Synthesis and Applications of Unquaternized C‑Bound Boron Enolates

A general
and facile method to prepare unquaternized C-bound boron
enolates by a ligand-controlled O-to-C isomerization is reported.
Using this protocol, C-bound pinacolboron enolates have been isolated
in pure form for the first time, and have been fully characterized
by NMR spectroscopy and X-ray crystallography. In contrast to the
general perception, such C-boron enolates are stable without coordinative
saturation at the boron. Moreover, C-boron enolates present reactivities
that are distinct from the O-boron enolates, and their applications
in C–O and C–C bond formations are demonstrated.