Hydrogenation Dihydride mechanism

Figure 1.3. Catalytic hydrogenation of A-acylated dehydroamino esters via an unsatu-rated/dihydride mechanism the p substituents in the substrates are omitted for clarity [P-P = (i ,R)-DIPAMP, (i ,i )-CHIRAPHOS, or (R)-BINAP S = solvent or a weak ligand].

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... 

The homogeneous hydrogenation of alkenes is explained by two mechanisms. The first is the dihydride mechanism, in which the dihydride 1 is formed by oxidative addition of H2, and the hydrogenation proceeds by the insertion of alkene to the metal hydride bond, followed by reductive elimination (Scheme 10.1). The other hydrogenation is explained by the formation of the monohydride 2 (Scheme 10.2). Insertion of alkenes... 

These studies, although they did not deliver the much desired direct characterisation of a dihydride intermediate with the coordinated alkene, opened the door to the alternative mechanism, known as dihydride mechanism. An example of this mechanism, the hydrogenation of MAC with Rh/t-Bu-BisP complex, is depicted in Scheme 7.8. As with the unsaturated mechanism, this particular case is presented in the scheme, the mechanism is discussed in a broader context. 

Scheme 7.8 Dihydride mechanism for the hydrogenation of MAC with Rh/t-Bu-BisP precatalysts.

Imamoto and co-workers studied the mechanism of the reaction, concluding that it is similar to the dihydride mechanism of Rh-catalysed hydrogenations (Section 7.2.2), with the enantioselection occurring at the migratory insertion step. 

Some of the most detaOed studies on this process were conducted by Gridnev and Imam-oto on reactions catalyzed by bidentate, P-chiral ligands containing alkyl substituents on phosphorus. ° Two pieces of data together imply that this catalyst reactsby the hydrogen-first mechanism in Scheme 15.8, instead of the alkene-first mechanism. First, the solvated bisphosphine dihydride complex [Rh(Lj)(S)2(H)J " is formed at -90 C, and this complex reacts with MAC at this temperature to form Thus, this complex is... 

This is illustrated in Scheme 1.8. Hydrogenation of BenzP -Rh catalyst 8 is characterized by relatively high activation barrier (Table 1.1, entry 2), and in the case of the substrate with unsubstituted p-carbon atom, the achvahon of dihydrogen proceeds via unsaturated mechanism with lower achvahon barrier. Nevertheless, with the substrate having p-subshtuent, the achvahon barrier for oxidahve addihon to a chelating catalyst-substrate complex becomes very high, and H2 achvahon proceeds via dihydride mechanism. ... 

Gridnev, I. D. Higashi, N. Asakura, K. Imamoto, T. Mechanism of asymmetric hydrogenation catalyzed by a rhodium complex of (S, S)-bis(f-butylmethylphosphino)ethane. Dihydride mechanism of asymmetric hydrogenation. J. Am. Chem. Soc. 2000,122, 7183-7194. 

In the previous Sections, bulk specimens were alloyed with hydrogen from the gas phase. It was interesting to see whether hydrogen affects the mechanical properties of titanium in a similar way if metal is in a powder state and hydrogen is introduced by mechanical mixing of the metal powder with titanium dihydride, or the interparticle boundaries axe an insurmountable obstacle for hydrogen an eliminate the effects observed in bulk specimens. 

RhCl(PPhi)i as a homogenous hydrogenation catalyst [44, 45, 52]. The mechanism of this reaction has been the source of controversy for many years. One interpretation of the catalytic cycle is shown in Figure 2.15 this concentrates on a route where hydride coordination occurs first, rather than alkene coordination, and in which dimeric species are unimportant. (Recent NMR study indicates the presence of binuclear dihydrides in low amount in the catalyst system [47].)... 

The racemization mechanism of sec-alcohols has been widely studied [16,17]. Metal complexes of the main groups of the periodic table react through a direct transfer of hydrogen (concerted process), such as aluminum complexes in Meerwein-Ponn-dorf-Verley-Oppenauer reaction. However, racemization catalyzed by transition metal complexes occurs via hydrogen transfer processes through metal hydrides or metal dihydrides intermediates (Figure 4.5) [18]. 

The reversible hydrogenolysis of trfl -(Ph2MeSi)2Pt(PMe2Ph)2 has been discussed in Sections II,B,4 and II,B,7. A mechanism similar to that postulated for hydrogen halide reactions [e.g., Eq. (78)] has been suggested 71, 114) such a dihydride intermediate has not been detected, but some support comes from the identification of H2XSiPtl2(H)(PEt3)2 (X = Cl, I), as shown in Eq. (79) 27). Further discussion is in Section III. 

Scheme 1 Fundamental steps in Rh-catalyzed alkene hydrogenation involving a dihydride-based mechanism...

The hydrogenation of simple alkenes using cationic rhodium precatalysts has been studied by Osborn and Schrock [46-48]. Although kinetic analyses were not performed, their collective studies suggest that both monohydride- and dihydride-based catalytic cycles operate, and may be partitioned by virtue of an acid-base reaction involving deprotonation of a cationic rhodium(III) dihydride to furnish a neutral rhodium(I) monohydride (Eq. 1). This aspect of the mechanism finds precedent in the stoichiometric deprotonation of cationic rhodium(III) dihydrides to furnish neutral rhodium(I) monohydrides (Eq. 2). The net transformation (H2 + M - X - M - H + HX) is equivalent to a formal heterolytic activation of elemental... 

Detailed aspects of the catalytic mechanism remain unclear. However, influence of basic additives on the partitioning of the conventional hydrogenation and reductive cyclization manifolds coupled with the requirement of cationic rhodium pre-catalysts suggests deprotonation of a cationic rhodium(m) dihydride intermediate. Cationic rhodium hydrides are more acidic than their neutral counterparts and, in the context of hydrogenation, their deprotonation is believed to give rise to monohydride-based catalytic cycles.98,98a,98b Predicated on this... 

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