Chiral Bicyclic NHC/Cu Complexes for Catalytic Asymmetric Borylation of α,β-Unsaturated Esters
The potential of using chiral bicyclic NHC ligands that exhibit modularity was investigated in the Cu-catalyzed asymmetric borylation reaction of α,β-unsaturated esters. After screening for ligands and optimization of the reaction conditions, the corresponding products were afforded with good enantioselectivities (up to 85% ee).
Efficient Access to Chiral β-Borylated Carboxylic Esters via Rh-Catalyzed Hydrogenation
Rh/bisphosphine?thiourea ligand (ZhaoPhos)-catalyzed asymmetric hydrogenation of (Z)-β-substituted-β-boryl-α,β-unsaturated esters was successfully developed, furnishing a variety of chiral β-borylated carboxylic esters with high yields and excellent enantioselectivities (up to 99% yield and >99% ee). The gram-scale asymmetric hydrogenation was performed efficiently in the presence of only 0.05 mol% (S/C=2 000) catalyst loading with full conversion, 99% yield and 99% ee. Moreover, the hydrogenation product was easily converted to other versatile synthetic intermediates, such as methyl (S)-3-hydroxy-3-phenylpropanoate and methyl (S)-3-(furan-2-yl)-3-phenylpropanoate. (Figure presented.).
Paracyclophane-based carbene-copper catalyst tuned by transannular electronic effects for asymmetric boration
A series of planar chiral carbene-copper complexes based on the [2.2]paracyclophane backbone with a pseudo-ortho substitution pattern have been synthesized and applied to asymmetric β-boration of α,β-unsaturated esters. As a result, transannular electronic effects of the substituent of the chiral catalyst have significant influence on the catalytic performance. A variety of chiral β-hydroxyl esters were obtained in excellent enantioselectivities (up to 97% ee) and yields (up to 99%).
Asymmetric β-boration of α,β-unsaturated carbonyl compounds with chiral Rh[bis(oxazolinyl)phenyl] catalysts
Chiral rhodium[bis(oxazolinyl)phenyl] complexes exhibited high catalytic activity for the β-boration of α,β-unsaturated esters, ketones, and amides with bis(pinacolato)diboron in the presence of sodium tert-butoxide to attain high enantioselectivity of up to 97%. The substrate scope and catalytic mechanism were discussed.
Asymmetric β-boration of α,β-unsaturated carbonyl compounds promoted by chiral rhodium-bisoxazolinylphenyl catalysts
Chiral rhodium-bisoxazolinylphenyl acetate complexes exhibited high catalytic activity for the β-boration of α,β-unsaturated carbonyl compounds with bis(pinacolato)diboron in the presence of sodium t-butoxide with enantioselectivity up to 97%.