Electropolymerization reaction scheme

Fig. 4.1 The reaction scheme for the electropolymerization of aniline. (Reproduced from [49] with the permission of Elsevier Ltd.)...

One also obtains analogous findings with trace-crossing effects for the electropolymerization of thiophene and pyrrole. This cannot be explained by a simple linear reaction sequence, as presented in Scheme I, because it indicates competing homogeneous and heterogeneous electron transfer processes. Measurements carried out in a diluted solution of JV-phenylcarbazole provide a more accurate insight into the reaction mechanism (Fig. 2). 

The ideal electropolymerization scheme (Eq. (5.5.39)) is further complicated by the fact that lower oligomers can react with nucleophilic substances (impurities, electrolyte anions, and solvent) and are thus deactivated for subsequent polymerization. The rate of these undesired side reactions apparently increases with increasing oxidation potential of the monomer, for example, in the series ... 

Another type of polymer-supported chiral catalyst for asymmetric cyclopropanation was obtained by electropolymerization of the tetraspirobifluorenylporphyrin ruthenium complex [143]. The cyclopropanation of styrene with diazoacetate, catalyzed by the polymeric catalyst 227, proceeded efficiently at room temperature with good yields (80-90%) and moderate enantioselectivities (up to 53% at -40 °C) (Scheme 3.75). PS-supported versions of the chiral ruthenium-porphyrin complexes 231 (Scheme 3.76) were also prepared and used for the same reaction [144]. The cyclopropanation of styrene by ethyl diazoacetate proceeded well in the presence of the polymeric catalyst to give the product in good yields (60-88%) with high stereoselectivities (71-90% ee). The highest ee-value (90%) was obtained for the cyclopropanation of p-bromostyrene. 

Recently, Schulz was carried out electropolymerization, an original methodology to prepare polymer supported salen 113 [109][110]. It consisted in the introduction of a thiophene moiety, an electropolymerizable functionality, on the salen backbone (Scheme 62) followed by electropolymerization imder cyclic voltammetry conditions on a platinum grid. These polymeric catalyst 120a and 120b were then evaluated in hetero Diels-Alder reaction (HTR) between several aldehydes and l-methoxy-3-[(methylsilyl)oxy]-l,3-butadiene... 

Polymer RuCO-porphyrins 102 and 103 (Scheme 48) were used for the cyclopropanation reaction of styrenes but they were also tested in the epoxidation reaction. Contrary to cyclopropanation for which moderate enantioselectivities and low activities were observed, these Ru porphyrin complexes gave good ee (up to 76%) and activity (up to 89%) for AE of unfimctionalized olefines [204], Recently, Simmoneaux showed that iron catalyst 326 derived from electropolymerized tetraspirofluorenyl porphyrin (Scheme 138) led to moderate yields without chiral induction [205],... 

Applying a similar reaction procedure as in Scheme 1.22, two polymerizable bithiophene-functionalized dyads 2.130 and 2.131 (Chart 1.25) were prepared as mixtures which were separated by chromatography in 13-24% yield [232]. Electropolymerization of the two compounds showed enhanced conjugation and improved stability in the resulting polymer under redox cycUng. Significant enhancement of the photocurrent which was measured for these polymers revealed their potential utility for OSCs. 

Polymerization of thiophenes can be carried out in many different ways and the most commonly used methods can be generalized into three categories (i) electropolymerization, (ii) metal-catalyzed coupling reactions and (iii) chemical oxidative polymerization. Electropolymerization is a widely used method to prepare insoluble films of PTs and represents a simple and efficient way to study the optical and electronic properties of PTs [4], although it is rarely used in the preparation of electroluminescent materials. In 1980, Yamamoto et al. reported the Ni-catalyzed polycondensation of 2,5-dibromothiophene 1. The latter was allowed to react with Mg in THF, affording 2-magnesiobromo-5-bromothiophene 2, which in the presence of Ni(bipy)Cl2 (bipy = 2,2 -bipyridyl) produced PT 3 (Scheme 19.2) [15], In the same year, Lin and Dudek described another example of a metal-catalyzed route to unsubstituted PT 3, exploiting acetylacetonates of Ni, Pd, Co and Fe as catalysts [16]. 

From this scheme, electropolymerization proceeds through successive electrochemical and chemical steps. In the terminology of electrochemical reaction mechanisms, this chain-propagation process corresponds to a cascade of ECE steps. The chain growth is terminated either when the radical cation of the growing chain becomes too unreactive or, more likely, when the reactive end of the chain becomes sterically blocked from further reaction [61]. 

The monomers were easily prepared from bromo-3 thiophene and 2,4-dibromothiophene by classical silylation reactions. Polymers prepared by electropolymerization of these monomers (Scheme 14.34) are denoted pSiTh-/i (n being defined in Scheme 14.34)... 

A primary goal of this experiment was to use diffuse reflectance for evaluation of thicker porphyrin films which cannot be studied by using transparent conductive electrodes and absorption spectroscopy. The main concern was whether the integrity of the porphyrin system had been retained in the electropolymerization scheme and whether the reactions taking place in the film would be comparable to those taking place in the bulk. The cyclic voltammograms of the films in solution suggested that both of these facts were true. 

Supported metalloporphyrins have also been synthesized by electropolymerization leading to films of polymers. They were formed on the platinium working electrode during the oxidative electrosynthesis (Scheme 49) [98], Contrary to the preceding catalysts 102 and 103, these electropolymerized catalysts led to low enantioselectivities for the bench-mark reaction between styrene and EDA. Up to 53% ee at - 40°C was reached in the presence of 105 a. Moreover, at room temperature, the reactions proceeded efficiently (yields 80-90%). Seven recycling of polymer 105a were carried out without a significant decrease in enantioselectivity and activity. 

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