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A repertoire of pyridinium-phenyl-methyl cross-talk through a cascade of intramolecular electrostatic interactions

A repertoire of pyridinium-phenyl-methyl cross-talk through a cascade of intramolecular electrostatic interactions

Acharya, P. ORCID: 0000-0003-2521-9968, Plashkevych, O., Morita, C., Yamada, S. and Chatopadhyaya, J. (2003) A repertoire of pyridinium-phenyl-methyl cross-talk through a cascade of intramolecular electrostatic interactions. Journal of Organic Chemistry, 68 (4). pp. 1529-1538. ISSN 0022-3263 (Print), 1520-6904 (Online) (doi:https://doi.org/10.1021/jo026572e)

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Abstract

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 (pKa 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 (pKa 2.92) compared to that in the standard 2a (pKa 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 pKa 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 moleculethe pyridinyl-nitrogen. The relative chemical shift changes and the pKa 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 pKa 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.

Item Type: Article
Uncontrolled Keywords: aromatic interaction, electrostatic force, stacking, nucleic acid
Subjects: Q Science > QD Chemistry
Faculty / Department / Research Group: Faculty of Engineering & Science
Faculty of Engineering & Science > Natural Resources Institute
Faculty of Engineering & Science > Natural Resources Institute > Food & Markets Department
Last Modified: 10 Jul 2020 13:44
Selected for GREAT 2016: None
Selected for GREAT 2017: None
Selected for GREAT 2018: None
Selected for GREAT 2019: None
Selected for REF2021: None
URI: http://gala.gre.ac.uk/id/eprint/28724

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