Abstract
Selective borylation of azines-nitrogen-containing aromatic heterocycles used in the synthesis of many pharmaceuticals-is made possible by forming a radical from an aminoborane using a photocatalyst.
Boron functional groups are often introduced in place of aromatic carbon-hydrogen bonds to expedite small-molecule diversification through coupling of molecular fragments(1-3). Current approaches based on transition-metal-catalysed activation of carbon-hydrogen bonds are effective for the borylation of many (hetero)aromatic derivatives(4,5) but show narrow applicability to azines (nitrogen-containing aromatic heterocycles), which are key components of many pharmaceutical and agrochemical products(6). Here we report an azine borylation strategy using stable and inexpensive amine-borane(7) reagents. Photocatalysis converts these low-molecular-weight materials into highly reactive boryl radicals(8) that undergo efficient addition to azine building blocks. This reactivity provides a mechanistically alternative tactic for sp(2) carbon-boron bond assembly, where the elementary steps of transition-metal-mediated carbon-hydrogen bond activation and reductive elimination from azine-organometallic intermediates are replaced by a direct, Minisci(9)-style, radical addition. The strongly nucleophilic character of the amine-boryl radicals enables predictable and site-selective carbon-boron bond formation by targeting the azine's most activated position, including the challenging sites adjacent to the basic nitrogen atom. This approach enables access to aromatic sites that elude current strategies based on carbon-hydrogen bond activation, and has led to borylated materials that would otherwise be difficult to prepare. We have applied this process to the introduction of amine-borane functionalities to complex and industrially relevant products. The diversification of the borylated azine products by mainstream cross-coupling technologies establishes aromatic amino-boranes as a powerful class of building blocks for chemical synthesis.