

For the rational design of transition metal catalyzed reactions, as well as for fine-tuning, it is vital to know about the catalytic mechanism in as much detail as possible. Apart from kinetic measurements, the only way to learn about mechanistic details is direct spectroscopic observation of reactive intermediates. In this chapter, we have demonstrated that NMR spectroscopy is an invaluable tool in this respect. In combination with other physicochemical effects (such as parahydrogen induced nuclear polarization) even reactive intermediates, which are present at only very low concentrations, can be observed and fully characterized. Therefore, it might be worthwhile not only to apply standard experiments, but to go and exploit some of the more exotic techniques that are now available and ready to use. The successful story of homogeneous hydrogenation with rhodium catalysts demonstrates impressively that this really might be worth the effort. 

Asymmetric reduction with very high ee values has also been achieved with achiral reducing agents and optically active catalysts. The two most important examples are (1) homogeneous catalytic hydrogenation with the catalyst 2,2 -bis(diphenylphosphino)-1,1 -... 

Hydrogenation of aromatics with homogeneous catalysts is not easy to achieve, and only relatively few complexes were found to be effective. 

Hydrogenation with homogeneous catalysis involves a soluble catalyst rather than the more common heterogeneous catalysis with, say, Pd metal dispersed on an insoluble charcoal support as in Chapter 24. In general terms homogeneous catalysts are those that are soluble in the reaction mixture. 

SCCO2 is largely used to process food (extraction or fractionation), but other applications, such as the fluoropolymer synthesis by DuPont, hydrogenation or alkylation by Thomas Swan, coatings by Union Carbide, and polyurethane processing by Crain Industries, are still in development [111]. The application of supercritical fluids (SCFs) as reaction media with homogeneous catalysts has been mainly investigated on a laboratory scale. 

Selective hydrogenation of polyenes to monoenes is possible with homogeneous catalysts. Dichlorodicarbonylbis(triphenylphosphine)ruthenium(II) catalyzed the selective reduction of 1,5,9-cyclododecatriene to cyclododecene, as well as dichloro-tris(triphenylphosphine)ruthenium(II) ... 

The use of CO + HjO as an in situ source of H2 via the shift reaction or as a source of reducing electrons with concomitant oxidation of CO to CO2 has recently been explored with homogeneous catalyst solutions. Equations (a) and (b) are modifications (Reppe) of hydroformylation (see 14.6.3) and olefin hydrogenation, respectively. The most effective catalysts for equation (a) are Ru3(CO),2, Rh fCOlig, Ir4(CO)i2 in alkaline THE or MeOH. The first of these shows great selectivity in the formation of linear vs. branched aldehyde . With Rhg(CO),5, the aldehyde is reduced further to the alcohol. A different catalyst based on Co2(CO)g/diphos in polar ether solvents has also been used to catalyze equation (a) with propylene as substrate. ... 

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