Pyridinium salts, charge-transfer donors

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. 

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. 

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... 

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). 

Using the empirical parameter of the solvent polarity Z based on the molar energy of the transition in E, in kilocalorie per mole (kcal/mol) for the CT band, the Ep values of sodium salt of l-methyl-4-[2-(4-hydroxyphenyl)ethenyl)]pyridinium] hydrogensquarate are shown in Table 6.3. These data illustrate the analogy between the Z values of the sodium salt of l-methyl-4-[2-(4-hydroxyphenyl)ethenyl)]pyridinium] hydrogensquarate with Ep(30), which is the empirical solvent polarity parameter, based on the intramolecular charge transfer absorption of a pyridinium-N-phenolate betaine dye. The Z values are practically equal in the solvents acetone, pyridine, and cyclohexane, which means that for a difference in the values of 8, solvent donor number (DN) and solvent acceptor number (AN) of 35.3, 33.1, and 18.9 (kcal/mol), the... 

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