Dissociation isomerism mechanism

To distinguish between simple, reversible slow binding (scheme B) and an enzyme isomerization mechanism (scheme C), one can examine the dependence of kobs on inhibitor concentration. If the slow onset of inhibition merely reflects inherently slow binding and/or dissociation, then the term kobs in Equations (6.1) and (6.2) will depend only on the association and dissociation rate constants k3 and k4 as follows ... 

More recently, a new mode of cis-trans isomerization of a disilene has been suggested for the extremely hindered disilene 27. As will be detailed in Section VIII. B, 27 undergoes thermal dissociation into the corresponding silylenes. Monitoring the thermolysis of (Z)-27 at 50°C by H and 29Si NMR reveals a competitive formation of the isomerized ( >27 and benzosilacyclobutene 37, which is most likely formed by intramolecular insertion of silylene 36 into the C—H bond of the o-bis(trimethylsilyl)-methyl group (Scheme 3).22,59 This suggests the possible occurrence of cis-trans isomerization via a dissociation-association mechanism. 

The energy minima between the energy barriers for the monomer coordination and insertion correspond to alkene-bound intermediates of the kind simulated by our molecular mechanics calculations (Figures 1.7 and 1.9). The possible dissociation of the monomer coordinated with the wrong enantioface can lead back to the alkene-free intermediate or, directly, to the alkene-bound intermediate with the right enantioface (through some isomerization mechanism, for which the monomer does not leave the coordination sphere of the metal). 

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... 

By analogy with thermal processes (92), photoinduced isomerization s may occur either by intramolecular or intermolecular mechanisms. Intramolecular isomerizations are further classified either as "twisting" mechanisms, which involve no metal-ligand bond breaking, or as bond-rupture mechanisms. The latter term applies to reactions of chelate compounds in which one of the ligands is for a short time partially dissociated. Intermolecular mechanisms involve species other than those that make up the reactant complex. 

Based on these results, intervention of pseudorotation is not excluded but it represents a higher energy process. A similar conclusion was reached in the isomerization of related pentacoordinate tin compounds 227 and 228 (Figure 45) (359). The intramolecular mobility of these compounds was shown to involve a dissociation-inversion mechanism with a free energy of activation, AC = 15 kcal/mol. 

The thermodynamics and kinetics associated with Scheme 8 have been partially quantified. At 25°C, AH for the isomerization of/ac-[MnCl(CO)3-(dppm)] + to the mer cation is 56 ( 4) kJ mol 1, and the rate of the reaction is 2.4 ( 0.2) sec-1 (138) -, the solvent independence of the rate implies a non-dissociative twist mechanism. For the neutral species, mer-[MnX(CO)3-(L-L)] does not convert to the fac isomer, and the complexes of mono-dentate ligands isomerize only over extended periods (10), probably by a dissociative process. 

Spectroscopic and kinetic studies give considerable insight into the sequence and rates of the individual steps of the isomerization mechanism. Ni[P(OE03]4 itself does not dissociate to the NiL3 complex to a detectable extent, even at 70°. Using the very sensitive vis/uv method, is estimated to be less than at 70°, and must be considerably less than... 

Chemical kinetics studies the rate of chemical reactions and factors that influence them. A chemical process can be divided into a sequence of one or more elementary reactions that typically involve collisions between two molecules (bimolecular reaction) or dissociation/isomerization of a reagent molecule (uni-molecular reaction). A trimolecular reaction, which is the collision of three reactant molecules, occurs less frequently. Usually, the mechanisms are described in a single global step, but in reahty they occur in a series of elanentary steps [1]. 

A previously reported process has been generalized to provide a flexible and directed synthesis of azoxyalkanes (Scheme 130). The Z-E thermal isomerization of aliphatic azodioxy-compounds (dimeric nitroso-alkanes) has been shown to occur exclusively by a dissociation-recombination mechanism. 

In this chapter we describe the estimation and interpretation of rate coefficients for thermal dissociation, isomerization, and recombination reactions. While such reactions are only a small fraction of the elementary reactions of combustion mechanisms, some of them do play essential roles. Their kinetic behavior is governed by the competition of unimolecular chemical changes in molecular structure with bimolecular physical collisional energization and deenergization processes. 

K has been identified as CFl200I-I from its chemistry the reaction mechanism is insertion [115], Collision-induced dissociation (in a SIFT apparatus, a triple-quadnipole apparatus, a guided-ion beam apparatus, an ICR or a beam-gas collision apparatus) may be used to detemiine ligand-bond energies, isomeric fomis of ions and gas-phase acidities. 

The best understood compounds are cis- and fra s-RuX2(DMSO)4 (X = Cl, Br). The fra s-isomers are thermodynamically less stable and isomerize in DMSO solution to the m-isomer, with first-order kinetics, probably via a dissociative mechanism. The reverse process, cis to trans, is catalysed by light. Syntheses for these and other DMSO complexes are shown in Figure 1.37 [108],... 

The evidence presented so far excludes the formation of dissociated ions as the principal precursor to sulfone, since such a mechanism would yield a mixture of two isomeric sulfones. Similarly, in the case of optically active ester a racemic product should be formed. The observed data are consistent with either an ion-pair mechanism or a more concerted cyclic intramolecular mechanism involving little change between the polarity of the ground state and transition state. Support for the second alternative was found from measurements of the substituent and solvent effects on the rate of reaction. 

Baechler and coworkers204, have also studied the kinetics of the thermal isomerization of allylic sulfoxides and suggested a dissociative free radical mechanism. This process, depicted in equation 58, would account for the positive activation entropy, dramatic rate acceleration upon substitution at the a-allylic position, and relative insensitivity to changes in solvent polarity. Such a homolytic dissociative recombination process is also compatible with a similar study by Kwart and Benko204b employing heavy-atom kinetic isotope effects. 

---