Electrochemical Initiation

In previous chapters we already discussed some aq ects of electroinitiatton of cationic polymerisation, namely the anodic production of relatively stable radical-cation salts used in situ as initiators and the application of hi -electric fields to prmnote flie formation of highly active radical-cations in liquid monomers. In this ch ter we will consider the more traditional type of electroinitiated polymerisation in which the electrolysis of a solution containing monomer and a supporting electrolyte produces the cationic polymerisation of the former. Of cour, radical and anionic polymerisatimi can also be initiated by this technique, but these processes are outside the scope of the present review. 

The pioneering work of Breitenbach s group establidied for the first time in an unequivocal manner the cationic nature of the electroinitiated pdymerisations of styrene, N-vinylcarbazole and vinyl ethers with perchlorate supporting electrolytes. Since then, a considerable amount of work has been devoted to this topic and various reviews have summed up the major achievements of this type of studies In view of this, we will limit our discussion to the fundamental aspects most relevant to the present context, and more particidarly to an analysis of recent work. 

In 1970, Funt and Blain published a thorough study of the electroinitiated polymerisation of styrene in the presence of tetrabutylammonium perchlorate. The kinetics of these reactions were followed in methylene chloride with a cell provided with a sintered-glass membrane and a sampling device at the anode compartment. S-shaped time-conversion curves were obtained indicating the accumulation of chain carriers during the electrolysis. The results were treated assuming that the concentration of active species was directly proportional to the time-integrated current flow, and that no termination oc- 

The formation of styrene radical cations was supported by the observation of a green colour at the inner region of the anode. Funt and Blain argued that these primary initiating species wcmld probably soon couple through their radical ends and therefore the polymerisation would proceed mostly through a cationic propagation. The similarity of the situation achieved after initiation with that of the system styrene-perdiloric acid is obvious. 

Pistoia investigated the electroinitiated polymerisation of styrene in propylene carbonate-lithium perchlorate solutions at 25°C. Mechanistic evidence was obtained for the formation of perchloric acid at the anode and the cationic nature of the process thus proved. The kinetic analysis yielded a kp value of 0.5 M sec . Although no comparisons can be made between this result and previous ones in other solvents, the presence of lithium perchlorate was here a source of homocorgugation for the acid produced and thus the cause of considerable deactivation of its initiating power. As in previous cases, this was not recognised by the author. A simflar study by Pistoia and Scro-sati in dimethylsulphate gave an insoluble polymer at the anode and the nature or the initiator was not elucidated, but it did not seem to be perchloric acid. The cationic properties of this process was however proved 

---

In certain cases, Michael reactions can take place under acidic conditions. Michael-type addition of radicals to conjugated carbonyl compounds is also known.Radical addition can be catalyzed by Yb(OTf)3, but radicals add under standard conditions as well, even intramolecularly. Electrochemical-initiated Michael additions are known, and aryl halides add in the presence of NiBr2. Michael reactions are sometimes applied to substrates of the type C=C—Z, where the co-products are conjugated systems of the type C=C—Indeed, because of the greater susceptibility of triple bonds to nucleophilic attack, it is even possible for nonactivated alkynes (e.g., acetylene), to be substrates in this... 

The potential of ECL in analytical chemistry has only more recently been investigated, but has rapidly gained recognition as both a sensitive and selective method of detection. Most reported applications have utilized the tris(2,2 -bipyri-dyl) ruthenium(II) [Ru(bpy)32+] ECL reaction, or else the electrochemical initiation of more conventional CL reactions, but many other potentially useful systems have been investigated. The applications of ECL in analytical chemistry have recently been the subject of comprehensive reviews [12-16],... 

The review below deals with anodic and cathodic reactions of organic derivatives of sulfur, which have interesting potentialities for the electrosynthesis and electrochemically initiated functionalization of organosulfur and related organic compounds. 

Fig. 6.20. Effect of polymerisation potential (Epoi) on the percentage conversion in electrochemically initiated copolymerisation of isoprene with a-methylstyrene o with ultrasound without ultrasound.

The method of electrochemical initiation of these reactions has some limitations, the main limitation being that the probability of substitution of a leaving group by a nucleophile depends on the nature of the substrate. Let us compare two reactions similar in the solvent employed (DMSO) and the nucleophile used (Bu4NSPh), but different in the chosen substrates (4-bromobenzophenone or bromobenzene)... 

Pinson and Saveant 1978, Swartz and Stenzel 1984). On electrochemical initiation (Hg cathode), 4-bromobenzophenone gives rise to 4-(phenylthio)benzophenone in the 80% yield, whereas bromoben-zene yields diphenyldisulfide with the yield of only 10% and unsubstituted benzene with the yield of more than 95%. In the bromobenzene case, this means that the substitution is a minor reaction, whereas the main ronte is ordinary debromination. According to Swartz and Stenzel (1984), the substrate anion-radicals are initially formed in the preelectrode space. Stability of these anion-radicals are different. The less stable anion-radicals of bromobenzene do not have enough time to go into the catholyte pool. They give rise to the phenyl radicals in the vicinity of the cathode. The phenyl radicals are instantly reduced into the phenyl anions. They tear protons from the solvent and yield benzene. 

The known tendency of 2-vinylindole to homopolymerize is sufficiently low on the electrode surface. Therefore, an electrochemical initiation of the reaction under potentiostatic control is very favorable. The yield is high (90%), and the product formed is especially interesting because it incorporates the skeleton of the indole alkaloid goniomitine. 

The anion-radical mechanism for these syntheses is based on the following facts. The reactions require photo- or electrochemical initiation. Oxygen inhibits the reactions totally, even with photoirradiation. Indoles are formed from o-iodoaniline only the meta isomer does not give rise to indole. Hence, the alternative aryne mechanism (cine-substitution) is not valid. What remains as a question is the validity of the ion-radical mechanism exclusively to the substitution of the acetonyl group for the halogen atom in o-haloareneamine or also for intramolecular condensation. 

It is logical to consider the nncleophile, Nu-, as a source of the electron to be transferred onto the snbstrate molecnle, RX. However, in most cases, the nucleophile is such a poor electron donor that electron transfer from Nn- to RX is extremely slow, if it is possible at all. These reactions reqnire an external stimulation in which a catalytic amount of electrons is injected. Such kinds of assistance to the reactions from photochemical and electrochemical initiations or from solvated electrons in the reaction mediums have been pointed out earlier. Alkali metals in liquid ammonia and sodium amalgam in organic solvents can serve as the solvated electron sources. Light initiation is also used widely. However, photochemical initiation complicates the reaction performance. 

Breitenbach, J. W., and C. Srna Electrochemical initiation of polymerization. Pure AppL Chem. 4, 245 (1962). 

Russian chemists have obtained polyphenylisocyanate by electrochemical initiation of polymerization in dimethyl formamide with tetra-butylammonium at 58° C (25). They believe that initiating... 

Electrochemical Initiation of Radical-Ion Reactions of Organofluorine Compounds ... 

Aprotic solvents, such as DMSO and acetonitrile generally give good results, and the latter appears to be the solvent of choice (with appropriate supporting electrolytes) in electrochemically initiated reactions. 

Cathodic reduction of 1,1,1 -trimethyl-2,2,2-triphenyldisilane under constant current in MeCN provides triphenylsilane quantitatively (equation 55). The reaction involves an electrochemically initiated chain reaction73. The source of oxygen in (Me3Si)20 may be the residual water in acetonitrile. 

Although less common, ketyl anions can also be generated by removal of an a-hydrogen from an alkoxide (Figure 1, reaction 3). An interesting example where a ketyl anion is formed as an intermediate in this manner is provided by the electrochemically-initiated reduction of an aryl halide by an alkoxide anion via the free radical chain process illustrated in Scheme l6. 

Some electrochemically initiated organometallic reactions of rhodium and iridium porphyrins have been explored and reviewed by Kadish et al. [178,306], For more recent papers concerning this matter, the reader is referred to Sect. 5. 

The application of ECL per se or in combination with enzyme IM to the detection of explosives has been reported and has extended the possibilities of CL in the field of explosives. ECL can be defined as the luminescence generated by the relaxation of excited state molecules that are produced during an electrochemically initiated reaction [64],... 

This chain process requires an initiation step. In a few systems a thermal (spontaneous) initiation is observed. One of the methods consists in the formation of the radical anion of the substrate by reaction with alkali metals in liquid ammonia. Electrochemical initiation is an approach that has been successful in a considerable number of cases with aromatic and heteroaromatic substrates [14]. Other possibilities include the use of Fe+2 [15-18], Sml2 [19], or Na(Hg) [20]. However, the most extensively used method is the photoinitiation, and it has proven extremely suitable for synthetic purposes Even though it is a widely used initiation method, there are not many mechanistic studies of this step. 

In the S l mechanism of aromatic substitution the initiating step is the formation of a radical anion. In order to distinguish the process from the route described above (SR+N1) in which a radical cation plays a crucial role, the symbol S l has been used17. Creation of the radical anion can occur by several procedures. The reaction can be electrochemically initiated, a solvated electron in a solution of alkali metal in liquid ammonia may be involved or a radical anion may be used as the source of electrons. The most common source of electrons is, however, the nucleophile itself involved in the substitution reaction. In many cases the electron transfer from nucleophile to substrate is light-catalysed and the process is then sometimes referred to as S l Ar. Although the nucleofugic group in S l... 

Complexes of the type [Rh(TPP)(RX)] [RX = C H X (n = 3-5, X = Cl or Br n = 3-6, X = I) TPP = dianion of tetraphenylporphyrin] were prepared by Anderson et al. (179). The nature of RX was found to determine the overall electrochemical behavior for the reduction of [Rh(TTP)(RX)]. For some complexes, specifically those where X = Br and I, the bound alkyl halide could be reduced without cleavage of the metal-carbon bond. This resulted in the electrochemically initiated conversion of [Rh(TPP)(RX)] to a [Rh(TPP)(R)] complex. The E. value for this reduction was dependent on the chain length and halide of the RX group and followed the trend predicted for alkyl halides. The reduction of the bound RX occured at Ei values significantly less negative that those for reduction of free RX under the same solution conditions. 

---