New Shelf-Stable Halo- and Alkoxy-Substituted Pyridylboronic Acids and their Suzuki Cross-Coupling Reactions to Yield Heteroarylpyridines

Abstract

New shelf-stable pyridylboronic acids have been synthesized: bromine-lithium exchange followed by reaction with triisopropylborate (TIPB) yielded 2-fluoro-5-pyridylboronic acid (4), 3-bromo-5-pyridylboronic acid (5) and 2-ethoxy-5-pyridylboronic acid (6); directed lithiation followed by reaction with trimethylborate (TMB) or TIPB afforded 2-methoxy-3-pyridylboronic acid (8), 3-bromo-6-methoxy-4-pyridylboronic acid (11) and 3-bromo-6-ethoxy-4-pyridylboronic acid (12). Cross-coupling of pyridylboronic acids 4, 6, 8, and 11 with 3-bromoquinoline [Cs₂CO₃, Pd(PPh₃)₂Cl₂, 1,4-dioxane, 95 °C] gave pyridinylquinoline derivatives 13, 15-17 in 50-77% yields: the analogous reaction of 5 was low yielding due to further in situ reactions of the product 14. Cross-coupling of 12 with 2-bromo-5-nitrothiophene gave 3-bromo-4-(5-nitro-2-thienyl)-6-ethoxypyridine (18).

pyridine - boronic acids - cross-coupling - Suzuki reaction - heterobiaryl

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