Pyrylium fluoroborates

Nitro compounds, ar. Nitrobenzene, m-Dinitrobenzene o-Nitrophenol N-Nitramines Oxonium fluoroborate Trialky loxonium salt Trialky loxonium fluoroborate Pyrylium fluoroborate Carboxylic acid esters HCOOC Hs, p-Nitro-phenyl formate Isopropenyl acetate Salicylic acid esters Ethyl cyanoacetate Chloroformic acid esters Ethyl chloroformate, ClCOOC Hg-i Methyl trichloroacetate Diethyl oxalate Ethyl malonate... 

Nitro compounds, ar. Nitrobenzene, m-Dinitrobenzene o-Nitrophenol N-Nitramines Oxonium fluoroborate Trialky loxonium salt T rialky loxonium fluoroborate Pyrylium fluoroborate Carboxylic acid esters... 

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... 

Tri- butylpyrylium can be obtained by dehydrogenating the corresponding 1,5-diketone with triphenylmethyl fluoroborate, It was shown by Farcasiu that 1,5-diketones can also he dehydrogenated and dehydrated to pyrylium salts by triphenylmethyl hexachloroantimonate generated in situ from chlorotriphenyl-methane and antimony pentachloride. Even pentaphenylpyrylium may thus be prepared at room temperature. ... 

Pyridyl)tetrahydropyran (32), when treated with ammonia over alumina at about 400°C, also gives 2,2 -bipyridine. In another related synthesis, the pyrylium salt 4,6-diphenyl-2-(2-pyridinio)pyrylium bis(tetra-fluoroborate) was converted to 4,6-diphenyl-2,2 -bipyridine by anhydrous ammonia in ethanol. ... 

The unsubstituted benzo[c]pyrylium-4-oxide 19 was formed from iso-chroman-4-one derivatives 17 [76ACS(B)619] or from 18 (84CC702). All attempts to isolate oxide 19, for instance as a fluoroboric acid salt, failed because it reacted further by polymerization and dimerization [76ACS(B)619]. Even in substituted systems 15, in spite of added stabili-... 

The two target molecules, which illustrate cyclisation reactions involving substituted 1,5-dicarbonyl compounds in the presence of appropriate reagents, are diethyl 2,6-dimethylpyridine-3,5-dicarboxylate (85) and 2,4,6-triphenylpyrylium fluoroborate (86). The latter is of specific interest since it provides a simple example to illustrate a general procedure for the conversion of pyrylium salts into pyridinium derivatives these latter compounds are important reagents for a variety of functional group interconversions (see Sections 5.5.6, p. 574, and 5.15.3, p. 768). 

For the preparation of spirobi(pyrans), the pyrylium salt (preferably the easily purified perchlorate or fluoroborate) may be used, but it is often more convenient to merely condense the appropriate hydroxyaldehyde(s) with the requisite aliphatic ketone either directly using gaseous HC1 in ethanol, or via a base-catalyzed condensation to first give a 2-hydroxychalcone intermediate.5... 

H Chemical Shifts" (ppm) and H, H Coupling Constants (Hz) of Pyrylium, Thiopyrylium, and Selenopyrylium Fluoroborates in CD,CN ... 

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). 

Like trityl perchlorate and trityl fluoroborate (1, 1256-1258 2, 454), this salt can be used for hydride abstraction one advantage over the perchlorate salt is that it is not explosive. It has been used for the conversion of cycloheptatriene into tropylium hexachloroantimonate1 and for conversion of 1,5-diketones into pyrylium salts.2... 

A detailed study of this dehydrogenative condensation in the presence of triphenylmethyl perchlorate or fluoroborate was made by Simalty-Siemiatycki and Fugnitto. The reaction is best carried out in refluxing acetic acid nitromethane or acetonitrile give less satisfactory results. Chalcone reacts in these conditions with phenylacet-aldehyde yielding 2,4-diphenylpyrylium with an unsubstituted a-position. This and similar 2-unsubstituted pyrylium salts prepared by this method are so reactive that they do not afford pyridines on treatment with ammonia in the usual conditions this behavior is similar to that of the unsubstituted pyrylium perchlorate. The reaction of... 

Recently, Tsao and Ketley have reported that thiopyrylium salts, VI, are active photoinitiators for the cationic polymerization of epoxides These compounds were prepared by established techniques from the corresponding pyrylium tetrafluoro-borate salts, which can be obtained by treating benzaldehydes and acetophenones with phosphoryl chloride followed by fluoroboric acid as shown in Eq. (89). 

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