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.