Regioselective Transition-Metal-Free C(sp 2 )-H Borylation: A Subject of Practical and Ongoing Interest in Synthetic Organic Chemistry.

Considerable advances have been made in the area of C-H functionalization in the last few decades. A number of approaches including both directed and nondirected strategies have been developed thus far. Among the various C-H functionalizations, C-H borylation is of special interest due to the wide applications of organoboron compounds. In this regard, various transition-metal-catalyzed regioselective strategies have been developed. However, the major concern regarding metal-catalyzed C-H borylation procedures is the requirement of a precious metal as well as the contamination by metal precursors in the desired products, which limit the application of this process in large-scale synthesis. Therefore, recent trends have involved the use of transition-metal-free systems. We summarize recent developments in transition-metal-free regioselective C-H borylation. We believe that this Review will help to increase interest in this field and stimulate further progress.