modes of activation and origin of stereoselectivity
Xue, Hansong; Tan, Choon-Hong and Wong, Ming Wah
Canadian Journal of Chemistry 2016, 94, 1099-1108
Abstract: D. functional theory calculations were employed to study the catalytic mechanism, modes of activation, and origin of enantioselectivity of guanidine-catalyzed asym. Strecker reaction of N-benzhydryl imine with hydrogen cyanide. Two types of bifunctional activation mode were identified, conventional bifunctional Broensted acid activation and unconventional bifunctional Broensted-Lewis acid activation. The lowest-energy transition states correspond to the conventional bifunctional mode of activation. The calculated enantiomeric excess, based on eight lowest-energy C-C bond forming transition states, is in good accord with observed enantioselectivity. NCI (noncovalent interaction) anal. of the key transition states reveals extensive noncovalent interactions, including aromatic interactions and hydrogen bonds, between the guanidinium catalyst and substrates. Multiple aryl-aryl interactions between the Ph groups of guanidine catalyst and the Ph rings of N-benzhydryl imine are the key stabilizations in the most stable (R)-inducing transition state. Differential attractive aryl-aryl stabilization is the major factor for stereoinduction.
Paper download: https://www.nrcresearchpress.com/doi/pdf/10.1139/cjc-2016-0307
