Kutta Roger Jan, Großkopf Johannes, van Staalduinen Nils, Seitz Antonia, Pracht Philipp, Breitenlechner Stefan, Bannwarth Christoph, Nuernberger Patrick, Bach Thorsten
Institut für Physikalische und Theoretische Chemie, Universität Regensburg, Universitätsstr. 31, RegensburgD-93053, Germany.
Department Chemie and Catalysis Research Center (CRC), School of Natural Sciences, Technische Universität München, D-85747Garching, Germany.
J Am Chem Soc. 2023 Feb 1;145(4):2354-2363. doi: 10.1021/jacs.2c11265. Epub 2023 Jan 20.
Upon irradiation in the presence of a chiral benzophenone catalyst (5 mol %), a racemic mixture of a given chiral imidazolidine-2,4-dione (hydantoin) can be converted almost quantitatively into the same compound with high enantiomeric excess (80-99% ). The mechanism of this photochemical deracemization reaction was elucidated by a suite of mechanistic experiments. It was corroborated by nuclear magnetic resonance titration that the catalyst binds the two enantiomers by two-point hydrogen bonding. In one of the diastereomeric complexes, the hydrogen atom at the stereogenic carbon atom is ideally positioned for hydrogen atom transfer (HAT) to the photoexcited benzophenone. Detection of the protonated ketyl radical by transient absorption revealed hydrogen abstraction to occur from only one but not from the other hydantoin enantiomer. Quantum chemical calculations allowed us to visualize the HAT within this complex and, more importantly, showed that the back HAT does not occur to the carbon atom of the hydantoin radical but to its oxygen atom. The achiral enol formed in this process could be directly monitored by its characteristic transient absorption signal at λ ≅ 330 nm. Subsequent tautomerization leads to both hydantoin enantiomers, but only one of them returns to the catalytic cycle, thus leading to an enrichment of the other enantiomer. The data are fully consistent with deuterium labeling experiments and deliver a detailed picture of a synthetically useful photochemical deracemization reaction.
在一种手性二苯甲酮催化剂（5摩尔%）存在下进行辐照时，给定手性咪唑烷-2,4-二酮（乙内酰脲）的外消旋混合物几乎可以定量地转化为具有高对映体过量（80 - 99%）的同一化合物。通过一系列机理实验阐明了这种光化学消旋反应的机理。核磁共振滴定证实，催化剂通过两点氢键与两种对映体结合。在其中一种非对映体配合物中，手性碳原子上的氢原子处于理想位置，可用于向光激发的二苯甲酮进行氢原子转移（HAT）。通过瞬态吸收检测到质子化的酮基自由基，表明仅从一种乙内酰脲对映体而非另一种对映体发生了氢原子抽取。量子化学计算使我们能够可视化该配合物内的氢原子转移，更重要的是，表明反向氢原子转移不是发生在乙内酰脲自由基的碳原子上，而是发生在其氧原子上。在此过程中形成的非手性烯醇可以通过其在λ≅330 nm处的特征瞬态吸收信号直接监测。随后的互变异构导致两种乙内酰脲对映体的形成，但只有其中一种回到催化循环，从而导致另一种对映体的富集。这些数据与氘标记实验完全一致，并给出了一个合成上有用的光化学消旋反应的详细情况。
