Theoretical analysis of Lewis basicity based on local electron-donating ability. Origin of basic strength of cyclic amines
Theoretical analysis of Lewis basicity based on local electron-donating ability. Origin of basic strength of cyclic amines
复制标题
DOI：
10.1021/jo0486589
复制
发表时间：
2004-10-29
影响因子：
3.6
通讯作者：
Ogawa, A
中科院分区：
文献类型：
--
作者：
Ohwada, T;Hirao, H;Ogawa, A
实验证明，氮杂环丁烷的质子亲合势（PA）和溶液碱度（pK（BH）（+））远小于相应的吡咯烷和哌啶，但氮杂环丁烷的碱性强度与吡咯烷和哌啶相近。简单地认为基本强度依赖于键角的观点似乎是不成立的。由于环状胺的碱性是有机化学中的一个基本性质，我们重新审视了这个主题，以定量地澄清相关胺的刘易斯碱性强度的内在起源，特别是基于局域反应性杂化轨道（RHO）概念中评估的胺氮原子的局部给电子能力。在代表性的N-取代基，如氢，甲基和苯基的情况下，给电子的氮中心，通过最大化一种superdelocalizability，得到的能级，被证明是相关的实验和计算的气相质子亲和力的大小。本研究结果有力地支持了C-N-C键角，即，环胺中的角应变不是这些胺的碱性强度差异的主要来源，而是氮原子周围的非对称化程度对氮孤对轨道的供电子能力具有显著影响。
It has been experimentally established that the proton affinities (PA), as well as the solution basicities (pK(BH)(+)), of aziridine derivatives are much smaller than those of the corresponding pyrrolidines and piperidines, though the basic strength of azetidines is close to those of pyrrolidines and piperidines. A simple idea of dependence of the basic strength on bond angles seems to be invalid. Because the basicity of cyclic amines is a fundamental property in organic chemistry, we revisited this topic in order to clarify quantitatively the intrinsic origin of the strength of Lewis basicity of the relevant amines, in particular, based on the local electron-donating ability of the amine nitrogen atoms evaluated in terms of the localized reactive hybrid orbital (RHO) concept. In the cases of representative N-substituents such as hydrogen, methyl, and phenyl groups, the electron-donating energy level of the nitrogen center, obtained by maximizing a kind of superdelocalizability, was shown to be correlated with the magnitudes of experimental and calculated gas-phase proton affinities. The present results strongly support the view that the C-N-C bond angle, i.e., angle strain, in the cyclic amines is not the major source of the difference in strength of basicity of these amines, but rather, the degree of pyramidalization around the nitrogen atom has a significant impact on the electron-donating ability of the nitrogen lone-pair orbital.