Lithium and magnesium complexes by using pyridyl-pendanted unsymmetrical β-diketiminates: syntheses and application as catalysts for the hydroboration of carbonyl compounds.

Abstract

The push for environmentally benign and sustainable chemical processes has reinforced the demand to displace transition metals with cheap, nontoxic and naturally abundant metals. To fulfil this requirement, we endeavored to synthesize alkali- and alkaline earth-metal complexes and employ them as catalysts for organic transformations. Two lithium and one magnesium complexes, which are supported by pyridyl-pendanted unsymmetrical β-diketiminates, have been synthesized and characterized. They are formulated as [Li(L1)]2 (1), [Li(L2)]2 (2) and [Mg(L2)2]·0.5C6H14 (3) (HL1 = N{-4-[(2,6-diisopropylphenylamino)pent-3-en-2-ylidene]-6-methyl}pyridin-2-amine and HL2 = N-{4-[(2,6-diisopropylphenyl)imino]pent-2-en-2-yl}pyridin-2-amine). Complex 1 features two inequivalent Li(I) sites. One Li(I) center exhibits tetrahedral geometry, while the other Li(I) ion adopts a triangular coordination sphere. The catalytic activities of 1-3 toward the hydroboration of aldehydes and ketones by pinacolborane (HBpin) were investigated. Both dinuclear lithium complexes and homoleptic magnesium compound were found to be highly efficient catalysts for the hydroboration of aldehydes and ketones, affording the corresponding borate esters in good yields within a very short time. Synthetic application of the complexes was demonstrated by the gram-scale reduction of ketones via a one-pot two step reaction with only 0.2 mol% loading of 2. Mechanistic details for the hydroboration were revealed by DFT calculations.