Pyridinyl Radicals

The ESR hyperfine coupling constants have been established experimentally (67MI20402) for the pyridinyl radical (134 R = H) and deuterated analogues, produced by y irradiation of a solid solution of pyridine in ethanol at 77 K, but the signs of the couplings are not known experimentally and are made solely on the basis of Huckel MO calculations. INDO MO calculations on this radical, together with the radical anions of quinoline, isoquinoline and acridine h ve also been carried out (740MR(6)5). 

An example from a recent report on arylations with aryl and heteroaryl radicals is depicted in Scheme 38. Within this series of experiments, in which the substrate was used as solvent, remarkable regioselectivities were observed for some examples. The arylation of chlorobenzene with a 2-pyridinyl radical derived from bromide 103 gave a ratio of ortho para arylation products 104 and 105 of 13 1 [156, 157]. 

Scheme 38 Arylation of chlorobenzene with 2-pyridinyl radicals [156]...

Kosower s radical (66) reacts with alkyl halides by a mechanism which transfers a halogen atom to the pyridinyl radical in the rate determining step (Py is 66) ... 

Many 1-alkyl-l-hydropyridinyl radicals are not persistent in aqueous medium. The bimolecular decay reaction has been investigated for 66 and 70 and a mechanism consistent with products and kinetics advanced.239 The reactions of 70, its 3-carboxamide isomer, and the pyridinyl radical derived from nicotinamide adenine dinucleotide (NAD) with cytochrome c have been investigated by pulse radiolysis and rates established.240... 

From a study of the consequences of 7t-complex formation for the UV spectra of pyridinyl radicals a geometry of the rc-complex was proposed. As usual for such complexes, a parallel disposition of the aromatic planes was suggested, mutually oriented to minimize steric interactions between substituents.246... 

Molecular complexes frequently exhibit two intermolecular charge-transfer transitions.256 The energy difference between the two such transitions in 1-alkylpyridinium iodides has been correlated with the HOMO-LUMO transition in the corresponding pyridinyl radicals, supporting the contention that the charge-transfer transitions have their origin in dual electron-acceptor levels.257... 

A PET in intramolecular CPs between pyridinium ions and bromide, chloride or thiocyanate ions for polymerization initiation is described, too [137-139]. As expected, an equilibrium exists among free ions, ion pairs, and CT, which is shifted to the free ions in polar solvents and to the complex in a less polar solvent That complex serves as the photosensitive species for the polymerization (see Scheme 10). The photodecomposition of the CT yields radicals of the former anion and N-alkylpyridinyl radicals. Probably, the photopolymerization is initiated only by X- radicals, whereas latter radicals terminate the chain reaction. By addition of tetrachloromethane, the polymerization rate is increased owing to an electron transfer between the nucleophilic pyridinyl radical and CC14 (indirect PET). As a result, the terminating radicals are scavenged and electrophilic -CQ3 radicals are produced. 

Various compounds were shown to sensitize the photochemical decomposition of pyridinium salts. Photolysis of pyridinium salts in the presence of sensitizers such as anthracene, perylene and phenothiazine proceeds by an electron transfer from the excited state sensitizer to the pyridinium salt. Thus, a sensitizer radical cation and pyridinyl radical are formed as shown for the case of anthracene in Scheme 15. The latter rapidly decomposes to give pyridine and an ethoxy radical. Evidence for the proposed mechanism was obtained by observation of the absorption spectra of relevant radical cations upon laser flash photolysis of methylene chloride solutions containing sensitizers and pyridinium salt [64]. Moreover, estimates of the free energy change by the Rehm-Weller equation [65] give highly favorable values for anthracene, perylene, phenothiazine and thioxanthone sensitized systems, whilst benzophenone and acetophenone seemed not to be suitable sensitizers (Table 5). The failure of the polymerization experiments sensitized by benzophenone and acetophenone in the absence of a hydrogen donor is consistent with the proposed electron transfer mechanism. 

A nice example is the disproportionation reaction of the l-ethyl-4-(meth-oxycarbonyl)pyridinyl radical (43), which leads to the ion pair (43a)l(43b) [215, 216]. 

Halogen abstraction by the stable free radical 1 -ethyl-4-(methoxycarbonyl)pyridinyl (Py ) proceeds by the mechanism shown in Eq. (5-66) [214, 570]. The first step, which is rate-determining, is a transfer of the halogen atom to the pyridinyl radical. 

The negligible solvent effect of this radical reaction with dibromomethane demonstrates that the activated complex for bromine atom-transfer has the same charge separation as the initial reactants. The dipole moment expected for a molecule like the pyridinyl radical is probably (0... 10) 10 ° Cm (0...3 D). Dibromomethane has a modest dipole moment of 5 10 Cm (1.5 D). Consequently, in view of the negligible solvent effect upon rate, the activated complex must also have a dipole moment between (0... 10) 10-3 Cm [214, 570]. 

It has been established that for solvents in which specific solvation is not dominant, a small solvent effect implies an atom-transfer reaction and a large solvent effect suggests an electron-transfer reaction between neutral species. The high solvent sensitivity of electron-transfer reactions between neutral molecules should provide a useful test of their occurrence [215, 570]. From Table 5-11, it can be concluded that atom-transfer, according to Eq. (5-66), is the rate-limiting step in the reaction of pyridinyl radical with... 

Table 5-11. Rate constant solvent effects for the reaction of haloalkanes with l-ethyl-4-(methoxy-carbonyl)pyridinyl radicals at 25 °C [215, 570],...

Rate constant for the reaction of pyridinyl radical in acetonitrile divided by that for the reaction in 1,2-dimethoxyethane and the corresponding change in Gibbs activation energy. 

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