Six-Membered Saturated NHC-Stabilized Borenium Cations: Isolation of a Cationic Analogue of Borinic Acid.

The reaction of six-membered saturated NHC [1,3-di(2,6-diisopropylphenyl) tetrahydropyrimidine-2-ylidene; henceforth abbreviated as 6-SIDipp] with PhBCl 2 yields a Lewis base adduct, 6-SIDipp·PhBCl 2 ( 1 ), which readily undergoes nucleophilic substitution reaction with AgNO 3 , leading to the single ( 2 ) and double ( 3 ) substitution of both chlorides with ONO 2 moieties at the boron atom. The reaction of 1 with 1 equiv of AlCl 3 resulted in a borenium cation of composition [6-SIDipp·B(Ph)Cl] + ( 4 ) with AlCl 4 - as the counteranion. Although borenium cations with different substituents on boron have been reported, a structurally characterized phenylchloroborenium cation remains unknown. Similarly, the reaction of 1 with triflic acid provides the first representative of a new class of borenium cations bearing one hydroxyl and one phenyl group on boron ( 5 ), a cationic analogue of borinic acid.