N-Alkylpyridinium Ions

The application of the concept of a reactive-ion micelle is illustrated by addition of cyanide ion to N-alkylpyridinium ions (Fig. 3) and rate constants are compared in Table 4. Second-order rate constants are essentially independent of substrate hydrophobicity and are only slightly affected by added inert salts. They are also very similar to second-order rate constants in water. 

We note in this context that k /kw is greater than unity both for nucleophilic attack upon neutral aromatic substrates, e.g. 2,4-dinitrochloro-benzene and cationic N-alkylpyridinium ions (Tables 3 and 4). 

Hydrophilic anions will stay in the outer, water-rich regions of micelles and the more hydrophobic organic substrates may be located more deeply in the micelle. These effects, and those due to the preferred orientation of substrates in the micelle, do not seem to be of major importance in determining k /k, because values are not very different for reactions of substrates which have the same reactive groups but different hydrophobici-ties. This conclusion is illustrated by comparing values of k lk, for reactions of OH with 4-nitrophenyl acetate and octanoate or of CN with a series of N-alkylpyridinium ions (Tables 3 and 4). Despite large differences in hydro-phobicities, as shown by variations in K there is little change in (Il is... 

Therefore, alkylpyridines and N-alkylpyridinium ions undergo base- or acid-catalyzed reactions with electrophilic reagents preferably at the 2- and 4- heterobenzylic positions. For instance, the CH3 group of 2- or 4-picohne can be alkylated (—> 71), carboxylated —> 72), and acylated (—> 74) by a CiAisEN-like condensation aldol addition 73), multiple aldol addition (—> 76), and aldol condensation 75) are also possible. 

Reduction of pyridine derivatives can be accomphshed by metals. Sodium in alcohol solvents reduces pyridine to piperidine (Ladenburg reaction), and N-alkylpyridinium ions are converted to N-alkylpiperidines by metals (such as Sn or Zn) in acidic medium or by electrochemical reduction. 

Ionic liquids have been known for nearly a century the first to be discovered was ethylammonium nitrate, CH3CH2NH3 " N03, with a melting point of 12 °C. More generally, however, the ionic liquids in use today are salts in which the cation is unsymmetrical and in which one or both of the ions are bulky so that the charges are dispersed over a large volume. Both factors minimize the crystal lattice energy and disfavor formation of the solid. Typical cations are quaternary ammonium ions from heterocyclic amines, either 1,3-dialkylimidazolium ions, N-alkylpyridinium ions, or ring-substituted N-alkylpyridinium ions. 

N-Alkylpyridinium salts give mainly N,N -dialkyltetrahydro-4,4 -dipyridyl derivatives on reduction in neutral and slightly alkaline aqueous solution [76]. These products can be oxidised to the N,N -dialkyl-4,4 -dipyridyl. The radical-zwitterion derived from 4-cyano-l-methylpyridine couples and then loses cyanide ion to form N,N -dimethyl-4,4 -dipyridyl in 39 % yield [77]. 

Alkyl halides and related alkylating agents react with pyridines to form N-alkylpyridinium salts (Scheme 2.7). These compounds are much more stable than their 7V-acylpyridinium equivalents and can often be isolated as crystalline solids, particularly if the halide ion is exchanged for perchlorate, tetrafluoroborate or another less polarizable counter anion. 

Radius can be generated by reduction of carbonium ions or onium salts. Reduction of the N-alkylpyridinium salt 203 yields in either buffered aqueous KC1 or CH3CN/Bu4N+C104 the radical 204 in 100%efficiency 597). 

For scientific studies primary alkylammonium ions RNH and alkylpyridinium ions n-r are often used. 

Characteristic for the fragmentation of the title compounds (237-243) are a very facile N—S bond cleavage (route A in equation 50 giving rise to 244) as in sulfonylhydrazides (Section V.C) and skeletal rearrangements accompanied by the extrusion of SO2 from the M and [M — 1] ions to the ionized JV-aryliminopyridinium betaines (245, compounds 237-239) or AT-imino-2-benzylpyridinium betaines (247, compounds 240-243 in equation 52) and azacarbazoles (246), respectively . Ion 244 decomposes further by elimination of HCN and via the loss of R N and HCN, as shown in equation (51). The sequence M " (240-243)- 247 ->248 (equation 52) is supported by the fact that AT-imino-a-alkylpyridinium betaines can lose an NH2 radical due to the operation of an ortho effect . 

Like alkylpyridinium derivatives, dialkyldimethylammonium derivatives show basal spacings that increase linearly with alkyl-chain length from 4 nm (n = 14) to 5 nm (n = 18). (Fig. 20). The slope Ad/An = 0.180 nm indicates that the alkylammonium ions form bilayers with the chains tilted 45° to the layer [42]. The tetrahedral orientation of ammonium C-N bonds causes the alkyl chains in all-trans conformations to form a V (Fig. 21a), and space filling in mono-or bimolecular films subsequently would be poor. Transformation of two trans bonds into two gauche bonds (Fig. 21b) brings the alkyl chains in parallel orientation (Fig. 21c). This configuration is adopted by dialkyldimethylammonium ions... 

Fig. 4.2.3 Apparent standard ion transfer potentials of some hydrophobic ions across nitrobenzene and water interface for preparing moderately hydrophobic ionic liquids 25 °C [43] Cnmim" l-alkyl-3-methylimidazoliiim (alkyl = butyl (n = 4), pentyl(n = 5), hexyl(n = 6), heptyl(n = 7), octyl(n = 8), decyl ( = 10), dodecyl(n = 12)) PyCiCj A-alkyl-W-methylpyrrolidinium (aMyl = butyl( = 4), octyl(n = 8)) C Py iV-alkylpyridinium (alkyl hexyl (n = 6), octyl( = 8), decyl(n = 10), dodecyl(n = 12)) Cnlq" Af-alkyllisoquinolinium (alkyl = butyl(n = 4), dodecyl( = 12), hexadecyl(n = 14)) C CJST (bis (perfluoroalkanesulfonyl)amide m and n stand for the number of difluoromethylene moiety). The value after each abbreviated name of ion indicates the value of inV...

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