Substituted pyrylium cations

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... 

Barltrop et alf° have reported a study of the phototranspositions of alkyl-substituted pyrylium cations. 

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. 

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. 

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. 

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... 

Apparently, the aromatization of the heterocyclic cation serves as a driving force of the Cope rearrangement in the transformation of the 3-formyl-4-allyl-4//-pyrane (481) into poly-substituted pyrylium salt 483 which presumably proceeds via 482 (equation 183)242. 

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. 

Band group V is recorded at about 400 nm in the spectra both of 1-aryl-[64ACH(40)225] and 1,3-diaryl-substituted benzo[c]pyrylium salts (87RRC417). The batho- and hypochromic effects are discussed in detail in the case of multiple substitution in l-aryl-benzo[c]pyrylium cations. 

The chemical shift of the methyl group in methyl-substituted pyrylium, thiopyrylium, and selenopyrylium cations is reported in Table V. Whereas for pyrylium and thiopyrylium the order of chemical shift of methyl group is a > y > /3, in the case of selenopyrylium the order is a > 13 > y. The presence of additional methyl groups causes only small... 

These photorearrangements are known to occur not only in substituted benzenes but also in substituted naphthalenes, pyridines, pyrylium cations, and pyridinium salts.786 For example, irradiation of 2,4,6-trimethylpyrylium perchlorate (184) results in the formation of the oxabenzvalene cation 185, which rearranges to the 2,3,5-trimethyl phototransposition isomer 186 (Scheme 6.77).800... 

An advantageous method for the conversion of 2- and 4-methyl substituted pyridinium and quinolinium salts (120) into their corresponding 2,4,6-triaryl-phenyl derivatives (121) in good yields has been developed by Zimmermann [158]. The anhydrobases of 121 were discussed as the key intermediates of this pyrylium ring transformation they attack the pyrylium cation 122 in the initial step as carbon nucleophiles of the enamine type (see Scheme 56). The reaction... 

The properties of pyrylium cations are best compared with those of pyridinium cations the system does not undergo electrophilic substitution nor, indeed, are benzopyrylium cations substituted in the benzene ring. This is a considerable contrast with the chemistry of quinolinium and isoquinolinium cations and is a comment on the stronger deactivating effect of the positively charged oxygen. 

A new class of seven-membered cations with 87c-electrons, the 1,2-oxazepinium salts (61), has been prepared by reaction of 2,4,6-triaryl substituted pyrylium salts (e.g., 60) with nitrones in boiling DMF. Reaction of (60b) with A-phenylhydroxylamine in DMF also gives (61b). The proposed intermediates in the nitrone reaction, (62), (63), and (64), are shown in Scheme 6 <8lZ0R88i>. Extension to other pyrylium salts has been described <82KG457>. 

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