Synthesis of Chemically and Configurationally Stable Monofluoro Acylboronates: Effect of Ligand Structure on their Formation, Properties, and Reactivities

The
recent disclosures of two classes of acylborons, potassium
acyltrifluoroborates (KATs) and N-methyliminodiacetyl
(MIDA) acylboronates, demonstrated that certain acylboron species
can be both remarkably stable and uniquely reactive. Here we report
new classes of ligands for acylboronates that have a significant influence
on the formation, properties, and reactivities of acylboronates. Our
systematic investigations identified a class of neutral, monofluoroboronates
that can be prepared in a one step, gram-scale fashion from readily
accessible KATs. These monofluoroboronates are stable to air, moisture,
and silica gel chromatography and can be easily handled without any
special precautions. X-ray crystallography, NMR spectroscopy, and
HPLC studies showed that they are tetravalent, configurationally stable B-chiral acylboronates. Significantly, the ligands on the
boronate allow for fine-tuning of the properties and reactivity of
acylboronates. In amide-forming ligation with hydroxylamines under
aqueous conditions, a considerable difference in reactivity was observed
as a function of ligand structure. The solid-state structures suggested
that subtle steric and conformational factors modulate the reactivities
of the acylboronates.