Redox-substituted pyrroles

Cyclic voltammetry is most commonly used to investigate the polymerization of a new monomer. Polymerization and film deposition are characterized by increasing peak currents for oxidation of the monomer on successive cycles, and the development of redox waves for the polymer at potentials below the onset of monomer oxidation. A nucleation loop, in which the current on the reverse scan is higher than on the corresponding forward scan, is commonly observed during the first cycle.56,57 These features are all illustrated in Fig. 3 for the polymerization of a substituted pyrrole.58... 

Due to its electronic conductivity, polypyrrole can be grown to considerable thickness. It also constitutes, by itself, as a film on platinum or gold, a new type of electrode surface that exhibits catalytic activity in the electrochemical oxidation of ascorbic acid and dopamine in the reversible redox reactions of hydroquinones and the reduction of molecular oxygen iV-substituted pyrroles are excellent... 

Table 1. N-substituted pyrroles employed in the preparation of redox-Modified polypyrrole films on electrodes...

Many redox enzymes can be denatured if the pH of the solution is taken too far from neutral or if nonaqueous solvents are used. It is therefore desirable, although not always essential, to use neutral aqueous solutions in the electropolymerization process, thus restricting the choice of monomers that can be used. One way to overcome this problem is to incorporate, or immobilize, the enzyme after deposition of the polymer film. This allows a much wider choice of deposition conditions. In addition to adsorbing the enzyme onto preformed poly(pyrrole) films, " GOx has also been covalently immobilized onto a wide range of preformed 3- and N-derivatized pyrrole carboxylates or onto N-amino-substituted pyrroles. This technique may increase current densities at a given glucose concentration by as much as twentyfold when compared to the best results obtained elsewhere. The authors make no claims for direct electron transfer between enzyme... 

A method for the synthesis of substituted pyrroles 33 via a [3+2]-cycloaddition, skeletal rearrangement, and redox cascade reaction was reported which resulted in the installation of allyl groups in the P-position of pyrroles. The starting materials are easily accessible Michael acceptors 31 and HCl salts 32 of amino acid-type derivatives (14OL3580). 

More recently, Llobet and coworkers reported the anodic electropolymerization of A -substituted pyrroles as a convenient method of anchoring a redox-active dinuclear ruthenium catalyst onto conducting solid supports, like vitreous carbon sponges (VCS) and fluorine-doped tin oxide (FTO). In the presence of Ce(IV) as the sacrificial oxidant, turnover numbers up to 76 have been achieved. A major improvement of the system is accomplished by the copolymerization with a robust non active redox species, able to further separate the catalytically active species on the solid support, obtaining up to 250 catalytic cycles. 

Downard et al. [123] reported the synthesis of thin films based on the oxidative electropolymerization of substituted pyrroles. The films contained redox couples involving pyrrole derivatives of either the chromophore [Ru(2,2 -bipyridine>3], the electron transfer acceptor paraquat, or the electron transfer donor phenothiazine. 

The formation of a polymer film obtained by electrochemical oxidation of pyrrole was first reported by Diaz and Kanazawa [542, 543]. At least initially, electrooxidation of various substituted pyrroles surprisingly resulted in no polymers. A relationship between the oxidation potential and the dipole moment of the pyrroles was found [544]. Nevertheless, redox active films were obtained from A7-methylpyrrole and N-phenylpyrrole [545]. Figure 58 shows the molecular formula of most pyrroles discussed in the following section. An early review of PPy was provided by Street [546]. 

Iron Porphyrins. Porphyrias (15—17) are aromatic cycHc compouads that coasist of four pyrrole units linked at the a-positions by methine carbons. The extended TT-systems of these compounds give rise to intense absorption bands in the uv/vis region of the spectmm. The most intense absorption, which is called the Soret band, falls neat 400 nm and has 10. The TT-system is also responsible for the notable ring current effect observed in H-nmr spectra, the preference for planar conformations, the prevalence of electrophilic substitution reactions, and the redox chemistry of these compounds. Porphyrins obtained from natural sources have a variety of peripheral substituents and substitution patterns. Two important types of synthetic porphyrins are the meso-tetraaryl porphyrins, such as 5,10,15,20-tetraphenylporphine [917-23-7] (H2(TPP)) (7) and P-octaalkylporphyrins, such as 2,3,7,8,12,13,17,18-octaethylporphine [2683-82-1] (H2(OEP)) (8). Both types can be prepared by condensation of pyrroles and aldehydes (qv). 

Furthermore, the utilization of preformed films of polypyrrole functionalized by suitable monomeric ruthenium complexes allows the circumvention of problems due to the moderate stability of these complexes to aerial oxidation when free in solution. A similar CO/HCOO-selectivity with regards to the substitution of the V-pyrrole-bpy ligand by an electron-with-drawing group is retained in those composite materials.98 The related osmium-based redox-active polymer [Os°(bpy)(CO)2] was prepared, and is also an excellent electrocatalyst for the reduction of C02 in aqueous media.99 However, the selectivity toward CO vs. HCOO- production is lower. 

Several approaches have been undertaken to construct redox active polymermodified electrodes containing such rhodium complexes as mediators. Beley [70] and Cosnier [71] used the electropolymerization of pyrrole-linked rhodium complexes for their fixation at the electrode surface. An effective system for the formation of 1,4-NADH from NAD+ applied a poly-Rh(terpy-py)2 + (terpy = terpyridine py = pyrrole) modified reticulated vitreous carbon electrode [70]. In the presence of liver alcohol dehydrogenase as production enzyme, cyclohexanone was transformed to cyclohexanol with a turnover number of 113 in 31 h. However, the current efficiency was rather small. The films which are obtained by electropolymerization of the pyrrole-linked rhodium complexes do not swell. Therefore, the reaction between the substrate, for example NAD+, and the reduced redox catalyst mostly takes place at the film/solution interface. To obtain a water-swellable film, which allows the easy penetration of the substrate into the film and thus renders the reaction layer larger, we used a different approach. Water-soluble copolymers of substituted vinylbipyridine rhodium complexes with N-vinylpyrrolidone, like 11 and 12, were synthesized chemically and then fixed to the surface of a graphite electrode by /-irradiation. The polymer films obtained swell very well in aqueous... 

Pyrrole anion is unreactive in liquid ammonia under irradiation with PhBr or 1-chloro-naphthalene. However, the reactions of aryl chlorides (p-chlorobenzonitrile, 3- and 4-chloropyridines and 4-chlorodiphenyl sulphone) with 2,5-dimethylpyrrole anion under electrochemical inducement in the presence of a redox mediator gave the C3-substituted product in moderate yields (35-40%) (equation 120)225. The rate constant of the coupling reaction between this nucleophile and aryl radicals is about 5-8 x 109 M"1 s 1 determined by electrochemical methods225. 

As regards the electrooxidation of the corresponding monomers, they have less anodic Epa values than N-phenylpyrrole even with the nitro substituent and the reactions remain irreversible. The substituents influence the oxidation of these monomers much more than was observed with pentaphenylpyrrole. For example, substitution of a p-methoxy group in the N-phenyl of die latter produces a 20 mV cathodic shift in the Epa value (10). The dimethylaminophenyl and nitrophenyl groups show reversible redox behavior and appear to behave independent of the pyrrole moiety in these derivatives. 

---