Improved catalysis of nitrone 1,3-dipolar cycloadditions by solving the aggregation issue of the DBFOX/Ph-transition metal complexes

A new method has been demonstrated to solve the aggregation issue of metal complex catalysts; Steric protection of the metal center of catalysts was effective in the cases of DBFOX/Ph-transition metal complexes by structural modification of chiral ligand. The new ligands have been successfully applied to the nitrone cycloadditions to a variety of α,β-unsaturated aldehydes. Excellent enantioselectivities up to 99% ee have been demonstrated in the reactions at room temperature with a catalytic loading of 2 mol %.