Synthesis and Structural Characterization of the Bis(diisopropylamino)boron Enolate of <i>tert</i>-Butyl Methyl Ketone

Lithium, sodium, and potassium enolates reacted with bisaminoboron halides to give bisaminoboron enolates 1a−5c. Specifically, the potassium enolate of tert-butyl methyl ketone reacted with bis(diisopropylamino)boron chloride in THF at room temperature within 1 h to give the bis(diisopropylamino)boron enolate of tert-butyl methyl ketone (3b) in 61−84% isolated yields. Under similar conditions, the reactivity is highly dependent on the metal employed to generate enolates (K > Na > Li) as well as the nitrogen substituents in the bisaminoboron halides (iPr > Et > TMS). This latter observation is a compromise between the boron−nitrogen resonance and the steric effect. The structural information of enolate 3b was studied in detail. Unlike the aggregated alkali-metal enolates, this boron enolate exists exclusively as a monomer in both the solid and solution states, as identified by X-ray and diffusion-ordered NMR.