Pyridinium refers to the cation[C5H5NH]+. It is the conjugate acid of pyridine. Many related cations are known involving substituted pyridines, e.g. picolines, lutidines, collidines. They are prepared by treating pyridine with acids.[3]
When the acidic proton is replaced by alkyl, the compounds are called N-alkylpyridinium. A simple representative is N-methylpyridinium ([C5H5NCH3]+). These pyridinium intermediates have been used as electrophiles in synthetic organic chemistry to build dearomatized congeners called dihydropyridines, as demonstrated in one example from Smith in 2021.[5] Earlier, the same research group also delineated the rules surrounding regioselectivities associated with adding nucleophiles to pyridinium electrophiles with varying substituents.[6] From a commercial perspective, an important pyridinium compound is the herbicideparaquat.[7]
^Linnell, Robert (1960). "Notes – Dissociation Constants of 2-Substituted Pyridines". Journal of Organic Chemistry. 25 (2): 290. doi:10.1021/jo01072a623.
^Pearson, Ralph G.; Williams, Forrest V. (1953). "Rates of Ionization of Pseudo Acids.1V. Steric Effects in the Base-catalyzed Ionization of Nitroethane". Journal of the American Chemical Society. 75 (13): 3073. doi:10.1021/ja01109a008.
^George A. Olah; Michael Watkins (1978). "Fluorinations With Pyridinium Polyhydrogen Fluoride Reagent: 1-Fluoroadamantane". Org. Synth. 58: 75. doi:10.15227/orgsyn.058.0075.
^"Aromatic Compounds" (PDF). Alex Roche, Rutgers University.
^Grigolo, Thiago A.; Subhit, Ariana R.; Smith, Joel M. (2021-09-03). "Regioselective Asymmetric Alkynylation of N-Alkyl Pyridiniums". Organic Letters. 23 (17): 6703–6708. doi:10.1021/acs.orglett.1c02276. ISSN 1523-7060. PMID 34474575. S2CID 237401193.
^Knight, Brian J.; Tolchin, Zachary A.; Smith, Joel M. (2021-03-11). "A predictive model for additions to N-alkyl pyridiniums". Chemical Communications. 57 (21): 2693–2696. doi:10.1039/D1CC00056J. ISSN 1364-548X. PMID 33595047. S2CID 231945207.
^Shimizu, Shinkichi; Watanabe, Nanao; Kataoka, Toshiaki; Shoji, Takayuki; Abe, Nobuyuki; Morishita, Sinji; Ichimura, Hisao (2007). "Pyridine and Pyridine Derivatives". Ullmann's Encyclopedia of Industrial Chemistry. Weinheim: Wiley-VCH. doi:10.1002/14356007.a22_399. ISBN 978-3527306732.