Pyrylium cation reduction

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. 

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). 

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). 

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]. 

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... 

A 2,4)6-trisubstituted 2H or 4/f) pyran (38, R = R = Ph) was reported to result in low yield by catalytic reduction of 2,4,6-triphenyl-pyrylium salts by oxidation or by treatment with concentrated sulfuric acid it regenerated the triphenylpyrylium cation. There was no subsequent confirmation of this reaction. The reduction of pyrylium salts with sodium borohydride affords 1,5-diones by way of 4H-pyrans and 2,4-dien-l-ones by way of 2H-pyrans. ... 

Catalytic reduction of thiopyrylium salts under forcing conditions yields tetrahy-drothiopyrans, but oxidation with manganese dioxide of the unsubstituted thiopyrylium cation (first obtained by Pettit [100]) affords thiophene-2-carbaldehyde in a reaction without analogy to pyrylium salts [101], Like a- and y-benzylic positions in pyrylium salts, methyl(ene) groups of alkyl substituents in thiopyrylium cations are usefully acidic and as such the corresponding anhydrobases may be trapped by various nucleophiles. 

The electron-impact mass spectra of bromides, iodides, and fluorobo-rates of the 2,4,6-triphenyl-substituted cations 8 and 9 have the base peak at the mass number of the cation (74OMS80). No molecular ion peak of an adduct between the cation and the anion has been found the fluoroborates show also weak peaks with the elemental composition of an adduct between the cation and F". On the contrary, the spectra of perchlorates do not show the peaks at the mass number of the cation but peaks indicating the addition of an oxygen atom and the removal of a hydrogen atom. From ionization potential measurements it has been shown that the bromides, iodides, and fluoroborates of 8 and 9 are thermally reduced in the mass spectrometer to volatile free radicals 50 and 51 prior to evaporation, presumably with concomitant oxidation of the anion. In the presence of a nonoxidizable anion, e.g., perchlorate, reduction of the cations to free radicals does not take place. Interestingly, the order of ionization potentials of the radicals, 50 < 51, indicates that the LUMO energy level of pyrylium is higher than that of thiopyrylium, consistent with electrochemical studies (Section II,D). 

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