Dissociative reduction

It was first suggested that the reaction of an alkyl halide with a nickel(I) Schiff base complex yields an alkylnickel(III) intermediate (Equation (56)). Homolytic cleavage of RBr to give an alkyl radical R and a nickel(II) complex (Equation (57)) or, alternatively, one-electron dissociative reduction leading to R (Equation (58)) are possible pathways.254 A mechanism based on the formation of R via dissociative electron transfer of Ni -salen to RX (Equation (59)) has also been proposed.255... 

Kinetics, and mechanisms of reactions of organic cation radicals in solution, 20, 55 Kinetics and mechanism of the dissociative reduction of C—X and X—X bonds (X = O, S),... 

A simple diagram depicting the differences between these two complementary theories is shown in Fig. 1, which represents reactions at zero driving force. Thus, the activation energy corresponds to the intrinsic barrier. Marcus theory assumes a harmonic potential for reactants and products and, in its simplest form, assumes that the reactant and product surfaces have the same curvature (Fig. la). In his derivation of the dissociative ET theory, Saveant assumed that the reactants should be described by a Morse potential and that the products should simply be the dissociative part of this potential (Fig. Ib). Some concerns about the latter condition have been raised. " On the other hand, comparison of experimental data pertaining to alkyl halides and peroxides (Section 3) with equations (7) and (8) seems to indicate that the simple model proposed by Saveant for the nuclear factor of the ET rate constant expression satisfactorily describes concerted dissociative reductions in the condensed phase. A similar treatment was used by Wentworth and coworkers to describe dissociative electron attachment to aromatic and alkyl halides in the gas phase. ... 

Much effort has been put into understanding and defining the practical differences between the concerted and the stepwise dissociative reduction mechanisms and whether or not there is a smooth transition between them (mechanistic continuum) or simply a partitioning between competing processes. Saveant has described this transition in terms of three... 

Fig. 2 Energy factors that influence the transition from the concerted (left) to the stepwise (right) dissociative reduction.

Table 1 Effect of interaction between A and B on the kinetics of dissociative reduction of AB.

Bond dissociation energies, BDE, are required in order to understand dissociative reduction processes. While BDE values of many simple hydrocarbons have been measured using various gas phase methods, for practical purposes, reactions of interest to electrochemists occur in solution. Solution-... 

One of the key issues in understanding these stepwise dissociative reductions is to define the thermodynamics (or at least relative thermodynamics) of the fragmentation itself. It is equally important to understand on which side of the scissile bond the charge actually resides, as this has important mechanistic implications. This is most easily inferred by comparison of the measured standard (or peak) potential with standard potentials of similarly substituted aromatic molecules that make stable radical anions. 

Table 4 Summary of the voltammetric reduction peak potentials ( p), standard dissociative reduction potentials ( "roor/ro ,ro ) for 3 variety peroxides and endoperoxides in DMF/O.l M TBAP at T = 25°C. Also summarized are the BDFEs and the standard potentials of the corresponding leaving group."...

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