Alabugin Igor V, Kuhn Leah, Medvedev Michael G, Krivoshchapov Nikolai V, Vil' Vera A, Yaremenko Ivan A, Mehaffy Patricia, Yarie Meysam, Terent'ev Alexander O, Zolfigol Mohammad Ali
Department of Chemistry and Biochemistry, Florida State University, Tallahassee, FL 32306, USA.
N. D. Zelinsky Institute of Organic Chemistry, Russian Academy of Sciences, 47 Leninsky prosp., 119991 Moscow, Russian Federation.
Chem Soc Rev. 2021 Sep 20;50(18):10253-10345. doi: 10.1039/d1cs00386k.
Although carbon is the central element of organic chemistry, oxygen is the central element of stereoelectronic control in organic chemistry. Generally, a molecule with a C-O bond has both a strong donor (a lone pair) and a strong acceptor (, a σ* orbital), a combination that provides opportunities to influence chemical transformations at both ends of the electron demand spectrum. Oxygen is a stereoelectronic chameleon that adapts to the varying situations in radical, cationic, anionic, and metal-mediated transformations. Arguably, the most historically important stereoelectronic effect is the anomeric effect (AE), , the axial preference of acceptor groups at the anomeric position of sugars. Although AE is generally attributed to hyperconjugative interactions of σ-acceptors with a lone pair at oxygen (negative hyperconjugation), recent literature reports suggested alternative explanations. In this context, it is timely to evaluate the fundamental connections between the AE and a broad variety of O-functional groups. Such connections illustrate the general role of hyperconjugation with oxygen lone pairs in reactivity. Lessons from the AE can be used as the conceptual framework for organizing disjointed observations into a logical body of knowledge. In contrast, neglect of hyperconjugation can be deeply misleading as it removes the stereoelectronic cornerstone on which, as we show in this review, the chemistry of organic oxygen functionalities is largely based. As negative hyperconjugation releases the "underutilized" stereoelectronic power of unshared electrons (the lone pairs) for the stabilization of a developing positive charge, the role of orbital interactions increases when the electronic demand is high and molecules distort from their equilibrium geometries. From this perspective, hyperconjugative anomeric interactions play a unique role in guiding reaction design. In this manuscript, we discuss the reactivity of organic O-functionalities, outline variations in the possible hyperconjugative patterns, and showcase the vast implications of AE for the structure and reactivity. On our journey through a variety of O-containing organic functional groups, from textbook to exotic, we will illustrate how this knowledge can predict chemical reactivity and unlock new useful synthetic transformations.
尽管碳是有机化学的核心元素，但氧却是有机化学中立体电子控制的核心元素。一般来说，含有碳 - 氧键的分子既有强供体（一对孤对电子）又有强受体（一个σ*轨道），这种组合为在电子需求谱两端影响化学转化提供了机会。氧是一种立体电子变色龙，能适应自由基、阳离子、阴离子和金属介导的转化中的各种情况。可以说，历史上最重要的立体电子效应是端基异构效应（AE），即糖的端基异构位置上受体基团的轴向偏好。尽管AE通常归因于σ受体与氧上孤对电子的超共轭相互作用（负超共轭），但最近的文献报道提出了其他解释。在这种背景下，及时评估AE与各种各样的氧官能团之间的基本联系是很有必要的。这种联系说明了与氧孤对电子的超共轭在反应性中的一般作用。从AE中吸取的经验教训可以用作将零散的观察结果组织成一个逻辑知识体系的概念框架。相反，忽视超共轭可能会产生严重的误导，因为它移除了立体电子基石，正如我们在本综述中所展示的，有机氧官能团的化学很大程度上是基于此。由于负超共轭释放了未共享电子（孤对电子）“未充分利用”的立体电子能量以稳定正在形成的正电荷，当电子需求较高且分子偏离其平衡几何形状时，轨道相互作用的作用就会增加。从这个角度来看，超共轭端基异构相互作用在指导反应设计中起着独特的作用。在本手稿中，我们讨论了有机氧官能团的反应性，概述了可能的超共轭模式的变化，并展示了AE对结构和反应性的巨大影响。在我们遍历从教科书到奇特的各种含氧有机官能团的过程中，我们将说明这些知识如何能够预测化学反应性并开启新的有用的合成转化。
