Lewis Base Activation of Lewis Acids
Although the concepts of Lewis acid-base interactions have
been developed and understood for nearly 80 years, the implications
for catalysis of organic reactions have still not been fully
realized. In recent years we have outlined the conceptual framework
for the counter-intuitive ability of electron-pair donors (Lewis
bases) to enhance the electrophilic character of the electron-pair
acceptors (Lewis acids) along with the unique opportunities
for enantioselective reactions promoted by the catalytically
active species thus generated. The main themes of this program
are the conceptual development, structural basis, and preparative
application of chiral Lewis base catalysis in the main group
of the Periodic Table.
This project encompasses three major components of research:
methodology, mechanism, and total synthesis. Methodologically,
we are investigating the scope of reactions that can be catalyzed
by a combination of weak Lewis acids (such as silicon tetrachloride)
and a chiral Lewis base donor (such as phosphoramides or N-oxides).
Good success has been recorded for directed aldol reactions
of enoxysilane derivatives of esters, ketones, chiral ketones,
aldehydes and unsaturated esters, amides and ketones in combination
with aldehydes electrophiles. Also, the addition of allyltin
reagents and isocyanides (Passerini reaction) to aldehydes
has been successful. Total synthesis of selected natural products
is frequently undertaken as a vehicle to illustrate and test
the limits of a newly developed method.
We actively employ the tools of physical organic chemistry
to investigate the foundations of these reactions. We have
describe structural (X-ray, NMR) and mechanistic investigations
(kinetics, stereochemistry and computations) on the origin
of catalysis and stereoselection in a number of Lewis-base
catalyzed processes. In addition, we have constructed a unique
Rapid Injection NMR system to allow for in-situ monitoring
of fast reactions directly at any temperature, for any combination
of reactants, and observing any nucleus.
Read the most recent review:
245, Chapter 8
Catalytic
Enantioselective Aldol
Reactions with Trichlorosilyl
Enolates and Ketene Acetals
Ligand-Accelerated Reactions of Allylic Trichlorosilanes
with Chiral Lewis Bases
Chiral Lewis Base-Activation of Lewis Acids
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