Dery Shahar, Bellotti Peter, Ben-Tzvi Tzipora, Freitag Matthias, Shahar Tehila, Cossaro Albano, Verdini Alberto, Floreano Luca, Glorius Frank, Gross Elad
Institute of Chemistry and The Center for Nanoscience and Nanotechnology, The Hebrew University, Jerusalem 91904, Israel.
Organisch-Chemisches Institut, Westfälische Wilhelms-Universität Münster, Münster 48149, Germany.
Langmuir. 2021 Aug 24;37(33):10029-10035. doi: 10.1021/acs.langmuir.1c01199. Epub 2021 Aug 9.
Adsorption of chiral molecules on heterogeneous catalysts is a simple approach for inducing an asymmetric environment to enable enantioselective reactivity. Although the concept of chiral induction is straightforward, its practical utilization is far from simple, and only a few examples toward the successful chiral induction by surface anchoring of asymmetric modifiers have been demonstrated so far. Elucidating the factors that lead to successful chiral induction is therefore a crucial step for understanding the mechanism by which chirality is transferred. Herein, we identify the adsorption geometry of OH-functionalized N-heterocyclic carbenes (NHCs), which are chemical analogues to chiral modifiers that successfully promoted α-arylation reactions once anchored on Pd nanoparticles. Polarized near-edge X-ray absorption fine structure (NEXAFS) measurements on Pd(111) revealed that NHCs that were associated with low enantioselectivity were characterized with a well-ordered structure, in which the imidazole ring was vertically positioned and the OH-functionalized side arms were flat-lying. OH-functionalized NHCs that were associated with high enantioselectivity revealed a disordered/flexible adsorption geometry, which potentially enabled better interaction between the OH group and the prochiral reactant.
手性分子在多相催化剂上的吸附是诱导不对称环境以实现对映选择性反应性的一种简单方法。尽管手性诱导的概念很简单，但其实际应用却远非易事，迄今为止，通过不对称修饰剂的表面锚定实现成功手性诱导的例子寥寥无几。因此，阐明导致成功手性诱导的因素是理解手性转移机制的关键一步。在此，我们确定了羟基官能化的氮杂环卡宾（NHCs）的吸附几何结构，它们是手性修饰剂的化学类似物，一旦锚定在钯纳米颗粒上就能成功促进α-芳基化反应。对Pd(111)进行的偏振近边X射线吸收精细结构（NEXAFS）测量表明，对映选择性较低的NHCs具有有序结构，其中咪唑环垂直定位，羟基官能化的侧臂平躺。对映选择性较高的羟基官能化NHCs显示出无序/灵活的吸附几何结构，这可能使羟基与前手性反应物之间具有更好的相互作用。
