CO scission and reductive coupling of organic carbonyls by a redox-active diboraanthracene

Abstract

The boron-centered reactivity of the diboraanthracene-auride complex [Au(B₂P₂)][K(18-c-6)]; (B₂P₂, 9,10-bis(2-(diisopropylphosphino)-phenyl)-9,10-dihydroboranthrene) with a series of organic carbonyls is reported. The reaction of [Au(B₂P₂)]⁻ with formaldehyde or paraformaldehyde results in a head-to-tail dimerization of two formaldehyde units across the boron centers. In contrast, the reaction of [(B₂P₂)Au]⁻ with two equivalents of benzaldehyde yields the pinacol coupling product via C–C bond formation. Careful stoichiometric addition of one equivalent of benzaldehyde to [Au(B₂P₂)]⁻ enabled the isolation of an adduct corresponding to the formal [4+2] cycloaddition of the CO bond of benzaldehyde across the boron centers. This adduct reacts with a second equivalent of benzaldehyde to produce the pinacol coupling product. Finally, the reaction of [Au(B₂P₂)]⁻ with acetone results in a formal reductive deoxygenation with discrete hydroxo and 2-propenyl units bound to the boron centers. This reaction is proposed to proceed via an analogous [4+2] cycloadduct, highlighting the unique small molecule activation chemistry available to this platform.