Mechanism dissociative pathway

The non-cleavage pathway would remove most cohesin during prophase/ pro-metaphase by an as yet obscure mechanism. This pathway could involve phosphorylation of a cohesin subunit by mitotic protein kinases, because vertebrate cohesins rebind to chromatin in telophase when mitotic kinases are inactivated and chromosomes decondense (Losada et al 1998). The dissociation of cohesin from chromatin during prophase coincides with, but does not depend on, the association of condensin with chromosomes. This first phase of cohesin removal may be crucial (possibly along with the arrival of... 

The rate constant for the k term equals that for reaction of [Ca(parv)] with cydta, consistent with rate-determining dissociation of [Ca(parv)] in both cases the k2 term may be assigned to an associative (adjunctive) process (497). This mechanism parallels that of parallel associative and dissociative pathways established for displacement of edta from Ca(edta)2 by Ttr+ (cf. Section II.D.3 (334)). 

It was found that CO exchange in (diphosphine)Rh(CO)2H complexes proceeds via the dissociative pathway [60], The decay of the carbonyl resonances of the (diphosphine)Rh(13CO)2H complexes indeed followed simple first-order kinetics. The experiments with ligand 20 at different 12CO partial pressure show that the rate of CO displacement is independent of the CO pressure. Furthermore, the rate is also independent of the (diphosphine)Rh(13CO)2H complex concentration, as demonstrated by the experiments with ligand 18. It can therefore be concluded that CO dissociation for these complexes obeys a first-order rate-law and proceeds by a purely dissociative mechanism. 

Example More extensive substitution at the oxirane system brings additional dissociation pathways for the molecular ions. Nevertheless, one of the main reaction paths of molecular ions of glycidols gives rise to enol radical ions by loss of a aldehyde (R = H) or ketone molecule. [218] The reaction mechanism can be rationalized by the assumption of a distonic intermediate (Scheme 6.78) ... 

The mechanism of replacement of benzylideneacetone (PhCH=CHCOMe, bza) in [Fe(CO)3-(bza)] by diimines is determined by the nature of the incoming ligand, with bipy or diacetyldianil reacting by parallel associative and dissociative pathways, but 2-acetylpyridine anil reacting solely by a limiting dissociative mechanism. ... 

Schemes Mechanism of oxygenation of Pd(PPh3)4 proceeds by competitive associative and dissociative pathways...

A useful method to probe whether the reaction mechanism involves an associative or dissociative pathway is to measure AV (the volume of activation) for the reaction. High pressure kinetics in methanol give AV 1 —12 cm3 mol-1 for an associative first step, and +7.7 cm3 mol"1 for the isomerization reaction. It is proposed that the faster reaction is a solvolytic replacement of Cl" followed by a dissociative isomerization step with [PtR(MeOH)(PEt3)2]+ (R = alkyl, aryl equation 210).580 Since isomerization and substitution reactions are mechanistically intertwined, it is useful to note here that for the rates of substitution of both cis- and frara,-PtBr(2,4,6-Me3C6H2)(PEt3)2 by I" and thiourea, the volumes of activation are negative, in support of associative processes.581 Further support for associative solvation as the first step in the isomerization of aryl platinum(II) complexes has been presented,582 and the arguments in favor summarized.583... 

Catalyst cycle of Rh(I)-phosphine system. Most mechanistic studies on ligand-modified rhodium catalysts have been performed with HRh(CO)(PPh3)3. Extensive mechanistic studies have revealed that HRh(CO)2(PPh3)2 (18-electron species) is a key active catalyst species, which readily reacts with ethylene at 25°C [43]. Two mechanisms, an associative pathway and a dissociative pathway, were proposed [43-46], depending on the concentration of the catalyst. 

The first process involves electron ionization to form radical M-1"1 molecular ions. This process has been observed primarily for nonpolar molecules. The proposed mechanisms are charge-exchange transitions between sputtered ions and the neutral organic molecules or electron attachment of low-energy secondary electrons to neutral molecules. The fragmentation reactions of the M ions usually follow the dissociation pathways for odd-electron gas-phase ions. 

In this dissociative pathway (which is assumed to be the major one today) first a phosphine is displaced from the metal center to form an active 14-electron-intermediate 42. After alkene coordination cis to the alkylidene fragment the 16-electron-olefine-complex 43 undergoes [2 + 2]-cycloaddition to give a metallacylobutane 44. Compound 44 breaks down in a symmetric fashion to form carbene complex 45. The ethylene is replaced in the conversion to complex 46. In the next steps (they are not further discribed above), another intramolecular [2 + 2]-cycloaddition joins up the eight-membered ring 11 regenerating the catalyst 42. Each step of the reaction is thermodynamically controlled making the whole RCM reversible. With additional excess of phosphine added to the reaction mixture an associative mechanism is achieved, in which both phosphines remain bound. 

Stereochemical studies by Morandini and Consiglio and colleagues, on the reaction of Grignard reagents with the separate epimers of (tj5-C5H5)-[(R)-PROPHOS]RuC1 (34 and 35) support the mechanisms shown in Scheme 1 since they indicate that the formation of both alkyl and hydride products proceeds with retention of configuration at the ruthenium center [Eqs. (37) and (38)] (35). It should be noted, however, that dissociative pathways to the alkyl complexes via coordinatively unsaturated ruthenium... 

Figure 5.4.3 Molecular modeling simulation of different CO dissociation pathways on an iron (100) surface, along with relative energies in electron volt (1 eV 96 kJ/mol). The direct dissociation of CO (i.e. reaction [10] in the text), yields the lowest activation barrier, but the two-step mechanism in which CO first reacts with H to an HCO intermediate yields an overall barrier that is only slightly higher. Hence, under conditions where the CO is coadsorbed with H atoms, this route might compete with the direct dissociation (adapted from Elahifard et al. [20]).

Substitution reactions may take place by mechanisms that represent two ideal or limiting types. In the first of these processes, the leaving group departs before the entering ligand becomes attached. The rate of such a substitution process is limited only by the concentration of the starting complex. In this mechanism, the coordination number of the metal is reduced by one in forming the transition state. Such a mechanism is called a dissociative pathway, and the process can be illustrated by the equation... 

A process following the rate law shown in Eq. (20.65) is said to be an SN1 (substitution, nucleophilic, unimolecular) process. The term unimolecular refers to the fact that a single species is required to form the transition state. Because the rate of such a reaction depends on the rate of dissociation of the M-X bond, the mechanism is also known as a dissociative pathway. In aqueous solutions, the solvent is also a potential nucleophile, and it solvates the transition state. In fact, the activated complex in such cases would be indistinguishable from the aqua complex [ML H20] in which a molecule of H20 actually completes the coordination sphere of the metal ion after X leaves. This situation is represented by the dotted curve in Figure 20.1 where the aqua complex is an intermediate that has lower energy than [ML,]. The species [ML H20] is called an intermediate because it has a lower energy than that of the activated complex, [MLJ. 

It thus appears that substitution of OH- in the cobalt complex follows a different mechanism than the dissociative pathway usually observed for substitution reactions of such complexes. Therefore, either OH- behaves differently than other nucleophiles or another way of explaining the observed rate law must be sought. 

The values for the rate constant 0 are nearly the same for the mixed complexes when L = picolinate, sulfosalicylate and 8-oxyquinoline-5-sulfonate indicating and hence it appears that they fit Model 1 (i.e.) Diebler-Eigen mechanism involving a dissociative pathway. This is true in the case of lighter lanthanides. In the case of heavier lanthanides,... 

To illustrate the approach, let us consider some of the data and deductions for the system Co[CH3COCHCOCH(CH3)2]3, measured in C6H5C1. It was found that both the isomerization and the racemization are intramolecular processes, which occur at approximately the same rate and with activation energies that are identical within experimental error. It thus appears likely that the two processes have the same transition state. This excludes a twist mechanism as the principal pathway for racemization. Moreover, it was found that isomerization occurs mainly with inversion of configuration. This imposes a considerable restriction on the acceptable pathways. Detailed consideration of the stereochemical consequences of the various dissociative pathways, and combinations thereof, leads to the conclusion that for this system the major pathway is through a tbp intermediate with the dangling ligand in an axial position as in Fig. l-12(c). 

This pathway is energetically favored by the gain in aromaticity of the six-membered ring in the -intermediate, a stabilization not available to Cp. It should be noted, however, that this indenyl effect is specific to an associative mechanism substitution reactions of RuCl(rf-ind)(PPh3)2, which proceed via a dissociative pathway, are only one order of magnitude faster than those of RuCl(Cp)(PPh3)2.6... 

Saddle-node bifurcations taking place for the reasons just described have been observed for HOBr [41], HOCl [36,38,39], and HCP [34-36]. For HOBr and HOCl, the stable PO bom at the saddle-node bifurcations is called [D] for dissociation, because this PO stretches along the dissociation pathway and scars OBr- or OCl-stretch quantum mechanical wavefunctions (see Fig. lie of Ref. 38, Figs. 3b and 3g of Ref. 41, or Section III.B). In the case of HCP, the stable PO born at the bifurcation is better called [I], for isomerization, because this PO stretches along the isomerization pathway and scars bending quantum mechanical wavefunctions (see Figs. 6b and 6d of Ref. 35 or Figs. 7b and 7d of Ref. 36). 

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