Pyridinium-3-olates

Pyridinium-3-olates (86) have been extensively studied by Katritzky and co-workers who in particular have investigated the relationship between 

3-dipolar reactivity and the nature of the N-substituent. Comprehensive accounts of this work have been published - and so only general features will be described here. 

The earliest reference to a pyridinium-3-olate (86) is the description of a viscous oil obtained from A-methyl-3-hydroxypyridinium iodide (87 R = Me, = H, X = I) and aqueous silver oxide.The iodide 87 (R = Me, R = H, X = I) (QH7NO HI) with triethylamine gives a complex salt [(C6H7N0)2 HI] which also yields the betaine 86 (R = Me, R = H) (C6H7NO) with silver oxide. A similar procedure using vitamin Bg (89) gives betaine 90 which is also made from compound 89 and diazomethane.Other betaine derivatives of vitamin Bg have been encountered. 

A number of salts of the pyridinium-3-olates (86) have been prepared from 3-hydroxypyridine and alkyl halides. Whether the products are 

A-aminopyridinium-3-olates 93 (R = H or Me). Acetylation of the A -amino compound 93 (R = H) gives the A -imide 94 (R = COMe), and not the tautomer 93 (R = COMe), whereas the A-methylamino derivative gives the pyridinium-3-olate 

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Dimerization of pyridinium-3-olates (86) is also induced photochemically. In accord with theory the structure of the photodimers differs from that of the thermal dimers. A-Phenylpyridinium-3-olate (101) upon irradiation gives dimer 103 together with the bicyclic valence tautomer 102 (Scheme 4). This valence tautomerism (101 - 102) is analogous to that of pyrylium-3-... 

The triaryl compounds 290 (R = Ar) are prepared by condensation of 2-aminophenol with triarylpyrylium salts followed by treatment with alkali. The triphenyl betaine (290 R = Ph) is obtained as a purple solid, mp 165 C (decomp), which shows large thermo/solvatochromic effects. Oxidation of the betaine 290 (R = Ph) with hydrogen peroxide gives the triphenyl-pyridinium-3-olate 291 (R = Ph) (see Section III,A,2) and the pyrrole 292 (R = Ph). The mechanism of this unusual reaction has not yet been Established. 

Considering the proliferation of MO calculations in recent years, it is remarkable how few calculations have been reported for these heterocycles. Frontier orbitals of several betaines have been calculated using the Hiickel method and electroselectivity correctly predicted on the basis of the orbital symmetry.Similar results have been obtained for pyridinium-3-olates (427) using the Pariser- Parr Pople (PPP) method. The CNDO... 

Fig. 6. Frontier orbital interactions in (a) thermal and (b) photochemical dimerization of pyridinium-3-olates. Sites of addition are indicated by arrows.

Irradiation of pyridinium-3-olates gives a dimer of different structure (454). In a photochemical process the important frontier orbital interactions are those between HOMO-HOMO and LUMO-LUMO. Figure 6b demonstrates that for photodimerization, a different orientation favors transition-state stabilization and leads to the novel adduct 454. 

Simitar reactivity is observed between betaines and electron-deficient alkenes (E CH CHj) and a high degree of regiospecificity is often observed. Pyridinium-3-olates (Section III,A,2) react with acrylonitrile or methyl-... 

Cycloadditions of betaines are not restricted to electron-deficient alkenes. Pyridinium-3-olates also react with conjugated olefins (e.g., styrenes) and with electron-rich olefins (e.g., ethyl vinyl ether). In the latter case, the betaine LUMO/alkene HOMO interaction becomes dominant and reaction is only observed with pyridinium-3-olates having a low-energy LUMO... 

The study of reactions between mesomeric betaines and 1,3-dienes has so far been restricted to pyridinium-3-olates (Section III,A,2) and quinolinium-3-olates (Section III,B, 18). The mode of addition of dienes differs from that of 23T-electron addends in two respects (i) Dienes add across a different... 

Further studies on the 1,3-dipolar cycloadditions of these molecules (86) have been reported. Addition of allyl alcohol gives endo adducts (505) which are not isolated but spontaneously cyclize to tricyclic products (506). Similar tricyclic products were also obtained using A-allylbenzenesulfonamide, triethylammonium acrylate, and vinylpyridines as dipolarophiles. It has previously been shown that the pyridinium-3-olates with chloroketenes (RCC1=C=O) give 2-oxofuro [2,3-c] pyridine 507 (see p. 22). Further studies demonstrate that when bromoketenes (RCBr=C=O) are used as dipolarophiles, a mixture of 2-oxofuro[2,3-c]pyridines (507) and isomeric... 

Treatment of the l-(4,6-dimethylpyrimidin-2-yl)pyridinium-3-olate dimer 510 with l-dimethylaminobuta-l,3-diene and subsequent oxidation leads to the benzo dimer 511 which is converted to the regioisomer 512 by heating in an inert solvent. Thermal dissociation of the adduct 511 into the two betaines 513 and 514 was demonstrated by trapping these species with several... 

Polymethylene-bridged bis (pyridinium-3-olates) 509 and their bis-adducts have been prepared. The C NMR spectra of a number of pyridinium-... 

The rates of cycloaddition of methyl acrylate with six 1-substituted pyridinium-3-olates have been studied, A Hammett plot of the second-order rate constants against tr values of the substituents gave satisfactory correlation. 272.373... 

In aqueous solution 3-hydroxypyridine 176 equilibrates with the mesomeric betaine 177a for which no uncharged structure can be written. Since these pyridinium-3-olates 177a undergo 1,3-dipolar cycloadditions, it is reasonable to assume that there is also a contribution of the one form 177b to the overall structure. 

Pyridinium-3-olates 33 (Z = NR, Y = O) are converted photochemically into the bicycle 34 (Z = NR, Y = O) corresponding pyrylium-3-olates and especially isochromylium-4-olates isomerize more easily (cf. 5556). 

Pyridinium-3-olates and pyrylium-3-olates react with a variety of monoenes, dienes, and trienes. Each of these systems react readily with dienophiles to yield cycloadducts of type 433. For Z = NMe, an electron-withdrawing X group is required in the dienophile, but with Z = 0 or A-(2-pyridyl) even unactivated alkenes react. 1-Phenylpyridi-nium-3-olate and benzyne give 434 dienes give adducts of type 435. Fulvenes behave as trienes to give adducts across the 2,6-positions 436. 2-Benzothiopyrylium-4-olate 437 gives a thermal dimer across the 1,3-positions. 

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