Pyrylium cation

The mass spectrum of 2-pyrone shows an abundant molecular ion and a very prominent ion due to loss of CO and formation of the furan radical cation. Loss of CO from 4-pyrone, on the other hand, is almost negligible, and the retro-Diels-Alder fragmentation pathway dominates. In alkyl-substituted 2-pyrones loss of CO is followed by loss of a hydrogen atom from the alkyl substituent and ring expansion of the resultant cation to the very stable pyrylium cation. Similar trends are observed with the benzo analogues of the pyrones, although in some cases both modes of fragmentation are observed. Thus, coumarins. 

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... 

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. 

The pyrylocyanine obtained by Strzelecka and Simalty from 2,6-diphenyl-4-phenacylpyrylium and orthoformic ester (cf." Section II, B, 1, a) has the structure of a pseudo base. Accordingly, its protonation is accompanied by dehydration leading to a triple pyrylium cation (see Scheme 4). 

Phosphorus. Substituted phosphorus analogues of pyridine (phosphinines, A3-phosphabenzenes, also called phosphonins or phosphorins) were first prepared by Markl starting from pyrylium cations their chemical properties suggest that their aromaticity is lower than that of pyridine (e.g. phosphinine 186, Scheme 72).230-232 Molecular calculations for other six-membered jr-systems with planar tricoordinate phosphorus, such as phospininines 186 and 187, have evidenced their aromaticity (Scheme 72).156... 

The pyrylium cation is isoelectronic with pyridine it has the same number of electrons and, therefore, we also have aromaticity. Oxygen is normally divalent and carries two lone pairs. If we insert oxygen into the benzene ring structure, then it follows that, by having one electron in a p orbital contributing to the aromatic sextet, there is a lone pair in an sp orbital,... 

A problem arises with trivial names when a sp hybridized atom is present in an otherwise unsaturated ring. A good example is pyran, a heterocycle that is formally the product of the addition of a single hydride ion to the pyrylium cation. However, as this addition could occur either at C-2 or C-4, two isomers of pyran are possible so the question is, how can you distinguish between them The solution is to call one compound 2/f-pyran and the other 4/f-pyran, using the number of the ring atom and the letter H, in italics, to show the position of the hydrogen (see Box 1.2). This system of nomenclature works tolerably well in many related cases and is widely used other examples will be found in this book. 

As oxygen is divalent, no strict equivalent of benzene exists, although the pyrylium cation does achieve aromaticity (see Chapter 1). Both 2/f-pyrans and 4/f-pyrans are known, but are encountered more frequently as their carbonyl analogues pyran-2-one and pyran-4-one (see Box 4.1). In addition, reduced forms such as 3,4-dihydro-2//-pyran and 3,4,5,6-... 

Although pyrylium cations in combination with an anion of a strong acid are stable, the presence of a formally charged oxygen atom renders them susceptible to reactions with nucleophiles and the valence bond description indicates that C-2(6) and C-4 are the potential targets for attack (Scheme 4.1). 

A Piperidine acts both as a nucleophile and as a base. First it combines with the pyrylium cation at C-2, forming an adduct which then ring opens, deprolonates and tautomerizes to an enamino ketone (Schema 4.3), This product cyclizes through an intramolecular reaction between the enamine unit and the carbonyl group, followed by dehydration to form the phenylpiperidine. 

Disubstituted and 2,4,6-trisubstituted pyrylium cations of type 158a,b are capable of a one-electron reduction to reactive 4-pyranyl radicals 162, dimerizing easily to bis-4//-pyrans 163a,2,8-220 163b,22 163c,221,222 and 163d223,224 in variable yields. 

The quinolizinium ion, the parent compound of the aromatic quinolizines, is a cationic aromatic system like the pyrylium or thia-pyrylium cation. It is isoelectronic with naphthalene. The parent... 

Replacement of CH in benzene by an oxonia group (0+) gives the pyrylium cation, but no neutral oxygen analogue of pyridine is possible. Both 2H- and 4//-pyran contain -hybridized carbon atoms. Many trivial names exist for oxygen heterocycles and the more important of these are shown in Scheme 3. 

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. 

Pyridinium and pyrylium cations, pyridones and pyrones are all readily hydrogenated e.g. flavylium ion (220) and coumarin yield (387) and (388), respectively. 

Related reactions include preparations of furans from pyrylium cations (Section 3.2.1.6.3.v). 1,4-Dithiins (56JA850) and 1,2-dithiins (67AG(E)698) readily lose sulfur on heating, yielding the corresponding thiophene (191 — 192 193 — 194). 

All of the compounds discussed are based on three molecules 2/f-pyran (1), 4//-pyran (2) and the pyrylium cation (3). Names which have been used for the benzologue (4) of 2//-pyran include 2H- 1-benzopyran, benzo-a-pyran, chrom-3-ene and 2//-chromene. A similar situation exists for the corresponding derivative (5) of 4/f-pyran. The unambiguous and simplest name chromene is used in the present work. The benzologue (6) of pyrylium is known both as benzopyrylium and chromylium the former name is preferred here. Higher benzologues are referred to as naphthopyrans, such as 2H-naphtho[ 1,2-6 jpyran (7), but the names xanthene and xanthylium are used for (8) and (9). 

The two features of interest in the structure of 6-(2-hydroxyprop-l-enyl)-2,4-dimethyl-pyrylium cation (179) are the enolic nature of the 6-substituent and the shortness of the C(6)—C(l ) bond. The latter is considered to indicate a significant contribution from the methylenepyran structure (180) (75CC284). 

The substitution of a CH unit in benzene by 0+ (the oxonia group) gives rise to the pyrylium cation (3). Since this ring still possesses 6 ir-electrons, it may be expected to exhibit aromatic properties. As the oxygen is primarily tricovalent, the pyrylium ring may be formally regarded as a cyclic oxonium ion. However its enhanced stability relative to aliphatic and alicyclic oxonium salts is doubtless due to its aromatic nature. 

Total charge densities for the pyrylium cation have been calculated by the INDO MO method and show an increase of positive charge at the 2-, 4- and 6-positions. A linear relationship with 13C chemical shifts was shown to exist (770MR(9)l6). 

Disubstituted pyrylium salts, such as (41), are attacked at C-4 only (74JOU2015) and in the presence of perchloric acid a new pyrylium salt (42) is formed. Flavylium (43) and xanthylium (44) salts also react at the y-position, for instance, with CH-acidic reagents such as pentane-2,4-dione, malonic acid derivatives or aromatic electron-rich compounds like IV V-dimethylaniline, 1,3-dimethoxybenzene or IV-methylindole (59CB46, 74CHE1019) some examples are shown in Scheme 1. 2,4-Disubstituted pyrylium cations, e.g. (45), react at C-6 (80JOC5160). 

Pyrylium cations may be converted into benzene rings by a long-established reaction (14LA(407)332) with carbanions this can be a valuable synthetic route. Such reactions may... 

Methylene groups of pyrylium cations also react under mild conditions with nitrosoben-zenes in the presence of acetic anhydride and an alkali metal salt, which plays an essential role in the formation of Schiff s bases (116). The latter are hydrolyzed to the carbonyl compound (117) (71BSF3603). It is possible to form potential dyes by treatment of pyrylium salts with ortho esters such as triethyl orthoformate. The reactants are heated together in acetic anhydride or pyridine and trimethine dyes of type (118) and (119) are formed (70JHC1395, 71JOC600, 59CB2309). 

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