Relative Abundances of Surface Diastereomeric Complexes Formed by Two Chiral Modifiers That Differ by a Methyl Group

Enantioselectivity in heterogeneous catalysis can be induced through the adsorption of optically active compounds known as chiral modifiers. The modifiers typically stereodirect prochiral reactants on the metal surface through the formation of diastereomeric complexes. Surface science measurements on model systems show several examples where such complexes display multiple abundant binding configurations. Insights into the factors determining the relative populations of complexation states can be potentially gained from comparing slightly variant chiral modifiers. Here, we compare scanning tunneling microscopy (STM) data for chirality transfer complexes formed by (R)-1-(1-naphthyl)­ethylamine and (R)-1-(8-methyl-1-naphthyl)­ethylamine coadsorbed with the prochiral substrate 2,2,2-trifluoroacetophenone in order to probe for changes induced by the methyl substituent. The comparison shows changes in relative abundances beyond those predicted by only direct steric interaction. Density functional theory (DFT) methods are used to calculate complexation energies. A number of rationalizations are proposed for the experimentally determined differences in relative abundances and also for differences between DFT-predicted and STM-measured populations.非均相催化中的对映选择性，可通过吸附一类被称为手性修饰剂（chiral modifiers）的光学活性化合物来实现诱导。这类修饰剂通常通过形成非对映异构配合物，在金属表面上对前手性反应物进行立体定向调控。针对模型体系开展的表面科学测量研究已观测到多例此类配合物存在多种高丰度结合构型。通过对比结构略有差异的手性修饰剂，有望揭示决定配合物构型相对占比的关键影响因素。本研究对比了两种手性修饰剂——(R)-1-(1-萘基)乙胺与(R)-1-(8-甲基-1-萘基)乙胺——分别与前手性底物2,2,2-三氟苯乙酮共吸附形成的手性转移配合物的扫描隧道显微镜（scanning tunneling microscopy, STM）数据，以探究甲基取代基诱导的变化。对比结果表明，配合物的相对丰度变化远超仅通过直接空间位阻相互作用预测的结果。本研究采用密度泛函理论（density functional theory, DFT）方法计算配合物的结合能，并针对实验观测到的相对丰度差异，以及DFT预测结果与STM实测的构型占比之间的偏差，提出了多种合理解释。