Pyridinium compounds leaving groups

In contrast, reactions of organomagnesium compounds with pyridinium salts are more useful than those of organolithium compounds. Unsubstituted pyridinium salts have a somewhat greater tendency towards 4-alkylation than pyridine itself [5], and with a bulky N-substituent, 4-alkylation predominates. If the N-substituent is also a good leaving group, a viable general synthesis of 4-substituted pyridines results [6],... 

In a similar way, pyridine phosphonium salts and phosphonates can be prepared by reaction of trivalent phosphorus compounds with the more accessible iV-trifluoromethanesulfonyl-pyridinium salts, when tri-fluoromethanesulfone is the leaving group from nitrogen (as sulfinate anion) attack is normally at C-4, as illustrated below. The A -trifluoromethanesulfonyl-pyridinium salts also react with ketones or with electron-rich aromatic compounds to give 1,4-dihydropyridine adducts. Subsequent treatment with potassium t-butoxide brings about elimination of trifluoromethanesulfinic acid, and thus aromatisation. It is also possible to utilise phosphonates in reaction with aldehydes, leading finally to 4-substimted pyridines. ... 

Sodium and potassium borohydrides are above all used for reducing aldehydes and ketones (Sections 3.2.1, 3.2.2) a,p-ethylenic ketones are converted to mixtures [W3]. In alcoholic media or THF, they leave epoxides, esters and lactones, acids, amides, and most nitro compounds unreacted, but they reduce halides (Section 2.1), anhydrides (Section 3.2.6), quartemary pyridinium salts (Section 3.3), double bonds conjugated to two electron-withdrawing groups (Sections 3.2.9, 4.4), and CUPd... 

The pyridinium group leaving with its lone pair, acts as a built-in oxidizing agent2i2nitroso compounds produce a similar pattern of anil and nitrone formation si and anils of the type (52) have been pre-... 

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