Pyrroles radical

Quantitative investigations of the kinetics of these a-coupling steps suffered because rate constants were beyond the timescale of normal voltammetric experiments until ultramicroelectrodes and improved electrochemical equipment made possible a new transient method calledjhst scan voltammetry [27]. With this technique, cyclic voltammetric experiments up to scan rates of 1 MV s are possible, and species with lifetimes in the nanosecond scale can be observed. Using this technique, P. Hapiot et al. [28] were the first to obtain data on the lifetimes of the electrogenerated pyrrole radical cation and substituted derivatives. The resulting rate constants for the dimerization of such monomers lie in the order of 10 s . The same... 

Pyridone undergoes fragmentation by loss of CO and formation of the pyrrole radical cation. 3-Hydroxypyridine, on the other hand, loses HCN to give the furan radical cation while 4-pyridone... 

The reactions of t with pyrrole and 1-methylpyrrole have been investigated by Kubota and Sakurai (117,135). The formation of adducts 71 and 72 upon irradiation of t-1 in pyrrole solvent is proposed to occur via a nonfluorescent singlet ex-ciplex, which undergoes N-H atom transfer from pyrrole to t-1 to yield a radical pair (117). Radical pair combination occurs at C-2 or C-3 of the pyrrole radical to yield adducts 71 and 72, which isomerize to yield the isolated products, 73 and 74... 

FIGURE 1 The electrochemical polymerization mechanism for the formation of polypyrrole begins when a pyrrole radical cation forms from the oxidation of the monomer. Then two radical cations react to create a dimer and split off two protons. The dimer undergoes further oxidation to generate a radical, which reacts with another radical to propagate chain growth and eventually produce polypyrrole. 

Therefore, coupling is believed to occur between oligomer radical cations and pyrrole radical cations at the electrode surface. The growth of polypyrrole chains probably terminates when the ends of the growing chains become sterically blocked... 

Figure 4.6. Coupling of pyrrole radical cation to polypyrrole (spin densities at radical cation are given).

Conducting polymer-based immobilization or wired enzymes is a global enzyme immobilization method that differs in many respects from those just described. In one example, a redox polymer is formed on the surface by the oxidation of pyrrole molecules to pyrrole radical cations, which then polymerize on the surface to form conductive polypyrrole [60,68]. Other conducting polymers include polyvinylpyridine, polythiophene, polyaniline, and polyindole. If enzymes are present in the solution as polymerization takes place, they are entrapped within the polymer. When these polymers are cross-linked with redox mediators such as [Os(bpy)2Cl]+ 2 the resulting amperometric (or potentiomet-ric) biosensors are referred to as wired enzyme electrodes [5-7]. The distance between the redox centers of the polymer and the FADH2 centers of the reduced enzyme is reduced sufficiently for electrons to be transferred and, therefore, for the mediated electro-oxidation of glucose on conventional electrodes. These electrodes do not require diffusing redox mediators or membranes to contain the enzyme and the redox polymer. 

During the vinylation of 2-arylpyrroles in the presence of the same radical trap, the ESR signals of vinyl-tert-butylnitroxyl and spin adducts of f-BuNO with the adducts of 2-substituted pyrrole radicals to acetylenes have been detected. N-Centered radicals of 2-arylpyrrole are quite stable and are observed directly in the ESR spectra (Scheme 2.40) [496]. 

The analysis of the hyperiine structure constants indicates the formation of spin adduct C (consecutive addition of two molecules of ONBu-t to benzoylacetylene radical) and spin adduct D (capture of the pyrrole radical by the spin trap). 

---