Reactions of N‑Heterocyclic Carbene Boranes with 5‑Diazo-2,2-dimethyl-1,3-dioxane-4,6-dione: Synthesis of Mono- and Bis-hydrazonyl NHC-Boranes

N-Heterocyclic carbene boranes (NHC-boranes) react with 5-diazo-2,2-dimethyl-1,3-dioxane-4,6-dione at 40 °C in dichloromethane to provide NHC-boryl hydrazone derivatives of 2,2-dimethyl-1,3-dioxane-4,6-dione. These hydrazones disproportionate to bis-hydrazones on treatment with diiodine in dichloromethane at room temperature. The mono- and bis-hydrazones are yellow solids that are stable to chromatography and storage.