Dimerization radical-substrate mechanism

There are two mains aspects of the role of dimerization of intermediates on the electrochemical responses that are worth investigating in some detail. One concerns the effect of dimerization on the primary intermediate on the current-potential curves that corresponds to the first electron transfer step, the one along which the first intermediate is generated. Analysis of this effect allows the determination of the dimerization mechanism (radical-radical vs. radical-substrate). It is the object of the remainder of this section. 

The simplest electrodimerization mechanism occurs when the species formed as the result of a first electron transfer reaction reacts with itself to form a dimer (Scheme 2.7). This mechanism is usually termed radical-radical dimerization (RRD) because the most extensive studies where it occurs have dealt with the dimerization of anion and cation radicals formed upon a first electron transfer step as opposed to the case of radical-substrate dimerizations, which will be discussed subsequently. It is a bimolecular version of the EC mechanism. The bimolecular character of the follow-up reaction leads to nonlinear algebra and thus complicates slightly the analysis and numerical computation of the system. The main features of the cyclic voltammetric responses remain qualitatively similar, however. Unlike the EC case, however, the dimensionless parameter,... 

The radical-substrate dimerization (RSD) mechanism is as depicted in Scheme 2.8, involving, as a first follow-up reaction, coupling of the electron transfer intermediate with the substrate. There are, in fact, several versions of the RSD mechanism according to the nature of the electron transfer step,... 

The ease of cyclobutane cleavage and the detailed mechanism can be affected by the nature of the substituents and the substitution pattern. While the above cyclobutanes are cleaved without a discernible intermediate, the n//-head-to-head dimer of dimethylindene shows a significantly different behavior. This substrate is cleaved in an apparent two-step process, involving a ring-closed radical cation (with spin and charge localized on one indan system) and a ring-opened 1,4-bifunctional radical cation. Apparently, the cleavage of the doubly benzylic cyclobutane bond is reversible. The involvement of more than one dimer radical cation is indicated by a unique polarization pattern (Fig. 13), which is incompatible with any one intermediate, but can be simulated on the basis of two successive radical cations (see Sect. 5.2) [256]. 

Electrochemical methods were used to obtain kinetic information concerning the cation-radical dimerization of anisole (and related compounds). Two mechanisms were consistent with data A radical-radical coupling (RRC) mechanism and a radical-substrate coupling (RSC) mechanism (Fig. 42) [198]. 

Another important set of reactions are the dimerizations, which are usually discussed within the frames of the two mechanisms shown in Scheme 3. If the formation of C results from the coupling of two radicals or radical ions (ii), the reaction is referred to as a radical-radical process (RR). Another route to C, the radical-substrate process (RS), includes the coupling between A and B (iii) resulting in the formation of an intermediate I that is further reduced to C by reaction with B (iv). The direct reduction of I to C at the electrode is usually without importance. Finally, the intermediate C reacts with a reagent X to the product D. These dimerization mechanisms belong to a more general scheme to be discussed in some detail later (Sec. II.C.5). 

Cinnamonitrile (9) and substituted analogues are easier to reduce than the simple nitriles and form less reactive radical anions. The mechanism and kinetics of the EHD of 9 has therefore been much studied. In DMF n 1, and RRDE measurements were in agreement with the RR mechanism at substrate concentrations <8 mM [55]. The rate constant for the rate-determining dimerization step (RR mechanism) has been determined by several methods in DMF (Table 3). In liquid ammonia T = —43°C), dimerization is considerably slower (Table 3), and, significantly, even in the presence of /-PrOH the coulometric n-value was small (0.05-0.27), indicating considerable polymerization [52]. 

Finally, the formation of biaryls by C—C coupling can take place through two different mechanisms referred to as radical-radical (RRD) and radical-substrate (RSD) dimerization. A mechanism involving le oxidation of the phenol by the peroxide and biaryl coupling by preferential addition of the phenol radical cation to the ort/io-positions to the alkoxy group of the diaroyl peroxide has been suggested. 

That this mechanism can take place under suitable conditions has been demonstrated by isotopic labeling and by other means. However, the formation of disproportionation and dimerization products does not always mean that the free-radical abstraction process takes place. In some cases these products arise in a different manner.We have seen that the product of the reaction between a carbene and a molecule may have excess energy (p. 247). Therefore it is possible for the substrate and the carbene to react by mechanism 1 (the direct-insertion process) and for the excess energy to cause the compound thus formed to cleave to free radicals. When this pathway is in operation, the free radicals are formed after the actual insertion reaction. 

The half-order of the rate with respect to [02] and the two-term rate law were taken as evidence for a chain mechanism which involves one-electron transfer steps and proceeds via two different reaction paths. The formation of the dimer f(RS)2Cu(p-O2)Cu(RS)2] complex in the initiation phase is the core of the model, as asymmetric dissociation of this species produces two chain carriers. Earlier literature results were contested by rejecting the feasibility of a free-radical mechanism which would imply a redox shuttle between Cu(II) and Cu(I). It was assumed that the substrate remains bonded to the metal center throughout the whole process and the free thiyl radical, RS, does not form during the reaction. It was argued that if free RS radicals formed they would certainly be involved in an almost diffusion-controlled reaction with dioxygen, and the intermediate peroxo species would open alternative reaction paths to generate products other than cystine. This would clearly contradict the noted high selectivity of the autoxidation reaction. 

Development of the industrial process for electrochemical conversion of acrylonitrile to adiponitrile led to extensive investigation into the mechanism of the dimerization process. Reactions of acrylonitrile radical-anion are too fast for investigation but the dimerization step, for a number of more amenable substrates, has been investigated in aprotic solvents by electrochemical techniques. Pulse-radiolysis methods have also been used to study reactions in aqueous media. 

---