Subject pyridinium salts

Some nucleophiles add to iV-fluoro-pyridinium salts to give dihydropyridines in which elimination of fluoride occurs in situ to give the 2-substimted pyridine. However, the preparation of the pyridinium salts requires the use of elemental fluorine (31.1) and also, some carbanions are subject to competitive reactions such as C-fluorination. However, silyl enol ethers do react efficiently stabilised heteronucleophiles (phenolate, azide) can also be used, and isonitriles produce picolinamides. ... 

Interestingly, when lower temperature and energy microwave is used, loss of water does not occur, and die imidazolin-pyridinium salt forms. When this intermediate is subjected to the hydrolysis conditions, a Dimroth-type rearrangement is proposed to occur which leads to the alkyl substituent that was at die 1-position of the imidazole ring to now be on the 2-amino-substituent as shown. 

There are relatively few pyridinium salts in this class. Several pyridinium salts with heavier p-block elements in side chains (Si, S or Se) have been crystallographically characterised and are discussed in Section 11.17. One or two melt below 100°C, though none contain the triflate counter-anion. However, in one of these reports [454], some of the starting materials have been isolated as triflate salts. [2,4-Dichloro-l-methylpyridinium][OTf], [4-chloro-2-dimethylamino- 1-methylpyridinium] [OTf] and [2-chloro- l-methyl-4-phenylaminopyridinium] [OTf] were isolated, with mpt. of 96-98, 57-59 and 98.5-102°C, respectively, though only the latter was subjected to single-crystal study (L7l 427 kJ mor ). 

The pyridinyl substituent in position 1 of 1,10-bis- 3-phenyl-1-(pyridin-3-yl)imidazo[l,5-cj]quinoxalin-4-on-5-yl decane 149, which then would be subjected to alkylation with alkyl dihalides with the formation of pyridinium salts, opens the possibility of building imid-azoquinoxaline macrocycles with two pyridinium salt fragments. The interaction of l,10-bis(imidazoquinoxaline)decane 149 with the m-xylylenediamine in acetonitrile solution at high dilution brings about the P,7 -diphenyl,-l, 7 -dioxa-l,7(l, 5)-diimidazo[l,5-a]qunoxalina,-2(3,1),6(1,3)-dipyridina,-4(l,3)-benzenacycloheptadecaphane-2, 6, -ylium dibromide 150 in a 60% yield. Unfortunately, the analytically pure sample of 150 when separated from the reaction mixture was only 25% (2013EJMC345). 

The pyridinyl substituent in position 1 of l,10-bis- 3-phenyl-l-(pyridin-3-yl) imidazo[l,5-a]quinoxalin-4-on-5-yl decane 170, which then would be subjected to alkylation with alkyl dihalides with the formation of pyridinium salts, opens the... 

Benzo-l,4-thiazines and Related Compounds.— The chemical behaviour of the Vilsmeier product (111), derived from l,4-benzothiazin-3(4H)-one, has been the subject of detailed studies. Hydrolysis under different conditions affords the acid (112), the aldehyde (114), or the chloro-aldehyde (113), depending on the method of hydrolysis. Treatment of the perchlorate of (111) with DMSO leads to the displacement of the chlorine atom to give the dimethylsulphoxonium derivative (115), while reaction with pyridine proceeds with selective attack at the aminomethylene function, yielding the pyridinium salt (116). This latter reacts readily with aniline in acetic acid medium to give, along with 2-formyl-3-anilino-l,4-benzothiazine (117), the anilinomethylene derivative (120), whose formation probably involves an intramolecular rearrangement of the intermediate pyridinium salt (118), as depicted. 

The aminocyclopentene products of these pyridinium salt photoreactions were shown to arise by acid-catalyzed methanolytic aziridine ring opening of bicycHc aziridine intermediates. Accordingly, treatment of the bicyclic aziridines 24 and 25, produced by irradiation of basic MeOH solutions of the salts 20 and 21, with catalytic quantities of HCIO4 in MeOH led to smooth formation of the respective aminocydo-pentenes 22 and 23. Thus, when base is not included in pyridinium salt photoreaction mixtures, the initially formed photocyclization products are N-protonated bicycHc aziridines, which are subject to nucleophile-induced cleavage at their allylic aziridine centers. 

H and C NMR studies on neutral azoles and pyridinium quaternary salts is by now a well-documented subject, and to a lesser extent, azolium quaternary salts. In contrast, only few studies have been devoted to azo-late ions and practically all the reported data for the anion species have been generated in situ using the appropriate NMR solvent in basic medium, often because the azolate anions themselves are unknown. 

Katritzky and coworkers have extensively developed the activation of amines by reaction with pyry-lium salts to provide (V-alkyl (or N-aryl) pyridinium compounds. When buttressing substituents were present to discourage attack on the pyridine ring, the N-alkyl substituent was subject to displacement and elimination processes. In general, primary alkyl substituents reacted with most nucleophiles in a normal 5n2 process as shown in Scheme 12, whereas competition between substitution and elimination took place with the secondary analogs, with elimination dominating the reactions starting from cycloalkyl-amines. 

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