Enantioselective Aziridination Using Copper Complexes of Biaryl Schiff Bases

Racemic 2,2‘-diamino-6,6‘-dimethylbiphenyl is resolved using simulated moving bed chromatography, and the absolute configuration of the enantiomers is confirmed via the X-ray crystal structure
of a derivative. The diamine is condensed with a range of aldehydes to give bidentate aldimine
proligands L. Molecular structures of the complexes formed between L and Cu(I) fall into two classes;
bimetallic double helices ([Cu2L2]2+) and monometallic ([CuL]+). The latter are strikingly more
efficient in the aziridination of alkenes than are the former in terms of rate, turnover, and
enantioselection. In particular, the imine ligand formed from the diamine and 2,6-dichlorobenzaldehyde gives, in combination with Cu(I) or Cu(II), up to 99% ee in the aziridination of 6-acyl-2,2-dimethylchromene and 88−98% ee for a range of cinnamate esters. Styrenic and other alkenes are
converted with lower selectivities (5−54%). The catalytic system shows a linear response in product
ee to catalyst ee, and the product ee does not vary significantly during the reaction. UV
spectrophotometric investigations indicate that conversion of Cu(I) to Cu(II) is not essential for
catalysis but that Cu(II) is probably also a competent system.