Rhodium-catalyzed hydrogenation, reaction pathway

Transition metal catalysts generally operate via elementary steps that can occur at the metal center, typically oxidative addition, substrate coordination, migration (insertion), reductive eUmination, and so on. This creates a reaction pathway by breaking down the reaction in several different steps. As a typical example, the reaction pathway of rhodium-catalyzed hydrogenation is depicted. The reaction profile of such a metal-catalyzed reaction therefore consists of various transition states and intermediates (Figure 4.7). Often one of the transition states has the highest energy barrier and represents the most difficult step, and it is this step of... 

The reaction pathway for rhodium-catalyzed asymmetric hydrogenation of enamides is described and intermediates are defined in solution by P-31, C-13, and H-l NMR. The stereochemical relationship of bound enamide to rhodium alkyl and to the product of hydrogenation is demonstrated. Experiments involving the addition of HD to a variety of olefins in the presence of rhodium biphosphine catalysts suggest that a concerted addition of hydrogen to olefin and metal may occur in appropriate cases. 

Landis, C. R., Hilfenhaus, P., Feldgus, S. Structures and Reaction Pathways in Rhodium(l)-Catalyzed Hydrogenation of Enamides A Model DFT Study. J. Am. Chem. Soc. 1999,121, 8741-8754. 

Rhodium-Diphosphine Catalysts. The mechanism of rhodium-catalyzed asymmetric hydrogenation is one of the most intensively investigated and best understood. Reaction pathways have been accurately studied both experimentally and theoretically (138,162,213-221). In early studies, Halpern (222) and Brown (214) established that the hydrogenation proceeds according to the reaction sequence presented in Figure 51 for the hydrogenation of a dehydroamino acid with a chiral diphosphine-rhodium complex. Many variants on both catalyst and reactant have been described. Stereoselectivity takes place via the difference in reactivity of the involved diastereomeric square-planar... 

In general, rhodium-catalyzed hydroformylation of alkynes proceeds much slower than the reaction with olefins. It should be remembered that homogeneously catalyzed hydroformylation of olefins with unmodified rhodium catalysts can be irreversibly poisoned by the presence of even trace quantities of alkynes. As Liu and Garland [94, 95] found by means of in situ IR spectroscopy, the reason is likely the formation of dinuclear rhodium-carbonyl complexes I, which are stable even in the presence of hydrogen (Scheme 4.17). Therefore, alternative pathways for the production of a,P-unsaturated aldehydes have been suggested, consisting of Ni-catalyzed hydrocyanation followed by chemoselective hydrogenation [96]. 

We have already seen in Section 2.2.2 that metal-alkyl compounds are prone to undergo /3-hydride elimination or, in short, /3-elimination reactions (see Fig. 2.5). In fact, hydride abstraction can occur from carbon atoms in other positions also, but elimination from the /8-carbon is more common. As seen earlier, insertion of an alkene into a metal-hydrogen bond and a /8-elimination reaction have a reversible relationship. This is obvious in Reaction 2.8. For certain metal complexes it has been possible to study this reversible equilibrium by NMR spectroscopy. A hydrido-ethylene complex of rhodium, as shown in Fig. 2.8, is an example. In metal-catalyzed alkene polymerization, termination of the polymer chain growth often follows the /8-hydride elimination pathway. This also is schematically shown in Fig. 2.8. 

The intramolecular insertion of a hydride into a coordinated olefin is a crucial step in olefin hydrogenation catalyzed by late transition metal complexes, such as those of iridium, rhodium, and ruthenium (Chapter 15), in hydroformylation reactions catalyzed by cobalt, rhodium, and platinum complexes (Chapter 16), and in many other reactions, including the initiation of some olefin polymerizations. The microscopic reverse, 3-hydride elimination, is the most common pathway for the decomposition of metal-alkyl complexes and is a mechanism for olefin isomerizations. 

Primary amines can be used as substrates for C-C bond activation reactions that consist of four independent transformations [29]. This process is exemplified by reaction of 3-phenylpropan-l-amine (49) with 3,3-dimethylbut-l-ene (47) in the presence of 16 and 21, which produces both the symmetric dialkyl ketone 51 and tmsymmetric ketone 50 (Scheme 10a). The route followed in this reaction (Scheme 10b) begins with rhodium mediated transfer hydrogenation between amine 49 and alkene 47 to generate phenethylimine 52, which then undergoes transimination with 21 to yield the aminopicoline derived imine 53. Chelation-assisted hydroimination of 53 with the olefin then forms ketimine 54, which upon acid promoted hydrolysis produces ketone 50. In a competing pathway, Rh(I)-catalyzed C-C bond activation of ketimine 54, followed by subsequent addition of 47, affords the symmetric dialkyl ketimine 55, which is converted to symmetric dialkyl ketone 51 upon hydrolysis. 

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