Direct deoxygenative borylation of carboxylic acids

Jianbin Li,Rustam Z Khaliullin,Chia-Yu Huang,Mohamad Ataya,Chao-Jun Li

doi:10.1038/s41467-021-25229-8

Abstract

Carboxylic acids are readily available, structurally diverse and shelf-stable; therefore, converting them to the isoelectronic boronic acids, which play pivotal roles in different settings, would be highly enabling. In contrast to the well-recognised decarboxylative borylation, the chemical space of carboxylic-to-boronic acid transformation via deoxygenation remains underexplored due to the thermodynamic and kinetic inertness of carboxylic C-O bonds. Herein, we report a deoxygenative borylation reaction of free carboxylic acids or their sodium salts to synthesise alkylboronates under metal-free conditions. Promoted by a uniquely Lewis acidic and strongly reducing diboron reagent, bis(catecholato)diboron (B2cat2), a library of aromatic carboxylic acids are converted to the benzylboronates. By leveraging the same borylative manifold, a facile triboration process with aliphatic carboxylic acids is also realised, diversifying the pool of available 1,1,2-alkyl(trisboronates) that were otherwise difficult to access. Detailed mechanistic studies reveal a stepwise C-O cleavage profile, which could inspire and encourage future endeavours on more appealing reductive functionalisation of oxygenated feedstocks.

Highlights

Carboxylic acids are readily available, structurally diverse and shelf-stable; converting them to the isoelectronic boronic acids, which play pivotal roles in different settings, would be highly enabling
The same group, collaborating with Blackmond’s laboratory, reported two other practical protocols based on the inexpensive copper catalysis and electrochemical means[22,23]. Aggarwal and his colleagues disclosed a photoinduced decarboxylative borylation manifold with the same redox-active esters (RAEs), in which the strategic assembly of a light-absorbing boron complex could avoid the usage of transition metal catalysts[24]
Consecutive removals of C–O bonds through boron group transfer could give a gem-diboron intermediate, which was susceptible for protodeboronation to furnish the final product

Summary

Introduction

Carboxylic acids are readily available, structurally diverse and shelf-stable; converting them to the isoelectronic boronic acids, which play pivotal roles in different settings, would be highly enabling. These benign features made them ideal starting materials for the alkylboronate synthesis and encouraged continuing exploration in the chemical space of carboxylic-to-boronic acid transformation (Fig. 1a)[20]. An important alternative to convert carboxylic acids into their boronic analogues is through deoxygenative borylation, ideally in the absence of metal catalysts and exogenous additives.

Results

Conclusion