Origin of Enantioselectivity in the Propargylation of Aromatic Aldehydes Catalyzed by Helical N-oxides

The enantioselective propargylation of aromatic aldehydes with allenyltrichlorosilanes catalyzed by bipyridine N-oxides was explored using density functional theory. Low-lying transition states for a highly enantioselective helical bipyridine N-oxide catalyst [Org. Lett. 2011, 13, 1654] were characterized at the B97-D/TZV(2d,2p) level of theory. Predicted free energy barrier height differences are in agreement with experimental ee's for the propargylation of benzaldehyde and substituted analogues. The origin of enantioselectivity was pinpointed through distortion-interaction analyses. The stereoselectivity arises in part from through-space electrostatic interactions of the carbonyl carbon with the Cl ligands bound to Si, rather than noncovalent aryl-aryl interactions between the aromatic aldehyde and the helix as previously proposed. Moreover, aryl-aryl interactions between the aldehyde and helix are predicted to favor transition states leading to the R enantiomer, and ultimately reduce the enantioselectivity of this reaction. (S)-2,2'-bipyridine N-oxide was studied as a model catalyst in order to quantify the inherent enantioselectivity arising from different chiral arrangements of ligands around the hexacoordinate silicon in the stereocontrolling transition state for these reactions. The predicted selectivities arising from different chiral octahedral silicon complexes provide guidelines for the development of transition state models for N-oxide-based alkylation catalysts.