Pyrylium tetrafluoroborate

Although cyclic azoalkanes are well known as biradical precursors [159] they have been used as 1,2- and 1,3-radical cation precursors only recently [160-164]. Apart from the rearrangement products bicyclopentane 161 and cyclopentene 163, the PET-oxidation of bicyclic azoalkane 158 yields mostly unsaturated spirocyclic products [165]. Common sensitizers are triphenyl-pyrylium tetrafluoroborate and 9,10-dicyanoanthracene with biphenyl as a cosensitizer. The ethers 164 and 165 represent trapping products of the proposed 1,2-radical cation 162. Comparison of the PET chemistry of the azoalkane 158 and the corresponding bicyclopentane 161 additionally supports the notion that the non-rearranged diazenyl radical cation 159 is involved (Scheme 31). 

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. 

However, radical intermediates cannot be definitely excluded, at least not in the reaction of pyiylium salts in pyridine. Steuber showed that such pyrylium salts as 2.4.6-triphenyl-pyrylium-or 2.4.6-tri-tert-butyl-pyrylium-tetrafluoroborate can be reduced to stable pyryl radicals 32 by pyridine this reduction proceeds particularly smoothly if traces of copper powder are added. 

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

Either the pyrylium tetrafluoroborate or the perchlorate salt may be converted into the corresponding halide by initial conversion into the pseudobase [(90), l,3,5-triphenylpent-2-en-l,5-dione] with dilute alkali, followed by recycli-sation in the presence of the hydrogen halide.24... 

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. 

Calculations relating to the aromatic nature of selenabenzene derivatives such as 3 have been reported <2001HCA1578> (see Section 7.11.2). Crystal structure studies confirming the aromatic nature of 2-benzoseleno-pyrylium tetrafluoroborate 19 and 2-benzotelluropyrylium tetrafluoroborate 20 have been described <2002J(P1)606> (see Section 7.11.3.1). Spectroscopic studies on Se-methylselenabenzenes 26 also indicate the aromatic nature of these molecules <1999J(P1)1155> (see Section 7.11.3.2). 

Interesting rearrangements have been observed when the phenylnitromethane anion is added to 2,4,6-arylated pyrylium salts. A sensitive intermediate, probably the dihydrobenzene derivative 1 was isolated from 2,6-diphenyl-4-(p-tolyl)-pyrylium-tetrafluoroborate and phenylnitromethane in t-butanol with only one mole potassium-t-butylate. When heated in toluene, the dihydrobenzene derivative 2 which is sub-... 

When 2,4,6-tri- or 2,3,4,6-tetraarylated pyrylium tetrafluoroborate is treated with nitromethane under different conditions rearrangement is also observed. No rearrangement occurs when the two components are heated together with an excess of potassium t-butylate in t-butanol (route a). Under strong basic conditions the intermediate 11 probably loses the acidic proton at position 1 to give the aromatic nitro compound 12. 

When the pyrylium tetrafluoroborate is reacted in ethanol or 1,2-dichlorobenzene with nitromethane, however, and only one mole of triethylamine is used, a much smaller yield of the normal nitrobenzene derivative 12 is found. Two different... 

The pyrylium tetrafluoroborate (20 mmol) and the primary amine (20 mmol) were stirred in abs EtOH (50 mL) for 6h. The mixture was diluted with Et O (100 mL) to give the pyridinium salt as a w hite. crystalline material, which was recrystallizcd from abs EtOH. 

A photocycloaddition of a different type has been reported between indole and substituted cyclohexa-1,3-dienes triaryl-pyrylium tetrafluoroborates are used as sensitisers in this electron-transfer process and the [ 4 + 2] adducts are isolated as their W-acyl derivatives (111). High regioselectivity is observed. 

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