Helical Multi‐Coordination Anion‐Binding Catalysts for the Highly Enantioselective Dearomatization of Pyrylium Derivatives

Abstract

A general and highly enantioselective synthesis of oxygen heterocycles from readily available in situ generated pyrylium derivatives has been realized by embracing a multi‐coordination approach with helical anion‐binding tetrakistriazole catalysts. The high activity of the tetrakistriazole (TetraTri) catalysts, with distinct confined anion‐binding pockets, allows for remarkably low catalyst loadings (down to 0.05 mol %), while providing a simple access to chiral chromanones and dihydropyrones in high enantioselectivities (up to 98:2 e.r.). Moreover, experimental and theoretical studies provide new insights into the hydrogen‐donor ability and key binding interactions of the TetraTri catalysts and its host:guest complexes, suggesting the formation of a 1:3 species.

Insight into the Folding and Cooperative Multi‐Recognition Mechanism in Supramolecular Anion‐Binding Catalysis

“…Based on the difference between the calculated gas‐phase entropies and the measured Δ S , the solvation contribution is ≈+66.5 cal mol −1 . However, the most surprising observation was the nearly nonexistent binding to triflate, even though this counter‐anion was also proven to be efficient in asymmetric catalysis leading to a highly reactive and enantioselective outcome [14d] . Therefore, it is clear that the standard titration anion‐binding analysis is not always appropriate to fully describe and, even more importantly, predict the performance and efficiency of supramolecular binders in catalysis.…”

Chalcogen Bond Mediated Enhancement of Cooperative Ion‐Pair Recognition

“…Thec ations are coordinated by the five oxygen atoms from the crown ether host, with Na + -O distances ranging from 2.302(10) to 2.477 (10) and their coordination spheres are completed by additional interactions with water solvate molecules and iodide anions.T he triazole-bound iodide anion is encapsulated within each of the 3,5-bis-triazole pyridine anionbinding clefts via an array of CH•••I À hydrogen bonding interactions. [29,30]…”

Chalcogen Bond Mediated Enhancement of Cooperative Ion‐Pair Recognition

“…The cations are coordinated by the five oxygen atoms from the crown ether host, with Na + –O distances ranging from 2.302(10) Å to 2.477(10) Å and their coordination spheres are completed by additional interactions with water solvate molecules and iodide anions. The triazole‐bound iodide anion is encapsulated within each of the 3,5‐bis‐triazole pyridine anion‐binding clefts via an array of CH⋅⋅⋅I − hydrogen bonding interactions …”