Main Group Catalyzed Arene Borylation: Challenges and Opportunities
Michael J. Ingleson
Abstract
RecommendationsC -H borylation is an efficient method to generate organoboranes that are powerful intermediates in synthesis, due in large part to the Suzuki-Miyaura reaction. 1remendous advances have been made using transition metal based catalysts for arene C-H borylation. 2 These catalysts generally utilize the variable oxidation states available to the transition metals to effect C-H cleavage and C-B bond formation.Iridium based C-H borylation catalysts in particular are established as one of the foremost methods in the field of C-H functionalization. 3 Main group compounds also have been applied as catalysts for arene C-H borylation, with the greater abundance, lower cost, and higher permitted exposure limits of most main group elements relative to precious metals one key driver for this research. 4A second driver stems from the fact that main group catalysis generally proceeds at a fixed oxidation state thus requiring distinct borylation mechanisms.This can lead to complementary regioselectivity and functional group tolerance to that using transition metal based catalysts.However, significant challenges remain in main group catalyzed arene borylation, particularly around expanding the substrate scope and generating operationally simple methodologies. 5Most systems reported to date use (pre)catalysts that are challenging to handle (due to their sensitivity to protic species such as H 2 O) and that are limited in substrate scope to nucleophilic (hetero)arenes (defined herein as substrates with Mayr nucleophilicity values (N) > 1, for reference, 2-Me-thiophene and furan have N ≈ 1.3). 6 Identifying a single catalytic system that addresses both of these challenges is essential for the wider uptake of main group catalyzed arene C-H borylation.This viewpoint summarizes the general approaches used to date in main group catalyzed intermolecular arene C-H borylation.It is not a comprehensive survey of the area; instead, it focuses on analyzing common key steps and through this seeks to highlight some of the current challenges and future opportunities in the field.For stoichiometric (in strong Lewis acid) 7 and intramolecular C-H borylation, the reader is directed to recent reviews.