Pyridinium salts, charge-transfer

Despite considerable efforts, the formulation in equation (42) remains incomplete owing to the high reactivity of organocuprates as well as their oligomeric nature. Accordingly, we select organoborates as stable electron donors to study alkyl additions to various pyridinium acceptors (by thermal and photoinduced electron transfer) via charge-transfer salts as follows. 

In another approach to the estimation of solvent polarities the effect of a solvent on the absorbance maximum in the visible-ultraviolet region of the charge-transfer band of a salt such as 1 -ethyl-4-carbomethoxy pyridinium iodide is measured 147). A shift of the maximum to shorter wavelengths occurs as solvent polarity increases. The wavelength, expressed in kcal, is called the Z value of the solvent. This method provides a simple and rapid measure of solvent polarity at the molecular level. 

When pyridinium hydrochloride was treated at 250°C with iodine monochloride in the presence of aluminium chloride, a 2-pyridylpyridinium salt was formed. Both 1 1 and 1 2 complexes formed when iodine monochloride or iodine reacted with pyridine (80AJC1743). Formation constants for the charge-transfer complexes between iodine and pyridine (and pico-lines) have been measured [84JCS(P2)731], and solvent effects on the... 

Spectroscopic methods can be used to specify the position of donors and acceptors before photoexcitation [50]. This spatial arrangement can obviously influence the equilibrium eomplexation in charge transfer complexes, and hence, the optical transitions accessible to such species [51]. This ordered environment also allows for effective separation of a sensitizing dye from the location of subsequent chemical reactions [52], For example, the efficiency of cis-trans isomerization of A -methyl-4-(p-styryl)pyridinium halides via electron transfer sensitization by Ru(bpy) + was markedly enhanced in the presence of anionic surfactants (about 100-fold) [53], The authors postulate the operation of an electron-relay chain on the anionic surface for the sensitization of ions attached electrostatically. High adsorptivity of the salt on the anionic micelle could also be adduced from salt effects [53, 54]. The micellar order also influenced the attainable electron transfer rates for intramolecular and intermolecular reactions of analogous molecules (pyrene-viologen and pyrene-ferrocene) solubilized within a cationic micelle because the difference in location of the solubilized substances affects the effective distance separating the units [55]. 

The molecular (space-filling) models in Fig. 1 illustrate the location of the anionic donors I- and Co(CO)4- relative to the cobalticenium acceptor for optimal orbital overlap with the LUMO in the equatorial plane (34). For the pyridinium salts of Co(CO)4", the analogous charge-transfer interaction of the tetracarbonylcobaltate donor places it above the aromatic acceptor planes for optimal orbital overlap with the ti-LUMOs of Q+ and NCP+. Such X-ray crystallographic structures indicate that these charge-transfer salts consist of contact ion pairs that are directionally constrained for optimum CT interaction in the crystal lattice. 

Cationic polymerizations induced by thermally and photochemically latent N-benzyl and IV-alkoxy pyridinium salts, respectively, are reviewed. IV-Benzyl pyridinium salts with a wide range of substituents of phenyl, benzylic carbon and pyridine moiety act as thermally latent catalysts to initiate the cationic polymerization of various monomers. Their initiation activities were evaluated with the emphasis on the structure-activity relationship. The mechanisms of photoinitiation by direct and indirect sensitization of IV-alkoxy pyridinium salts are presented. The indirect action can be based on electron transfer reactions between pyridinium salt and (a) photochemically generated free radicals, (b) photoexcited sensitizer, and (c) electron rich compounds in the photoexcited charge transfer complexes. IV-Alkoxy pyridinium salts also participate in ascorbate assisted redox reactions to generate reactive species capable of initiating cationic polymerization. The application of pyridinium salts to the synthesis of block copolymers of monomers polymerizable with different mechanisms are described. 

It is well known that cyano derivatives of anthracene form charge transfer (CT) complexes with certain aromatic compounds. It was reported [67] that the radical cations formed upon irradiation of these complexes played an important role in initiation of cationic polymerization of cyclic ethers. Pyridinium salts were also found [68] to form CT complexes with hexamethyl benzene and trimethoxy benzene which result in the formation of a new absorption band at longer wavelengths where both donor and acceptor molecules have no absorption. This way the light sensitivity of the pyridinium salts may be extended towards the visible range. According to the results obtained from the... 

Evaluation of solvent-sensitive properties requires well-defined referena i ran eis. A macroscopic parameter, dielectric constant, does not always give interpretable correlations of data. The first microscopic measure of solvent polarity, the Y-value, based on the solvolysis rate of t-butyl chloride, is particularly valuable for correlating solvolysis rates. Y-values are tedious to measure, somewhat complicated in physical basis, and characterizable for a limited number of solvents. The Z-value, based on the charge-transfer electronic transition of l-ethyl-4-carbomethoxy-pyridinium iodide , is easy to measure and had a readily understandable physical origin. However, non-polar solvent Z-values are difficult to obtain b use of low salt solubility. The Et(30)-value , is based on an intramolecular charge-transfer transition in a pyridinium phenol b ne which dissolves in almost all solvents. We have used the Er(30)-value in the studies of ANS derivatives as the measure of solvent polarity. Solvent polarity is what is measured by a particular technique and may refer to different summations of molecular properties in different cases. For this reason, only simple reference processes should be used to derive solvent parameters. 

Tetraalkylborates are mild and selective alkylation reagents [186, 187], and they are commonly considered as sources of nucleophilic alkyl groups (R ) just as borohy-drides are depicted as hydride (H ) sources. However, since organoborates represent excellent electron donors (see Table 5, Section 2.2.1), the question arises as to what role electron donor-acceptor interactions play in the nucleophilic alkyl transfer. Phenyl- and alkyl-substituted borate ions form highly colored charge-transfer salts with a variety of cationic pyridinium acceptors [65], which represent ideal substrates to probe the methyl-transfer mechanisms. Most pyridinium borate salts are quite stable in crystalline form (see for example Figure 5C), but decompose rapidly when dissolved in tetrahydrofuran to yield methylated hydropyridines (Eq. 65). 

Charge-transfer activation of the charge-transfer salts effects a spontaneous electron transfer [18] from the borate donor to the pyridinium acceptor which results in the formation of a radical pair (Eq. 67). 

Charge-transfer complexes with pyridinium cations 82UK185. Cycloaddition reactions of pyridinium N-methylides 82WCH231. Nucleophilic reactions of pyridinium salts 83MI13. 

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