Rhodium hydrogen transfer reaction

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. 

Recent mechanistic studies on transition metal-catalysed hydrogen transfer reactions have been reviewed. Experimental and theoretical studies showed that hydrogen transfer reactions proceed through different pathways. For transition metals, hydridic routes are the most common. Within the hydridic family there are two main groups the monohydride and dihydride routes. Experimentally, it was found that whereas rhodium and iridium catalysts favour the monohydride route, the mechanism for ruthenium catalysts proceeds by either pathway, depending on the ligands. A direct hydrogen transfer mechanism has been proposed for Meerwein-Ponndorf-Verley (MPV) reductions.352... 

It is well known that hydride derivatives of transition metals can be prepared by hydrogen-transfer reactions from primary and secondary alcohols. Penta-methylcyclopentadienyl (/i-pyrazolate)-( t-hydrido) complexes of iridium and rhodium were obtained by using propan-2-ol as the hydride source (128, 150). 

Dehydrogenation of alcohols to aldehyde or ketone allows subsequent bond construction steps which would not be possible for the parent alcohols. Hence, a variety of iridium, rhodium or ruthenium phosphine, pincer and related complexes, that are efficient catalysts for the dehydrogenation of alcohols, can potentially be appHed for the related hydrogen-transfer reactions, thus leading to new added-value compounds. The hydrogen atoms transfer to a sacrificial hydrogen acceptor, such as a carbonyl compound or an olefin which is reduced to the corresponding alcohol or alkane. 

Finally, the use of S/P ligands derived from (i )-binaphthol has been considered by Gladiali et al. in the asymmetric rhodium-catalysed hydrogen-transfer reduction of acetophenone performed in the presence of i-PrOH as the hydrogen donor.It was noted that racemisation occurred when the reaction time increased and consequently the corresponding alcohol was obtained in only low enantioselectivities (< 5% ee) as shown in Scheme 9.21. Similar results were more recently reported by these authors by using iridium combined with the same ligands. ... 

Transition-metal catalysts are, in general, more active than the MPVO catalysts in the reduction of ketones via hydrogen transfer. Especially, upon the introduction of a small amount of base into the reaction mixture, TOFs of transition-metal catalysts are typically five- to 10-fold higher than those of MPVO catalysts (see Table 20.7, MPVO catalysts entries 1-20, transition-metal catalysts entries 21-53). The transition-metal catalysts are less sensitive to moisture than MPVO catalysts. Transition metal-catalyzed reactions are frequently carried out in 2-propanol/water mixtures. Successful transition-metal catalysts for transfer hydrogenations are based not only on iridium, rhodium or ruthenium ions but also on nickel [93], rhenium [94] and osmium [95]. It has been reported that... 

In a closely related study, complex 26, where L = PPh3, has been prepared in the absence of water by Nicholas and its structure has been determined by X-ray crystallography (Fig. 12)(104). Investigations employing a Rh-D complex, coupled with the lack of sensitivity of the reaction to H20, led these researchers to propose a route involving rhodium promoted reductive disproportionation of C02 followed by hydrogen transfer from Rh to the oxygen of the coordinated carbonate (Scheme 9) (105). 

Compared to the rhodium-catalyzed stereoselective reactions, studies on the iridium-catalyzed reactions have been limited until recently. Usually lower selectivities have been observed in the Ir(i)-catalyzed reactions.459,460 The asymmetric hydrosilylation of imines affords optically active secondary amines. These are very valuable compounds, but the studies on that reaction are quite limited.461 Close examinations of these reactions revealed that they proceed via a transfer hydrogenation. Other conditions such as the 2-propanol/base system in the presence of an appropriate metal complex have been employed as well, but only low selectivities were obtained.462... 

To complete the alkene hydrogenation reaction sequence, the first hydrogen transfer must be followed by a second, which results in the reductive elimination of the alkane product. This proceeds through a three-centered transition state. The catalytic cycle is shown in Fig. 22-1 but the process is actually more complicated since the equilibria are dependent on phosphine, alkene, rhodium concentrations, temperature, and pressure. 

Catalytic hydrogen transfer from a hydrogen donor molecule to an unsaturated substrate sometimes presents advantages over hydrogenation by molecular hydrogen. This type of reaction can be catalyzed by a number of ruthenium or rhodium catalysts. Cycloocta-1,5-diene and hexa-1,5-diene can be selectively reduced to cyclooctene and a mixture of hexenes, respectively, by Rh6(CO)16 via hydrogen transfer from isopropanol.The reaction proceeds at 145°C and a CO pressure of 45 bar. For cycloocta-1,5-diene... 

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