Rhodium-catalyzed hydrogenation mechanism

The influence of the electronic properties of the alkene on the insertion rate has been evaluated in the kinetic study of the reaction of a Rh(III) dihydrido complex and / ara-substituted styrenes (Scheme 6.8) [45]. The process is part of the proposed mechanism for the rhodium catalyzed hydrogenation of alkenes. Under pseudo-first order conditions (excess of PR3 and alkene) in benzene the rate law is -d[RhH2Cl(PR3)]/dt = A obs[RhH2Cl(PR3)] with Us = i k[alkene]/ [PR3] + fir [alkene]. The values of K and k determined for different para-substituted styrenes show that more electron withdrawing substituents in the substituted styrene ring increase the value of K (better alkene coordination), but decrease k (slower insertion). These opposite trends tend to cancel each other and the overall rates of the process vary little for different styrenes. K values are in the range... 

Figure 4.7 (a) General mechanism of the rhodium-catalyzed hydrogenation and typical metal... 

Abstract The application of QM/MM methods to the study of the reaction mechanisms involved in chemo-, regio-, and enantio- selective processes has been a very productive area of research in the last two decades. This review summarizes basic general ideas in both QM/MM methods and the computational study of selectivity and presents selected results on the study of three of the most representative examples of these applications rhodium-catalyzed hydrogenation, rhodium-catalyzed hydroformylation, and copper-catalyzed cyclopropanation. 

Fig. 2 Proposed mechanism for the rhodium-catalyzed hydrogenation of enamides...

Halpern J, Okamoto T and Zakhariev A 1976 Mechanism of the chlorotris(triphenylphosphine)rhodium(l)-catalyzed hydrogenation of alkenes J. Mol. Catal. 2 65-9... 

Because of the complexity of the rhodium-catalyzed reduction of benzaldehyde to benzyl alcohol with CO and H20, it is not possible to fully elucidate the mechanism of catalytic reduction given the extent of the kinetic studies performed to date. However, the results do allow us to draw several important conclusions about the reaction mechanism for benzaldehyde hydrogenation and several related reactions. 

Fig. 35.4 Outline mechanism for the rhodium-catalyzed enantioselective transfer hydrogenation reaction.

TPR of supported bimetallic catalysts often reveals whether the two metals are in contact or not. The TPR pattern of the 1 1 FeRh/SiOi catalyst in Fig. 2.4 shows that the bimetallic combination reduces largely in the same temperature range as the rhodium catalyst does, indicating that rhodium catalyzes the reduction of the less noble iron. This forms evidence that rhodium and iron are well mixed in the fresh catalyst. The reduction mechanism is as follows. As soon as rhodium becomes metallic it causes hydrogen to dissociate atomic hydrogen migrates to iron oxide in contact with metallic rhodium and reduces the oxide instantaneously. 

Thus the two plausible catalytic cycles have been considered, one via an Ir dihydride complex A and the other via an IrH2(ri -H2) complex B (Fig. 3). The first is analogous to the well-established mechanism for rhodium diphosphine-catalyzed hydrogenation of olefins going through Ir(I) and Ir(III) intermediates [26-29]. 

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.

The mechanism operating in rhodium-catalyzed and iridium-catalyzed hydrogen transfer reactions involves metal hydrides as key intermediates. Complexes such as [ M(p.-C1)(L2) 2], [M(cod)L2](Bp4) (M = Rh, Ir L2 = dppp, bipy), and [RhCl(PPh3)3] are most likely to follow the well-established mechanism [44] via a metal alkoxide intermediate and elimination to generate the active hydride species, as shown in Scheme 2. 

A multistep pathway analogous to the mechanism of alkene hydrogenation has been shown to be operative in the rhodium-catalyzed hydroboration of alkenes.363 Deuterium labeling studies furnished evidence that the reversibility of the elementary steps is strongly substrate-dependent. The key step is hydride rather than boron migration to the rhodium-bound alkene. 

Industrially the straight chain isomer is generally the most desired product and hence the normal/iso product ratio obtained for a given catalyst is of importance. Further, the hydrogenation activities of catalysts vary considerably such that alcohols can in some cases be obtained in a single step (222). The first catalysts developed for this reaction were based on cobalt carbonyl and later cobalt carbonyl phosphine complexes. However, more recently attention has been focused on the intrinsically much more active rhodium catalysts (222, 223). A simplified mechanism for (223) cobalt- and rhodium-catalyzed hydroformylation has been proposed which involves the following steps ... 

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