On the mechanism of hydrogenation

The mechanism of enantioselective hydrogenation on nickel catalysts modified with optically active compounds was suggested for the first time by Balandin s group They proposed that the reaction proceeded through an intermediate complex involving catalyst metal atom, (Ni), modifier molecule, (M), and substrate molecule, (S) Ni-M-S. Similar models were proposed later by Yasumori et al and Izumi and Tai 

The aims of studies to elucidate the mechanism have been to account for the observed enantioselectivity and the reaction rate enhancement. The 

Site A asymmetric hydrogenation site according to the Template model resulting (./ )-(-)-MeLa. 

Site C racemic hydrogenation site, because there is no interaction 

Site B shows another orientation of MePy leading to (S)-(+)-MeLa, the adlayer of Cnd molecules exert steric hindrance for adsorption of the methyl and methoxy groups and therefore make this model less probable. 

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Since the first use of catalyzed hydrogen transfer, speculations about, and studies on, the mechanism(s) involved have been extensively published. Especially in recent years, several investigations have been conducted to elucidate the reaction pathways, and with better analytical methods and computational chemistry the catalytic cycles of many systems have now been clarified. The mechanism of transfer hydrogenations depends on the metal used and on the substrate. Here, attention is focused on the mechanisms of hydrogen transfer reactions with the most frequently used catalysts. Two main mechanisms can be distinguished (i) a direct transfer mechanism by which a hydride is transferred directly from the donor to the acceptor molecule and (ii) an indirect mechanism by which the hydride is transferred from the donor to the acceptor molecule via a metal hydride intermediate (Scheme 20.3). 

Fig. 2.4 A possible pathway for rhodium-catalyzed hydroboration by RhCI(PPh3)3, based on the mechanism of hydrogenation and the contributions of Evans and others [17], This takes into account the theoretical support for an associative pathway [13] and the observed irreversibility of the H-transfer step with styrenes.

Frey, P. A., Essenberg, M. K., and Abeles, R. H., 1967, Studies on the mechanism of hydrogen transfer in the cobamide coenzyme-dependent dioldehydrase reaction. J. Biol. Chem. 242 5369n5377. 

In a study by Okabe and McNesby the photolysis of ethylene was studied at 1236,1470 and 1849 A, with emphasis on the mechanism of hydrogen elimination. The products found agreed with the results of Sauer and Dorfman, and the above mechanism was confirmed. The examination of the isotopic distribution of hydrogen and acetylene from the photolysis of CH2CD2 permitted a comparison of the relative importance of the following reactions... 

No other detailed studies on the mechanisms of hydrogenation of aromatics have been reported to date and are clearly desirable. Although this area of research has remained relatively dormant, in comparison with other homogeneous hydrogenation reactions, the current urgent need for better processes aimed at reducing the aromatic... 

HG Nelson, On the mechanism of hydrogen-enhanced crack growth in ferritic steels , in Proceedings of the Second International Conference on Mechanical Behavior of Materials, ASM, Metals Park, OH, 1976, pp. 690-4. 

The nature of the adsorbed intermediate (Hajs) and its impact on the mechanism of hydrogen evolution has been a major concern in HER electrocatalysis. Platinum has long been known to form adsorbed hydrogen [4], and the adsorption of hydrogen holds a particular significance in the overall development of modern electrochemical surface science. 

FIGURE 10.9 A first-guess mechanism for the reaction of an alkene and bromine. For analogy, it draws on the mechanism of hydrogen bromide addition to an alkene. 

Joo, F. Csiba, P. Benyei, A. (1993) Effect of water on the mechanism of hydrogenations catalyzed by rhodium phosphine complexes, Chem. Commun., 1602-4. 

In 2002, Sakaki and coworkers carried out a theoretical study on the mechanism of hydrogenation of CO2 catalyzed by the three-coordinate, 14e d monohydride model complex RhH(PH3)2 [13]. It was found that CO2 insertion into the Rh-H bond is almost barrierless. This result suggests that the 14e rhodium(I) hydride is very reactive toward CO2. The COj insertion leads to formation ofthe rhodium (I) formate intermediate Rh(PHg)2(0C(0)H). To the rhodium(I) formate intermediate, oxidative addition of H2, which leads to formation of a dihydride complex, requires a barrier of 7.3 kcal mol". Then the reductive elimination of formic acid in the dihydride complex via a five-membered ring transition state (1) was calculated to have a very small barrier of 1.9 kcal mol . In the o-bond metathesis pathway, a barrier of only 6.1 kcal mol" was calculated. The o-bond metathesis... 

In 2002, Sakaki also carried out theoretical studies on the mechanism of hydrogenation of CO2 catalyzed by the square pyramidal, 16e d dihydride complexes cfs-[RhH2(PH3)2(H20)] [13]. In the cfs-[RhH2(PH3)2(H20)] -catalyzed reaction, CO2 inserts into a Rh-H bond to give a formate intermediate with a barrier of 28.4kcal mol . Then the total barrier calculated for o-bond metathesis followed by release of formic acid was 5.9 kcal mol . ... 

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