An Acid-Free Anionic Oxoborane Isoelectronic with Carbonyl: Facile Access and Transfer of a Terminal BO Double Bond

We disclose the synthesis
and structural characterization of the
first acid-free anionic oxoborane, [K­(2.2.2-crypt)]­[(HCDippN)2BO] (1) (Dipp = 2,6-iPr2C6H3), which is isoelectronic
with classical carbonyl compounds. 1 can readily be accessed
from its borinic acid by a simple deprotonation/sequestration sequence.
Crystallographic and density functional theory (DFT) analyses support
the presence of a polarized terminal BO double bond. Subsequent
π bond metathesis converts the BO bond to a heavier BS containing system, affording the first
anionic thioxoborane [K­(2.2.2-crypt)]­[(HCDippN)2BS] (2), isoelectronic with thiocarbonyls. Facile BO bond
cleavage can also be achieved to access B–H and B–Cl
bonds and, via a remarkable oxide (O2–) ion abstraction,
to generate a borenium cation [(HCDippN)2B­(NC5H5)]­[OTf] (4). By extension, 1 can act as an oxide transfer agent to organic substrates,
a synthetic role traditionally associated with transition-metal compounds.
Hence we show that B–O linkages, which are often considered
to be thermodynamic sinks, can be activated under mild conditions
toward bond cleavage and transfer, by exploiting the higher reactivity
inherent in the BO double bond.