Achiral Inorganic Gypsum Acts as an Origin of Chirality through Its Enantiotopic Surface in Conjunction with Asymmetric Autocatalysis

Abstract

Achiral inorganic gypsum (CaSO 4 ·2 H 2 O) triggers the asymmetric autocatalysis of pyrimidyl alkanol on its twodimensional enantiotopic faces to give highly enantioenriched alkanol products with absolute configurations corresponding to the respective enantiotopic surfaces. This is the first example of highly enantioselective synthesis on the enantiotopic surface of an achiral mineral.The origins of the homochirality of biological compounds such as l-amino acids and d-sugars have been the subject of great interest in chemistry, biology, physics, astrobiology, and studies on the origin of life. [1] There have been several theories proposed for the origins of the chirality of organic compounds.Meanwhile, increasing attention has been focused on twodimensional surface chirality. [1n,o, 2] Although chiral inorganic minerals have been considered to be an origin of chirality, achiral natural minerals with an enantiotopic surface have rarely been considered as an origin of chirality. A spectacular example was reported by Hazen et al. [3] They reported the enantiomer-selective adsorption of racemic amino acids on the enantiotopic faces of the achiral mineral calcite (CaCO 3 ) with moderate enantiomeric excess (Scheme 1 a). Certain metal surfaces, such as artificially cut Cu(643), become enantiotopic, and the enantiomer-selective decomposition of chiral compounds have been reported. [4] However, it has not yet been possible to control the enantioselective synthesis induced on the enantiotopic surface of achiral inorganic minerals to generate chiral molecules from achiral molecules.We have been studying asymmetric autocatalysis of pyrimidyl alkanol with amplification of chirality. [5][6][7][8] Various chiral factors, [9] including circularly polarized light, [10] isotope chirality, [11] and chiral inorganic crystals [12] trigger asymmetric autocatalysis. However, to the best of our knowledge, no definitive report has appeared on asymmetric synthesis using achiral inorganic crystals. [13] Gypsum is a common mineral that has been used in a wide range of applications, including materials for sculpture, plasterboard in buildings, and plaster casts for medical use. The crystal structure of gypsum is achiral, but its large grown face and its habit of cleavage make it enantiotopic. Although Cody and Cody have reported asymmetric crystal growth of the enantiotopic surface of gypsum in the presence of a chiral organic compound, [14] and Viedma, Cintas, et. al. have reported oriented aggregation growth of gypsum crystals, [15] no example has been reported of asymmetric synthesis on gypsum enantiotopic surfaces.Herein, we report enantioselective synthesis using the two-dimensional enantiotopic face of an achiral inorganic crystal (Scheme 1 b). The achiral inorganic mineral gypsum (calcium sulfate dihydrate, CaSO 4 ·2 H 2 O) triggers asymmetric autocatalysis on its enantiotopic face, thereby providing highly enantioenriched alkanol product with chirality corresponding to that of the enantiotopic face of the gypsum.Gypsum exhibits a p...

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Asymmetric autocatalysis. Chiral symmetry breaking and the origins of homochirality of organic molecules

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Chemical Mechanisms of Prebiotic Chirality Amplification

“…The criticism of the asymmetric autocatalytic model focuses on the high sensitivity of the outcome of the Soai reaction (which so far has been the only autocatalytic reaction ensuring significant enhancement of enantiomeric excess) to external additives (even not necessarily chiral [43]), which again puts "chance" in the forefront, bringing into question the capability of the Soai system to function unidirectionally under nonideal reaction conditions. Another point is that organometallic Soai reaction, yet extremely efficient, requires strictly nonaqueous environment, which is not compatible with the presumably partially aqueous environment of the early Earth by the end of the abiotic period of its history.…”