A Repertoire of Pyridinium−Phenyl−Methyl Cross-Talk through a Cascade of Intramolecular Electrostatic Interactions

• Published in: Journal of organic chemistry Vol. 68; no. 4; pp. 1529 - 1538
• Main Authors: Acharya, P, Plashkevych, O, Morita, C, Yamada, S, Chattopadhyaya, J
• Format: Journal Article
• Language: English
• Published: Washington, DC American Chemical Society 21.02.2003
• Subjects: Chemistry; Exact sciences and technology; Heterocyclic compounds; Heterocyclic compounds with only one n hetero atom and condensed derivatives; Organic chemistry; Preparations and properties; Organic cation; Nitrogen heterocycle; Ab initio method; Electrostatic interaction; Aromatic compound; NMR spectrometry; Experimental study; Ketone; Oxazole derivatives; Intramolecular interaction; Oxygen nitrogen heterocycle; Pyridine derivatives
• ISSN: 0022-3263; 1520-6904
• DOI: 10.1021/jo026572e

Direct intramolecular cation−π interaction between phenyl and pyridinium moieties in 1a + has been experimentally evidenced through pH-dependent 1H NMR titration. The basicity of the pyridinyl group (pK a 2.9) in 1a can be measured both from the pH-dependent chemical shifts of the pyridinyl protons as well as from the protons of the neighboring phenyl and methyl groups as a result of electrostatic interaction between the phenyl and the pyridinium ion in 1a + at the ground state. The net result of this nearest neighbor electrostatic interaction is that the pyridinium moiety in 1a becomes more basic (pK a 2.92) compared to that in the standard 2a (pK a 2.56) as a consequence of edge-to-face cation (pyridinium)−π (phenyl) interaction, giving a free energy of stabilization (ΔΔ ) of −2.1 kJ mol-1. The fact that the pH-dependent downfield shifts of the phenyl and methyl protons give the pK a of the pyridine moiety of 1a also suggests that the nearest neighbor cation (pyridinium)−π (phenyl) interaction also steers the CH (methyl)−π (phenyl) interaction in tandem. This means that the whole pyridine−phenyl−methyl system in 1a + is electronically coupled at the ground state, cross-modulating the physicochemical property of the next neighbor by using the electrostatics as the engine, and the origin of this electrostatics is a far away point in the moleculethe pyridinyl-nitrogen. The relative chemical shift changes and the pK a differences show that the cation (pyridinium)−π (phenyl) interaction is indeed more stable (ΔΔ = −2.1 kJ mol-1) than that of the CH (methyl)−π (phenyl) interaction (ΔΔ = −0.8 kJ mol-1). Since the pK a of the pyridine moiety in 1a is also obtained through the pH-dependent shifts of both phenyl and methyl protons, it suggests that the net electrostatic mediated charge transfer from the phenyl to the pyridinium and its effect on the CH (methyl)-π (phenyl) interaction corresponds to Δ of the pyridinium ion (∼17.5 kJ mol-1), which means that the aromatic characters of the phenyl and the pyridinium rings in 1a + have been cross-modulated owing to the edge-to-face interaction proportional to this Δ change.