Pyrylium tetrafluoroborate, reaction with

Triphenylpyrylium tetrafluoroborate is a versatile and useful stable starting material. Its reaction with nitromethane under basic conditions has made 2,4,6-triphenylnitrobenzene easily available. In addition, pyrylium salts are readily converted to a variety of pyridine derivatives i i . 20 including alkyl- and arylpyridinium salts, to thiopyrylium salts," and to substituted azulenes. ... 

Of the many PET Diels-Alder reactions of potential synthetic utility we mention two reactions of vinylindole (103) catalyzed by 2,4,6-tris(4-methoxyphenyl)-pyrylium tetrafluoroborate. With cyclohexadiene, 103 reacts as a diene, giving rise to tetrahydrocarbazole (104) with exocychc dienes, 103 serves as dienophile generating a different tetrahydrocarbazole (105). Molecular orbital calculations provide a rationale for the regio- and diastereoselectivities of these reactions. 

The direct reaction of4.4 -bis-2.6.2/6 -tetraphenyl-pyrylium tetrafluoroborate with tris-hydroxymethylphosphine did not lead to the desired product. 

NMA+) and 2,4,6-triphenyl-pyrylium tetrafluoroborate (TPP+) in the presence of biphenyl as cosensitizer were suitable for this reaction [174], The assumed mechanism of formation of do by this cosensitization is shown in Scheme 7. Reaction of do with H-donors such as te/t-butylmethylether, propionaldehyde and alcohols results in the formation of 1 1 adducts, the 1-substituted 1,2-dihydro-[60]fullerenes. Product structure support a H-abstraction process [212,213] rather than nucleophilic addition. In Scheme 8, the general formation of 1-substituted l,2-dihydro-[60]fullerenes is shown. Selected examples of the products obtained by this method are summarized in Table 10. 

The series of equilibria allow a change in the anion of a pyrylium salt to be accomplished. The readily available pyrylium perchlorates and tetrafluoroborates are converted into the pseudobases on treatment with weak bases. Reaction of these diketones with a wide variety of acids leads to the new pyrylium salt generally in high yield (Table 9) (80T679). In some... 

Tris(trimethylsilyl)phosphine (Me3Si)3P readily reacts with pyrylium salts, giving substituted phosphabenzenes12, and the reaction can be conveniently extended to include the preparation from 3-azapyrylium tetrafluoroborate of 1,3-azaphosphorines which, with acetylene carboxylates, gives the phosphabenzene with nitrile elimination (equation 6)13. Both 1,3,4-oxadiazolium and oxazolium salts react similarly to form the five-membered azaphospholes (equation 7)14. 

By 1916 Dilthey had synthesized 2,4,6-triphenylpyrylium ferrichlo-ride (tetrachloroferrate) from acetophenone and benzaldehyde in acetic anhydride in the presence of ferric chloride. He discovered other syntheses of pyrylium salts having aryl and alkyl substituents, as aryl groups enhanced the stability of pyrylium salts. Most often, the anions were perchlorate and tetrachloroferrate, but tetrafluoroborate began to be appreciated. A simple synthesis of alkyl-substituted pyrylium salts had to wait till the treatment of mesityl oxide with acetic anhydride was described by Schneider and Sack using sulfoacetic acid, and by Diels and Alder using perchloric acid. Then a lack of interest followed till the late 1950s, when Karl Dimroth and Klaus Hafner in Germany described new reactions of pyrylium salts, and new synthetic approaches were developed in Bucharest and in London. 

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