Department of Chemistry, Organic Chemistry, University of Cologne , Greinstraße 4, 50939 Cologne, Germany.
Radboud University , Institute for Molecules and Materials, FELIX Laboratory, Toernooiveld 7c, 6525 ED Nijmegen, The Netherlands.
J Am Chem Soc. 2017 Apr 26;139(16):5779-5786. doi: 10.1021/jacs.6b10348. Epub 2017 Apr 13.
While hydrogen tunneling at elevated temperatures has, for instance, often been postulated in biochemical processes, spectroscopic proof is thus far limited to cryogenic conditions, under which thermal reactivity is negligible. We report spectroscopic evidence for H-tunneling in the gas phase at temperatures around 320-350 K observed in the isomerization reaction of a hydroxycarbene into an aldehyde. The charge-tagged carbene was generated in situ in a tandem mass spectrometer by decarboxylation of oxo[4-(trimethylammonio)phenyl]acetic acid upon collision induced dissociation. All ion structures involved are characterized by infrared ion spectroscopy and quantum chemical calculations. The charge-tagged phenylhydroxycarbene undergoes a 1,2-H-shift to the corresponding aldehyde with an half-life of about 10 s, evidenced by isomer-selective two-color (IR-IR) spectroscopy. In contrast, the deuterated (OD) carbene analogue showed much reduced 1,2-D-shift reactivity with an estimated half-life of at least 200 s under the experimental conditions, and provides clear evidence for hydrogen atom tunneling in the H-isotopologue. This is the first spectroscopic confirmation of hydrogen atom tunneling governing 1,2-H-shift reactions at noncryogenic temperatures, which is of broad significance for a range of (bio)chemical processes, including enzymatic transformations and organocatalysis.
虽然在生化过程中经常假设高温下的氢隧穿，但到目前为止，光谱证据仅限于低温条件下，在低温条件下，热反应可以忽略不计。我们报告了在 320-350 K 左右的气相中观察到的 H 隧穿的光谱证据，该反应是羟基亚基异构化为醛的反应。带电荷的卡宾在串联质谱仪中通过羰基[4-（三甲基铵基）苯基]乙酸的碰撞诱导解离原位生成。涉及的所有离子结构都通过红外离子光谱和量子化学计算进行了表征。带电荷的苯基亚基经历了 1,2-H 迁移到相应的醛，半衰期约为 10 s，这通过异构体选择性双色（IR-IR）光谱得到证实。相比之下，氘代（OD）卡宾类似物的 1,2-D 迁移反应性大大降低，在实验条件下半衰期至少为 200 s，为 H-同位素的氢原子隧穿提供了明确的证据。这是首次在非低温条件下对控制 1,2-H 迁移反应的氢原子隧穿进行的光谱确认，这对一系列（生物）化学过程具有广泛的意义，包括酶转化和有机催化。
