Pyrylium ion

Pyrylium ion should be even less reactive than the corresponding pyri-dinium cation in fact, no simple electrophilic substitutions are known. 

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Pyrylium ions react with ammonia or primary amines to give pyridinium ions (see p. 447). ... 

The formation of the acetal 271 is evidence for a polar nature of 258 in terms of the zwitterions 258-Zi, whose pyrylium-ion character explains the attack of the tert-butoxide ion to give the vinyl anion 265 (Scheme 6.58). In contrast to 260/260-Zi, 258 does prefer cycloadditions if it is generated in the presence of an activated... 

This is manifest in the reactivity of 180/180-Z1 which was generated from 3-bromo-41-f-pyran (283) by /3-elimination of hydrogen bromide with KOtBu (Scheme 6.61). Whether or not this reaction was conducted in the presence of styrene or furan, the only product identified was tert-butyl 4H-pyran-4-yl ether (284). This is in line with the relationship of the intermediate to a pyrylium ion. Thus, the addition of the tert-butoxide ion to 180/180-Zj has to be expected at the 4-position with formation of the vinyl anion 285, which is then protonated to give 284. Likewise, the attack of the nucleophile is predicted at C2and C6 leading to the vinyl anions 286, which... 

The amount of a given isomer in reaction mixtures is affected by substitution patterns of the starting pyrylium ions and by the structure of a reagent, in accordance with quantum chemical interpretations using various LCAO-MO methods.201-2028 Almost all additions to 2,6-disubstituted pyrylium ions occur at position 4 to give 4//-pyrans. Unsubstituted or 2,4,6-trisubstituted substrates may be attacked, on the other hand, at a or y positions providing 2H- and/or 4//-pyrans. Limited information on tetrasubstituted and penta-substituted pyrylium ions leads to the conclusion that the former afford AH-pyrans, whereas the latter tend to be attacked by nucleophiles only at positions 2 or 6. 

Two isomeric 2//-pyrans 167 and 169 together with a 4//-pyran 168 may be formed from substituted pyrylium ions 166 and organometallic reagent RM (Eq. 9). 

If a pyrylium ion possesses hydrogen atoms sufficiently active toward a given organometallic agent, elimination may occur instead of addition as was found in the case of 170.236... 

The regioselectivity of the addition of organometallics to 2,4,6-trisub-stituted pyrylium ions 166 (R2 = R4 = H) is mainly determined by the structure of the entering group R. Thus 2,4,6-trimethylpyrylium perchlorate 177 was found to react with MeMgl, MeLi, or MeNa exclusively to give 2,2,4,6-tetramethyl-2Z/-pyran (176),212,220 237 238,246 whereas i-PrMgX, /-PrLi,... 

However, if a tetrasubstituted pyrylium ion possessed one or two condensed six-membered carbocyclic rings, then only 4//-pyrans 191, 192, 27, and 193 are isolable (35 to 92%) after reaction with PhCH2MgCl236 or with other Grignard reagents RMgX.210... 

Hydroxide ion adds to 2,4,6-TriphenylpyryIium ion in the presence of sodium acetate260,261 or in aqueous solution at pH 14 to give 2//-pyran 40, which isomerizes to the corresponding dienone at lower pH.88 The same reaction was discovered for 2,6-dimethyl-4-(/j-methoxyphenyl)pyrylium ion.2614... 

Equilibrium aromatizations of 4//-pyran (5), 4//-thiopyran (7), and 4H-selenopyran (8) with appropriate pyrylium ions were explored for the estimation of relative stabilities of the latter.351... 

Disproportionation has been observed frequently with thiopyrans and rarely with 4//-pyrans, and all cases involve a tetragonal carbon center (position 2 or 4) bearing at least one C—H bond. Some molecules of the substrate are aromatized to corresponding thiopyrylium or pyrylium ions and others reduced to dihydro or tetrahydro products. The relative abilities of pyrans and thiopyrans to disproportionate were interpreted within a proposed hydride transfer mechanism by a CNDO/2 method.45... 

The formation of 2,6-dimethyl-4-methoxy-l-nitrobenzene (551) from intermediate 2,6-dimethyl-4-methoxy-2//-pyran (323) and nitromethide ion319 involves aromatization to pyrylium ion 373 followed by the expected process shown in Eq. (35). 

More reports on the mass spectrometric behavior of 4//-pyrans are available.36,6J 86 112 121,333 The fragmentation patterns were studied in detail for 4//-pyran carboxylates of 68 (X = EtO, Y = Me) and 87b-d, using high resolution measurements and detection of metastable ions.86,112 The main mode of fragmentation involved the loss of an alkyl or aryl 4-substituent to generate very stable pyrylium ions, as shown in Scheme 40. 

Electrochemical investigations show that 4//-pyranyl radicals 162 and 633 as well as corresponding radical anions 197 are reversibly oxidized to appropriate pyrylium ions at about -0.3 to -0.4 y.224,225,228,239... 

Irreversible electrochemical oxidation of dimeric 4//-pyrans 163 to monomeric pyrylium ions at about +0.6 V was observed.225,227,228 The anodic oxidation of the dimers in the presence of aromatic hydrocarbons caused electroluminiscence of the latter.228,472 The polarographic oxidation of carbo-ranyl 4//-pyran 174a (R = Ph) at a platinum microelectrode was found to... 

Fused benzene rings aid nucleophilic attack on pyridines, pyridinium and pyrylium ions, and pyrones the loss of aromaticity involved in the formation of the initial adduct is less in monobenzo derivatives and still less in linear dibenzo derivatives than in monocyclic compounds. For the same reason, the tendency for this initial adduct to re-aromatize is less for benzopyridines. Fused benzene rings also influence the point of attack by nucleophilic reagents attack rarely occurs on a carbon atom shared with a benzene ring. Thus, in linear dibenzo derivatives, nucleophilic attack is at the y-position (157). 

Pyrylium cations form pyridines with ammonia and pyridinium salts with primary amines (B-82MI 505-02). For example, 2,4,6-triphenylpyrylium cation (261 Z=0) yields 2,4,6-triphenylpyridine with ammonia, the corresponding 1-methylpyridinium salt with methylamine, and pyridine 1-arylimines with phenylhydrazine. Xanthylium ions (210), where ring opening cannot readily occur, form adducts (262) with ammonia, amines, amides, ureas, sulfonamides and imides. Similar adducts (e.g. 263) are formed by benzo[( ]pyrylium ions. 

Derivatives of 2,3-epoxycyclopentadienone also undergo heterolytic cleavage. The coloured species is believed to be a pyrylium ion (134), illustrated in Scheme 11 by the photochromic reaction of 2,3-diphenyIindenone oxide (133) (21CB2327). 

Condensation occurs in a basic, neutral or acidic medium and high yields are often obtained at moderate temperatures. An electron-releasing group in the aryl ring(s) attached to the pyrylium ion decreases the reactivity of the methylene group (66JGU1724). Similar reactions occur in the benzopyrylium and xanthylium series and the products sometimes have potential as dyes. 

Another characteristic of electrophilic reactions of pseudoazulenes is the application of numerous cations as the electrophile, for example, diazonium salts and Vilsmeier- Haack s reagent (see Table VI), tropylium ion,135 triphenylmethyl cation,"4 pyrylium ion,119 and dithiolium ion.166 Very stable cations are formed (e.g., 120) addition of base releases the substituted pseudoazulene (see example in Eq. 10). Generally reactions of this type are thermodynamically favored (see also Section IV,B). The site of substitution... 

The transformation of a reactant to another form by changing the pH of the solution can be used as a chromogenic sensor. The 1,3,5-triaryl-1,5-pentane-dione derivative 89 [69] can be readily transformed into the corresponding pyrylium ion, as shown in Fig. 17. Addition of nitric acid to a solution of 89 in... 

Pyrylium ions are six-membered heterocycles in which a positively charged sp2-hybridized oxygen replaces the nitrogen in pyridine. The pyrylium ring appears in many naturally occurring flower pigments. 

Six-membered unsaturated oxygen heterocyclic compounds are based on three molecules 2/7-pyran 1,4/7-pyran 2, and the pyrylium ion 3. Based on this, the benzo analog of 2/7-pyran is named 277-1-benzopyran (commonly 2H-chromene) and the benzo analog of 477-pyran is called 4/7-1-benzopyran (commonly 477-chromene). The benzo analog of 3 is known as 1-benzopyrylium 6 (sometimes chromylium). Related naphthyl analogs are exemplified by 2/7-naphtho[l,2,A]pyran 7, the xanthylium ion 8, and xanthene 9. 

A [Mn(GO)5] mediated condensation of methyl propiolate and 1,2-diphenylethyne affords the organometallic pyrylium ion complex 1215 in poor yield (Equation 467) <2002OM3434>. 

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