1.2-Shift/1,1 -elimination mechanism

Wu et al. [78] found no direct path on the potential surface from the adsorbed state to the transition state. The structure of the transition state (Fig. 3) is reminiscent of a dihydride. Indeed, the steepest descent path from this transition state led back to a dihydride local minimum. This dihydride is weakly stable, but is connected by a direct path to the doubly occupied dimer. This isomerization/desorption path, from the doubly occupied dimer to the dihydride and then to the transition state, is the 1,2-shift/1,1-elimination mechanism previously suggested by Nachtigall et al. [75] on the basis... 

The two mechanisms may result in substantial and characteristic differences in deuterium distribution. The metal hydride addition-elimination mechanism usually leads to a complex mixture of labeled isomers.195 198 208-210 Hydride exchange between the catalyst and the solvent may further complicate deuterium distribution. Simple repeated intramolecular 1,3 shifts, in contrast, result in deuterium scram-bling in allylic positions when the ir-allyl mechanism is operative. ... 

Depending on the nature of the catalyst, the transition-metal-catalysed isomerisation will proceed by an addition-elimination mechanism or by the formation of a it allyl complex followed by a 1,3-hydrogen shift (Scheme 2.7b). The equilibrium of the isomerisation lies strongly in favour of the propenyl ether because of resonance stabilisation between the oxygen lone pair and the it orbital of the double bond. Isomerisation can also be performed under strongly basic conditions by using potassium f-butoxide in dimethylsulfoxide (DMSO).18... 

The fundamental differences between these two mechanisms are that 1) the jr-allyl metal hydride mechanism involves a 1,3-hydrogen shift while the metal hydride addition-elimination mechanism involves a 1,2-hydrogen shift and 2) the hydrogen shift in the Jt-allylhydride mechanism proceeds in an intramolecular fashion while that in the metalhydride addition-elimination mechanism proceeds in an intermolecular fashion. 

The crossover product, propionaldehyde-l,3-d-3- C 12, clearly demonstrated that the isomerization occurred via intermolecular 1,3-hydrogen shift. These results are consistent with a modified metal hydride addition-elimination mechanism which involves exclusive 1,3-hydrogen shift through oxygen-directed Markovnikov addition of the metal hydride to the carbon-carbon double bond (Scheme 12.2). The directing effect of functional groups on the selectivity of transition metal catalysis is well presented [9], and an analogous process appears to be operative in the isomerization of allylamines to enamines [10]. 

Since the expulsion of the leaving group is largely dependent on the mesomeric effect of the nucleophilic moiety incorporated in the molecule, one might expect that the use of thio nucleophiles should shift the mechanism to the addition-elimination mode. Indeed, when the reaction of 113 with thiophenoxide was performed in dimethoxyethane, the addition-elimination product 117 was exclusively obtained (equation 161). However, in methanol the ionic bicyclobutane mechanism was found to be operative in spite of the weaker mesomeric effect of the thio group (equation 162). ... 

The polarity of the medium has a drastic effect on the mechanism of the reaction. Variations of the polarity obtained by adding methanol to DME, had no effect on the reaction course up to ca. 2.5 M of MeOH. The fraction of the elimination reaction then rises steeply, from 0.1 at 3 M to 0.5 at 6 M of MeOH. Experiments with other solvent systems indicated that the shift in mechanism occurs at Ej(30) values around 47... 

These predictions are consistent with the experimental value of 58 kcal/ mol for the activation barrier to desorption, but it permits only a small barrier for the reverse adsorption reaction. Nachtigall et al. [75] pointed out that if there is a substantial activation barrier to adsorption, these calculations could not be consistent with desorption from a prepaired state. They suggested that a more complex, multistep mechanism might be responsible for desorption. For example, they drew an analogy with gas-phase elimination of H2 from disilane, in which the 1,1-elimination mechanism has a lower activation barrier than the 1,2-elimination [75]. They suggested that desorption may occur by a 1,2-hydrogen shift (to form a dihydride) followed by 1,1-elimination (desorption of the dihydride). This speculation was supported... 

Toste and coworkers [165] have implicated a bimetallic reductive elimination mechanism in the oxidative heteroarylation of olefins. Electrochemical and stereochemical studies were made of mono-and di-gold(l) precatalysts binuclear gold species had superior catalytic properties. Electrochemical markers of Au(l)---Au(l) aurophilic interactions were discovered, including a 140-mV cathodic shift in the (irreversible) oxidation potential of di-gold species relative to mononuclear gold(I) analogs. 

The other paths are initiated by a bimolecular interaction of the excited state of the reagent with the nucleophile. The S 2 Ar reaction (path c) matches the usual addition-elimination mechanism of the thermal 8 2 Ar reaction, with the difference that is the electron distribution in the excited state, singlet or triplet according to the case, that now governs the process and thus dictates the orientation rules. When the nucleophile is a good donor, this process shifts toward electron transfer (path d), which may lead to the same type of products, but possibly with a different regiochemistry, since here is the electron distribution in the radical anion, not in the excited state, that matters. 

The following mechanisms in corrosion behavior have been affected by implantation and have been reviewed (119) (/) expansion of the passive range of potential, (2) enhancement of resistance to localized breakdown of passive film, (J) formation of amorphous surface alloy to eliminate grain boundaries and stabilize an amorphous passive film, (4) shift open circuit (corrosion) potential into passive range of potential, (5) reduce/eliminate attack at second-phase particles, and (6) inhibit cathodic kinetics. 

In contrast to acetals, which are base-stable, hemiacetals undergo base-catalyzed hydrolysis. In the alkaline pH range, the mechanism shifts toward a base-catalyzed elimination. 

Conversion of Amides into Carboxylic Acids Hydrolysis Amides undergo hydrolysis to yield carboxylic acids plus ammonia or an amine on heating in either aqueous acid or aqueous base. The conditions required for amide hydrolysis are more severe than those required for the hydrolysis of add chlorides or esters but the mechanisms are similar. Acidic hydrolysis reaction occurs by nucleophilic addition of water to the protonated amide, followed by transfer of a proton from oxygen to nitrogen to make the nitrogen a better leaving group and subsequent elimination. The steps are reversible, with the equilibrium shifted toward product by protonation of NH3 in the final step. 

The basic advantages of this process are (a) elimination of a mechanical device (recycle gas compressor) for controlling the adiabatic temperature rise, (b) combination of CO shift with methanation, (c) significant increase in byproduct steam recovery, and (d) significant capital advantages. 

The propagation centers also react with the inhibitors inevitably present in the reaction medium. The interaction with coordination inhibitors may stabilize the transition metal-carbon bond, as the elimination of the coordinative insufficiency of the transition metal ion makes it impossible for the metal-carbon bond to rupture through the mechanism of the /3-hydride shift. 

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