Benzopyrylium cations

The benzopyrylium cation is aromatic in nature, bul the heterocyclic ring is readily reduced and reacted with nucleophiles. 

The distribution of charge in the resonance forms (28)-(31) suggests that nucleophiles may attack at C-2, C-4 or C-6 (or at C-2 or C-4 in 1-benzopyrylium cations, and at C-l, C-3 or C-4a in 2-benzopyrylium ions) but they most commonly add at C-2 for example, attack by cyanide ion gives a 2//-pyran (37) which exists partly or wholly as the acyclic isomer (38). Steric and electronic effects in the reactants probably have a role in determining the course of the reaction of trisubstituted pyrylium salts with nucleophiles. A mixture of both 2H- and 4H-pyrans is sometimes produced, for example, from methoxide ion and 2,4,6-triphenylpyrylium perchlorate (39) no acyclic product was detected in this reaction... 

Benzopyrylium salts have not been found in nature, and in spite of their ability to take part in recyclization reactions, until recently it was considered that there were no specific features in their transformations relative to monocyclic pyrylium salts (79MI2), and the scope of these transformations has been considered as rather restricted (71CB2984). The reason for such a conclusion was based on the incorrect interpretation of the effects of benzoannelation on reactions of 2-benzopyrylium cations so far no review on this subject has been published. 

The unsubstituted 2-benzopyrylium cation 1 was first synthesized by Blount and Robinson (33JCS555) starting with indene. Its oxidation by lead tetraacetate affords homophthalic dialdehyde, which is cyclized to 2-benzopyrylium tetrachloroferrate on treatment with hydrochloric acid and ferric chloride. 

The formation of 2-benzopyrylium cations 30 by acid-catalyzed acylation of benzyl carbonyl compounds 28 may be described by several equilibrium processes as shown in Scheme 2. 

The acylation of homoveratric aldehyde gives rise to the stable perchlorates 46 (84KGS702 85KPS815) in spite of the prevailing opinion about the instability of 2-benzopyrylium cations with an unsubstituted 3-position [64ACH(40)225]. 

The structure of the final 2-benzopyrylium cation depends on the nature of the substituents at the methylene group in the initial nitrile (85MI2 86KGS999). 

On the basis of data obtained in the previous paragraphs, one can propose that formation of 5-oxoniachrysenes 67 (R2 = H) occurs via the primary O-formylation of 66 followed by cyclization of esters 70 (R2 = R3 = H), which is similar to the Pictet-Gams reaction (80T1279). Obviously, this mechanism can also be applied to the acylation of 66 with R2COOH (R2 does not equal H) in PPA in contrast to the scheme of formation of 2-benzopyrylium cations 30 from benzyl ketones 28 (Section II, C, 2). 

Nucleophilic addition reactions which are the most common. Depending on the nature of nucleophile, the structure of 2-benzopyrylium cation, and the reaction conditions, such additions may result in isolable adducts, in products of ring opening, or in formation of a new ring. 

Deprotonation reactions if the 2-benzopyrylium cation has an a-alkyl substituent or a group with a mobile hydrogen atom in any other fragment of the molecule. This conversion takes place over a wide range of conditions, and it often accompanies nucleophilic additions. Moreover, the problem of the acid-base and nucleophilic-electrophilic interactions for 2-benzopyrylium salts concerns not only the primary step of their reactions with nucleophiles, but also the ring-open forms, as shown in Section III,C,4. 

If the 2-benzopyrylium cation has no substituent in position 1, the interaction of the salt 159 with amines having a primary a-carbon atom or with aniline leads to the formation of isoquinolinium salts 160 in high yield (81KGS313 89KPS75). However, the use of amines with secondary a-car-... 

However, this reaction has only one pathway if the 2-benzopyrylium cation 30 has a single substituent able to take part in the aldol condensation (71KGS1437 81KGS1608). 

At the same time, it was impossible to suppress the deprotonation completely if the 2-benzopyrylium cation had a benzyl group in position 1 (76KGS745 77KGS1176) a stable anhydrobase results. As expected, the... 

The conversion under acidic conditions of 2-benzopyrylium salts into naphthols is not observed for isoquinolinium or monocyclic pyrylium salts, which become converted into phenols only in alkaline medium. Moreover, the ring opening of the heterocycle in 211, determined by its protonation, may be considered as the specific type of 1,4-addition of R2OH to the heterodiene fragment of the 2-benzopyrylium cation. This is a new version of the corresponding step in the ANRORC scheme. 

When 2-benzopyrylium cations have a substituent with a fairly mobile hydrogen atom (a-alkyl group or heteroatom group in any other position of the cation), deprotonation of such a substituent occurs even under mild conditions by an acid-base interaction as the primary step (Section III,A). Although deprotonation in both cases leads to compounds whose structures can be depicted by two resonance formulas, either with charge separation (betaines) or without (anhydrobases), on discussing products of C-deprotonation, the term (and the corresponding formula ) anhydro-base is more often used, whereas products of O-deprotonation are called betaines. ... 

Examples are known for deprotonation of hydroxyl groups attached either to the 2-benzopyrylium cation itself or to its subsituents. In both cases, the stability of the betaine thus formed is determined by the position of the deprotonated group. Thus, unstable betaine 15, obtained as described in Section II, A, may be formally considered as products of deprotonation of the nonisolated 4-hydroxybenzo[c]pyrylium salts (84CC702 86NKK1622). A valence-isomeric structure 14 is possible, and photochemical processes were discussed in Section II,A. 

Obviously, the formation of dimer 269 is due to the negative charge at the exo methine group of 267, which is not screened by substituents, and because of this fact, the a-methine carbon atom can add to position 1 of another unreacted 2-benzopyrylium cation. Such an interaction may be called a-1 dimerization it leads to dimeric salt 268 and then to pseudobase 269. 

Therefore, most probably, the formation of chrysenes 246 from 2-benzopyrylium salts occurs via the process of a-1 dimerization. At the same time, one cannot exclude the action of anhydrobase 267 as dieno-phile, which leads to its addition to positions 1 and 4 of the initial 2-benzopyrylium cation by analogy with [4 + 2] cycloadditions in reaction with ethyl vinyl ether (Scheme 14) (cf. Section III,D,1). 

Since compounds 10 and 30 are rather complicated, the authors could not attempt to determine the normal frequencies of the molecule, but had to restrict the analysis of spectra to the determination of characteristic frequencies. Thus, six frequencies have been found that can be used to identify 2-benzopyrylium cations. Band I appears between 1650-1610 cm-, and the pyrylium ring is responsible for this vibration [8a band according to Wilson s notation (34MI1)]. The position of this band and its intensity are dependent on the nature and position of substituents in the cation, and these changes are similar to data of monocyclic pyrylium salts [82AHC(Suppl)]. 

Although some radicals and cation radicals are postulated for chemical and electrochemical transformations of 2-benzopyrylium cations (Sections III,F,1 and IV,B)> attempts to record their electron spin resonance (ESR) spectra failed, obviously because of a low stability of these radicals. However, the structural combination of hydroxy aryl and 2-benzopyrylium fragments favors the formation of radical cations 301-303, and their ESR spectra were recorded on oxidation of the corresponding 2-benzopyrylium salts with lead tetraacetate (87RRC417). 

The application of the CNDO/2 method with standard geometry for various substituted 2-benzopyrylium cations results, as one could expect, in the preference for the resonance structures b-f with close values of charge densities on Ct and C3, and attempts to rationalize on the basis of these calculations yielded statistical agreement for correlations between change densities and H-NMR spectral data (90MI3) (cf. Section IV,A,2). 

The platinum-catalysed intramolecular domino annulation reaction of o-alkynylben-zaldehydes has been described as a versatile approach to naphthalenes with annulated carbocycles or heterocycles of various sizes (Scheme 32).94 A plausible mechanism for the platinum(II)-catalysed annulation reaction shows that the double annulation process most probably proceeds through the benzopyrylium cation (117), which results from the nucleophilic attack of the carbonyl oxygen at the alkyne, activated by the Lewis-acidic platinum salt. A subsequent intramolecular Huisgen-type 3 + 2-cycloaddition of the second alkyne is assumed to generate intermediate (118). Rearrangement to (119) and the formal 4 + 2-cycloaddition product (118) leads to the aromatized final (116), liberating the active catalyst. In the case of FeCl3 as the Lewis acid, we assume that intermediate (118) is oxidatively transformed to (121). 

In a spectral study of aqueous solutions of a series of benzopyrylium cations 46, Canalini et al.93 were able to detect significant ring opening of... 

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