Mechanism-Based Design of Quinoline Potassium Acyltrifluoroborates for Rapid Amide-Forming Ligations at Physiological pH

Potassium acyltrifluoroborates (KATs)
undergo chemoselective amide-forming
ligations with hydroxylamines. Under aqueous, acidic conditions these
ligations can proceed rapidly, with rate constants of ∼20 M–1 s–1. The requirement for lower
pH to obtain the fastest rates, however, limits their use with certain
biomolecules and precludes in vivo applications. By mechanistic investigations
into the KAT ligation, including kinetic studies, X-ray crystallography,
and DFT calculations, we have identified a key role for a proton in
accelerating the ligation. We applied this knowledge to the design
and synthesis of 8-quinolyl acyltrifluoroborates, a new class of KATs
that ligates with hydroxylamines at pH 7.4 with rate constants >4
M–1 s–1. We trace the enhanced
rate at physiological pH to unexpectedly high basicity of the 8-quinoline-KATs,
which leads to their protonation even under neutral conditions. This
proton assists the formation of the key tetrahedral intermediate and
activates the leaving groups on the hydroxylamine toward a concerted
1,2-BF3 shift that leads to the amide product. We demonstrate
that the fast ligations at pH 7.4 can be carried out with a protein
substrate at micromolar concentrations.