Download (direct link):
19 W. Schroth and G. Fischer, Z. Chem. 4, 281 (1964).
20 W. Dilthey and B. Burger, Ber. Deut. Chem. Ges. 54, 825 (1921).
21 H. Decker and T. von Fellenberg, Ann. Chem. 356, 281 (1907); 364, 1 (see p. 37) (1908).
B. Nomenclature and Formulation
(they actually proposed phenopyrylium, for what we now call benzo-pyrylium) in 1907-1908. Two years later, Baeyer tried to replace this name by pyroxonium because “pyrylium offers no fulcrum to the memory. ”2 2 Later, Dilthey proposed several other names: pyrylenium, pyronium, and pyrenium. He argued23 the pyrenium is the best name because pyronium or pyroxonium suggest pyrone salts, while pyrylium or pyrylenium suggest similarity with the pyrryl (pyrrole) radical. However, the name pyrylium had already gained general acceptance.
The Chemical Abstracts nomenclature will be adopted, but when common trivial names are available, these will also be employed, e.g., 4-pyrone along with 4//-pyran-4-one. Unlike Chemical Abstracts, thio will be used to designate a thione and thia will imply a sulfur heteroatom.
According to Chemical Abstracts, the pyrylium ring is numbered as shown in formula 14. Positions 2 and 6 may also be denoted by a, positions 3 and 5 by j8, and position 4 by y, as in pyridine.
The commonly employed benzenelike oxonium formula (la) will be used in preference to other proposed formulations (15 or 16), although all reactions of pyrylium salts involve carbonium structures (lb or lc).
'IT 'O 'U +
(14) (15) (16)
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 reactivity24,25 and the carbon basicity26-27 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
22 A. Baeyer, Ber. Deut. Chem. Ges. 43, 2337 (1910)/
23 F. Quint and W. Dilthey, Ber. Deut. Chem. Ges. 64, 2082 (1931).
24 J. O. Edwards and R. G. Pearson, J. Am. Chem. Soc. 84, 16 (1962).
25 J. F. Bunnett, Ann. Rev. Phys. Chem. 14, 271 (1963).
26 A. J. Parker, Proc. Chem. Soc. p. 371 (1961).
27 R. Gompper, Angew. Chem. 76, 412 (1964).
(softer bases28-30) such as hydroxyl, cyanide, carbonate, or acetate, the pyrylium salts react rapidly in two ways. If the cations possess an acid proton in a benzylic position, this is split off yielding an anhydro base (pyrones, pyronimines, methylenepyrans). Otherwise, addition occurs at a- or ó-positions, and a pyran is formed. In the particular case of the hydroxyl anion (17), the pseudo base thus formed may be a 4- or a 2-pyranol (nomenclature after Biilow and Wagner,31 and Baeyer and Piccard32) or a ring-opened product. The
nucleophilic reactivity of the 2-position usually exceeds that of the 4-position, but the 2-pyranol (18) is unstable because in these un-safcurated hemiketals the equilibrium is displaced toward the open-chain diketone (19 === 20).
Under the influence of acids, cM-2-ene-l,5-diones (20) or their enol forms (19) are converted by dehydration back into pyrylium salts. Accordingly, any synthetic method resulting in 1,5-enediones is, in fact, a method for the synthesis of pyrylium salts.
28 R. G. Pearson, J. Am. Chem. Soc. 85, 3533 (1963); Science 151, 172 (1966); Chem. Brit. 3, 103 (1967).
29 R. G. Pearson and J. Songstad, J. Am. Chem. Soc. 89, 1827 (1967).
30 B. Saville, Angew. Chem. 79, 966 (1967).
31 C. Biilow and H. Wagner, Ber. Deut. Chem. Ges. 34, 1189 (1901).
32 A. Baeyer and J. Piccard, Ann. Chem. 384, 208 (1911).
The first chemists to prepare a monocyclic pyrylium salt were von Kostanecki and Rossbach33 who in 1896 described the fluorescence in dilute aqueous solution of the reaction product obtained from
1,3,5-triphenylpcntane-1,5-dione (benzylidene-diacetophenone) and sulfuric acid. However, they failed to isolate the compound which caused the fluorescence and did not suspect that it was a pyrylium salt. It was only in 1916-1917 that Dilthey34 recognized that this fluorescence resulted from 2,4,6-triphenylpyrylium (3).
The puzzling discovery of Collie and Tickle85 in 1899 that 2,6-dimethyl-4-pyrone (8) affords crystalline salts (9) with acids, which were the first monocyclic pyrylium salts to be isolated, was interpreted by a formula (21) with tetracovalent oxygen.36’36 An active period of research followed. The methosulfate37 or methiodide88 of
2,6-dimethylpyrone was converted by ammonia into 4-methoxy-2,6-lutidine,22 therefore, the exocyclic oxygen of the pyrone must be involved in the salt formation. Thus, formula 21 was disproved and formula 22 was demonstrated for these salts.