Pyridine resonance forms

Tire and NMR parameters of some 1-alkyl-4-benzimidazolyl-2-idene- (type 72) and l-alkyl-4-(5-methylpyrazolyl-3-idene)-l,4-dihydro pyridines (type 73) were discussed in 89CC1086 and 91JOC4223. Comparison of the shifts for DMSO-dg and CDCI3 solutions with data reported for quaternary pyridinium compounds as well as anionic species in the azole series and data obtained for mesoionic betaines of the azinium azolate class left no doubt that these heterofulvalenes have a betaine character and, therefore, the NMR signals correspond to their dipolar resonance form. 

For (XX), L py, it is likely that the major reaction path involves initial skeletal isomerization to give (XXI) followed by rapid solvolysis of this isomer. The solvolysis of this isomer is strongly metal-assisted since the intermediate carbonium ion is stabilised by the metal-alkene resonance form as shown in the Scheme. The product is the 1-D2 isomer. Now, the skeletal isomerization of (XX) is expected to be retarded by free pyridine and cannot occur when L2 = 2,2 -bipyridyl C7). Hence under these conditions the reaction must occur by solvolysis of (XX) giving largely the 3-D2 isomer. However, the product formed under these conditions is still about 30% of the 1-D2 isomer (Table I). 

Figure 1-11. The resonance forms of some ligands that cannot be represented by a single valence bond structure (acetate anion, acetylacetonate anion and pyridine).

The 6th rank in terms of acylation reactivity that is attributed to the acyl imidazolides in Table 6.1 (entry 10) is also plausible. In the acyl imidazolides, the free electron pair of the acylated N atom is essentially unavailable for stabilization of the C=0 double bond by resonance because it is part of the -electron sextet, which makes the imidazole ring an aromatic compound. This is why acyl imidazolides, in contrast to normal amides (entry 2 in Table 6.1) can act as acylating agents. Nevertheless, acyl imidazolides do not have the same acylation capacity as acylpyridinium salts because the aromatic stabilization of five-mem-bered aromatic compounds—and thus of imidazole—is considerably smaller than that of six-membered aromatic systems (e. g., pyridine). This means that the resonance form of the acyl imidazolides printed red in Table 6.1 contributes to the stabilization of the C=0 double bond. For a similar reason, there is no resonance stabilization of the C=0 double bond in N-acylpyridinium salts in the corresponding resonance form, the aromatic sextet of the pyridine would be destroyed in exchange for a much less stable quinoid structure. 

Mesoionic systems may be readily substituted by electrophiles. Thus the thiazolo mesoion (342) will couple with diazonium salts despite their relatively weak electrophilicity (80KGS621). Substitution in a fused heteroaromatic betaine azine ring, e.g. (343), also takes place with ease. The resonance form (344) of the mesoion (343) shows that the electrophile will attack at C-6. The substitution in this position is also predicted by MO calculations (73JHC487). Similarly the pyridine ring in pyridinium olates is active towards electrophiles and is substituted in the positions ortho and para to the olate function. Bromination of the 5-methyl derivative (321 R = Me) occurs exclusively in the 7-position which is rationalized via the intermediate (345). In the absence of a 5-substituent, attack in either the 5- or 7-position occurs the dibromide is readily formed. No bromination in the thiazole ring is observed. The 2-bromo derivative (346) has been made, however, by condensation between the appropriate mercaptopyridine and 1,1,2,2-tetrabromoethane. 

Ficken and Kendall " discuss the relative basicities of the pyridine and pyrrolenine nitrogen atoms in the azaindolenines (Scheme 7) based on Brooker deviation measurements from light-absorption data of their cyanine dyes. These indicate that N(7 in the 7-azaindolenine (31) is considerably more basic than N(d in the sense that the 7-quaternary salt (117) displays less tendency to be stabilized by a isomerization than does the 1-quaternary salt (120). They relate this to the formation of an unstable o-quinonoid resonance form. In the case of the 4-azaindolenine (32), it was suggested that the pyridine N(4) is expected to be more basic than N(X), although reaction 12 J. Clark and D. D. Perrin, Quart. Rev. London) 18, 295 (1964). 

Sigma complex resonance forms that have a heteroatom with an incomplete octet are especially bad no attack on the ortho or para positions of pyridine occurs. 

Any electrophilic attack, including sulfonation, is preferred at the 3-position of pyridine because the intermediate is more stable than the intermediate from attack at either the 2-position or the 4-position. (Resonance forms of the sulfonate group are not shown, but remember that they are important )... 

Begin by drawing the resonance forms of pyridine N-oxide r... 

Fig. 32a-d. Resonance Raman spectra of Rps. sphaeroides reaction centers as a function of potential of the silver electrode a) —0.7 V vs SCE b) 0.0 V vs SCE c) Blank (buffer, electrolyte, and LDAO) d) BChl (10 M) dissolved in CH2CI2 with sufficient pyridine to form BChl pyridine laser excitation wavelength 457.9 nm laser power 20 mW monochromator bandpass 5cm (Cotton and Van Duyne, Ref. 

On the surface, it might seem that the resonance stabilization of pyridine is lost on conversion to a pyridone. However, as is true for all amides, pyridones can be expressed in a resonance form, and as shown in hybrid 6.6, it retains the six pi-electron system of an aromatic species. 

The explanation once again can be found in resonance theory. This is illustrated in Scheme 6.23 with the nitration of pyridine N-oxide. Resonance form 6.11 has tetravalent N bearing a positive charge, which is a stable form of N. 

Write two different resonance fornns for pyrrole in which nitrogen has a formal charge of + 1. Are comparable resonance forms possible for pyridine ... 

Electrophilic attack on pyridine at the 2-position gives an unstable intermediate, with one of the resonance structures showing a positive charge and only six electrons on nitrogen. In contrast, all three resonance forms of the intermediate from attack at the 3-position place the positive charge on the less electronegative carbon atoms. 

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