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Asymmetric Hydroamination of Aminoallenes Catalyzed by Metal
Complexes with Electron-Withdrawing Sulfonamide Ligands
Johnson Laboratory - Harvey Mudd College
Hydroamination, the addition of an
N-H bond across a C-C double
bond, can be catalyzed by metal
complexes with a variety of ligands.
Electron-withdrawing ligands have
been shown to increase reactivity1, and
were predicted to improve
stereoselectivity. This project focused
specifically on sulfonamide ligands,
which have electron-withdrawing
properties. The sulfonyl group is highly
electronegative and withdraws
electrons from the nitrogen, which then
becomes a poor electron donor to the
metal center.2
Hydroamination was performed using
Ti(NMe2)4 and Ta(NMe2)5 at 125 °C
and 135 °C using each of the six
sulfonamide ligands:
It was predicted that at lower
temperatures, the stereoselectivity
would improve. If the two
enantiomers have the same energy
but their transition states have
different energies, the reaction
favors the product with the lower
energy transition state. At higher
temperatures, energy is sufficient to
achieve either transition state, so
selectivity is decreased. It was also
predicted that the most electronwithdrawing sulfonamides would be
the most reactive and that these
reactions would be the most
stereoselective.
The expected trends were not
observed for the Titanium
complexes.
Reactions using Tantalum
complexes were more
stereoselective than those with
Titanium complexes.
Stereoselectivity improved at lower
temperatures, but did not increase
for more electron-withdrawing
ligands.
(1) Schafer, L. L., Bexrud, J. A., Li, C., Organometallics 2007, 26,
6366.
(2) Walsh, P. J., Pritchett, S., Woodmansee, D. H., Gantzel, P., J.
Am. Chem. Soc. 1998, 120, 6423.