Hydrogenation, catalytic, alkene heterogeneous, mechanism

Heterogeneous catalyst and reactants of a catalyzed reaction remain in two different phases. Heterogeneous catalysis affects the rate of a reaction, which occurs at the interface of two distinct phases — for example, the hydrogenation of alkenes in the presence of solid catalysts such as Pt C, Pd/C, and Ni. The details of the mechanisms of catalytic hydrogenation and related reaction are not yet well understood. 

The chiral sites which are able to rationalize the isospecific polymerization of 1-alkenes are also able, in the framework of the mechanism of the chiral orientation of the growing polymer chain, to account for the stereoselective behavior observed for chiral alkenes in the presence of isospecific heterogeneous catalysts.104 In particular, the model proved able to explain the experimental results relative to the first insertion of a chiral alkene into an initial Ti-methyl bond,105 that is, the absence of discrimination between si and re monomer enantiofaces and the presence of diastereoselectivity [preference for S(R) enantiomer upon si (re) insertion]. Upon si (re) coordination of the two enantiomers of 3-methyl-l-pentene to the octahedral model site, it was calculated that low-energy minima only occur when the conformation relative to the single C-C bond adjacent to the double bond, referred to the hydrogen atom bonded to the tertiary carbon atom, is nearly anticlinal minus, A- (anticlinal plus, A+). Thus one can postulate the reactivity only of the A- conformations upon si coordination and of the A+ conformations upon re coordination (Figure 1.16). In other words, upon si coordination, only the synperiplanar methyl conformation would be accessible to the S enantiomer and only the (less populated) synperiplanar ethyl conformation to the R enantiomer this would favor the si attack of the S enantiomer with respect to the same attack of the R enantiomer, independent of the chirality of the catalytic site. This result is in agreement with a previous hypothesis of Zambelli and co-workers based only on the experimental reactivity ratios of the different faces of C-3-branched 1-alkenes.105... 

AlcoholK and alkenes are also primary products and are not shown in the simplified Eq. 15.182. The overall reaction is complicated and, as a result, its mechanism has been the subject of considerable debate.The reaction may be viewed as the reductive polymerization of carbon monoxide, with molecular hydrogen as the reduc-ii agent. A variety oT heterogeneous catalysts, such as metallic iron and cobalt on alumina, have been used. It is believed that carbon monoxide di.ssociaies on the catalytic surface to ve carbides and that these are in tura hydrogenated to give sur ce carbenes ... 

The mechanism of homogeneous catalysis invoives the same steps as heterogeneous catalysis. An initial tt complex is formed with the reactant. Metal-hydride bonds then react with the complexed alkene to form a C-H bond and a bond between the metal and alkyl group. There can be variation in the timing of formation of the M—H bonds. The metal carbon bond can be broken by either reductive elimination or protonolysis. Note that reductive elimination changes the metal oxidation state, whereas protonolysis does not. The catalytic cycle proceeds by addition of alkene and hydrogen. 

In Section 5.9, we saw that alkenes can be converted to alkanes by catalytic hydrogenation by a variety of catalysts, such as palladium and platinum. These are heterogeneous catalysts. We also noted that homogeneous catalytic hydrogenation can be carried out by Wilkinsons catalyst, Ru[(PPh3)3Cl. We now return to that subject to discuss the reaction mechanism. We will find that hydrogenation by Wilkinson s catalyst occurs in a catalytic cycle that is strikingly similar to the catalytic cycles of the reactions we have discussed thus far in this chapter. The transition metal in the Wilkinson catalyst, however, is ruthenium, not palladium. 

---