# Modulating halide leaving-group trends through recognition by bisboranes

## TL;DR

This paper shows how bisboranes can reverse the usual order of halide leaving-group ability in chemical reactions.

## Contribution

The study introduces a bidentate Lewis acid platform that dynamically binds halides and reverses their leaving-group tendency.

## Key findings

- Bisboranes selectively bind halides based on cavity size and ion radius.
- Dynamic binding of halides influences nucleophilicity and reaction outcomes.
- Halide leaving-group order is reversed in diborane-catalyzed transformations.

## Abstract

Modulating the intrinsic leaving-group tendency of halides remains a long-standing challenge in synthetic chemistry. Herein, we reported dynamic anion recognition and demonstrated its application in nucleophilic substitution reactions, enabling an apparent reversal of the halide leaving-group tendency sequence. A bidentate Lewis acid-based platform was developed to selectively bind halide ions, forming host–guest complexes that were characterized by NMR spectroscopy and X-ray crystallography. Competitive experiments with the host molecule have revealed tunable binding affinities for the halides based on the cavity size of the bisboron center. Moreover, anion exchange experiments have demonstrated that the dynamic binding of halides is primarily influenced by their nucleophilicity and ion radius. The recognition of organohalides by diborane hosts induced a reversal of the leaving ability of Br− and Cl− in diborane-catalyzed transformations, which deviated from the conventional sequence.

We report highly Lewis-acidic diboranes that reversibly bind halides via cavity-controlled recognition, display dynamic host–guest behavior, and invert the conventional halide (Br/Cl) leaving-group order in diborane-catalyzed reactions.