Pyridinium cations nucleophilic attack

A ring carbon can also be involved, however, as in the reaction of the thianthrene and phenothiazine radical cations in neat pyridine or with pyridine in an anhydrous solvent. In this reaction the 1-pyridinium group is inserted on to the benzo ring (43), apparently via nucleophilic attack on di-cations 42, in turn resulting from oxidation of the initially formed radical cation adducts (Scheme 27). In the presence of moisture the sulfoxides are again formed [84]. 

The intermediate l-aza-2-azoniallene cations can undergo 1,3-dipolar cycloaddition reactions with inverse electron demand [119]. Thus, oxidation of a A -phenyl hydrazone in the presence of pyridine leads to the formation of a. y-triazolo[4,3-a]-pyridinium salt by attack of pyridine as a nucleophile on the intermediate nitrilimine [Eq. (17)] [120]. Other examples are reported by Jugelt [121]. 

The Pyridinium Cation. The pyridinium cation (83) is readily attacked by nucleophiles at C-2 and C-4. The total electron deficiency37 at C-2 of + 0-241 and at C-4 of +0-165 indicates that charge control (in other words with hard... 

These reactivity trends clearly show that polar effects are involved in these radical substitution reactions. The transition state is thought to include a charge transfer 9) from the radical (electron donor) to the pyridinium ion (electron acceptor) [13], Frontier Molecular Orbital Theory (FMO) [14] has been applied to explain the reactivity differences which have been observed upon varying the substituents at the pyridinium ion and upon altering the nucleophilicity of the attacking radical. Moreover, FMO can be used to explain the regioselectivities obtained in these homolytic aromatic substitutions. The LUMO of the substituted pyridinium cation... 

Molecular orbital calculations for tetrazolo[l,5-a]pyridinium cation (2) showed that the highest coefficient (chjmo) values can be found, except the bridgehead N-4, for C-5 and C-8a atoms which satisfactorily explained the observed selective nucleophilic attacks at these centers <86T5415>. 

Poly(3-alkylthiophene)s are chemically robust, withstanding strong reductants including boranes [67] and LiAlH4 [72]. The electron-rich backbone is, however, readily functionalized by oxidative methods. Li and co-workers exploited this to replace the 4-proton with Cl, Br or NO2 functionality [73-75]. Reaction at the a-methylene was noted in some instances. Subsequent Pd-catalyzed cross-coupling of the perbrominated polymer could effect >99% derivatization. Oxidation renders the backbone susceptible to nucleophilic attack. Li et al. found that pyridine derivatives efficiently reacted at the 4-position of the radical cation, functionalizing up to 60 % of the putative polaron pentads. Use of l-methyl-4-(4 -pyridyl)pyridinium salts yielded viologen substituents [76]. 

The mechanism for substitution of a nucleophile at a P-position of the ZnTPP macrocycle is a little different, since an EiCNmesoE2CNpCB process occurs in this case with two different successive nucleophilic attacks [110]. After electrogeneration of the porphyrin radical cation (ZnTPP step Ei), a first nucleophilic attack takes place at a wieso-position (step CNmeso)- This is kinetically more favorable because there is a larger charge density on the mesocarbons as compared to that on the P-carbons [123,124]. However, there is no proton that can be removed from the substituted me o-position after the second oxidation step (step E2) in this case, and a second nucleophilic attack takes place at the P-position (step Cnp) which simultaneously leads to the loss of the pyridinium attached to the /new-position. Then, the spare proton at the P-position can be removed in a last step in order to recover the aromaticity of the macrocycle (step Cb). As before, the global reaction can be written as shown in Eq. 2 ... 

Concerning the first variant, Bew et al. reported the use of AT-fluoropyridinium triflate as an F organocatalyst for the aziridine synthesis from imines and ethyl diazoacetates [114]. Catalytic amounts of the fluorinated pyridinium salt 14 are claimed by the authors to function as a source for the fluorenium cation F which presumably activates the imine component, thus facilitating the following nucleophilic attack of ethyl diazoacetate leading to the aziridine system (Scheme 11). 

A positive charge facilitates attack by nucleophilic reagents at positions or to the heteroatom. Amines, hydroxide, alkoxide, sulfide, cyanide and borohydride ions, certain carbanions, and in some cases chloride ions react with pyridinium, pyrylium, and thiopyrylium cations under mild conditions to give initial adducts of types 12 and 13. 

Nucleophiles, such as hydroxide, cyanide, and Grignard reagents attack the 2-position of pyridinium and the 1-position of isoquinolinium cations. Quinolinium salt reacts with nucleophiles at the 2- and 4-positions. Hydroxide attacks the 2-position, and cyanide attacks the 4-position. These results support the theoretical expectation. 

Practically all the reactions of quinolizinium ions are similar to those of pyridinium salts, thus they are resistant to electrophilic attack, but readily undergo nucleophilic addition, the initial adducts undergoing spontaneous electrocyclic ring opening to afford, finally, 2-substituted pyridines however the susceptibility of the cations to nucleophiles is not extreme - Uke simpler pyridinium salts they are stable to boiling water. 

An advantageous method for the conversion of 2- and 4-methyl substituted pyridinium and quinolinium salts (120) into their corresponding 2,4,6-triaryl-phenyl derivatives (121) in good yields has been developed by Zimmermann [158]. The anhydrobases of 121 were discussed as the key intermediates of this pyrylium ring transformation they attack the pyrylium cation 122 in the initial step as carbon nucleophiles of the enamine type (see Scheme 56). The reaction... 

---