Readily available rhodium(II) salts catalyze reactions between NHC-boranes (NHC-BH3) and diazocarbonyl compounds (N2CRCOR'). Stable alpha-NHC-boryl carbonyl compounds (NHC-BH2-CHRCOR') are isolated in good yields. The reaction is a reliable way to make boron-carbon bonds with good tolerance for variation in both the NHC-borane and diazocarbonyl components. It presumably occurs by insertion of a transient rhodium carbene into a boron-hydrogen bond of the NHC-borane. Competitive experiments show that a typical NHC-borane is highly reactive toward rhodium carbenes.