Synthesis, characterisation and reactivity studies of chiral β-diketiminate-like supported aluminium Lewis acid complexes towards difficult Diels Alder cycloadditions

Synthesis and characterisation of several chiral, oxazoline containing beta-diketiminate type ligand supported-aluminium compounds are reported. Together with 1 equiv. of Na(BAr4Cl) (Ar-Cl = 3,5-Cl-2-C6H3), these chiral Lewis acid complexes, which possess an achiral end and chiral end have been successfully utilised as catalysts in asymmetric Diels-Alder reactions of 1,3-cyclohexadiene and several different chalcones. Systematic increase in the steric demand of the ligand's achiral end of these complexes resulted in enhanced enantioinduction for the cyclisation of 1,3-cyclohexadiene and chalcone. Further structural modifications of the chiral end clearly established that having a tert-butyl group connected to the stereogenic centre of the oxazoline fragment yielded the highest enantioselectivity value for the examined cyclisation. A substrate scope was then expanded using several different dienophiles (i.e. chalcones) generating an enantiomeric excess range of 24-68%.

Automated summary

This research reports the synthesis and characterisation of several chiral aluminium compounds with oxazoline-containing beta-diketiminate type ligands. These chiral Lewis acid complexes, in combination with 1 equiv. of Na(BAr4Cl), were successfully used as catalysts in asymmetric Diels-Alder reactions between 1,3-cyclohexadiene and various chalcones. Increasing the steric demand of the ligands' achiral end resulted in improved enantioinduction for the cyclisation of 1,3-cyclohexadiene and chalcone. Furthermore, modifying the chiral end by introducing a tert-butyl group to the stereogenic center of the oxazoline fragment yielded the highest enantioselectivity value. The substrate scope was expanded to include several different dienophiles, generating enantiomeric excess values ranging from 24-68%.