Catalyst dihydride

The reaction of Cp2HfClH with Mes2SnH2 gives the dihydrodimer, HMes2SnSnMes2H,451 but, with other catalysts, dihydrides usually react to give stannane oligomers or polymers (see below). 

With this system, we finally succeeded in characterizing the first rhodium dihydride species in the asymmetric hydrogenation of enamides. Additionally, we succeeded afterwards in the characterization of all the possible catalyst dihydride species [39]. In subsequent work, now knowing what to look for and where to look, all transient complexes in the asymmetric enamide hydrogenation with the Rh(PHA-NEPHOS) catalyst could also be observed with classical NMR techniques [37]. 

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].)... 

Transition-metal catalyzed photochemical reactions for hydrogen generation from water have recently been investigated in detail. The reaction system is composed of three major components such as a photosensitizer (PS), a water reduction catalyst (WRC), and a sacrificial reagent (SR). Although noble-metal complexes as WRC have been used [214—230], examples for iron complexes are quite rare. It is well known that a hydride as well as a dihydrogen (or dihydride) complex plays important roles in this reaction. 

OsHCl(CO)(P Pr3)2 is also a very active and selective catalyst for the addition of triethylsilane to phenylacetylene at 60°C, to yield trans- and m-PhHC= CH(SiEt3).50 The reaction of OsHCl(CO)(P Pr3)2 with triethylsilane is the ratedetermining step. The catalytic reaction proceeds via a silyl dihydrogen intermediate of the formula Os(SiEt3)Cl(r)2-H2)(CO)(P,Pr3)2 and its dihydride tautomer, OsH2(SiEt3)Cl(CO)(P,Pr3)2 (Scheme 53). 

The related dihydride-dichloro complex OsH2Cl2(P Pr3)2 is also an active catalyst for the hydrogenation of olefins, diolefms, and a-(3-unsaturated ketones,14 but attempts to hydrogenate phenylacetylene show a rapid deactivation of the catalyst due to formation of a hydride-carbyne complex.54... 

Since the first reports on Wilkinson s catalyst,19,20 many transition-metal-based catalytic systems for hydrogenation of unsaturated organic molecules have been developed. Two major pathways seem to occur, one involving monohydride (M—11) species, and the other, dihydride (MH2)... 

Cofacial ruthenium and osmium bisporphyrins proved to be moderate catalysts (6-9 turnover h 1) for the reduction of proton at mercury pool in THF.17,18 Two mechanisms of H2 evolution have been proposed involving a dihydride or a dihydrogen complex. A wide range of reduction potentials (from —0.63 V to —1.24 V vs. SCE) has been obtained by varying the central metal and the carbon-based axial ligand. However, those catalysts with less negative reduction potentials needed the use of strong acids to carry out the catalysis. These catalysts appeared handicapped by slow reaction kinetics. 

The bis-DIOP complex HRh[(+)-DIOP]2 has been used under mild conditions for catalytic asymmetric hydrogenation of several prochiral olefinic carboxylic acids (273-275). Optical yields for reduction of N-acetamidoacrylic acid (56% ee) and atropic acid (37% ee) are much lower than those obtained using the mono-DIOP catalysts (10, II, 225). The rates in the bis-DIOP systems, however, are much slower, and the hydrogenations are complicated by slow formation of the cationic complex Rh(DIOP)2+ (271, 273, 274) through reaction of the starting hydride with protons from the substrate under H2 the cationic dihydride is maintained [cf. Eq. (25)] ... 

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... 

The use of the diphosphine PHANEPHOS (see Scheme 1.24) permitted Bar-gon, Brown and colleagues to detect and characterize a dihydrido intermediate in the hydrogenation of the enamide MAC by a rhodium-based catalyst The PH IP NMR technique was employed, and showed one of the hydrogen atoms to be agostic between the rhodium center and the /1-carbon of the substrate [85]. By using the same diphosphine and technique it was also possible to detect two diastereomers of the dihydride depicted in Scheme 1.25, which may also be detected using conventional NMR measurements [86]. 

Scheme 3.8 Generation of the active dihydride catalyst by transfer hydrogenation by reductive elimination of the product to give a ruthenium(O) intermediate ([Ru] = Ru(PPh3)3).

Evidence for a major mode of catalyst deactivation in this system came from the observation of phosphonium cations (HPR3) in the reaction mixture, which could form through the pro to nation of free PR3 by the acidic dihydride complex. It is not known which species decomposes to release free PR3, but the decomposition pathway is exacerbated by the subsequent reactivity in which protonation of phosphine removes a proton from the metal dihydride, effectively removing a second metal species from the cycle. 

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