Designed binaphthyl-derived titanium complexes: a new type of asymmetric catalyst for the carbonyl-ene reaction with glyoxylate

Abstract

A new type of titanium complex (N) derived from modified binaphthyl, 1,1′-bi-6-bromo-2-naphthol (6-Br-BINOL, 1) (98% e.e.) is found to be an extremely efficient asymmetric catalyst for the glyoxylate-ene reaction with α-methylstyrene to give the ene product with 97.5% e.e. in 94% yield. A certain level of positive non-linear effect (asymmetric amplification) is also observed in the present glyoxylate-ene reaction. The 6-Br-BINOL-derived titanium complex (98% e.e.) affords the glyoxylate-ene product of methylenecyclohexane with more than 99% e.e.! in 82% yield. Furthermore, 70% e.e. of 6-Br-BINOL leads eventually to 85% e.e. of the glyoxylate-ene product with α-methylstyrene. A higher level of asymmetric induction is observed in a less polar solvent, toluene, suggesting that dipolar repulsion is operative in controlling the conformation of the ligand 6-Br-BINOL ( 1). Unfortunately, however, the titanium complex N′ derived from the binaphthyl ligand 2 with the sterical demanding trifylamine moiety gave a disappointingly low level of asymmetric induction in the glyoxylate-ene reaction.