Recent Advances in C-B Bond Formation through a Free Radical Pathway

The development of a methodology for the preparation of arylboronic acids or arylboronates is of significant interest to organic chemists. Classical synthetic methods to prepare these organoboron compounds are based on the reaction of Grignard or lithium reagents with trialkyl borates. In the past few decades, the transition- metal-catalyzed borylation of aryl halides, or pseudohalides, and C-H bonds of hydrocarbons has been a powerful tool for the synthesis of arylboronates in modern organic synthesis. These transformations are generally considered to proceed via organometallic intermediates generated by oxidative addition or transmetalation processes from the boron reagent. Several reviews on this type of borylation catalyzed by transition metals have been published in the literature. Interestingly, there has been a novel recognition that the boron reagent can participate in free-radical coupling via the homolytic cleavage of the boron-boron bond in recent years. In this review, recent advances in this new area of boron chemistry are summarized and the reaction mechanisms are also discussed. 1 Introduction 2 Borylation of Arylamine Derivatives 3 Borylation of Aryl (Pseudo) Halides 4 Borylation of Carboxylic Acid Derivatives 5 Diboration of Alkynes 6 Conclusions and Perspectives