Benzo 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. 

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. 

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). 

B. Reactions That Conserve the Benzo[c]pyrylium Cation. 177... 

Two benzoannelated analogues of the pyrylium cation are known ben-zo[b]pyrylium (chromylium or 1-benzopyrylium) and benzo[c]pyrylium (isochromylium or 2-benzopyrylium) salts. So far, the 1-benzopyrylium system is considered to be the more interesting (79MI2 84MI1) because it is the basic heterocyclic system of important plant pigments (antho-cyanins), and because there are major differences between properties of its heterocyclic ring and those of monocyclic pyrylium systems as a result of benzoannelation. Thus, for instance, 1-benzopyrylium salts do not possess the ability to have recyclization reactions with heteroatom exchange. 

The search for benzo[c]pyrylium systems and the elaboration of synthetic approaches to its development could lead to the discovery of novel regularities that might be applied in syntheses of different types of heteroaromatic and aromatic compounds. On the other hand, this route could give a possible systematic study of the influence of benzo[c]annelation on reactions of the pyrylium cation. 

The benzo[c]pyrylium cation 30, in contrast to the monocyclic analog 96, has two nonequivalent a-positions as already mentioned (Section II1,B). Evidently, pathways a or b (Scheme 5) are determined by the... 

The further behavior of benzo[c]annelated adducts 100 depends on the structure of substituents in the initial cation 30, nature of the nucleophile, thermodynamic parameters of the final products, and conditions of the experiment. The reaction may be stopped at the step of adduct 100 (b, 1 in Scheme 5) or may be continued with the formation of ring-opened intermediate 101 (b,2). However, the latter step has some specific features, in comparison with monocyclic pyrylium salts, as a consequence of the presence of the annelated benzenoid ring in benzo[c]pyrylium cations. 

Interestingly, the addition of the hydride ion to benzo[c]pyrylium cations with the formation of adducts of type 107 occurs not only in reactions with lithium aluminum hydride (54JOC1533) or sodium boro-hydride (89KGS750), but also on short-time heating of the formate 30 (R1 = ME, R2 = Et, R3 = Ver, R4 = OMe) in formic acid [67ACH-(51)107] (Scheme 7). 

AH attempts to convert dimer 263 into a dimeric 2-benzopyryIium salt, on treatment with triphenylmethyl or acetyl perchlorate, lead only to the rupture of the newly formed C—C bond and to the regeneration of the initial monomeric salt 261, unlike the behavior of dimers of monocyclic pyrylium cations [73DOK(212)370]. Dimerization may be considered a typical reaction for benzo[c]pyrylium-4-oxides of type 19, which react in dimerizations as 1,3-dipoles by analogy with their behavior in cycloadditions (Section III,E,2). 

The reaction of 4-1 dimerization is similar to the primary step of recycli-zation of 2-benzopyrylium salts in acidic nucleophilic media (cf. Section III,C,4,b,i), but the reactive electrophile is the initial cation in this case, and not a proton. Probably for this reason, the 4-1 dimerization of 2-benzopyrylium perchlorates is not observed in acidic nucleophilic media, in contrast to a-1 dimerization (cf. Section III,F,2,b). At the same time, the scope of 4-1 dimerizations is less restricted in terms of structural requirements for 2-benzopyrylium salts in comparison with a a-1 dimerization. Thus, in the latter case, the presence of a methyl group in position 1 of benzo[c]pyrylium cation is compulsory, whereas for 4-1 dimerizations, the nature of the substituent in this position may be different, leading to a variety of 4-1 dimers and, as a consequence, to a wide variety of their transformations. 

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. 

At the same time, dimerization of one-electron reduction product was indicated by oscillopolarographic studies in acid for l-R-3,4-diphenyl-6,7-dimethoxybenzo[c]pyrylium perchlorates (R = Me, Et, Ph) (76MI1). As was noted in Section III,F, 1, the benzo[c]pyrylium cation 261, unsubstituted in position 1, undergoes l, 1 -dimerization via the postulated intermediate radical, and the bisisochromene 263 thus formed is readily converted into the initial monomeric salt, but not into the corresponding biscation on treatment with chemical electron acceptors (76KGS999). 

For explanation of experimental results and for correlation of charge densities with NMR data, semiempirical quantum-chemical calculations of benzo[c]pyrylium cation have been employed. Interestingly, the first calculation of 1,3-dimethyl-benzo[r]pyrylium cation by the simple linear combination of atomic orbitals/molecular orbital (LCAO/MO) method (70KGS1308) revealed a preference for the resonance from a in which the value of the charge density at C was three times as much as at C3. 

It is proposed that the benzo[c]pyrylium cation 39, produced from o-alkynylbenzaldehydes by AuBra catalysis, behaves as the 4n component in an inverse electron demand DA reaction with enols. Dehydration and bond rearrangement leads to naphthalene derivatives. Simple a, 3-unsaturated aldehydes can also be benzannulated in this way <04JA7458>. 

Interest in the synthesis and reactivity of the six-membered, potentially aromatic, phosphinine ring system has also continued, but at a much lower level than in recent years. New synthetic work includes the application of the pyrylium salt route to phosphinine synthesis, this time starting from pyrylium salts bearing chiral substituents to give the related chiral phosphinines," and the development of new routes to the 2-phosphanaphthalene (137)" ° and the phosphinine-2-aldehyde (138). Also reported is an approach to the synthesis of l,2-dihydro-l,4,2-benzo-diazaphosphinines," cationic gold(I) complexes of 2,4,6-tri-t-butyl-l,3,5-triphosphabenzene," and the synthesis of some X -phosphinines from phosphonium-iodonium ylides." " ... 

The goal of the present review is to demonstrate the wide potential for using newly developed methods to synthesize 2-benzopyrylium salts and to discuss in detail the numerous and unusual transformations of this class of oxygen cations in comparison with monocyclic pyrylium salts, i.e., the study of influence of benzo[c]annelation. 

The interaction between ammonia and l-R1-3-aryl-substituted ben-zo[c]pyrylium salts 30, as well as 3,4-diaryl-substituted indeno[e l,2-]-benzo[c]pyrylium, and other substituted cations having a similar fragment is more complicated, especially in hydroxyl-containing solvents. In these cases, mixtures in different ratios are obtained that contain isoquinolines 138, 3-hydroxy-3,4-dihydroisoquinolines 137, OR-adducts 109, anhydro-bases 119, diketones 29, a-naphthylamines 140, and a-naphthols 141 (88UP2). 

Tetralone 213 may be considered as the specific analog of 3-hydroxy-3,4-dihydroisoquinolines 137 (Section III,C,4,a,i). Its isolation is more evidence of the lower tendency toward aromatization in recyclization products of benzo[c]annelated pyrylium salts in comparison with monocyclic cations. 

Similar to aroxyl-benzo[b]pyrylium systems (78ZOR1643), there is no detectable delocalization of the unpaired electron on the benzo[c]pyrylium ring in 301-303 (87RRC417). The stability of the radical cation is determined in this case by the position of the aroxyl substituent in the cation, and the most stable is 301. The formation of diradical species 303 together with the monoradical cation, was indicated on oxidation of the corresponding 2-benzopyrylium salt. 

Pyridine, the diazines, 1,3,5-triazine, 1,2,4,5-tetrazine, quinoline, isoquinoline, quinolizinium ion and the benzodiazines are HtiCKEL-aromatic systems (heteroarenes). 2//-Pyrane, 4//-pyrane and the corresponding benzo compounds, 1,4-dioxin, 1,4-dithiin and 1,4-oxathiin are not aromatic. However, the cations derived therefrom, like pyrylium and thiinium cations or dioxinium and dithiinium dications are aromatic the same is true for the corresponding benzo or dibenzo systems (e.g., benzopyrylium). Phosphinin is characterized by a specific heteroaromatic situation. 

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