Comparison of Anionic and Lewis Acid Stabilized N-Heterocyclic Oxoboranes: Their Facile Synthesis from a Borinic Acid

摘要

There is great current interest in the synthesis and characterization of boron species containing multiple bonds. Oxo-boranes RBO, compounds containing a boron-oxygen multiple bond, have long been known to be highly reactive and tend to form oligomers owing to the oxophilicity of the unsaturated boron atoms. Consequently, for a long time oxoboranes were only invoked as intermediatesand studied in the gas phase or low-temperature matrix. In 2005, Cowley and co-workers reported the first isolable Lewis acid stabilized oxoborane supported by a β-diketiminato ligand. More recently, Braunschweig and co-workers reported the platinum oxoboryl complex trans-[(PCy3)2PtBr(BO)] (Cy = cyclohexyl), in which the low-coordinate boron center is stabilized by a late-transition-metal fragment. It has been shown that three-coordinate anionic boron species may also possess double-bonding character, as demonstrated by the anionic (Mes2BCH2)~- and dianionic (R2BBR2)~(2-) species reported by Power and others, which contain a boron-carbon and a boron-boron double bond, respectively. Inspired by the pioneering work on the stabilization of elusive oxoboranes and anionic boron π-bonding species, we anticipated that free monomeric oxoboranes (R2BO)~ would be accessible and may exhibit boron-oxygen double-bond character if suitable ligand frameworks are employed. A fair number of main-group and transition-metal complexes supported by diaryl and dialkyl boroxides have been synthesized and structurally characterized by several groups. However, metal-free anionic oxoboranes (R2BO)~- are still elusive. Herein, we report on the synthesis and structural characterization of the anionic oxoboranes 3a,b and the Lewis acid stabilized oxoboranes 4a,b prepared from the borinic acid 2 by deprotonation of the hydroxy group with N-heterocyclic carbenes (NHCs) and the hydrogen migration promoted by Lewis acids, respectively, as outlined in Scheme 1 and Scheme 2.