Preparation of catechol boronate esters enabled by N → B dative bond as efficient, stable, and green catalysts for the transfer hydrogenation of various ketones

Abstract

A series of new structurally related catechol boronate esters enabled by N → B dative bond of general composition (B1-B4) and (B1N-B4N) were designed and synthesized for the transfer hydrogenation of various ketones, as catalysts. The structures of the synthesized trigonal-planar and tetrahedral catechol boronate esters forming dative bonds with neutral base were well-elucidated by NMR (1H, 13C, and 11B), FT-IR, UV–Vis, fluorescence, and LC-MS/MS spectroscopy, melting point and microanalysis. With the development of a more active and stable catalytic system for transfer hydrogenation of ketones in mind, all catechol boronate esters were tested as catalysts for the transfer hydrogenation of aromatic ketones, showing good activity in this reaction. Particularly, it was proved that the mononuclear molecules can afford an efficient catalytic conversion compared to the corresponding dinuclear boronate esters in transfer hydrogenation catalytic studies. Among the catalysts, (B4) and (B4N) are the most active catalysts and catalytic activity of (B4) was generally much higher in the studied hydrogen transfer reactions than that of (B4N) under mild conditions in isoPrOH. This clearly suggests that catechol boronate esters are also one of the propitious catalyst candidates and are worthy of further investigations.