Achiral Organic, Inorganic, and Metal Crystals as Auxiliaries for Asymmetric Transformations

Abstract

The use of achiral crystalline architectures as intermediate auxiliaries for the performance of “absolute” asymmetric transformations is reviewed. Such architectures are delineated, in some cases, by pairs of homochiral surfaces of opposite handedness. This phenomenon is more common among organic crystals that frequently appear in triclinic, monoclinic, orthorhombic, or tetragonal space groups. Consequently, the chiral surfaces of such crystals have been shown to display enantiomeric recognition for molecules of the environment, a process that has been instrumental in the conversion of achiral host crystals into enantiomorphous solid‐solutions, for a successful performance of “absolute” asymmetric transformations and for the control of crystal polymorphism. Mixed crystals of reduced symmetry display properties such as second harmonic generation or pyroelectricity. On the other hand, achiral faces delineate metals, which crystallize in cubic space‐groups of high symmetry. However, by slicing such crystals in particular directions, they might express homochiral high Miller index faces that contain homochiral kink sites, which have been successfully exploited in electrochemical separations of sugars and for the resolution of enantiomers by enantioselective desorption. Representative examples of each class of materials are described.

Enantioselective Separation on Naturally Chiral Metal Surfaces: d,l‐Aspartic Acid on Cu(3,1,17)R&S Surfaces

“…[1,2] Higher enantioselectivities have been observed for adsorption on chiral surfaces of organic crystals. [3] Highly enantioselective adsorption could be of enormous importance, if observed on catalytically active materials such as metals. Herein, we demonstrate that naturally chiral metal surfaces can yield much higher enantioselectivities than minerals and we provide the first definitive proof of enantioselective separation by a naturally chiral metal surface.…”

“…[1,2,4] Somewhat counter-intuitively, chiral surfaces can also be generated from achiral materials such as metals. [5] As an example, the two non-superimposable mirror images of the Cu (3,1,17) surface are illustrated in Figure 1. The kinked step structures of these surfaces lack symmetry and, therefore, they are chiral.…”

“…[2b,f] Here, we demonstrate the use of 13 C labeling for enantiodiscrimination of chiral adsorbed species. Enantioselective separation of gas phase d,l-aspartic acid by equilibrium adsorption on naturally chiral Cu- (3,1,17) R&S surfaces was demonstrated and quantified by measuring the ratios of the enantiospecific adsorption equilibrium constants, K D…”

“…then provide quantitative measurements of the enantiospecific difference in the adsorption free energy: This work has used the adsorption of d-and l-aspartic acid on the Cu (3,1,17) R&S surfaces under ultra-high vacuum conditions for quantitative measurement of enantiospecific adsorption equilibrium constants. For the purpose of enantiodifferentiation, we have used *l-aspartic acid-1,4-13 C 2 (HO 2 13 CCH(NH 2 )CH 2 13 CO 2 H, hitherto *l-Asp) and unlabeled d-aspartic acid (d-Asp).…”

“…Once the surface is saturated with adsorbed species, the components displace one another to establish equilibrium between the gas phase and the adsorbed phase. One of the critical characteristics of d-and *l-Asp adsorption on Cu (3,1,17) R&S is that they displace one another on time scales that allow equilibrium to be achieved. In Figure 4 A the Cu(3,1,17) S surface was saturated with *l-Asp at 400 K and then held at 460 K while being exposed to a flux of d-Asp for various lengths of time.…”