An Account of Chiral Metal Surfaces and Their Enantiospecific Chemistry
An Account of Chiral Metal Surfaces and Their Enantiospecific Chemistry
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DOI：
10.1021/accountsmr.1c00145
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发表时间：
2021-10-28
影响因子：
14.6
通讯作者：
Gellman, Andrew J.
中科院分区：
文献类型：
--
作者：
Gellman, Andrew J.
提要：自1848年巴斯德证明了分子手性与手性化合物溶液中的光的旋转有直接联系以来，分子手性一直是科学研究的热点。在20世纪60年代，药物化合物的手性和它们的生理影响之间建立了联系；一种对映体可以治疗，另一种是有毒的。这一认识促使人们在合成用于生物活性的对映体纯化合物方面做出了巨大努力(每年3000亿美元的市场)。直到最近，金属作为不对称表面化学的潜在底物一直被忽视，因为金属具有高度对称的非手性体相结构，前提是它们不能露出手性表面。1996年，我们证明了金属的高密勒指数表面可以是手性的，以两种对映体形式M(Hkl)(R&S)存在，并假设它们与手性吸附物表现出对映体特异性相互作用。大多数这类本质上手性的金属表面都有理想的结构基元，这些基元是由扭结的单原子台阶隔开的低米勒指数阶梯。本文从手性金属表面的理想和真实结构的简短教程开始，为理解手性金属表面手性的起源提供了坚实的基础。然后，它记录了我们对它们与手性吸附对映体特异性相互作用的理解的演变。检测、量化和理解本质上手性金属表面的对映专一性表面化学比认识到这样的表面存在要具有挑战性得多。第一个成功来自于对环氧丙烷和反式-1，2-二甲基环丙烷等手性小分子对映异构体吸附能的测量和模拟。这揭示了观察对映体专一性的核心挑战之一，即反应能和势垒的对映专一性往往很小，即几个kJ/mol。测定了R-3-甲基环己酮在7种不同的铜(Hkl)(R&S)表面上的对映吸附能，表明它们对表面结构很敏感，但也只有几个千焦耳/摩尔的变化。手性表面化学中最重要的进展之一是弱对映异构体相互作用的限制可以通过具有非线性动力学或平衡的过程来规避。例如，D-酒石酸和t，-酒石酸在铜(Hkl)(R&S)表面的表面爆炸机理导致对映异构化速率相差近2个数量级，尽管速率常数只是弱对映异构体。更令人惊讶的是，观察到D-和L-天冬氨酸的非外消旋混合物的平衡吸附可以导致对映体过量的自动放大，甚至在非手性铜表面也是如此。同样，这是由非线性吸附等温线引起的。最近，我们发展了一种高通量的方法，用于从铜(HKl)(R&S)表面取向的连续体中识别给定反应的最对映专一性的表面取向。这些进展，以及本报告中描述的其他进展，牢固地确立了手性表面化学的一些基本原理。
CONSPECTUS: Molecular chirality has been of scientific interest since 1848 when Pasteur demonstrated its direct connection to the rotation of light by solutions of chiral compounds. In the 1960s the connection was made between the chirality of pharmaceutical compounds and their physiological impact; one enantiomer can be therapeutic while the other is toxic. That realization prompted enormous effort in the synthesis of enantiomerically pure compounds for bioactive use (a $300B/yr market). Until relatively recently, metals were ignored as potential substrates for asymmetric surface chemistry because metals have highly symmetric, achiral bulk structures and the premise was that they could not expose chiral surfaces. In 1996, we demonstrated that the high Miller index surfaces of metals can be chiral, existing in two enantiomeric forms M(hkl)(R&S), and we hypothesized that they exhibit enantiospecific interactions with chiral adsorbates. Most such intrinsically chiral metal surfaces have ideal structural motifs based on low Miller index terraces separated by kinked monatomic steps. This Account begins with a short tutorial on the ideal and real structures of chiral metal surfaces to provide a firm basis for understanding the origin of their chirality. It then chronicles the evolution of our understanding of their enantiospecific interactions with chiral adsorbates.Detecting, quantifying, and understanding enantiospecific surface chemistry on intrinsically chiral metal surfaces has been far more challenging than coming to the realization that such surfaces exist. The first successes came from measurements and modeling of the enantiospecific adsorption energetics of small chiral molecules such as propylene oxide and trans-1,2-dimethylcyclopropane. These revealed one of the core challenges to observing enantiospecificity, the fact that the enantiospecificities of reaction energetics and barriers tend to be small, i.e., a few kJ/mol. Measurements of the enantiospecific adsorption energetics of R-3-methylcydohexanone on seven different Cu(hkl)(R&S) surfaces demonstrated their sensitivity to surface structure, but again revealed variations of only a few kJ/mol. One of the most important advances in our understanding of chiral surface chemistry is that the limitations imposed by weakly enantiospecific interactions can be circumvented by processes with nonlinear kinetics or equilibria. As an example, the surface explosion mechanism of D- and t,-tartaric acid decomposition on Cu(hkl)(R&S) surfaces leads to enantiospecific rates that differ by almost 2 orders of magnitude, in spite of the fact that the rate constants are only weakly enantiospecific. More surprising is the observation that equilibrium adsorption of nonracemic mixtures of D- and L-aspartic acid can lead to autoamplification of enantiomeric excess, even on achiral Cu(111) surfaces. Again, this arises from a nonlinear adsorption isotherm. Most recently, we have developed a high throughput method for identification of the most enantiospecific surface orientation for a given reaction from the continuum of Cu(hkl)(R&S) surface orientations. These developments, and others described in this Account, firmly establish some of the basic principles of chiral surface chemistry.