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

被引:24
作者
Acharya, P
Plashkevych, O
Morita, C
Yamada, S
Chattopadhyaya, J
机构
[1] Uppsala Univ, Dept Bioorgan Chem, Biomed Ctr, S-75123 Uppsala, Sweden
[2] Ochanomizu Univ, Dept Chem, Fac Sci, Bukyo Ku, Tokyo 1128610, Japan
关键词
D O I
10.1021/jo026572e
中图分类号
O62 [有机化学];
学科分类号
070303 ; 081704 ;
摘要
Direct intramolecular cation-pi interaction between phenyl and pyridinium moieties in la+ has been experimentally evidenced through pH-dependent H-1 NMR titration. The basicity of the pyridinyl group (pK(a) 2.9) in la 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 la+ at the ground state. The net result of this nearest neighbor electrostatic interaction is that the pyridinium moiety in la 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)-pi (phenyl) interaction, giving a free energy of stabilization (DeltaDeltaG(pKa)(o)) 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 la also suggests that the nearest neighbor cation (pyridinium)-pi (phenyl) interaction also steers the CH (methyl)-pi (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 pKa differences show that the cation (pyridinium)-pi (phenyl) interaction is indeed more stable (DeltaDeltaG(pKa)(o) = -2.1 kJ mol(-1)) than that of the CH (methyl)-pi (phenyl) interaction (DeltaDeltaG(pKa)(o) = -0.8 kJ mol(-1)). Since the pKa of the pyridine moiety in la is also obtained through the pH-Sependent 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)-pi (phenyl) interaction corresponds to DeltaG(pKa)(o) of the pyridinium ion (similar to17.5 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 DeltaG(pKa)(o) change.
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页码:1529 / 1538
页数:10
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