Proton concerted mechanism

Figure 13.12 A concerted mechanism for GTP hydrolysis by Ga in which transfer of a proton to the y phosphate is coupled to deprotonation of the attacking water by Gin 200. (Adapted from J. Sondek et al., Nature 372 276-279, 1994.)...

Fig. 8.1. Representation of transition states for the first stage of acetal hydrolysis, (a) Initial C—O bond breaking (b) concerted mechanism with C—O bond breaking leading O—H bond formation (c) concerted mechanism with proton transfer leading C—O bond breaking (d) initial proton transfer.

Fig. 8.2. Contour plot showing a favOTed concerted mechanism for the first step in acetal hydrolysis, in which proton transfer is more complete in the transition state than C—O bond breaking.

Finally, it should be noted that at alkaline and neutral pH a concerted mechanism, involving /S-carbon protonation of the enamine and a simultaneous addition of a water molecule, leading to the amino alcohol, can be rejeeted, since the immonium ion appeared to be a real and deteetable intermediate. 

More recently Hand et al. (ref. 9) have studied the decomposition reaction of N-chloro-a-amino acid anions in neutral aqueous solution, where the main reaction products are carbon dioxide, chloride ion and imines (which hydrolyze rapidly to amine and carbonyl products). They found that the reaction rate constant of decarboxylation was independent of pH, so they ruled out a proton assisted decarboxylation mechanism, and the one proposed consists of a concerted decarboxylation. For N-bromoamino acids decomposition in the pH interval 9-11 a similar concerted mechanism was proposed by Antelo et al. (ref. 10), where the formation of a nitrenium ion (ref. 11) can be ruled out because it is not consistent with the experimental results. Antelo et al. have also established that when the decomposition reaction takes place at pH < 9, the disproportionation reaction of the N-Br-amino acid becomes important, and the decomposition goes through the N,N-dibromoamino acid. This reaction is also important for N-chloroamino compounds but at more acidic pH values, because the disproportionation reaction... 

Figure 7-4. Excess coordination number plots for (a) MEP (three-water-bridge) [27] and (b) PMF (H64 in ) [14] simulations for the proton transfer in CAII. Note that the MEP calculation follows a very concerted mechanism while PMF simulation follows a step-wise proton hole mechanism...

Scheme 20.13 Concerted hydride-proton transfer mechanism.

The dehydration of neopentyl alcohol can best be explained by a concerted mechanism involving the removal of the proton from the y-carbon atom by the basic sites and of the hydroxyl group by the acid sites of the alumina, with migration of the methyl group ... 

Results of a PEPICO study of the dissociation dynamics of 2-bromobutane ions have been analysed with tunnelling-corrected RRKM statistical theory using vibrational frequencies obtained from ab initio MO calculations. It has been concluded that the slow rate of loss of HBr, to form the but-2-ene ion, occurs via a concerted mechanism in which tunnelling is a feature of the proton transfer. 

Stead of the concerted mechanism, in which the attacking or the leaving step is a rate-limiting that cover the other proton transfer effect, if occurred, in the faster step and Gs works as a general acid or base catalyst and an unidentified catalyst works in the other side. Although further investigation seems to be needed to elucidate the detailed chemistry, it is likely that, at least, Gs works as a catalyst in cleavage chemistry [121]. 

Another important family of elimination reactions has as the common mechanistic feature cyclic transition states in which an intramolecular proton transfer accompanies elimination to form a new carbon-carbon double bond. Scheme 6.16 depicts examples of the most important of these reaction types. These reactions are thermally activated unimolecular reactions that normally do not involve acidic or basic catalysts. There is, however, a wide variation in the temperature at which elimination proceeds at a convenient rate. The cyclic transition states dictate that elimination occurs with syn stereochemistry. At least in a formal sense, all the reactions can proceed by a concerted mechanism. The reactions, as a group, are referred to as thermal syn eliminations. 

Stereochemical considerations in the E2 reactions The E2 follows a concerted mechanism, where removal of the proton and formation of the double bond occur at the same time. The partial 7t bond in the transition state requires the parallel alignment or coplanar arrangement of the p orbitals. When the hydrogen and leaving group eclipse each other (0°), this is known as the 5 yn-coplanar conformation. 

A scheme for the imidazole catalysis of the mutarotation of glucose, similar to the concerted mechanism proposed by Swain and Brown (13), is shown below, in which a proton is transferred from the D-glucose to imidazole or benzimidazole and from the H20 (represented as an acid) to the D-glucose in the rate-determining step. 

Thermal decomposition of 2-azidotropone (8) gives a ketene (9) as primary product which cyclizes to o-salicylonitrile or reacts with protonic solvents to give the corresponding carboxylic add derivatives.1 167 The reaction occurs at a quite moderate temperature (60-80°) and is therefore best explained by a concerted mechanism. 

A fully concerted mechanism for reaction 299 has been eliminated as inconsistent with 14C and 15N KIEs and also with the observed inverse solvent D2O effect. The reaction path for the deamination of AMP has been formulated613 as a stepwise conversion involving the formation of tetrahedral intermediate 515 characterized by full-bonded hydroxyl and amino groups (equation 300). The TS for slow formation of 515, resulting from the attack of the hydroxyl from enzyme zinc-activated water at the C(6), is characterized by the C(6) OH bond order of 0.8 0.1 (late TS) and fully bonded NH2, that is by the nearly complete conversion to sp3 at C(6), and by nearly complete protonation of Nq), 516, The protonation of NH2 (in 515) and departure of NH3 (with TS 517) take place in the subsequent rapid steps as shown in equation 300, Zinc hydroxide is formed prior to attack514 at C(6). Enzymatic degradation of [6-14C]AMP has been carried out to prove the position of the radiolabel in 513 (equation 301). No radioactivity in the allantoin... 

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