posted on 2012-02-15, 00:00authored byTongxiang Lu, Rongxiu Zhu, Yi An, Steven E. Wheeler
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.