Hydroboration and reductive amination of ketones and aldehydes with HBpin by a bench stable Pd(ii)-catalyst

Abstract

A palladium(II) complex [(κ⁴-{1,2-C₆H₄(NCH–C₆H₄O)₂}Pd] (1) supported by a dianionic salen ligand [1,2-C₆H₄(NCH–C₆H₄O)₂]²⁻ (L) was synthesised and used as a molecular pre-catalyst in the hydroboration of aldehydes and ketones. The molecular structure of Pd(II) complex 1 was established by single-crystal X-ray diffraction analysis. Complex 1 was tested as a competent pre-catalyst in the hydroboration of aldehydes and ketones with pinacolborane (HBpin) to produce corresponding boronate esters in excellent yields at ambient temperature under solvent-free conditions. Further, the complex 1 proved to be a competent catalyst in the reductive amination of aldehydes with HBpin and primary amines under mild and solvent-free conditions to afford a high yield (up to 97%) of corresponding secondary amines. Both protocols provided high conversion, superior selectivity and broad substrate scope, from electron-withdrawing to electron-donating and heterocyclic substitutions. A computational study based on density functional theory (DFT) revealed a reaction mechanism for Pd-catalysed hydroboration of carbonyl species in the presence of HBpin. The protocols also uncovered the dual role of HBpin in achieving the hydroboration reaction.