Pyrylium radical dimerization

A typical example of such behavior is given by the reduction of pyrylium cations. Under steady state or moderate-scan cyclic voltammetry, their reduction involves the net consumption of one electron per molecule, which corresponds to the fast radical dimerization in Eq. (23). 

Dimerization Of Pyrylium Radicals. Determination Of Rate Constants Close To The Diffusion Limit. One electron reduction of 4-unsubstituted pyrylium cations affords radicals which are known to undergo dimerization with rate constants close to the diffusion limit. Thus, on the basis of flash-photolysis experiments, the dimerization of the 2,5-diphenyl-pyrylium radical, (1), has been estimated to proceed with a rate constant of 1.2 x 10 M... 

To establish the experimental achievability of this target the dimerization mechanism of 2,6-diphenyl-pyrylium radicals electrogenerated by reduction of the corresponding cation, in eqn.4, was selected. The mechanism in eqn.4 was first postulated for the zinc powder reduction of pyrylium cations in acetonitrile and further confirmed by... 

Disubstituted and 2,4,6-trisubstituted pyrylium cations of type 158a,b are capable of a one-electron reduction to reactive 4-pyranyl radicals 162, dimerizing easily to bis-4//-pyrans 163a,2,8-220 163b,22 163c,221,222 and 163d223,224 in variable yields. 

One-electron reduction of pyrylium salts, with dissolving metals or electrochemically, gives dimers (e.g. 382) via pyranyl radicals (80AHC(27)46). 

At the same time, dimerization of one-electron reduction product was indicated by oscillopolarographic studies in acid for l-R-3,4-diphenyl-6,7-dimethoxybenzo[c]pyrylium perchlorates (R = Me, Et, Ph) (76MI1). As was noted in Section III,F, 1, the benzo[c]pyrylium cation 261, unsubstituted in position 1, undergoes l, 1 -dimerization via the postulated intermediate radical, and the bisisochromene 263 thus formed is readily converted into the initial monomeric salt, but not into the corresponding biscation on treatment with chemical electron acceptors (76KGS999). 

Although the constant K includes a contribution from the reversible dimerization of the pyranyl radical in Eq. (41), the trend generally shows an increase with acceptor strength ( %) of the pyrylium cation and donor strength ( °,) of the oiganometalhc anion. 

In the early 1960s, it was found that pyrylium ions undergo single-electron reduction, either by chemical or electrochemical means, with the formation of pyranyl radicals which usually dimerize essentially entirely by coupling at the 4-position.Reduction of 2,4,6-triphenylpyrylium salts, conveniently by zinc dust in inert solvents, led to the red radical 29 which persists for several days in a sealed tube. ° The ESR spectrum of 29 was recorded and eventually analyzed by Degani et Their hyperfine splittings... 

Soviet workers have examined the polarographic properties of unsym-metrically substituted pyrylium ions (e.g., 34 and 35). As expected, pyranyl radicals intervene and dimerize. Adsorption phenomena at the dropping mercury electrode are important. The consequences for the electrode and homogeneous processes of variation in substitution, solvent, acidity, etc. were explored. 

Pyryliiim [181-183] and isobenzopyrylium [184] salts have been shown to be polaro-graphically reducible in a one-electron reduction. In cyclic voltammetry in aprotic solvents, 2,4,6-trisubstituted pyrylium salts show two peaks the shape of the peak depends on the rate of dimerization. This process occurs more rapidly at C-4 than at C-2 (C-6) [182, 183], and the dimerization takes place spontaneously for 4-unsubstituted pyrylium salts. The equilibrium between radicals and dimer is displaced in favor of the radicals on introduction of electron-withdrawing substituents such groups enhance the aromatic character of the radical [185]. If the reduction of pyrylium salts is made in the presence of an alkyl iodide a fair yield of the 4-alkylated 4i/-pyrane is isolated with the dimer [182]. 

The pyranyl radicals formed by reduction of pyrylium cations (108) dimerize at the 4-position, and anodic oxidation of the dimers leads to regeneration of the pyrylium cations [309 and refs, therein]. Where the 4-position is unsubstituted, the dimerization process is very fast and irreversible, and a rate constant of 2.5 x 10 M s has been measured for the dimerization step in the reduction of 2,6-diphenylpyrylium cation (108a) in MeCN by combination of LSV and fast CV using microelectrodes [310]. If a substituent other than H or Me is present in the 4-position, the dimerization process becomes reversible and in favor of the free radicals [309]. 

Stable cations as tropylium salts [144], cyclopropenylium salts [145], alkyl-pyridinium salts (Eq. 18) [146], and pyrylium salts [147] have been dimerized at the cathode and radicals are proposed as intermediates. 

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