Department of Chemistry, University of Missouri, Columbia, Missouri 65211, USA.
J Org Chem. 2010 Feb 19;75(4):1132-42. doi: 10.1021/jo902358c.
The uncatalyzed, thermal N-inversion reactions were studied of pyrimidin-4(3H)-imine (PMI), pyridin-2(1H)-imine (PYI), and 1H-purine-6(9H)-imine (PUI). Relevant regions of the potential energy surfaces were explored with second-order Moller-Plesset perturbation theory (MP2(full)/6-31G(d)) and with coupled cluster theory (CCSD/6-31G(d), CCSD/6-31+G(d)). The thermochemistry of stationary structures was evaluated at the MP2 level and their energies also were computed at the levels CCSD(T)/6-311+G(d,p) and CCSD(T)/6-311+G(2df,2p) and with structures optimized at lower CCSD levels. The best estimates for the (E)-preference free enthalpies DeltaG(298)(Z vs. E) are 2.6 (PMI), 2.3 (PYI), and 6.0 (PUI) kcal/mol and for the free enthalpies of activation DeltaG(298)(Z --> E) they are 21.6 (PMI), 21.1 (PYI) and 19.7 (PUI) kcal/mol. Nonplanar N-inversion transition state (ITS) structures occur along enantiomeric reaction paths and stationary structures for in-plane N-inversion correspond to second-order saddle points (SOSP) on the potential energy surface. The deformation energy DeltaE(def) = E(SOSP) - E(ITS) is less than 0.5 kcal/mol for PMI and PUI, but it is as high as DeltaE(def) approximately = 2 kcal/mol for PYI. The detailed study of structures and electronic structures along the entire N-inversion path of the isomerization (Z)-PMI <==> (E)-PMI revealed a remarkable stabilization due to asymmetry in the ascent region from the (E)-isomer to ITS. Structures in this region of the potential energy surface allow best for additional bonding overlaps in the HOMO, and this amidine effect predicts lower N-inversion barriers in analogous imines with (Z)-preference energies. The discussion of the halogen-bonded aggregate PMI x ClCH(3) exemplifies that the asymmetry in N-inversion paths is retained and perhaps even enhanced in chlorinated solvents of low polarity.
研究了嘧啶-4(3H)-亚胺（PMI）、吡啶-2(1H)-亚胺（PYI）和 1H-嘌呤-6(9H)-亚胺（PUI）的非催化热 N 反转反应。采用二阶 Moller-Plesset 微扰理论（MP2(full)/6-31G(d)）和耦合簇理论（CCSD/6-31G(d)、CCSD/6-31+G(d)）对势能面的相关区域进行了探索。在 MP2 水平上评估了稳定结构的热化学性质，并在 CCSD(T)/6-311+G(d,p)和 CCSD(T)/6-311+G(2df,2p)水平以及较低 CCSD 水平优化的结构上计算了它们的能量。（E）-构型优先自由焓 DeltaG(298)(Z vs. E)的最佳估计值分别为 2.6（PMI）、2.3（PYI）和 6.0（PUI）千卡/摩尔，（Z）-构型向（E）-构型的自由焓活化能 DeltaG(298)(Z --> E)分别为 21.6（PMI）、21.1（PYI）和 19.7（PUI）千卡/摩尔。非平面 N 反转过渡态（ITS）结构沿着对映反应路径发生，平面内 N 反转的稳定结构对应于势能面上的二阶鞍点（SOSP）。PMI 和 PUI 的变形能 DeltaE(def) = E(SOSP) - E(ITS)小于 0.5 千卡/摩尔，但 PYI 的 DeltaE(def)高达约 2 千卡/摩尔。对（Z）-PMI <==>（E）-PMI 异构化整个 N 反转路径上的结构和电子结构的详细研究表明，在从（E）-异构体到 ITS 的上升区域，由于不对称性而导致显著的稳定化。势能面这个区域的结构允许在 HOMO 中进行额外的键重叠，这种脒基效应预测在具有（Z）-优势能量的类似亚胺中，N 反转势垒较低。对卤键合聚集体 PMI x ClCH(3)的讨论表明，N 反转路径中的不对称性在低极性氯化溶剂中得到保留，甚至可能增强。
