Boronic Acids and Their Derivatives as Continuous-Flow-Friendly Alkyl Radical Precursors

Abstract

Since its recognition as an enabling tool to form challenging C–C and C–heteroatom bonds under mild and sustainable conditions, photoredox catalysis has been in the spotlight within the synthetic community. As a consequence, the interest in developing novel synthetic strategies has spiked together with the need to define suitable technologies to overcome scale-up issues dictated by the Bouguer–Beer–Lambert law. In this context, continuous-flow reactors play a major role in increasing the efficiency of a given photocatalyzed reaction, thus rendering scale-up processes more accessible. In the alkyl radical precursor landscape, boron-based species have begun to play a predominant role. Though the reactivity of trifluoroborates has been deeply investigated, the interest in using other boron species as radical precursors in photocatalyzed reactions has recently arisen. This late exploration lies in the fact that the high oxidation potential of boronic acids (BAs) hinders their possible applications. Nevertheless, to circumvent this issue, a diverse array of activation modes has been developed, exploiting in most cases the inherent Lewis acidity of the boronic acid. The aim of this Account is to highlight our recent contribution to this vibrant field with a focus on broad applicability, selectivity, and scalability via continuous-flow methodology. For the sake of clarity, the Account is discussed under the following sections.

1 Introduction

2 Why Photochemistry in Flow?

2.1 Preliminary Considerations

2.2 Batch vs. Flow Photochemical Reactions

2.3 Commercially Available Lab-Scale Solutions for Photoflow Chemistry

3 Organoboron Compounds

3.1 The Evolution of Organoboron Compounds as Radical Precursors in Photoredox Catalysis

3.2 Organoboron Compounds in Flow

4 Activation of Boronic Acids towards Radical Formation

4.1 Giese-Type Addition

4.2 Petasis Reaction

4.3 Light-Driven Four-Component Reaction

4.4 Minisci Reaction

5 Conclusion and Future Perspective

Key words

photoredox catalysis - continuous-flow chemistry - boronic acid - free radicals

Publication History

Received: 17 January 2023

Accepted after revision: 04 April 2023

Accepted Manuscript online:
04 April 2023

Article published online:
08 May 2023

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