Role of Association in Chiral Catalysis: From Asymmetric Synthesis to Spin Selectivity

• Published in: Chemistry : a European journal Vol. 24; no. 70; pp. 18587 - 18600
• Main Authors: Ageeva, Aleksandra A., Khramtsova, Ekaterina A., Magin, Ilya M., Purtov, Peter A., Miranda, Miguel A., Leshina, Tatyana V.
• Format: Journal Article
• Language: English
• Published: Germany Wiley Subscription Services, Inc 12.12.2018
• Subjects: Aldehydes; Alkylation; asymmetric catalysis; Asymmetric synthesis; Biomolecules; Catalysis; Chemical synthesis; Chemistry; chirality; Electron transfer; Enantiomers; enantioselectivity; Free radicals; Magnetic fields; Magnetic resonance spectroscopy; Naproxen; NMR spectroscopy; Organic chemistry; Photochemistry; Reagents; Selectivity; spin selectivity; Tryptophan; enantioselectivity; asymmetric catalysis; chirality; electron transfer; spin selectivity
• ISSN: 0947-6539; 1521-3765
• DOI: 10.1002/chem.201801625

The origin of biomolecules in the pre‐biological period is still a matter of debate, as is the unclarified nature of the differences in enantiomer properties, especially for the medically important activity of chiral drugs. With regards to the first issue, significant progress was made in the last decade of the 20th century through experimental confirmation of Frank's popular theory on chiral catalysis in spontaneous asymmetric synthesis. Soai examined the chiral catalysis of the alkylation of achiral aldehydes by achiral reagents. Attempts to model this process demonstrated the key role of chiral compounds associates as templates for chiral synthesis. However, the elementary mechanism of alkylation and the role of free radicals in this process are still incompletely understood. Meanwhile, the influence of external magnetic fields on chiral enrichment in the radical path of alkylation has been predicted. In addition, the role of chiral dyad association in another radical process, electron transfer (ET), has been recently demonstrated by the following methods: chemically induced dynamic nuclear polarisation (CIDNP), NMR spectroscopy, XRD and photochemistry. The CIDNP analysis of ET in two dyads has revealed a phenomenon first observed for chiral systems, spin selectivity, which results in the difference between the CIDNP enhancement coefficients of dyad diastereomers. These dyads are linked systems consisting of the widespread drug (S)‐naproxen (NPX) or its R analogue and electron donors, namely, (S)‐tryptophan and (S)‐N‐methylpyrrolidine. Because NPX is one of the most striking examples of the difference in the therapeutic properties of enantiomers, the appearance of spin selectivity in dyads with (S)‐ and (R)‐NPX and S donors can shed light on the chemical nature of these differences. This review is devoted to discussing the chemical nature of spin selectivity and the role of chiral associates in the chiral catalysis of an elementary radical reaction: ET in chiral dyads. Catalysed self‐formation: A new example of chiral catalysis is the influence of dimerisation on intramolecular electron transfer in dyads with two chiral centres. Dyad dimer formation acts in different ways on the conformations of the paramagnetic precursors of the (R,S) and (S,S) diastereomers (see figure). These results are in agreement with Frank's theory and the Soai reaction, which is used for experimental confirmation.