A Structurally Robust Chiral Borate Ion: Molecular Design, Synthesis, and Asymmetric Catalysis

Abstract

Catalysis by chiral weakly-coordinating anions (WCAs) remains underdeveloped due to the lack of a molecular design strategy for exploiting their characteristics, such as the non-nucleophilic nature. Here, we report the development of a chiral borate ion comprising an O,N,N,O-tetradentate backbone, which ensures hitherto unattainable structural robustness. Upon pairing with a proton, the hydrogen borate acts as an effective catalyst for the asymmetric Prins-type cyclization of vinyl ethers, providing access to structurally and stereochemically defined dihydropyrans. The key to selectivity control is the distinct ability of the borate ion to discriminate the prochiral faces of the acyclic oxonium ion intermediate and dictate the regiochemical outcome. We anticipate that this study paves the way for exploring the untapped potential of WCA catalysis for selective chemical synthesis.

A Structurally Robust Chiral Borate Ion: Molecular Design, Synthesis, and Asymmetric Catalysis

“…From this core . [26] Angewandte Chemie Forschungsartikel structure,f our phenyl groups spread outward to create ad iscrete chiral environment over the boron center. Importantly,o ne of the carbonyl oxygens of the amide moieties interacted with the pairing benzyldimethylammonium cation via aweak hydrogen bond (C=O•••HN + = 1.727 )sothat the cation is positioned between ap henyl group and an aphthyl group (Figure 1c-iii).…”

The Power of the Proton: From Superacidic Media to Superelectrophile Catalysis

“…Chiral Brønsted acids based on binaphthyl-derived disulfonimides (DSI) and imido­diphosphorimidates (IDPi), resembling HNTf 2 , as well as a binaphthyl-allyl-tetrasulfone (BALT) C–H acid have been successfully employed to asymmetric “silylium ion catalysis” under “tamed conditions”. Another chiral Brønsted acid featuring a borate anion with a binaphthyl-derived O,N,N,O-tetradentate backbone has been recently introduced by the Ooi group . However, the presence of coordinating oxygen- and nitrogen-donor atoms in these nonsuperacids makes them incompatible with truly “superelectrophile conditions”.…”

“…Another chiral Brønsted acid featuring a borate anion with a binaphthyl-derived O,N,N,Otetradentate backbone has been recently introduced by the Ooi group. 191 However, the presence of coordinating oxygen-and nitrogen-donor atoms in these nonsuperacids makes them incompatible with truly "superelectrophile conditions". Attempts with weakly coordinating anions on the basis of perfluorinated borate anions have not been fruitful so far, 192−194 and Brønsted acids with chiral carborate anions have not been described yet.…”

Access to Hexahydroindeno[2,1-c]pyran-Based Propellanes by a Domino Prins/Friedel–Crafts Cyclization

“…[10][11][12][13] Recent reports also describe the control of the configuration of the newly created stereogenic centers by using chiral phosphoric acids or borates. [14][15][16][17] Indenopyrans represent an interesting subclass of oxygenated heterocycles for which an indene and a pyran subunit are fused. Some of them have been reported to exhibit fluorescent properties and have been considered for the fabrication of OLEDs.…”