PyBidine-Cu(OTf)2 -catalyzed asymmetric [3+2] cycloaddition with imino esters: harmony of Cu-Lewis acid and imidazolidine-NH hydrogen bonding in concerto catalysis.

A bis(imidazolidine)pyridine (PyBidine)-Cu(OTf)2 complex catalyzing the endo-selective [3+2] cycloaddition of nitroalkenes with imino esters was applied to the reaction of methyleneindolinones with imino esters to afford spiro[pyrrolidin-3,3'-oxindole]s in up to 98 % ee. X-ray crystallographic analysis of the PyBidine-Cu(OTf)2 complex and DFT calculations suggested that an intermediate Cu enolate of the imino ester reacts with nitroalkenes or methyleneindolinones, which are activated by NH-hydrogen bonding with the PyBidine-Cu(OTf)2 catalyst.