Homogeneous hydrogenation, catalyzed mechanism

The mechanism of homogeneous hydrogenation catalyzed by RhCl(Ph3P)3 ° involves reaction of the catalyst with hydrogen to form a metal hydride (PPh3)2RhH2Cl (43), which rapidly transfers two hydrogen atoms to the alkene. 

To elucidate the mechanism of homogeneous hydrogenation catalyzed by Fe(CO)s, kinetic studies were carried out with mixtures of unsaturated fatty esters containing a radioactive label. A C-labeled methyl octadecadienoate-Fe(CO)3 complex was prepared to serve as a catalytic intermediate. Hydrogenation of methyl oleate (m-9-octa-decenoate) and palmitoleate (cis-9-hexadecenoate) and of their mixtures with methyl linoleate was also studied to determine the selectivity of this system, the function of the diene-Fe(CO)3 complex, and the mechanism of homogeneous isomerization. Mixtures of reaction intermediates with a label helped achieve unique simulation of the kinetic data with an analog computer. 

Catecholborane and pinacolborane are especially useful in hydroborations catalyzed by transition metals.163 Wilkinson s catalyst Rh(PPh3)3Cl is among those used frequently.164 The general mechanism for catalysis is believed to be similar to that for homogeneous hydrogenation and involves oxidative addition of the borane to the metal, generating a metal hydride.165... 

Based on the above reactions, an overall mechanism for the hydrogenation of NBR catalyzed by Wilkinson s catalyst was proposed (see Scheme 19.4), which is also applicable to the kinetic performance of the homogeneous hydrogenation of PB [88] and styrene-butadiene copolymers [89], where K2 and Ks vanish. 

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. 

The ability of solutions of salts and complexes of the Group VIII metals to catalyze homogeneous hydrogenation is also widespread once again hydridic species probably play an important role. For olefins, the general mechanisms may be written as follows. 

Mechanism of Homogeneously Catalyzed Hydrosilylation. A mechanism analogous to the homogeneous hydrogenation reaction has been proposed by Chalk and Harrod (200) (Scheme 47) for the platinum catalyzed hydrosilylation. In the first step, oxidative addition of the silane occurs and leads to an hexa-coordinate intermediate complex 126, in which both silane and olefin are cis oriented. Hydrogen migration produces a o-alkyl complex 127 from which the hydrosilylation products 128 are obtained by reductive elimination. 

Homogeneous hydrogenation of Q to THQ is also catalyzed by Os3(CO)j2 and its derivatives. These studies, together with the synthesis of osmium cluster complexes of quinoline, have led to another mechanism for quinoline hydrogenation which involves more than one metal center. It is likely that reactions catalyzed by Cp Rh(NCMe)3 and Os3(CO)i2 proceed by different mechanisms. Whether either of these mechanisms is consistent with the heterogeneously catalyzed conversion of Q to THQ is unknown. 

Scheme 5. Proposed mechanism for the homogeneous hydrogenation of ketones catalyzed by Ru(H)2(PPh3)2(diamine) complexes in neat substrate at 3 atm H2, 20 C.

Several mechanisms have been proposed for the platinum-catalyzed homogeneous hydrosilylation reaction. The most commonly invoked mechanism, proposed by Chalk and Harrod in 1965, consists of elementary steps similar to homogeneous hydrogenation, oxidative addition, migratory insertion, and reductive elimination (226). However, this mechanism fails to describe the indnction period or the presence of colloidal species at the end of the reaction. Lewis proposed an alternative mechanism based on the intermediacy of colloids that were detected by transmission electron microscopy after evaporation of catalsdically active solutions (227,228). 

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