Metal-Free Radical Borylations: Mechanisms, Catalytic Strategies, and Synthetic Applications

Abstract

Metal-free radical borylation has emerged as a powerful and sustainable alternative to transition-metal-catalyzed methods, addressing challenges such as residual metal contamination, functional group sensitivity, and the high cost of precious metals. This review provides a comprehensive analysis of the mechanistic underpinnings and catalytic innovations driving this rapidly evolving field. We delve into the fundamental steps of radical generation, including homolytic B-X bond cleavage and the formation of alkyl, aryl, and boryl radicals through single-electron transfer (SET), hydrogen atom transfer (HAT), and energy transfer (EnT) pathways. Recent strategic developments are critically evaluated, including photoinduced processes such as electron donor-acceptor (EDA) complexe activation, consecutive photoinduced electron transfer (ConPET), and polarity reversal catalysis, as well as electrochemical activation using organic mediators and thermal initiation via radical chain processes. Unlike previous reviews, we place a special emphasis on catalytic strategies, substrate scope limitations, and emerging approaches for achieving regiocontrol. The synthetic utility of metal-free radical borylation is highlighted through its application in the late-stage functionalization of pharmaceuticals, natural products, and other complex molecules, enabling transformations that are often difficult to achieve using traditional methods. Finally, we outline the key challenges that continue to shape the field, including selective borylation of unactivated C(sp³)–H bonds and the scalability of photochemical and electrochemical systems, while also identifying promising future directions in enhancing atom economy, deepening mechanistic understanding, and integrating method development with cutting-edge technology such as high-throughput screening and machine learning. By bridging fundamental radical principles with practical synthesis, this review aims to serve as a roadmap for the continued development of metal-free boron chemistry.