CAAC Boranes. Synthesis and characterization of cyclic (alkyl) (amino) carbene borane complexes from BF(3) and BH(3)

In situ formation of two cyclic (alkyl) (amino) carbenes (CAACs) followed by addition of BF(3)•Et(2)O provided the first two examples of CAAC–BF(3) complexes: 1-(2,6-diisopropylphenyl)-3,5,5-trimethyl-3-phenylpyrrolidin-2-ylidene trifluoroborane, and 2-(2,6-diisopropylphenyl)-3,3-dimethyl-2-azaspiro[4.5]decan-1-ylidene trifluoroborane. These CAAC–BF(3) complexes are robust compounds that are stable to ambient laboratory conditions and silica gel chromatography. They were characterized by spectroscopy and X-ray crystallography. In contrast, a CAAC complex with borane (BH(3)) was readily formed in situ according to (1)H and (11)B NMR analysis, but did not survive the workup conditions. These results set the stage for further studies of the chemistry of CAAC boranes.