Pyrylium ions, electrophilic substitution

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... 

Pyrylium ion should be even less reactive than the corresponding pyri-dinium cation in fact, no simple electrophilic substitutions are known. 

This basic reactivity of pyrylium ions is reversed by donor substituents in the 2-, 4- and 6-positions. For instance, 2,4,6-tris(dialkylamino)pyrylium ions 14 are stable towards nucleophilic attack, but are easily substituted by electrophiles (e.g. HNO3/H2SO4 or BrCN/AlCl3) to give 15 (SCHROTH 1989) ... 

A formally electrophilic substitution at the 4-position of pyrylium ions with a substitution pattern like 16 can be achieved using benzotriazole(Bt)-mediated methodology (Katritzky [3]). Pyrylium salts 16 are readily converted to 4-Bt-substituted 4//-pyranes 17 by addition of Bt-Na. Subsequent deprotonation, trapping of the Bt-stabilized anion by alkylation (to give 18) and acid cleavage of the benzotriazole moiety in 18 affords the 4-substituted pyrylium ions 19 [4] ... 

The reactivity of 18 may be understood in terms of the electron distribution in the donor-substituted pyrylium ion, which apparently does not correspond to a cyclically delocalized 6jt-system (18a), but rather to that of a localized trimethine cyanine (18b/c). (6) Due to the charge distribution in the pyrylium system, CH3 groups in the positions 2, 4, and 6 display marked C-H-addity, since they are deprotonated by base. In the resulting enol ethers 20 or 22, the methylene groups can take part in electrophilic reactions of the aldol or Claisen type (side-chain reactivity, compare p. 358) ... 

The pyrylium cation possesses, according to the substituents in positions 2, 4, and 6, a more or less pronounced electrophilic reactivity which enables it to add nucleophiles in these positions. According to the nucleophilic reactivity and the carbon basicity " of the anions, an ion pair (a substituted pyrylium cation and an anion halide, perchlorate, sulfate, fluoroborate, chloroferrate, etc.), or a covalently bonded 2H- or 4//-pyran may be formed. With the more basic anions... 

The attack of the acid (154) on the readily polarizable 1,2-dithiafulvene (155) corresponds to the extremely ready addition of electrophilic reagents to the simple and vinylogous heptafulvene derivatives, which are iso-n-electronic with 155. The opening of the dithiole rings in 156 and 158 under the pressure of the carbanionoid electron pair liberated by the proton abstraction and of the free electron pair on the sulfur, as well as the elimination of elementary sulfur and the intramolecular electrophilic attack of the mercaptide ion (157) on the 5-position to form 158, are simply the typical reactions of 1,2-dithioles that have already been discussed (Section II, B, 3). The reactivity of the 3-methyl group in 154 finds many parallels in the ease of condensation of the methyl-substituted pyridinium, pyrylium, thiopyrylium, and tropylium salts, and particularly... 

Heteroarenes show specific side-chain reactivity at heterobenzylic C-H-bonds in position 2 or 4 to the ring hetero atom. This is responsible for a large number of base-catalyzed electrophilic C-C forming processes like aldol, Claisen and Mannich reactions with appropriately substituted pyrylium and pyridinium ions, pyridines, benzazines and benzodiazines. 

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