Intermediates dihydrides

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. 

The proposed catalytic cycle, which is based on experimental data, is shown in Scheme 6. Loss of 2 equiv. of N2 from 5 (or alternatively 1 equiv. of N2 or 1 equiv. of H2 from complexes shown in Scheme 3) affords the active species a. Olefin coordination giving b is considered to be preferred over oxidative addition of H2. Then, oxidative addition of H2 to b provides the olefin dihydride intermediate c. Olefin insertion giving d and subsequent alkane reductive elimination yields the saturated product and regenerates the catalytically active species a. 

Nagashima reported the hydrogenation of di-, tri- and tetranuclear ruthenium complexes bearing azulenes below 100 °C revealed that only the triruthenium compounds reacted with H2 via triruthenium dihydride intermediates.398 This indicates that there exists a reaction pathway to achieve facile activation of dihydrogen on the face of a triruthenium carbonyl moiety.399... 

Cobalt(III) sepulchrate (l)8 and tetrazamacrocyclic complexes of cobalt(II) (2)9 and nickel(II) (3) (6)9-11 catalyze the electroreduction of water to dihydrogen, at potentials ranging from - 0.7 V (complex (1)) to — 1.5 V (complexes (4)-(6)) vs. SCE in aqueous electrolytes, with current efficiencies as high as 95% for complex (4).9 It is noteworthy that the binuclear nickel biscyclam complex (6) is 10 times more active (at pH 7) than the mononuclear nickel cyclam complex (5). This behavior tends to indicate that some cooperativity between the two metal centers occurs in complex (6), as depicted in the possible reaction (Scheme 3) involving a dihydride intermediate.11... 

The hydrogens within the octahedral olefin-dihydride intermediate are transferred consecutively with overall cis addition, and the rate-determining step (k9) is olefin insertion to give the alkyl- hydride. Kinetic and thermodynamic parameters for nearly all the steps of Fig. 1 have been estimated for the cyclohexene system. Because the insertion reaction is generally believed to require a cis disposition of the hydride and olefin... 

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 elimination of HC1 was proposed to occur also during the H2 activation with the [Pd(PNP)Cl]Cl complexes (PNP = bis-2-(diphenylphosphino)ethyl benzy-lamine, bis-2-(diphenylphosphino)ethyl amine or tris-2-(diphenylphosphino)ethyl amine) [24, 25]. Based on the findings of 31P 1H - and 1H-NMR investigations, the hydride [HPd(PNP)]Cl was detected under H2 atmosphere. The alternative mechanism which involves the oxidative addition of H2 with formation of a Pd(IV)-dihydride intermediate, appeared less likely on the basis of thermodynamic considerations. 

Scheme 12.3 Formation of dihydride intermediates of a cationic Rh complex via displacement of the NMD ligand in the DIPHOS-derived catalyst (S = solvent).

Table 12.2 NMR data and substituent effects of the observed dihydride intermediates.

Whereas, from all of these informative 1H-PHIP-NMR spectra, the structure of the dihydride intermediate (including geometric details about peculiar bonding therein) can be determined rather exactly and reliably, a degree of uncertainty remains as to whether this intermediate represents the major or the minor diastereomer according to the nomenclature of Halpern [27]. This is the consequence of different kinetic constants associated with the two alternative cycles with different stereochemistry, and which accounts for the major and minor reaction product (Fig. 12.18). In fact, it is the difference in the rate... 

Fig. 12.17 Agostic dihydride intermediate derived from a dehydroamino ester substrate.

Scheme 22.2 Formal heterolytic hydrogen activation via deprotonation of a dihydride intermediate.

The enamide dihydride intermediate that precedes migratory insertion has proved elusive, despite one earlier claim where the evidence is incomplete and possibly not correctly interpreted [36]. Hydrogenation by rhodium complexes of... 

First, solvent molecules, referred to as S in the catalyst precursor, are displaced by the olefinic substrate to form a chelated Rh complex in which the olefinic bond and the amide carbonyl oxygen interact with the Rh(I) center (rate constant k ). Hydrogen then oxidatively adds to the metal, forming the Rh(III) dihydride intermediate (rate constant kj). This is the rate-limiting step under normal conditions. One hydride on the metal is then transferred to the coordinated olefinic bond to form a five-membered chelated alkyl-Rh(III) intermediate (rate constant k3). Finally, reductive elimination of the product from the complex (rate constant k4) completes the catalytic cycle. 

In the examples studied, neither the dihydride intermediates nor the alkyl intermediates have been observed and therefore it seems reasonable to assume that addition of H2 is also the rate-determining step. bimolecular reaction and the other ones are monomolecular rearrangement reactions one cannot say in absolute terms that oxidative addition is rate-determining. >... 

In this work, we have compared the potential energy profiles of the model catalytic cycle of olefin hydrogenation by the Wilkinson catalyst between the Halpern and the Brown mechanisms. The former is a well-accepted mechanism in which all the intermediates have trans phosphines, while in the latter, proposed very recently, phosphines are located cis to each other to reduce the steric repulsion between bulky olefin and phosphines. Our ab initio calculations on a sterically unhindered model catalytic cycle have shown that the profile for the Halpern mechanism is smooth without too stable intermediates and too high activation barrier. On the other hand, the key cis dihydride intermediate in the cis mechanism is electronically unstable and normally the sequence of elementary reactions would be broken. Possible sequences of reactions can be proposed from our calculation, if one assumes that steric effects of bulky olefin substituents prohibits some intermediates or reactions to be realized. 

Tris(triphenylphosphine)chlororhodium(I). The homogenous catalysis of the hydrogenation of ethylene and other olefins by RhCl( PPha) a, recently discovered by Wilkinson and co-workers (6J), probably involves a dihydride intermediate. A plausible mechanism for this reaction, involving steps of the type already described, is shown in Reaction 36. 

One proposal for the catalytic cycle involves an Ir(III)-dihydride intermediate that forms after OA of H2 onto an Ir(I)-alkene complex. Experimental results seem to support this cycle,36 but computational studies suggest that the cycle involves Ir(III) and Ir(V) intermediates.37 The details of neither proposal have been elucidated. Work continues to expand the scope of this reaction to include asymmetric hydrogenation of any unfunctionalized alkene that could yield a chiral alkane. 

The net reaction in equation (a) for some systems must certainly occur via an initial oxidative addition of Hj, followed by reductive elimination of HX for the dihydride intermediate the second step may again be base-assisted. Of the systems listed in Table 1, some Pt phosphine complexes give Pt (H)2 species via the equilibrium outlined in (j),L = PEta l... 

Similarly, there are examples of rhodium(I) and iridium(I) tertiary phosphine complexes that form isolable dihydrides, which with separate treatment with external base yield monohydrides, equation (k) . Hydrogenations catalyzed by rran5-RhCl(CO)(PPh3)2 " may involve rrani-RhH(CO)(PPh3)2 formed according to equation (1) via an undetected dihydride intermediate. In some aminophosphine analogues, a coordinated N atom may act as proton acceptor s. 

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