Alkaline Earth Metal Compounds of Methylpyridinato β-Diketiminate Ligands and Their Catalytic Application in Hydroboration of Aldehydes and Ketones

Abstract

Ever increasing demand for green and sustainable chemical processes has set up a drive to replace transition metals with earth-abundant, nontoxic, and environmentally benign alternatives. In this regard, the alkaline earth metal complexes have attracted significant attention. Herein, we have used a β-diketiminato ligand with methyl-pyridine side arm to synthesize magnesium (1) and calcium (2) compounds. The constitutions of 1 and 2 have been confirmed by single crystal X-ray studies, which show that the magnesium and calcium atom in 1 and 2 possesses octahedral geometry. Subsequently, we have used them as catalysts (1 mol %) for hydroboration of a wide range of aldehydes using pinacolborane (HBpin) at room temperature. The strategy has further been extended to ketones with 2 mol % catalyst loading. DFT calculations have been performed to understand the mechanism.

Enantiopure Calcium Iminophosphonamide Complexes: Synthesis, Photoluminescence, and Catalysis

“…Such catalytic efficiency of 1 – 3 and fast conversion of the substrates may be related to their coordinatively unsaturated nature. Perhaps due to this unsaturation in coordination sites the conversion requires less time in comparison to previously reported homoleptic alkaline earth metal catalysts [12d] . Since the catalysts 1 – 3 are enantiopure, one might expect enantiomeric excess in the product.…”

“…The hydroboration catalysts based on alkaline earth metals reported so far are mostly heteroleptic in nature [12b,e,f, 17] . The homoleptic catalysts for this chemical transformation are extremely rare possibly due to saturation in coordination sites [12d] . With this background, one of the goals of our study was to probe a performance of tetra coordinated complexes 1 – 3 as catalysts for hydroboration of ketones.…”

“…The known catalysts based on alkaline‐earth‐metal complexes are mostly heteroleptic in nature [12b,e,f, 17] . However, catalysts based on homoleptic alkaline‐earth‐metal complexes are limited possibly due to saturation in the coordination sites [10b, 12d, 14b] …”

Homoleptic Zinc‐Catalyzed Hydroboration of Aldehydes and Ketones in the Presence of HBpin

“…In contrast to the traditional mechanism where the metal hydride acts as an active catalyst, very recently, various research groups have reported the use of a Lewis acid‐based mechanism for the hydroboration of carbonyl compounds ,. [18c], …”

“…There are reports that a limited catalyst was used for the chemoselective reduction of aldehydes over other active functional groups such as ‐NH 2 , ‐OH, esters, amides, and nitriles , ,. [33a], …”