Rothermel Kerstin, Žabka Matej, Hioe Johnny, Gschwind Ruth M
Institute of Organic Chemistry , University of Regensburg , D-93053 Regensburg , Germany.
J Org Chem. 2019 Nov 1;84(21):13221-13231. doi: 10.1021/acs.joc.9b01811. Epub 2019 Oct 2.
In Brønsted acid catalysis, hydrogen bonds play a crucial role for reactivity and selectivity. However, the contribution of weak hydrogen bonds or multiple acceptors has been unclear so far since it is extremely difficult to collect experimental evidence for weak hydrogen bonds. Here, our hydrogen bond and structural access to Brønsted acid/imine complexes was used to analyze BINOL-derived chiral disulfonimide (DSI)/imine complexes. H and N chemical shifts as well as coupling constants revealed for DSI/imine complexes ion pairs with very weak hydrogen bonds. The high acidity of the DSIs leads to a significant weakening of the hydrogen bond as structural anchor. In addition, the five hydrogen bond acceptors of DSI allow an enormous mobility of the imine in the binary DSI complexes. Theoretical calculations predict the hydrogen bonds to oxygen to be energetically less favored; however, their considerable population is corroborated experimentally by NOE and exchange data. Furthermore, an -alkylimine, which shows excellent reactivity and selectivity in reactions with DSI, reveals an enlarged structural space in complexes with the chiral phosphoric acid TRIP as potential explanation of its reduced reactivity and selectivity. Thus, considering factors such as flexibility and possible hydrogen bond sites is essential for catalyst development in Brønsted acid catalysis.
在布朗斯特酸催化中，氢键对反应活性和选择性起着至关重要的作用。然而，由于极难收集到关于弱氢键的实验证据，到目前为止，弱氢键或多个受体的贡献尚不清楚。在此，我们利用对布朗斯特酸/亚胺配合物的氢键和结构研究来分析联萘酚衍生的手性双磺酰亚胺（DSI）/亚胺配合物。氢和氮的化学位移以及耦合常数揭示了DSI/亚胺配合物中的离子对具有非常弱的氢键。DSI的高酸度导致作为结构锚定的氢键显著减弱。此外，DSI的五个氢键受体使得二元DSI配合物中亚胺具有极大的流动性。理论计算预测与氧形成的氢键在能量上较不利；然而，核磁共振核Overhauser效应（NOE）和交换数据在实验上证实了它们相当可观的数量。此外，一种在与DSI反应中表现出优异反应活性和选择性的α-烷基亚胺，在与手性磷酸TRIP形成的配合物中显示出扩大的结构空间，这可能是其反应活性和选择性降低的原因。因此，在布朗斯特酸催化中，考虑诸如柔性和可能的氢键位点等因素对于催化剂的开发至关重要。
