Development of Borylation Reactions Using N-Heterocyclic Carbene Boranes

N-Heterocyclic carbene boranes (NHC-boranes) exhibit high stability toward oxygen, water, weak acids, and heat. On the other hand, they serve as excellent hydride donors and precursors to boron-centered radicals (boryl radicals). These characteristics have enabled their application in a variety of borylation reactions in organic synthesis. In 2014, we discovered the hydroboration of arynes using NHC-boranes, which marked the beginning of our development of unique borylation reactions involving NHC-boryl radicals. We subsequently developed a range of transformations, including borylative transannular radical cyclization of cyclic diynes, trans-selective radical hydroboration, borylative lactonization of propargyl esters, synthesis and transformation of a 3-benzoborepin derivative, radical C-F borylation of polyfluoroarenes, and photochemical borylation of sulfonyl arenes. In this article, we provide an overview of these borylation reactions, including their discovery, representative results, and proposed mechanisms.