Mechanism of enantioselective Ti-catalyzed strecker reaction: Peptide-based metal complexes as bifunctional catalysts

Kinetic. structural, and stereochemical data regarding the mechanism of Ti-catalyzed addition of cyanide to imines in the presence of Schiff base peptide ligands are disclosed. The reaction is first order in the Ti-ligand complex; kinetic studies reveal DeltaS(double dagger) = -45.6 +/- 4.1 cal K-1 mol(-1), indicating a highly organized transition structure for the turnover-limiting step of the catalytic cycle. A mechanistic model consistent with the kinetic and stereochemical data is presented, where the Ti center is coordinated to the Schiff base unit of the ligand and the AA2 moiety of the peptidic segment of the chiral ligand associates and delivers HNC to the activated bound substrate. Thus, these studies illustrate that these non-C-2-symmetric catalysts likely operate in a bifunctional fashion.