Hydrides mechanisms

Another difference between the two mechanisms is that the former involves 1,2 and the latter 1,3 shifts. The isomerization of 1-butene by rhodium(I) is an example of a reaction that takes place by the metal hydride mechanism, while an example of the TT-allyl complex mechanism is found in the Fe3(CO)i2 catalyzed isomerization of 3-ethyl-l-pentene. " A palladium acetate or palladium complex catalyst was used to convert alkynones RCOCSCCH2CH2R to 2,4-alkadien-l-ones RCOCH= CHCH = CHCHR. ... 

The scope and limitations for transfer hydrogenation employing either the iron porphyrin system or the combination of iron compound/terpy/PPhs are listed in Table 8. In most cases, the FeCVterpy/PPhs system displays a higher activity. Except for chloromethyl- and cyclopropyl-acetophenone, the desired products were obtained in good to excellent yields. It should be noted that a ring opened product was not observed when cyclopropyl acetophenone was employed. Hence, a radical-type reduction pathway was excluded and a hydride mechanism appeared to be reasonable. 

Closer examination of the mechanism for the Rh catalyzed carbonylation of ethylene provides a rationale for the poor selectivity. The mechanism for the carbonylation of ethylene (Scheme 37.1) is well known (6) and proceeds via two simultaneously operating mechanisms which generate a common EtRh(CO)2l2 intermediate which rapidly reacts with iodide (Eqn. 10) to generate EtRh(CO)2l3 . The first, and predominant, mechanism is a hydride mechanism (Eqns. 6-8 below) in which the proton required for the formation of HRh(CO)2l2 and initiation of the... 

However, if we consider the alternative nucleophilic displacement, it is known that nucleophilic processes are accelerated by ionic liquids, but more pertinent is the fact that the Sn2 displacement of iodide from alkyl iodide (Mel) by Rh(CO)2l2 is slightly accelerated by ionic liquids (7). Unfortunately, ionic liquids would also be expected to accelerate the nucleophilic displacement of iodide from ethyl iodide by propionic acid to form ethyl propionate (Reaction 8). In fact, as an Sn2 Type II displacement (the interaction of two neutral species), the ester formation from propionic acid and ethyl iodide would be expected to be significantly increased compared to the reaction of Rh(CO)2l2 with EtI. Therefore, by operating in iodide containing ionic liquids, we had set up a situation in which we suppressed the normally predominant hydride mechanism, slightly accelerated the alternative nucleophilic mechanism, but dramatically increased the ethyl propionate by-product forming pathway. 

While the mechanism in the absence of Eti or HI is still a matter of conjecture, it is unlikely that a hydride mechanism was operable since, whereas we could possibly envision an imidazolium salt donating a hydrogen via carbene formation, there is no corresponding viable source of hydride when using pyridinium and phosphonium salts which are also effective solvents for the process. Therefore, by process of elimination, it was more likely that the process was operating via a nucleophilic process. 

Feldstein and Lancsek [30] measured plating rate, potential, and hydrogen evolution rate during the reduction of Ni2 + and Co2 + with H2P02 in the presence of various additives. They concluded that the deposition process could be described by a modified hydride mechanism. The basic steps of the process were identified as follows ... 

The conjugated diene (including the trans-trans, trans-cis, and cis-cis isomers) can further add ethylene to form Cg olefins or even higher olefins (/). The mechanism of isomerization is proposed to be analogous to butene isomerization reactions (4, 8), i.e., 1-butene to 2-butene, which involves hydrogen shifts via the metal hydride mechanism. A plot of the rate of formation of 2,4-hexadiene vs. butadiene conversion is shown in Fig. 2. 

In the direct transfer mechanism, the metal ion coordinates both reactants enabling an intramolecular reaction, and activates them via polarization. Consequently, strong Lewis acids including Alln and the Lnln ions are the most suitable catalysts in this type of reactions. In the hydride mechanism, a hydride is transferred from a donor molecule to the metal of the catalyst, hence forming a metal hydride. Subsequently, the hydride is transferred from the metal to the acceptor molecule. Metals that have a high affinity for hydrides, such as Ru, Rh and Ir, are therefore the catalysts of choice. The Lewis acidity of these metals is too weak to catalyze a direct hydride transfer and, vice versa, the affinity of Alm and Lnm to hydride-ions is too low to catalyze the indirect hydrogen transfer. Two distinct pathways are possible for the hydride mechanism one in which the catalyst takes up two hydrides from the donor molecule and another in which the catalyst facilitates the transfer of a single hydride. 

The selectivity of the hydrogen transfer is excellent When employing a catalyst with deuterium at the a-positions of the isopropoxide ligands (17), complete retention of the deuterium was observed. A computational study using the density functional theory comparing the six-membered transition state (as in Scheme 20.3, the direct transfer mechanism) with the hydride mechanism (Scheme 20.3, the hydride mechanism) supported the experimental results obtained [36]. A similar mechanism has been proposed for the MPV alkynylations [37] and cyanations [38]. 

Shift From the Hydride Mechanism to the Carbomethoxy Mechanism and Vice Versa... 

As already mentioned, Pd - H+ generated at the methanolysis step may transform into Pd-OCH3+ by the action of a quinone. Vice versa, Pd-OCH3+ generated in the protonolysis step may undergo - H elimination with formation of Pd - H+, as it occurs for the hydroacylation of ethene to DEK [33]. Direct shift from the hydride mechanism to the carbomethoxy mechanism has been demonstrated by Iggo et al. [58] (Sect. 3.3.2). 

Fig. 6 The carbomethoxy cycle (a), the hydride cycle (b) and the shift of the hydride mechanism to the other (c) (S = CH3CN). Adapted from [58]...

It is interesting to point out that with this catalyst formation of MP occurs also through the carbomethoxy cycle, whereas it will be shown that most catalysts that are highly selective to MP operate through the hydride mechanism (see Fig. 6). 

In the alkoxycarbonylation, the hydride mechanism initiates through the olefin insertion into a Pd - H bond, followed by the insertion of CO into the resulting Pd-alkyl bond with formation of an acyl intermediate, which undergoes nucleophilic attack of the alkanol to give the ester and the Pd - H+ species, which initiates the next catalytic cycle [35,40,57,118]. Alternatively, it has been proposed that a ketene intermediate forms from the acyl complex via /3-hydride elimination, followed by rapid addition of the alcohol [119]. In principle the alkyl intermediate may form also by protonation of the olefin coordinated to a Pd(0) complex [120,121]. 

Although the isolation and reactivity of acyl complexes strongly support the hydride mechanism, the other mechanism cannot be excluded. For example H20, the acid or molecular hydrogen, which can act as a hydride source, can promote the Pd - C splitting of the Pd-alkylcarboalkoxy intermediate in the alkoxy cycle as well. More convincing for the hydride route is the fact that the acid, which does not promote the formation of a Pd-OCH3+ species, has a promoting effect on the catalysis and can activate a Pd(0) complex, otherwise inactive, whilst a base, which not only promotes the formation of this species, but also deprotonates a Pd - H+ species to Pd(0), suppresses the catalysis. 

Fig. 9 Proposed hydride mechanism for the formation of d° 5 -MP. Adapted from [134]...

Because of its nature, DEK must form via a hydride mechanism. Up to the formation of a Pd-acyl intermediate, the paths leading to MP or DEK are similar. DEK forms if the insertion of a second molecule of ethene into the Pd-acyl bond is followed by protonolysis of the Pd - C bond of the resulting Pd-alkylacyl intermediate. 

Figure 4.28. Simplified scheme for the hydride mechanism for hydrogen transfer...

The classical Hoijtink mechanism and the dianion mechanism have been observed at electrodes with a high hydrogen overvoltage, such as mercury. If mercury is replaced by platinum with its low hydrogen overvoltage, a radical pathway seems to be favored [199], which is closely related to catalytic hydrogenations of hydrocarbons. Spectroelectrochemical experiments provided evidence for an additional hydride mechanism (Eqs. 25-27) [200]. ... 

Correct At least three mechanisms should be taken into consideration carbanion mechanism, hydride mechanism, and addition-elimination mechanism. 

As the solubility is exceeded in the zirconium, precipitation of zirconium hydride commences and ultimately the ductility of the alloy can be reduced leading to the possibility of cracks. Thus an additional requirement in fuel development was a thorough understanding of the hydriding mechanism, sources of hydrogen, rate controlling steps, protective methods, specifications of materials and processes, and quality assurance to achieve the required performance. ( 3)... 

The mechanism of 1,4-dihydropyridine reductions is actively being pursued (B-78MI20702). A mechanism involving hydride transfer is attractive because of its simplicity (80JA4198). However, many workers in the area prefer an electron transfer as the first step (79JA7402). The hydride mechanism can be completed by the transfer of a proton followed by an electron or by the transfer of a hydrogen atom (Scheme 29). It is unlikely that the mechanistic question will be resolved in the near future. It may be that the mechanistic pathway that these reactions follow is very sensitive to both the structure of the dihydropyridine and the compound being reduced. 

The metal hydride mechanism was first described for the cobalt-carbonyl-catalyzed ester formation by analogy with hydroformylation.152 It was later adapted to carboxylation processes catalyzed by palladium136 153 154 and platinum complexes.137 As in the hydroformylation mechanism, the olefin inserts itself into the... 

In general, any of several possibile mechanisms may be operative for complex reactions. For example, in the oxidation of isopropanol by RuIV the key redox step could involve initial outer-sphere electron transfer, initial H-atom transfer, or even two-electron hydride transfer. The hydride mechanism, which has been proposed to be the actual low-energy pathway in water at 25 °C, is illustrated in reaction (3).2... 

The metal hydride mechanism has been written particularly for hydrocarboxylation reactions with a palladium catalyst.67,68 In the reactions of propene in the presence of (PhaP dCk, the acyl complex (18) was isolated from the reaction mixture, and also shown to be a catalyst for the reaction. 

Both mechanisms are predicted to show syn addition of hydride and caiboxylate to the alkene. In the metal hydride mechanism (equation 36) alkene insertion is syn and CO insertion proceeds with retention of configuration at carbon. In the metal carboxylate mechanisms (equation 37) alkene insertion is syn and cleavage of the metal-carbon bond can take place with retention at carbon. The palladium-catalyzed hydroesterification reaction produces the erythro ester from (Z)-3-methyl-2-pentene (equation 38) and the threo ester from ( )-3-methyl-2-pentene (equation 39).w... 

To compare these two mechanisms, an NADH model without the recognition site was synthesised. The contribution of the flavin binding to the rate constant was thus evaluated and it was shown that the proximity of flavin and NADH model influenced the electron transfer rate. Mechanistic computations helped to show that with the appropriate NADH model system, both components were optimally arranged for the electron transfer. Although the exact mechanism of the reaction is still under debate, the kinetic isotope effect experiment indicated that in this case, the hydrogen at 4-position was transferred in the rate determining step which supported the hydride mechanism. 

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