Catalysis with Pnictogen, Chalcogen, and Halogen Bonds

Abstract

Halogen- and chalcogen-based σ-hole interactions have recently received increased interest in non-covalent organocatalysis. However, the closely related pnictogen bonds have been neglected. In this study, we introduce conceptually simple, neutral, and monodentate pnictogen-bonding catalysts. Solution and in silico binding studies, together with high catalytic activity in chloride abstraction reactions, yield compelling evidence for operational pnictogen bonds. The depth of the σ holes is easily varied with different substituents. Comparison with homologous halogen- and chalcogen-bonding catalysts shows an increase in activity from main group VII to V and from row 3 to 5 in the periodic table. Pnictogen bonds from antimony thus emerged as by far the best among the elements covered, a finding that provides most intriguing perspectives for future applications in catalysis and beyond.

Keywords: anion binding; catalysis; chalcogen bonds; halogen bonds; pnictogen bonds.

Figure 1

Pnictogen, chalcogen, and halogen bonds between the σ* orbital of a donor (D, with electron‐withdrawing groups (EWGs) to deepen the σ hole) and a lone pair n of a bound Lewis base (LB) afford bond angles around 180° (left) and are predicted to increase in strength with the polarizability of the donor in main groups V–VII (right; computational values based on MP2 calculations in a.u.).13

Figure 2

a) Structures of catalyst candidates 1–10 explored in this study, with dissociation constants of chloride complexes in THF. b) ¹⁹F NMR spectra of antimony donor 1 in the presence of increasing concentrations of TBACl (maroon to ultramarine). c) Molecular electrostatic potential surfaces (MEPs) of catalysts 3, 2, and 1 (from top to bottom; M06‐2X/6–311G**/aug‐cc‐pVTZ‐pp; isosurface: 0.001 a.u. (0.627 kcal mol⁻¹); red: −0.01327 a.u. (−8.3 kcal mol⁻¹), blue: +0.04527 a.u. (+28.4 kcal mol⁻¹)), and minimized structure of the chloride complex of 1, with relevant bond lengths and angles. C gray, Cl⁻ green, F green, Sb violet.

Scheme 1

Reactions tested with the potential σ‐hole catalysts 1–10, with the proposed mechanism for antimony catalyst 1. Substrate 11 (25 mm) was reacted with 12 (27 mm), nucleophiles 13 or 14 (38 mm), and the catalyst (20 mol %) in dry THF at −100 °C; substrate 17 (167 mm) was reacted with 14 (250 mm) and the catalyst (20 mol %) in dry THF at −78 °C, together with 1,4‐bis(trimethylsilyl)benzene (6 mm) as an internal standard. See the Supporting Information for details.