Response to “Comments on a Possible Transition to Solid‐Phase Homochirality”

Deracemisation of racemic or scalemic conglomerates of intrinsically chiral compounds appears to be a promising method of chiral resolution. By combining the established methods of asymmetric synthesis and the physical process of crystal growth, we were able to achieve a complete deracemisation (with 100% ee) of an asymmetric Mannich product conglomerate--vigorously stirred in its saturated solution--from a starting enantiomeric excess value of 15.8% in the presence of pyrrolidine (8 mol %) as an achiral catalyst for the CC bond-forming reaction. Strong activation of this deracemisation process was observed on mild isothermal heating to only 40 degrees C, resulting in dramatic acceleration by a factor of about 20 with respect to the results obtained at room temperature. Despite the fact that the racemisation half-life time of the nearly enantiopure Mannich product (with 99% ee) in the homogenous solution at the reaction temperature is eight days, the deracemisation process took only hours in a small-scale experiment. This apparent paradox is explained by a proposed rapid enantiomerisation at the crystal/solution interface, which was corroborated by a (13)C labelling experiment that confirmed the involvement of rapid enantiomerisation. Frequent monitoring of the solution-phase ee of the slowly racemising compound further revealed that the minor enantiomer dominated in solution, supporting an explanation based on a kinetic model. A generalisation of the process of "aymmetric autocatalysis" (resulting in automultiplication of chiral products in homogenous media) to encompass heterogeneous systems is also suggested.

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“…This contrasts with the anticipations in a recent report, though for a different reactive system 3e. 7…”

Section: Resultscontrasting

confidence: 97%

Autocatalytic Enantiomerisation at the Crystal Surface in Deracemisation of Scalemic Conglomerates

Wei

Mauksch

Tsogoeva

2009

Chemistry A European J

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“…Notably, Viedma et al later demonstrated that continuous grinding of a slurry of a racemic mixture of enantiomorphic crystals could cause its deracemization to produce an enantiopure product (Viedma ripening) 12–18 . Especially, Blackmond and coworkers for the first time experimentally supported a hypothesis of the emergence of solid-phase homochirality from racemic conglomerates composed of rapidly racemizing enantiomers 13,19,20 . Tsogoeva and coworkers achieved a complete deracemization (100% ee ) of an asymmetric reaction product by combining its reversible asymmetric reaction and physical grinding of crystals involved 21,22 .…”

Section: Introductionmentioning

confidence: 98%

Asymmetric catalysis mediated by a mirror symmetry-broken helical nanoribbon

et al. 2019

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Although chirality has been recognized as an essential entity for life, it still remains a big mystery how the homochirality in nature emerged in essential biomolecules. Certain achiral motifs are known to assemble into chiral nanostructures. In rare cases, their absolute geometries are enantiomerically biased by mirror symmetry breaking. Here we report the first example of asymmetric catalysis by using a mirror symmetry-broken helical nanoribbon as the ligand. We obtain this helical nanoribbon from a benzoic acid appended achiral benzene-1,3,5-tricarboxamide by its helical supramolecular assembly and employ it for the Cu 2+ -catalyzed Diels–Alder reaction. By thorough optimization of the reaction (conversion: > 99%, turnover number: ~90), the enantiomeric excess eventually reaches 46% (major/minor enantiomers = 73/27). We also confirm that the helical nanoribbon indeed carries helically twisted binding sites for Cu 2+ . Our achievement may provide the fundamental breakthrough for producing optically active molecules from a mixture of totally achiral motifs.

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“…While a stimulating scientific discussion on the fine details of the mechanism of symmetry breaking processes, both crystallization [37][38][39][40][41] and synthesis, [42][43][44][45] continues, theoretical reaction-type models have been developed that can describe the kinetics of the time evolution of a system toward homochirality. [46][47][48][49] Indeed, complete homochirality of products in a mirrorimage symmetry breaking process (whether crystallization or synthesis) can be designed as a result of non-linear autocatalysis coupled with back-reaction recycling of achiral substrates in a closed system.…”

Section: Introductionmentioning

confidence: 99%

Asymmetric autocatalysis: crystallization-induced highly enantioselective synthesis of a conglomerating oxo-rhenium(V) complex

Rybak

2008

Tetrahedron: Asymmetry

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Response to “Comments on a Possible Transition to Solid‐Phase Homochirality”

Cited by 18 publications

References 23 publications

Autocatalytic Enantiomerisation at the Crystal Surface in Deracemisation of Scalemic Conglomerates

Autocatalytic Enantiomerisation at the Crystal Surface in Deracemisation of Scalemic Conglomerates

Asymmetric catalysis mediated by a mirror symmetry-broken helical nanoribbon

Asymmetric autocatalysis: crystallization-induced highly enantioselective synthesis of a conglomerating oxo-rhenium(V) complex

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