异头效应、超共轭和静电：从经典立体电子现象的复杂性中吸取教训,Chemical Society Reviews

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异头效应、超共轭和静电：从经典立体电子现象的复杂性中吸取教训
Chemical Society Reviews ( IF 40.4 ) Pub Date : 2021-08-24 , DOI: 10.1039/d1cs00564b
Igor V Alabugin ₁ , Leah Kuhn ₁ , Nikolai V Krivoshchapov _{2,

3} , Patricia Mehaffy ₁ , Michael G Medvedev _{2,

4}

Affiliation

了解多种成分（立体、静电、立体电子、色散等）的相互作用。) 定义有机分子的整体能量、结构和反应性可能是一项艰巨的任务。当基于不同物理前提的多种方法在对多组分分子系统的分析中不一致时，任务变得更加困难。在这里，我们将使用经典的构象“奇数”，即异头效应，来讨论确定可指导化学预测的反应性关键因素的价值。在提供与相关类型超共轭相关的背景和异头效应起源的简要历史概述之后，我们概述了其模式的变化，并提供了异头效应在结构、稳定性和光谱性质中的作用的说明性例子。我们表明，完整的超共轭模型在解释结构和反应性之间的相互作用方面仍然具有优势。我们将利用最近关于异头效应起源的争议来开始与任何电子效应相关的更深入的讨论。为什么这些问题本质上是有争议的？如何使用“尽可能简单但不简单”的模型来描述复杂的量子系统？对这种模型的公平测试是什么？也许，而不是问“谁对谁错？” 人们应该问“为什么我们不同意？”。立体电子思维可以调和量子复杂性与化学直觉，并在结构和反应性之间建立概念桥梁。即使许多因素对观察到的结构和构象趋势有贡献，即，当分子从其平衡几何形状扭曲时，键正在断裂）。在这些情况下，轨道相互作用的作用增加到可以定义反应性的程度。例如，负超共轭可以释放未共享电子（即孤对电子）的“未充分利用”立体电子能力，以稳定异头碳上正在形成的正电荷。该分析为更广泛地讨论负超共轭在含氧官能团中无处不在的重要性铺平了道路。从这个角度来看，异头效应的立体电子成分在指导反应设计方面起着独特的作用。

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Anomeric effect, hyperconjugation and electrostatics: lessons from complexity in a classic stereoelectronic phenomenon

Understanding the interplay of multiple components (steric, electrostatic, stereoelectronic, dispersive, etc.) that define the overall energy, structure, and reactivity of organic molecules can be a daunting task. The task becomes even more difficult when multiple approaches based on different physical premises disagree in their analysis of a multicomponent molecular system. Herein, we will use a classic conformational “oddity”, the anomeric effect, to discuss the value of identifying the key contributors to reactivity that can guide chemical predictions. After providing the background related to the relevant types of hyperconjugation and a brief historic outline of the origins of the anomeric effect, we outline variations of its patterns and provide illustrative examples for the role of the anomeric effect in structure, stability, and spectroscopic properties. We show that the complete hyperconjugative model remains superior in explaining the interplay between structure and reactivity. We will use recent controversies regarding the origin of the anomeric effect to start a deeper discussion relevant to any electronic effect. Why are such questions inherently controversial? How to describe a complex quantum system using a model that is “as simple as possible, but no simpler”? What is a fair test for such a model? Perhaps, instead of asking “who is right and who is wrong?” one should ask “why do we disagree?”. Stereoelectronic thinking can reconcile quantum complexity with chemical intuition and build the conceptual bridge between structure and reactivity. Even when many factors contribute to the observed structural and conformational trends, electron delocalization is a dominating force when the electronic demand is high (i.e., bonds are breaking as molecules distort from their equilibrium geometries). In these situations, the role of orbital interactions increases to the extent where they can define reactivity. For example, negative hyperconjugation can unleash the “underutilized” stereoelectronic power of unshared electrons (i.e., the lone pairs) to stabilize a developing positive charge at an anomeric carbon. This analysis paves the way for the broader discussion of the omnipresent importance of negative hyperconjugation in oxygen-containing functional groups. From that point of view, the stereoelectronic component of the anomeric effect plays a unique role in guiding reaction design.

更新日期：2021-08-24

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