Hydrogen Bonding Shuts Down Tunneling in Hydroxycarbenes: A Gas-Phase Study by Tandem-Mass Spectrometry, Infrared Ion Spectroscopy, and Theory
Hydrogen Bonding Shuts Down Tunneling in Hydroxycarbenes: A Gas-Phase Study by Tandem-Mass Spectrometry, Infrared Ion Spectroscopy, and Theory
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氢键关闭羟基卡宾中的隧道效应：通过串联质谱、红外离子光谱和理论进行气相研究
DOI：
10.1021/jacs.3c01698
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发表时间：
2023
影响因子：
15
通讯作者：
Paul M
中科院分区：
文献类型：
--
作者：
Paul M
羟基卡宾可以通过α-酮羧酸的碰撞诱导脱羧反应在气相中生成并进行结构表征，然后进行红外离子光谱分析。使用这种方法，我们已经表明，量子力学氢隧穿（QMHT）帐户的异构化的电荷标记的苯基羟基卡宾在气相和室温以上的相应的醛。在这里，我们报告的结果，我们目前的研究脂肪族三烷基铵标记的系统。非常出乎意料的是，柔性3-（三甲基铵基）丙基羟基卡宾被证明是稳定的，没有发生H-移位到醛或烯醇。通过密度泛函理论的计算，这种新型的QMHT抑制作用是由于分子内弱酸性α-氨基C-H键与羟基卡宾的C-原子（C：···H-C）形成氢键。为了进一步支持这一假设，合成了（4-奎宁环基）羟基卡宾，其刚性结构阻止了这种分子内氢键。后一种羟基卡宾经历“常规”QMHT转化为醛，其速率可与例如，虽然QMHT已被证明用于许多生物H-位移过程，但本文公开的通过H-键合的其抑制可用于稳定高反应性中间体如卡宾，甚至作为偏置固有选择性模式的机制。
Hydroxycarbenes can be generated and structurally characterized in the gas phase by collision-induced decarboxylation of α-keto carboxylic acids, followed by infrared ion spectroscopy. Using this approach, we have shown earlier that quantum-mechanical hydrogen tunneling (QMHT) accounts for the isomerization of a charge-tagged phenylhydroxycarbene to the corresponding aldehyde in the gas phase and above room temperature. Herein, we report the results of our current study on aliphatic trialkylammonio-tagged systems. Quite unexpectedly, the flexible 3-(trimethylammonio)propylhydroxycarbene turned out to be stable─no H-shift to either aldehyde or enol occurred. As supported by density functional theory calculations, this novel QMHT inhibition is due to intramolecular H-bonding of a mildly acidic α-ammonio C–H bonds to the hydroxyl carbene’s C-atom (C:···H–C). To further support this hypothesis, (4-quinuclidinyl)hydroxycarbenes were synthesized, whose rigid structure prevents this intramolecular H-bonding. The latter hydroxycarbenes underwent “regular” QMHT to the aldehyde at rates comparable to, e.g., methylhydroxycarbene studied by Schreiner et al. While QMHT has been shown for a number of biological H-shift processes, its inhibition by H-bonding disclosed here may serve for the stabilization of highly reactive intermediates such as carbenes, even as a mechanism for biasing intrinsic selectivity patterns.