title: Reactivity in the nucleophilic aromatic substitution reactions of pyridinium ions.
abstract: The "element effect" in nucleophilic aromatic substitution reactions (SNAr) is characterized by the leaving group order, L = F > NO2 > Cl ≈ Br > I, in activated aryl substrates. A different leaving group order is observed in the substitution reactions of ring-substituted N-methylpyridinium compounds with piperidine in methanol: 2-CN ≥ 4-CN > 2-F ∼ 2-Cl ∼ 2-Br ∼ 2-I. The reactions are second-order in [piperidine], the mechanism involving rate determining hydrogen-bond formation between piperidine and the substrate-piperidine addition intermediate followed by deprotonation of this intermediate. Computational results indicate that deprotonation of the H-bonded complex is probably barrier free, and is accompanied by simultaneous loss of the leaving group (E2) for L = Cl, Br, and I, but with subsequent, rapid loss of the leaving group (E1cB-like) for the poorer leaving groups, CN and F. The approximately 50-fold greater reactivity of the 2- and 4-cyano substrates is attributed to the influence of the electron withdrawing cyano group in the deprotonation step. The results provide another example of β-elimination reactions poised near the E2-E1cB mechanistic borderline.
authors.affiliation: Department of Chemistry and Biochemistry, San Francisco State University, San Francisco, CA 94132, USA. keeffe@sfsu.edu wuw@sfsu.edu sgronert@vcu.edu.
authors.forename: Jeannette T
authors.initials: JT
authors.lastname: Bowler
authors.forename: Freeman M
authors.initials: FM
authors.lastname: Wong
authors.forename: Scott
authors.initials: S
authors.lastname: Gronert
authors.forename: James R
authors.initials: JR
authors.lastname: Keeffe
authors.forename: Weiming
authors.initials: W
authors.lastname: Wu
mesh_terms: D004583:Electrons; D006841:Hydrocarbons, Aromatic / Q000737:chemistry*; D006860:Hydrogen Bonding; D007477:Ions; D007700:Kinetics; D008968:Molecular Conformation; D010880:Piperidines / Q000737:chemistry; D011522:Protons; D011726:Pyridinium Compounds / Q000737:chemistry*
chemical_list: D006841:Hydrocarbons, Aromatic; D007477:Ions; D010880:Piperidines; D011522:Protons; D011726:Pyridinium Compounds