Hansen Poul Erik, Kamounah Fadhil S, Hansen Bjarke K V, Spanget-Larsen Jens
Department of Science, Systems and Models, Roskilde University, P.O. Box 260, DK-4000 Roskilde, Denmark.
Magn Reson Chem. 2007 Feb;45(2):106-17. doi: 10.1002/mrc.1925.
Tautomerism in aromatic systems with oxygen substitutents is rare. This is investigated in 2-acetyl-1,8-dihydroxy-3,6-dimethylnaphthalene (1) and in 2,7-diacetyl-1,8-dihydroxy-3,6-dimethylnaphthalene (2). The tautomeric nature of 2-acetyl-1,8-dihydroxy-3,6-dimethylnaphthalene is supported by long-range hydrogen-hydrogen coupling between the OH-1 and the OH-8 and by the isotope effects on 13C caused by deuteration at the CH3C==O methyl group. Compound 2 participates in a degenerate equilibrium between two equivalent nonsymmetrical rotamers (2A and 2B), each having two intramolecular O...HO hydrogen bonds: one involving an acetyl oxygen and the neighboring hydroxyl group, and the other between the oxygen centers at positions 1 and 8. In addition, each rotamer is involved in a tautomeric equilibrium, with a structure having an OH-substituted exocyclic double bond (2AT or 2BT).DFT calculations for a large set of compounds highlight the factors controlling the unusual rotational and tautomeric behaviors. A very important factor seems to be the repulsive interaction between the O-1 and O-8 centers, which is modulated by formation of an OH-1...O-8 or OH-8...O1 hydrogen bond. Steric interactions, mesomeric release of electrons from the oxygen at position 8, and a strong OH...O...C hydrogen bond are other factors.Solid-state 13C NMR spectra of 2,7-diacetyl-1,8-dihydroxy-3,6-dimethylnaphthalene at different temperatures demonstrated no averaging in the solid, whereas partially deuterated 2-acetyl-1,8-dihydroxy-3,6-dimethylnaphthalene showed an isotope effect at C-1 of 1.5 ppm, indicating tautomerism in the solid state.
具有氧取代基的芳香体系中的互变异构现象很少见。本文对2-乙酰基-1,8-二羟基-3,6-二甲基萘（1）和2,7-二乙酰基-1,8-二羟基-3,6-二甲基萘（2）进行了研究。2-乙酰基-1,8-二羟基-3,6-二甲基萘的互变异构性质通过OH-1与OH-8之间的远程氢-氢偶合以及CH3C==O甲基上的氘代对13C的同位素效应得到证实。化合物2参与两个等效的非对称旋转异构体（2A和2B）之间的简并平衡，每个异构体都有两个分子内O...HO氢键：一个涉及乙酰氧基和相邻的羟基，另一个在1和8位的氧中心之间。此外，每个旋转异构体都参与互变异构平衡，其结构具有一个OH取代的环外双键（2AT或2BT）。对大量化合物的密度泛函理论（DFT）计算突出了控制异常旋转和互变异构行为的因素。一个非常重要的因素似乎是O-1和O-8中心之间的排斥相互作用，这种相互作用通过形成OH-1...O-8或OH-8...O1氢键来调节。空间相互作用、8位氧的电子中介释放以及强OH...O...C氢键是其他因素。2,7-二乙酰基-1,8-二羟基-3,6-二甲基萘在不同温度下的固态13C核磁共振谱表明在固体中没有平均化，而部分氘代的2-乙酰基-1,8-二羟基-3,6-二甲基萘在C-1处显示出1.5 ppm的同位素效应，表明在固态下存在互变异构现象。
