Successive Protonation of an N-Heterocyclic Imine Derived Carbonyl: Superelectrophilic Dication Versus Masked Acylium Ion
Successive Protonation of an N-Heterocyclic Imine Derived Carbonyl: Superelectrophilic Dication Versus Masked Acylium Ion
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DOI：
10.1002/anie.201810709
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发表时间：
2018-12-10
影响因子：
16.6
通讯作者：
Aldridge, Simon
中科院分区：
文献类型：
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作者：
Loh, Ying Kai;Fuentes, M. Angeles;Aldridge, Simon
羰基阳离子是有机合成中最常用的反应中间体之一。虽然Olah开创了超强酸的先河，为原位研究提供了稳定的离子环境，但孤立的例子很少。本研究揭示了n -杂环亚胺（NHI）衍生羰基化合物（IDippN）(2)CO， 2连续质子化为单位化[(IDippN)(IDippNH) CO](+),[3]（+）和双质子化[(IDippNH)(2)CO](2+),[4]（2+）。[3]（+）是n -质子化羰基阳离子的罕见例子，[4]（2+）是超亲电羰基化的第一个例子。这三种化合物都通过x射线晶体学和红外光谱进行了表征，揭示了质子化作用下C=O键的逐步增强。这些体系的独特稳定性归功于NHI取代基提供的增强的碱度和空间分布。此外，我们还报道了相关的单一nhi稳定阳离子[IDippNCO](+),[5]（+）。晶体学和DFT分析揭示了羰基片段与NHI之间的相互作用，揭示了[CNCO]（+）单元（与CCCO等电子）可以被描述为一个被积云烯掩盖的酰基阳离子。
Carbonyl cations are among the most commonly invoked reactive intermediates in organic synthesis. While Olah pioneered superacids to provide a stable ion environment for their study in situ, isolated examples are rare. Here, we disclose successive protonation of an N-heterocyclic imine (NHI) derived carbonyl compound (IDippN)(2)CO, 2, to the monocation [(IDippN)(IDippNH) CO](+), [3](+), and the doubly protonated dication [(IDippNH)(2)CO](2+), [4](2+). [3](+) represents a rare example of an N-protonated carbonyl cation and [4](2+) the first example of a superelectrophilic carbonyl dication. All three compounds have been characterized by X-ray crystallography and IR spectroscopy, revealing stepwise strengthening of the C=O bond on protonation. The unique stability of these systems is attributed to the enhanced basicity and steric profile provided by the NHI substituents. In addition, we report the related singly NHI-stabilized cation [IDippNCO](+), [5](+). Crystallographic and DFT analyses provide insight into the interaction between the carbonyl fragment and the NHI, which reveals that the [CNCO](+) unit (isoelectronic to CCCO) can be described as an acylium cation masked as a cumulene.