Pyryliums reaction with amines

Pyrones, which are the ring-oxygen equivalents of pyridones, are simply a- and y-hydroxy-pyrylium salts from which an 0-proton has been removed. There is little to recommend that 2- and 4-pyrones be viewed as aromatic they are perhaps best seen as cyclic unsaturated lactones and cyclic p-oxy-a,P-unsaturated-ketones, respectively, for example 2-pyrones are hydrolysed by alkali, just like simpler esters (lactones). It is instructive that, whereas the pyrones are converted into pyridones by reaction with amines or ammonia, the reverse is not the case - pyridones are not transformed into pyrones by water or hydroxide. Some electrophilic C-substitutions are known for pyrones and benzopyrenes, the oxygen guiding the electrophile ortho or para, however there is a tendency for electrophihe addition to the C-C double bond of the heterocyclic ring, again reflecting their non-aromatic nature. Easy Diels-Alder additions to 2-pyrones are further evidence for diene, rather than aromatic, character. 

The addition of ammonia or primary and secondary amines to pyrylium salts is usually accompanied by a rapid ring opening without detection of the expected 2//-pyran intermediates. The exceptional behavior of the salt 160a was observed in its reaction with Af./V-dimethylaniline, affording 85% of 4H-pyran 210, whereas analogous addition products from toluene and methoxy-benzene were too unstable to be isolated.214... 

A 13C NMR study of the reaction of amines with pyrylium salts includes assignments of the resonances of a range of substituted pyrylium salts (81CS(18)256). 

The preparation of highly functionalized cyclopentadiene derivatives may by achieved by using oxalenes or their pyrylium salts (90 and 92, respectively), (see Scheme 14, where X = O). In this way, azalenes 26,51 29,8Z 88 32,51 or 3651 can be obtained by heating salts (90) with primary amines in dimethyl-formamide (Eq. 6). The reaction with sodium hydrogen sulfide gives thialenes... 

The effect of the structure of the alkyl substituent in primary amines on the course of reaction with these amines was observed also for monocyclic pyrylium salts [66T(57)9 87TL3143 89RRC1425]. The interaction of ben-zo[c]pyrylium-4-oxide 15 with aniline follows a different recyclization pathway, which is interpreted as shown here (64JA3814). 

A specific behavior in reactions with primary amines was described for l-aryl-3-carboxy-benzo[c]pyrylium salts 62 (89KGS454). While their monocyclic analogs undergo decarboxylation with heteroatom exchange,... 

It was shown by Katritzky (80T679) that acetic acid promotes recyclization of triaryl-pyrylium salts in reactions with primary amines. 

For benzo[c]pyrylium salts having different alkyl substituents in positions a(l) and a (3), the reaction with secondary amines loses its predictable regioselectivity. Thus, reaction with dimethylamine, independent of the solvent, gives rise to /3-naphthylamine 185, from l-ethyl-3-methyl-substituted salt 183 (81KGS1608), and a-naphthylamine 186 from 1-benzyl derivative 184 (88UP1). In the former case, the nucleophilic attack occurs in position 3, whereas in the latter case, it takes place in position 1. 

The outline procedure involves the initial reaction of the 2,4,6-triphenyl-pyrylium halide with the primary amine to yield the corresponding 2,4,6-triphenylpyridinium halide (see Section 8.4.1, and also Section 5.15.3, p. 768) this reaction proceeds either at room temperature in a suitable solvent, or more efficiently under reflux in benzene with azeotropic removal of water. Pyrolysis of the pyridinium halide under controlled conditions then yields the alkyl (or aralkyl) halide in good yield. The mechanism of the reaction in this case is probably of the Sn2 type. 

In the context of mechanistic studies, the electrochemical behavior and reactions with nucleophiles of 4-chloro-2,6-diphenylpyrylium and 4-chloro(bromo)flavylium have been studied <1999CHE653>. The proposed mechanism for nucleophilic substitution in halogen-substituted pyrylium and flavylium salts passes through formation of a charge-transfer complex that is converted into an ion-radical pair by simple electron transfer. Heterocyclic cleavage of the C-halogen bond occurs at the stage of the radical or the adduct from the reaction of the pyrylium salt and the nucleophile. In this study, an amine nucleophile was used however, the data are likely relevant for other types of nucleophiles as well (Scheme 5). 

As is the case for the reaction of secondary amines with alkylpyrylium salts to give dialkylanilines, benzopyrylium and pyrylium salts react with amines to give similar products (e.g., 113—>114 Equation 8) <1982AHC(32)128, 1970KGS1308, 1981KGS1351, 1987KGS889, 1998CHE159>. 

The first step involves the formation of a pyridinium ion by reaction of a pyrylium ion with a primary amine the second step (dequaternization) has been studied more extensively than the first (amine + pyrylium). This important work on dequaternization deserves special mention because, besides the value for synthesis and understanding of steric acceleration, it sheds new light on the mechanism of aliphatic nucleophilic substitution (84CSR47). 

The basis of a reusable colorimetric probe for cyanide ion in water is the opening of a polymer-bound pyrylium ring by the anion <05CC2790>. Oligomers derived from the reaction of a -phenylene-bis-4,4 -(2,6-diphenylpyrylium) salt with amines are rod-like and range in length from 2-9 nm <05JOC405>. 

It is important to realise that 1-benzopyrylium salts cannot be converted into quinolines or quinolinium salts by reaction with ammonia or primary amines (cf. pyryliums to pyridines, 11.1.2.2), whereas 2-benzopyrylinm salts are converted, efficiently, into isoqninolines or isoquinolinium salts, respectively."... 

Further extensions of the use of pyrylium salts as synthons have appeared for example, their reaction with hydrazines. The xanthyUum salt (27 A = 0) reacted with amines under very mild conditions and in good yield and it has been used to synthesize sulphones, sulphides, and ethers in the presence of phase-transfer catalysts for example, 2-hexyloxynaphthalene was obtained in 70% yield by heating the acridinium salt (27 A = NC Hia) with sodium 2-naphthoxide and Bu4N A phase-transfer catalyst has... 

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