Structure and Reactivity of a Preactivated sp2–sp3 Diboron Reagent: Catalytic Regioselective Boration of α,β-Unsaturated Conjugated Compounds

A novel sp2–sp3 diboron reagent has been developed for the copper-catalyzed β-boration of α,β-unsaturated conjugated compounds. The reaction proceeds under mild conditions with various substrates, i.e., α,β-unsaturated esters, ketones, nitriles, ynones, amides, and aldehydes, in the absence of additives such as phosphine and sodium tert-butoxide to provide β-borylhomoenolates in good to excellent yields. The presence of an sp3-hybridized boron center, unambigously confirmed by X-ray crystallography, sufficiently activates the unsymmetrical pinacolato diisopropanolaminato diboron (PDIPA diboron, 2) to transfer the sp2-hybridized boron moiety chemoselectively. These observations suggest that the activation of one of the boron atoms is an essential step in the Cu-catalyzed β-boration catalytic cycle.