Asymmetric Pnictogen-Bonding Catalysis: Transfer Hydrogenation by a Chiral Antimony(V) Cation/Anion Pair

Abstract

Pnictogen-bonding catalysis based on σ-hole interactions has recently attracted the attention of synthetic chemists. As a proof-of-concept for asymmetric pnictogen-bonding catalysis, we report herein an enantioselective transfer hydrogenation of benzoxazines catalyzed by a novel chiral antimony cation/anion pair. The chiral pnictogen catalyst library could be rapidly accessed from triarylstibine with readily available mandelic acid analogues, and the catalyst displays remarkable efficiency and enantiocontrol potency even at 0.05 mol % loading. Moreover, the properties of the catalyst and the mechanistic insights have been investigated by nonlinear effect studies, 1H NMR, LC-MS, and control experiments.

On the Importance of Pnictogen and Chalcogen Bonding Interactions in Supramolecular Catalysis

“…The authors observed a weak negative nonlinear relationship between ee of the catalyst and ee of the product. This phenomenon indicated the presence of ligand exchange and/or catalyst aggregation during the course of the reaction, which was further confirmed by 1 H NMR spectra [97].…”

“…In a similar way to the study of Zhang and collaborators [97], Yang et al used compounds 22 and 23 as catalysts for the transfer hydrogenation reaction between Hantzch ester and quinolines (see Figures 9b and 10). Furthermore, the authors also synthesized monostilbonium cations, which presented lower catalytic activity compared to bidentated Pn catalysts 22 and 23.…”

“…In the field of organocatalysis, the number of studies devoted to the development of new functional PnB-based catalysts has rapidly increased during recent years [93][94][95][96][97][98][99][100][101][102]. In this section, several selected examples involving PnB-based catalysts are discussed.…”

“…This ion pair promoted the reduction of benzoxazine efficiently in 2 h, obtaining the product with 99% yield with 80% ee. Zhang and collaborators [97] developed a new family of chiral PnB-based catalysts (see Figure 9a). Concretely, 20 was obtained through the reaction of triarylstibine and mandelic acid under oxidative conditions and used as a catalyst in the reduction of benzoxazine with Hantzch ester (Figure 9b).…”

Visible‐Light‐Induced Selective Photolysis of Phosphonium Iodide Salts for Monofluoromethylations

“…[8,7b] Most recently,T an and co-workers described an interesting asymmetric transfer hydrogenation with ac hiral antimony(V) cation/anion pair (Scheme 1B). [9] Despite these advances,all these strategies relied on the attraction of alone pair with the s-holes.W ee nvisioned that the s-hole effect may be able to facilitate/induce single electron transfer (SET) for the generation of useful radicals,e.g., R f radicals (Scheme 1C), under the irradiation of visible light.…”