Reactions of Pyrylium Cations

6-Triphenylpyrylium undergoes exchange at the 3- and 5-positions in hot deuterioacetic acid, but the process probably involves not protonation of the pyrylium cation, but formation of an equilibrium concentration of an adduct, with acetate added to C-2, allowing enol ether protonation and thus exchange.  

Nitration of 2,4,6-triphenylpyrylium proceeds on the benzene rings no nitrations of pyrylium rings are known. 

Pyrylium salts usually add nucleophiles at a carbon adjacent to the oxygen, and in many ways, such reactions are analogous with those of O-protonated carbonyl compounds. 

The degree of susceptibility of pyrylium salts to nucleophilic attack varies widely pyrylium cation itself is even attacked by water at 0°C, where 2,4,6-trimethylpyr-ylium is stable in water at 100 °C. Hydroxide anion, however, adds very readily to C-2 in all cases. 

The reaction of 2-methyl-4,6-diphenylpyrylium is typical the immediate 2-hydroxy-2-//-pyran, which is a cyclic enol hemiacetal, is in equilibrium with a dominant concentration of the acyclic tautomer, reached probably via a proton-catalysed process, since methoxide adducts remain cyclic. Treatment of such acyclic unsaturated diketones with acid regenerates the original pyrylium salt (section 8.3.1). 

6-Triphenylpyrylium undergoes exchange at the 3- and 5-positions in hot deuterioacetic acid, but the process probably involves not protonation of the 

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One electron transfer reactions of pyrylium cations and some related systems have been reviewed (94H116S). A single electron transfer is thought to be involved in the photochemical conversion of the diphenylcyclopropane (19) into the xanthylium salt (20) in the presence of an arene donor such as naphthalene (94CC1681). 

Phototransposition reactions of hydroxypyrylium salts have been studied extensively, and now the first report has appeared of the corresponding reactions of pyrylium salts with only alkyl substituents. The reaction is efficient only when both C-3 and C-5 bear an alkyl group, and this is interpreted in terms of either an oxoniabenzvalene primary product (14) that re-aromatizes thermally in a concerted manner, or an oxabicyclohexenyl cation (15) whose formation involves an activation energy that is sensitive to the stability of the cation. These results are consistent with others from the irradiation of the pyrylium salts in water, where alternative, photohydration products are formed. 

Recent important developments consist in the synthesis of the unsubstituted pyrylium cation by Klagcs and Trager, the preparation of pyrylocyanines by Wizinger, the development of simple syntheses for alkyl-substituted pyrylium salts by Balaban and Nenitzescu, Praill, Schroth and Fischer, Schmidt, and Dorofeenko, the discovery of a variety of reactions by Dimroth and Hafner, and the study of physical properties by Balaban. 

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. ... 

Although pyrylium cations in combination with an anion of a strong acid are stable, the presence of a formally charged oxygen atom renders them susceptible to reactions with nucleophiles and the valence bond description indicates that C-2(6) and C-4 are the potential targets for attack (Scheme 4.1). 

Related reactions include preparations of furans from pyrylium cations (Section 3.2.1.6.3.v). 1,4-Dithiins (56JA850) and 1,2-dithiins (67AG(E)698) readily lose sulfur on heating, yielding the corresponding thiophene (191 — 192 193 — 194). 

The distribution of charge in the resonance forms (28)-(31) suggests that nucleophiles may attack at C-2, C-4 or C-6 (or at C-2 or C-4 in 1-benzopyrylium cations, and at C-l, C-3 or C-4a in 2-benzopyrylium ions) but they most commonly add at C-2 for example, attack by cyanide ion gives a 2//-pyran (37) which exists partly or wholly as the acyclic isomer (38). Steric and electronic effects in the reactants probably have a role in determining the course of the reaction of trisubstituted pyrylium salts with nucleophiles. A mixture of both 2H- and 4H-pyrans is sometimes produced, for example, from methoxide ion and 2,4,6-triphenylpyrylium perchlorate (39) no acyclic product was detected in this reaction... 

Another characteristic of electrophilic reactions of pseudoazulenes is the application of numerous cations as the electrophile, for example, diazonium salts and Vilsmeier- Haack s reagent (see Table VI), tropylium ion,135 triphenylmethyl cation,"4 pyrylium ion,119 and dithiolium ion.166 Very stable cations are formed (e.g., 120) addition of base releases the substituted pseudoazulene (see example in Eq. 10). Generally reactions of this type are thermodynamically favored (see also Section IV,B). The site of substitution... 

Two benzoannelated analogues of the pyrylium cation are known ben-zo[b]pyrylium (chromylium or 1-benzopyrylium) and benzo[c]pyrylium (isochromylium or 2-benzopyrylium) salts. So far, the 1-benzopyrylium system is considered to be the more interesting (79MI2 84MI1) because it is the basic heterocyclic system of important plant pigments (antho-cyanins), and because there are major differences between properties of its heterocyclic ring and those of monocyclic pyrylium systems as a result of benzoannelation. Thus, for instance, 1-benzopyrylium salts do not possess the ability to have recyclization reactions with heteroatom exchange. 

Benzopyrylium salts have not been found in nature, and in spite of their ability to take part in recyclization reactions, until recently it was considered that there were no specific features in their transformations relative to monocyclic pyrylium salts (79MI2), and the scope of these transformations has been considered as rather restricted (71CB2984). The reason for such a conclusion was based on the incorrect interpretation of the effects of benzoannelation on reactions of 2-benzopyrylium cations so far no review on this subject has been published. 

The search for benzo[c]pyrylium systems and the elaboration of synthetic approaches to its development could lead to the discovery of novel regularities that might be applied in syntheses of different types of heteroaromatic and aromatic compounds. On the other hand, this route could give a possible systematic study of the influence of benzo[c]annelation on reactions of the pyrylium cation. 

The further behavior of benzo[c]annelated adducts 100 depends on the structure of substituents in the initial cation 30, nature of the nucleophile, thermodynamic parameters of the final products, and conditions of the experiment. The reaction may be stopped at the step of adduct 100 (b, 1 in Scheme 5) or may be continued with the formation of ring-opened intermediate 101 (b,2). However, the latter step has some specific features, in comparison with monocyclic pyrylium salts, as a consequence of the presence of the annelated benzenoid ring in benzo[c]pyrylium cations. 

Interestingly, the addition of the hydride ion to benzo[c]pyrylium cations with the formation of adducts of type 107 occurs not only in reactions with lithium aluminum hydride (54JOC1533) or sodium boro-hydride (89KGS750), but also on short-time heating of the formate 30 (R1 = ME, R2 = Et, R3 = Ver, R4 = OMe) in formic acid [67ACH-(51)107] (Scheme 7). 

The only competing reaction in interactions of 2-benzopyrylium salts with active methylene compounds is the formation of naphthalene 229, which is constructed with participation of the 1-methyl group of the ben-zo[c]pyrylium cation (90KGS315). 

AH attempts to convert dimer 263 into a dimeric 2-benzopyryIium salt, on treatment with triphenylmethyl or acetyl perchlorate, lead only to the rupture of the newly formed C—C bond and to the regeneration of the initial monomeric salt 261, unlike the behavior of dimers of monocyclic pyrylium cations [73DOK(212)370]. Dimerization may be considered a typical reaction for benzo[c]pyrylium-4-oxides of type 19, which react in dimerizations as 1,3-dipoles by analogy with their behavior in cycloadditions (Section III,E,2). 

The reaction of 4-1 dimerization is similar to the primary step of recycli-zation of 2-benzopyrylium salts in acidic nucleophilic media (cf. Section III,C,4,b,i), but the reactive electrophile is the initial cation in this case, and not a proton. Probably for this reason, the 4-1 dimerization of 2-benzopyrylium perchlorates is not observed in acidic nucleophilic media, in contrast to a-1 dimerization (cf. Section III,F,2,b). At the same time, the scope of 4-1 dimerizations is less restricted in terms of structural requirements for 2-benzopyrylium salts in comparison with a a-1 dimerization. Thus, in the latter case, the presence of a methyl group in position 1 of benzo[c]pyrylium cation is compulsory, whereas for 4-1 dimerizations, the nature of the substituent in this position may be different, leading to a variety of 4-1 dimers and, as a consequence, to a wide variety of their transformations. 

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