Hydrogen rate-limiting elimination

Ford and co-workers (7) have reported a first-order rate dependence on CO pressure in the Ru3(CO)-l2/KOH system and ascribed this effect to CO participation in a rate-limiting elimination of hydrogen from a cluster species. This explanation does not fit our observations, because if loss of H2 were rate-limiting, the use of KOH and NMe3 as bases would be expected to lead to comparable rates for the WGSR. A comparison of activities (Laine (9) 2.3 mol H2 per mol Ru3(CO) 2 Per using KOH/MeOH... 

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. 

When the attacking atom of the nucleophile carries a leaving group, replacement of hydrogen can take place via the VNS (vicarious nucleophilic substitution) reaction,15 as shown in Scheme 3.16 The key feature of this reaction is the rate-limiting base-induced 3-elimination from the cr-adduct. 

B is correct. The question stem presents a mechanism for an elimination reaction (the product gains a double bond) that relies on a rapid C—H bond dissociation as the rate-limiting step. When the heavier deuterium (D) is used instead of a pure hydrogen atom, the reaction rate decreases because of a stronger carbon—deuterium bond. 

First-order kinetics and the lack of dependence on [O2] rule out any mechanism that would require an initial bimolecular step, such as hydrogen atom abstraction or oxidative insertion. All of the observations can be accounted for by rate-limiting reductive elimination of PhCOOH, rapid addition of O2 to generate a side-on peroxo species, and protonation to yield the observed end-on hydroperoxo product (Scheme 8.2). 

The abbreviation El stands for Elimination, unimolecular. The mechanism is called unimolecular because the rate-limiting transition state involves a single molecule rather than a collision between two molecules. The slow step of an El reaction is the same as in the SN1 reaction unimolecular ionization to form a carbocation. In a fast second step, a base abstracts a proton from the carbon atom adjacent to the C+. The electrons that once formed the carbon-hydrogen bond now form a pi bond between two carbon atoms. The general mechanism for the El reaction is shown in Key Mechanism 6-8. 

In spite of the elimination of formic acid in a couple of steps changing the oxidation number of the rhodium metal center from -nl to -i-3 and vice versa, the reaction could take place by an alternative mechanistic pathway via cr-meta-thesis between the coordinated formate unit and the nonclassical bound hydrogen molecule [48, 49]. Initial rate measurements of a complex of the type 13 show that kinetic data are consistent with a mechanism involving a rate-limiting product formation by liberation of formic acid from an intermediate that is formed via two reversible reactions of the actual catalytically active species, first with CO2 and then with H2. The calculations provide a theoretical analysis of the full catalytic cycle of CO2 hydrogenation. From these results s-bond metathesis seems to be an alternative low-energy pathway to a classical oxidative addition/reductive elimination sequence for the reaction of the formate intermediate with dihydrogen [48 a]. 

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