Pyridine electrophilic aromatic

Both pyrimidine and purine aie planai. You will see how important this flat shape is when we consider the structure of nucleic acids. In tenns of their chemistry, pyrimidine and purine resemble pyridine. They are weak bases and relatively unreactive toward electrophilic aromatic substitution. 

The reactivity of pyridine relative to that of benzene has been measured using the competitive technique developed by Ingold and his schoool for corresponding studies of electrophilic aromatic substitution. The validity of the method applied to free-radical reactions has been discussed. Three sources of the phenyl radical have been used the results obtained are set out in Table II. 

Unlike benzene, pyridine undergoes electrophilic aromatic substitution reactions with great difficulty. Halogenation can be carried out under drastic conditions, but nitration occurs in very low yield, and Friedel-Crafts reactions are not successful. Reactions usually give the 3-substituted product. 

Problem 24.22 Electrophilic aromatic substitution reactions of pyridine normally occur at C3. Draw... 

Imidazo[l,5-tf]pyridines are aromatic systems in which the bridgehead nitrogen N-4 contributes to the aromaticity with its lone pair. Therefore, this nitrogen atom is not nucleophilic and electrophilic attacks occur at the N-2 position. SgAr occurs at C-l, but also sometimes at C-3, depending on the conditions used (Figure 3). 

The nitration reagents (NO2 Y) for electrophilic aromatic nitration span a wide range and contain anions Y such as nitric acid (Y = OH-), acetyl nitrate (Y = OAc-), dinitrogen pentoxide (Y = NO3-), nitryl chloride (Y = Cl-), TV-nitropyridinium (Y = pyridine) and tetranitromethane [Y = C(N02)3-]. All reagents contain electron-deficient species which can serve as effective electron acceptors and form electron donor-acceptor (EDA) complexes with electron-rich donors including aromatic hydrocarbons107 (ArH, equation 86). Excitation of the EDA complexes by irradiation of the charge-transfer (CT) absorption band results in full electron transfer (equation 87) to form radical ion... 

The effect of heteroatoms on electrophilic aromatic substitution, e.g. the reactions of pyridine, will be considered separately in Chapter 11. 

The empirical data for electrophilic aromatic substitution on benzocycloalkenes over a variety of reactions and conditions show a consistent trend of increased Cp selectivity due primarily to C deactivation, with some indication that Cp activation occurs in benzobicycloalkenes. Acidity work on the benzocycloalkenes and related pyridines demonstrates clearly the extent of deactivation. The rehybridization model of Finnegan and Streitweiser has been postulated to account for the deactivation. Thummel s correlation of C y -H P a provided the necessary link between rehybridization and deactivation. Theories involving bond fixation in the Wheland intmnediate deserve some further consideration but are not essential to an understanding of the present empirical data. 

Electrophilic aromatic substitution of the 4-aminobenzofuran 1103 with the complex salt 602 afforded the iron complex 1109 in quantitative yield. Cyclization of the complex 1109 with concomitant aromatization was achieved by oxidation with an excess of iodine in pyridine at 90 °C in air to afford directly furostifoline (224) (688,689) (Scheme 5.179). 

Electrophilic substitutions Pyridine s electron-withdrawing nitrogen causes the ring carbons to have significantly less electron density than the ring carbons of benzene. Thus, pyridine is less reactive than benzene towards electrophilic aromatic substitution. However, pyridine undergoes some electrophilic substitution reactions under drastic conditions, e.g. high temperature, and the yields of these reactions are usually quite low. The main substitution takes place at C-3. 

Electrophilic aromatic substitutions The chemistry of pyrimidine is similar to that of pyridine with the notable exception that the second nitrogen in the aromatic ring makes it less reactive towards electrophilic substitutions. For example, nitration can only be carried out when there are two ring-activating substituents present on the pyrimidine ring (e.g. 2,4-dihydroxypyrimidine or uracil). The most activated position towards electrophilic substitution is C-5. 

Electrophilic aromatic substitutions Quinoline and isoquinoline undergo electrophilic aromatic substitution on the benzene ring, because a benzene ring is more reactive than a pyridine ring towards such reaction. Substitution generally occurs at C-5 and C-8, e.g. bromination of quinoline and isoquinoline. 

Predict the product of electrophilic aromatic substitution reactions of pyridine and quinoline. 

Explain why pyridine is less reactive than benzene in electrophilic aromatic substitution reactions. 

The methyl groups are electron-donating and activate aromatic compounds toward electrophilic aromatic substitution (see Secs. 4.10 and 4.11). Therefore, 2,6-dimethylpyridine undergoes nitration under much milder conditions than required by pyridine. Notice that nitration occurs ortho and para to the methyl groups. 

The effect of the heteroatom is to make the pyridine ring very unreactive to normal electrophilic aromatic substitution. Conversely, pyridines are susceptible to nucleophilic attack. These topics are discussed later. 

Activated pyridines will do electrophilic aromatic substitution... 

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