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) 2 BO] ( 1 ) (Dipp = 2,6- i Pr 2 C 6 H 3 ), 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) 2 BS] ( 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 (O 2– ) ion abstraction, to generate a borenium cation [(HCDippN) 2 B(NC 5 H 5 )][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.