Pyranyl radicals

Radicals Containing a Six-Membered Heterocycle 1. Pyranyl Radicals 

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 

Other reductions of 2,4,6-triphenylpyrylium ion to give 29 have been examined. Tetramethyl-p-phenylene diamine (TMPD) transfers one electron and ESR spectroscopy shows 29 and TMPD to result. Chromium(II) ion was shown to reduce 2,4,6-triphenylpyrylium ion and other related cations. A comparison of chemical reactivity with reduction potential suggested an outer-sphere activated complex. A scale of the relative stabilities of the various radicals was deduced. Since 29 results when 2,4,6-triphenyl- 

Hiinig, G. Kiesslich, F. Linhart, and H. Schlaf, Justus Liebigs Ann. Chem. 752,196(1971). 

Radical 29 is formed from the precursor cation at different potentials in different solvents.Donor solvents lower the reduction potential the lowering has been suggested as a measure of the specific solvation of the triphenylpyrylium cation. 

The ECL observed when 29 is generated in the presence of rubrene cation-radical arises via the triplet state of rubrene. The triphenyl-pyranyl radical (29) has also been used in dynamic nuclear polarization experiments.  

Berberova, G. N. Dorofeenko, and O. Yu. Okhlobystin, Khim. Geteroisikl. Soedin., 

---

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. 

Electrochemical investigations show that 4//-pyranyl radicals 162 and 633 as well as corresponding radical anions 197 are reversibly oxidized to appropriate pyrylium ions at about -0.3 to -0.4 y.224,225,228,239... 

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

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. 

From the decay kinetics of the 550-nm band, in the absence and presence of oxygen, the rate constant for the process pyranyl radical TPF plus oxygen can be estimated to be ca. 107 dm3 mol-1 s 1. This latter result shows that TPF is far less efficient in generating superoxide ion than several cyanoaromatic radical-anion sensitizers. Finally, cyclic voltammetric experiments, under oxygen atmosphere, clearly show that the reversible CV curves, recorded for the sensitizer under inert atmosphere, become totally irreversible. This typical EC behavior [185] had also been observed in a case previously reported [186]. 

However, with triarylpyrylium acceptors the stable 17-electron radicals are generated in high yields due to a shift of the endergonic equilibrium by the homolytic dimerization of the pyranyl radicals (2 P P2) [183, 184]. 

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. 

As indicated in Section pyranyl radicals dimerize by coupling at... 

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]. 

UV-VIS measurements were also used to study the reversible coupling of the corresponding pyranyl radicals to 4-4 -bis-4//-pyran 96 (86NJC345 86TL4489 89BCJ2279 89RCI57). 

Dimethyl-2//-pyranyl radical ion and 2-methylpyrylium ion also were found among the products of intracluster polymerization reactions of acetylene and acetone (91JPC9625). Mass spectrometric fragmentations of 2,6-diaryl-4,4-diphenyl-4//-pyrans 8 have also been investigated in detail (87CP623). 

Pyranyl foam n. A type of rigid, pour-inplace, thermosetting foam similar to a polyurethane foam, but with superior resistance to high temperatures, it is formed in the same maimer as polyurethane foams, using as the monomer a pyranyl (radical) derived from polypropylene by heating and oxidation to form an Acrolein dimmer, which ultimately forms the pyranyl. 

---