Pyrylium salts alkylation

Pyrylium salts alkyl groups reactivity, 3, 662 aromaticity, 3, 640 arylammes from, 3, 657 benzenoid compounds from, 3, 656, 658 benzisoxazol-3-yl-synthesis, 6, 124 bicyclic... 

Pyrylium salts, 3-acetyl-2,4,6-trimethyl-crystallography, 3, 625 Pyrylium salts, 3-alkoxymethyl-synthesis, 3, 865 Pyrylium salts, alkyl-deprotonation, 2, 51 reactions, 2, 50 Pyrylium salts, 2-amino-reactions, 2, 55 Pyrylium salts, 4-amino-deprotonation, 2, 55... 

Selenolopyrylium salts, 4, 1034—1036 Selenolo[2,3-c]pyrylium salts synthesis, 4, 969 Selenolo[3,2-b]pyrylium salts synthesis, 4, 1035 Selenolo[3,2-c]pyrylium salts synthesis, 4, 969 Selenoloseknophenes electrophilic substitution, 4, 1057 NMR, 4, 13 synthesis, 4, 135 UV spectra, 4, 1044 Selenoloselenophenes, alkyl-synthesis, 4, 967 Selenolo[2,3-b]selenophenes ionization potentials, 4, 1046 Selenolo[3,2- bjselenophenes dipole moments, 4, 1049 ionization potentials, 4, 1046 structure, 4, 1038, 1039 Selenolo 3,4-f)]selenophenes H NMR, 4, 1042 synthesis, 4, 1067 Selenolo[3,4-c]selenophenes non-classical reactions, 4, 1062 synthesis, 4, 1076 Selenolothiophenes electrophilic substitution, 4, 1057 H NMR, 4, 1041 UV spectra, 4, 1044 Selenolo[2,3- bjthiophenes... 

Recent important developments consist in the synthesis of the unsubstituted pyrylium cation by Klagcs and Trager, the preparation of pyrylocyanines by Wizinger, the development of simple syntheses for alkyl-substituted pyrylium salts by Balaban and Nenitzescu, Praill, Schroth and Fischer, Schmidt, and Dorofeenko, the discovery of a variety of reactions by Dimroth and Hafner, and the study of physical properties by Balaban. 

It will be observed that most syntheses yield pyrylium salts in which positions 2,4, and 6 are substituted. Since according to formulas Ib-lc these positions have a partial positive charge, it can readily be understood why electron-donating substituents (hydroxy, alkoxy, alkyl, or aryl) in these positions stabilize the pyrylium salts. Only three pyrylium salts which do not have substituents in either a-position have been reported and few unsubstituted in y or in one a-position they are less stable toward hydrolysis, and in the case of perchlorates they explode more easily, than 2,4,6-trisubstituted compounds. In fact, the former are secondary, the latter tertiary carbonium ions. This fact also explains why the parent compound (1) was prepared only in 1953. 

Less reactive electrophilic reagents like those involved in acylation or alkylation apparently do not react with phenyl-substituted pyrylium salts the p-acylation of a phenyl group in position 3 of the pyrylium salt obtained on diacylation of allylbenzene (Section II, I), 3, a), and the p-l-butylation of phenyl groups in y-positions of pyrylium salts prepared by dehydrogenation of 1,5-diones by means of butyl cations (Section II, B, 2, f) probably occur in stages preceding the pyrylium ring closure. 

Such methyenepyrans afford still another possibility for obtaining new pyrylium salts, namely, electrophilic alkylation or acylation at the exocyclio methylene carbon atom. Thus, 2,6-diphenyl-4-iso-propylidene-4/I-pyran is converted into 2,6-diphenyl-4-i-butyl-pyrylium iodide on refluxing with methyl iodide (see Scheme 3). Unlike the protonation of methylenepyrans, this reaction is no longer... 

Triarylpyrylium salts react with hydrazine to give 4//-l,2-diazepines 472-1°s,ioo vja g unstable intermediates 3.110 The reaction fails with pyrylium salts containing alkyl groups in positions 2 and 6, with the exception of 2,4,6-tri-tert-butylpyrylium perchlorate. In some cases it is advantageous to use a thiapyrylium salt in place of the pyrylium salt. Selected examples are given. 

Method B involves the preparation of precursor of 2-alkyl-l-benzo-pyrylium salts, as shown in Scheme ll.50 2-Alkylbenzopyrylium salts have been prepared by condensation of salicylaldehyde with appropriate ketone in acetic acid or by alkylation or reduction of coumarin or chromone derivatives. Reaction of 2-alkylbenzopyrylium salts with salicylaldehyde gives directly a spirodibenzopyran or 2-vinynologue benzopyrylium salt 17 which then can be converted into the spirodibenzopyran by piperidine or pyridine. 

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. ... 

Beckmann rearrangement of ketoketoximes 288 (R,R = alkyl, aryl) with thionyl chloride unexpectedly afforded 2-aryl(or alkyl)amino-4,6-disubstituted pyrylium salts 289 (equation 124). This reaction is the first example of rearrangement/cyclization involving carbonylic oxygen as terminator. ... 

In contrast to the unsubstituted ring compounds 118 and 120,1,1-diaryl-, 1,1-dialkyl- or l-aryl-l-alkyl-X -phosphorins with three phenyl groups in positions 2,4 and 6 of the X -phosphorin ring 122 are much easier to prepare and to handle. They can be obtained either from 2.4.6-triphenyl-pyrylium salts or from 2.4.6-triphenyl-X -phosphorins. Most of these 2.4.6-tri-substituted X -phosphorins are very stable and can be isolated as well-defined crystalline compounds. They do not react with the above-mentioned cations. However, reversible protonation-deprotonation does take place in the presence of acids. 

Treatment of pyrylium salts with aryl-(or alkyl)-phosphines or their bis-hydroxymethyl derivatives. 

The conversion of pyrylium salts to pyrans via reductive alkylation or arylation may be accomplished with organometallic compounds or electrochemically. 

A ring transformation similar to the oxidative conversion of the pyrylium salts to the acylfurans is the formation of 5-alkyl-3,4-dichlorofuran-2-carbonyl chlorides from tetra-chloro-2//-pyrans by oxygen at room temperature. A hyperoxide is proposed as an inter-... 

There have been few mass spectral studies of pyrylium salts, possibly as a result of their low volatility. The examples reported so far have been concerned with the behaviour of alkyl- and aryl-substituted compounds under electron impact conditions. Information regarding the parent compound appears to be lacking. In the initial work, use was made of 2,4,6-trisubstituted pyrylium salts. The halides (iodides and bromides) were examined because of their enhanced volatility over the corresponding perchlorates and tetrafluorobo-rates <7lOMS(5)87>. 

The site of addition of organometallic compounds (for examples, see Scheme 1) to pyrylium salts is dependent on the bulk of the alkyl group R of RMgX and on that of the substituents already present on the ring, but when the latter factor is unimportant, attack occurs at C-2 and the products tautomerize to the dienones. The reaction of methylmag-nesium iodide with a number of pyrylium salts has been studied (Scheme 2) (72BSF707). Xanthylium salts react only at C-9 and this suggests that the contribution of the alternative ion (50) is small. 

Russian workers have advocated the use of orthoformates in the synthesis of pyrylium salts and their benzologues. In the presence of acids, alkyl orthoformates are converted into dialkoxycarbocations, which are efficient C-acylating agents. Even in the absence of a Friedel-Crafts catalyst and under mild conditions, acetophenone is converted into the pent-2-ene-l,5-dione (648), which cyclizes to the 2,6-diphenylpyrylium salt (Scheme 254) (71CHE147). Yields decrease with increasing alkyl chain length in the orthoformate. There is evidence that a 3-alkoxymethylpyrylium salt is the initial product. 

The first example of acylation of homoveratryl nitrile was reported to result in l-alkyl-3-acylaminobenzo[c]pyrylium salts 50 (71KGS730). 

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). 

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. 

Resonance, similar to that in pyrylium salts, was shown594,595 to exist between oxonium ion (299a) and carbenium ion (299b) forms in alkylated ketones, esters, and lactones that were obtained via alkylation with trimethyl- or triethyloxonium tetra-fluoroborates596 [Eq. (3.78)]. Ramsey and Taft597 used H NMR spectroscopy to investigate the nature of a series of secondary and tertiary carboxonium ions (300-302). 

All these various pyrylium salts are converted by ammonia, or alkyl, arylkyl or aryl primary amines into the corresponding pyridinium salts.25 The probable reaction sequence is given in the illustrative example 1-benzyl-2,4,6-triphenyl-pyridinium tetrafluoroborate26 (Expt 8.31). 

All physical properties of pyrylium salts (unsubstituted or substituted with alkyl and/or aryl groups) prove the aromaticity of these cations vibrational spectra [66], mass-spectral fragmentations [67], magnetic properties [68-70], and electronic absorption spectra [71]. It should be mentioned that there is a close similarity between the electronic absorption bands of pyrylium salts and those of benzene, easily recognized by the marked bathochromic effect of substituents in y-position of pyrylium salts on one of these bands. Two-photon absorption spectra of 2,4,6-triarylpyrylium cations [72] may be used in optical data storage, lasing, and photodynamic therapy. 

---