Methyl-transfer reactions Marcus theory

However, even for the simple methyl transfer reactions, there is considerable confusion and some disagreement about the details of the mechanism. Some authors (Sneen, 1973) have suggested that ionization of RX always precedes attack by the nucleophile, while others have maintained that the nucleophile attacks the covalent substrate. Extensive references to both points of view are given by McLennan (1976). In the present review the application of the Marcus theory of atom transfer (Marcus, 1968a) allows us to deduce values of the parameter a which describes the symmetry of the transition state. We shall compare this information about the transition state with that from changing the solvent, from isotope effects, and from Hammett relations. We shall then attempt to deduce a model for the transition state which is consistent for all the different types of data. 

We now explore whether the pattern of reactivity predicted by the Marcus theory is found for methyl transfer reactions in water. We use equation (29) to calculate values of G from the experimental data where, from (27), G = j(JGlx + AG Y). The values of G should then be made up of a contribution from the symmetrical reaction for the nucleophile X and for the leaving group Y. We then examine whether the values of G 29) calculated for the cross reactions from (29) agree with the values of G(27) calculated from (27) using a set of values for the symmetrical reactions. The problem is similar to the proof of Kohlrausch s law of limiting ionic conductances. 

As Bunnett has noted (4), the kinetic barrier to nucleophilic attack is affected by the thermodynamics of the reaction. If this thermodynamic contribution could be removed, then intrinsic nucleophilicities for substitution reactions could be obtained that would be independent of the leaving group. Pioneering work by Albery and Kreevoy (7), Pellerite and Brauman (8), and Lewis et al. (9) has shown that Marcus theory can be applied to methyl-transfer reactions to separate thermodynamic and kinetic contributions and provide intrinsic barriers to nucleophilic attack. One expression of Marcus theory is given in equation 1, where AE is the activation energy, AE° is the heat of reaction, and AE0 is the intrinsic activation energy or the barrier to reaction in the absence of any thermodynamic driving force. 

From the Pellerite and Brauman (8) application of Marcus theory to SN2 methyl-transfer reactions, the kinetic barrier is made up of an intrinsic barrier for the reaction that is decreased by the magnitude of the reaction exothermicity. Because the exothermicities of the reactions with 02 are the lowest (by upward of 20 kcal/mol) compared to the other anions of high kinetic nucleophilicity, the intrinsic barriers for these SN2 reactions with 02 are the smallest. Thus, 02 can be called a super SN2 nucleophile. 

Fig. 12 Test of the Marcus theory for methyl transfers in H,0. The graph compares the values of G(29) calculated by equation (29) from experimental free energies with C,7) calculated by equation (27) from the Cx x for the symmetrical reactions...

The purpose of this chapter is to give an introduction to the subject of nucleophilicity. The chapters of the present volume are collected into five groups (1) Marcus theory, methyl transfers, and gas-phase reactions (2) Br0nsted equation, hard-scft acid-base theory, and factors determining nucleophilicity (3) linear free-energy relationships for solvent nucleophilicity (4) complex nucleophilic reactions and (5) enhancement of nucleophilicity. The present chapter is divided in the same way, giving an introduction to each of the five topics followed by a description of key points in each chapter as they relate to current studies of nucleophilicity and the other chapters of the book. 

Marcus Theory, Methyl Transfers, and Gas-Phase Reactions... 

Electron transfer between hydrazine and methyl-substituted hydrazines and the cyano complexes of Fe(III), Mo(V), and W(V) in aqueous media proceed via an outer-sphere mechanism. The rate data obtained are consistent with the Marcus theory. The reactions are pH dependent with a rate law shown in equation... 

Chemical dynamics simulations of the gas phase 5 2 reactions of methyl halides have been carried out at many different levels of theory and compared with experimental measurements and predictions based on transition state theory and RRKM (Rice-Ramsperger-Kassel-Marcus) theory. Although many 5 2 reactions occur by the traditional pre-reaction complex, transition state, post-reaction complex mechanism, three additional non-statistical mechanisms were detected when the F -CH3-I reaction was analysed at an atomic level (i) a direct rebound mechanism where F attacks the backside of the carbon and CH3-F separates (bounces off) from the iodine ion, (ii) a direct stripping mechanism where F approaches CH3-I from the side and strips away the CH3 group, and (iii) an indirect reaction where the pre-reaction complex activates the C-I bond causing a CH3-I rotation and then the 5 2 reaction. The presence of these processes demonstrate that three non-statistical effects, (i) recrossing of the transition state is important, (ii) the transfer of the translational energy from the reactants into the rotational and vibrational modes of the substrate is inefficient, and (iii) there is... 

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