O,N-Heterocyclic germylenes as efficient catalysts for hydroboration and cyanosilylation of benzaldehyde

Abstract

New monomeric O,N-heterocyclic germylene ^AdAPGe (1) based on 4,6-di-tert-butyl-N-adamantyl-o-aminophenol is synthesized and structurally characterized. Germylene 1 in the crystal demonstrates weak Ge⋯Ge interactions between adjacent molecules. The redox activity of compound 1 and its known analogs ^PhAPGe (2), ^t-BuAPGe (3) was studied using cyclic voltammetry and chemical oxidation by the phenoxy radical. Germylene 1, as opposed to 2 and 3, was found to form a relatively stable o-iminosemiquinonate moiety during oxidation. The last one was detected by EPR spectroscopy in toluene solution at low temperatures. The catalytic activity of germylenes 1–3 and ^dippAPGe (4) (dipp – 2,6-di-isopropylphenyl) towards hydroboration and cyanosilylation of benzaldehyde was examined. Germylene 1 demonstrates the most potent promoter properties for such reactions, which allowed cyanosilylation and hydroboration of aldehyde under mild conditions with excellent conversion quickly. Theoretical studies were performed to establish the mechanism, and different reaction routes were examined. According to the DFT (B3LYP/def2-SVP) calculation, cyanosilylation and hydroboration processes are initiated by coordinating TMSCN or HBpin with the catalyst. The followed reaction with aldehyde leads to the resulting products.