Dissociative intermediate

Solvent paths and dissociate intermediates in substitution reactions of square planar complexes. R. J. Mureinik, Coord. Chem. Rev., 1978, 25,1-30 (133). 

Despite all the uncertainties in this calculation, it looks like sulfate diester hydrolysis should be stepwise or very close to it, because the dissociative intermediate 12 is 35 kcal/mol higher in energy than the associative intermediate 11. 

Low hydrogen pressures are favorable for the first two reactions with deeply dissociated intermediates. Hydrogen determines here the direction of the overall reactions, that is, the ratio of aromatic and coke formation. 

Effects of dissociative intermediate states on MP excita- 377 tion explored using as an example the MP excitation of... 

The formation of ion pairs in solvolysis reactions is now well documented75. The separation of the charged species during heteroly tic bond cleavage is not a continuous process but proceeds through a series of progressively more dissociated intermediates an intimate or internal ion pair (51), a solvent separated or external ion pair (52), and the dissociated carbocation and counterion (53). Attack by a nucleo-... 

An electron-transfer reduction of RX can he dissociative (no intermediate anion-radical RX ) or noil-dissociative (intermediate RX ). Where the noil-dissociative pathway is well documented (e.g.. aryl halides), only two reactions of the intermediate RX- arc generally posited, dissociation and electron-transfer oxidation [I7.18. However, product evidence suggests that R-X may undergo other reactions, including reduction. 

With respect to mechanism of action, the most extensive kinetic and equilibrium exchange studies have been carried out on monofunctional 10-formyl-H4-folate synthetase from Cl. cylindrosporum [84]. The data support a random sequential mechanism that does not involve the formation of freely dissociable intermediates. The most likely mechanism, however, is not concerted but probably involves the formation of a formyl phosphate intermediate, since the synthetase catalyzes phosphate transfer from carbamyl phosphate but not acetyl phosphate to ADP with H 4-folate serving as an activator. Carbamyl phosphate is an inhibitor of 10-formyl-H 4-folate synthesis - an inhibition that can be eliminated only when both ATP and formate are present in accord with the concept that it spans both sites [85]. It would be of considerable interest to attempt to demonstrate positional isotope exchange employing [, y- 0]ATP for this enzyme in order to further implicate an enzyme-bound formyl phosphate species [86]. 

Fig. 6.2.2 Reaction profile for the ructions (a) c/5 -[CoCl2(en)2] trans-[CoCl2(en)2] and (b) cw-[CoCl2(en)2] Cl trans-[CoCl2(Qn)2V + Cl". These values have been calculated from the other energies which have been measured independently. In the region of these transition states there must be at least one dissociative intermediate and one other transition state.

It may be significant that, except for the cases of catalytic inducement, the best authenticated examples of the fully developed dissociative intermediate are for anionic complexes, such as [Co(CN)5(H20)] . Here the water molecule dipole will normally be directed unfavourably for coordination and becomes more part of the solvent continuum than an integral part of the complex. Thus the retardation of entry, leading to the full formation of the five-coordinated intermediate, is probably the result of the strong solvent coherence even in the solvation sphere of the complex rather than something inherently different in the electronic properties of the complex itself. 

The rate of substitution varies over five orders of magnitude with the nature of the leaving group X, as shown in Table 17.3. The weaker the Co-X bond, the faster the reaction. These results support a dissociative intermediate, as the loss of X occurs in the RDS of the D mechanism. 

The ligand field activation energy (LFAE) is defined as the difference in the LFSE for the TS minus that of the reactants. Assuming a square pyramidal dissociative intermediate, the LFAEs for the different d" electron configurations are listed in Table 17.5. 

The products suggest activation as a phosphoryl rather than as a nucleotidyl derivative. Both succinyl phosphate and thiophosphoryl coenzyme A have been suggested as intermediates. However, neither is included, at least as a freely dissociable intermediate, in current formulations of this reaction. An enzyme-bound phosphoryl histidine intermediate is thought to be involved, as is some sort of activated enzyme-CoA complex. Many aspects of the enzyme mechanism are still in doubt, but the sequence below is consistent with most available data. 

Rossi and Hoffmann suggested that poor or non n acceptors prefer the equatorial site in a d square pyramidal dissociative intermediate. The same conclusion was reached by Lichtenberger and Brown for Mn(CO)jX systems. This prediction has been used in stereoselective labeling woiit. 

The higher A// observed when L is P(OMe)3 is attributed to a stabilizing effect caused by intramolecular hydrogen bonding, N-H 0-P. (Intermolecular hydrogen accelerates the reaction.) Experiments with CO demonstrate that the [Mo(CO)4P(OMe)3] unit retains its geometry during the reaction, from which it is inferred that the dissociative intermediate is square pyramidal. 

For catalytic reactions, the BEP relation is an essential element. A low activation energy benefits the efficiency of fhe reaction however, the accompanying high stability of the dissociated reaction prevents subsequent steps of the reaction (i.e., surface diffusion, bond formation, and desorption). In other words, a good catalyst has a low activation energy and weak bonding of the dissociated intermediates. 

The mechanism is considered to involve the reaction of hydroxylated and non-dissociated intermediates (Scheme 2), the observed rate law for forma-... 

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