Minimal Theoretical Description of Magnesium Halogen Exchanges

Although Grignard
exchange reactions have been known for about
a century, little beyond heuristics guides the prediction of when
and where an exchange will occur. Additionally, little is understood
from a modeling perspective about why the heuristics work or when
they might fail. We develop a simple quantum mechanical model for
predicting when and where an exchange will occur. We show that the
solvent-free model successfully predicts for reactions regioselectively
using only thermodynamic end points. After validating the model, we
employ it to explain some of the heuristic understandings, such as
preference for exchanging with halogens lower on the periodic table
and facilitation of the exchange with lithium chloride. Preference
for bromine over chlorine appears to be related to both kinetic barriers
and thermodynamic considerations, and addition of lithium chloride
to the exchange mixture appears to act almost exclusively in a catalytic
role. We then use the model to predict materials that should allow
ready exchange onto any carbon, including tertiary halide carbons.
We find that removable silicon-linked directing ethers attached to
the molecule of interest and, potentially more versatilely, silyl
Grignard reagents could serve in this role.