The mechanism of three important aldol reactions and a biomimetic
transamination is investigated using a combination of experimental kinetic isotope
effects (KIEs), standard theoretical calculations and dynamics trajectory
simulations. This powerful mechanistic probe is found to be invaluable in
understanding intricate details of the mechanism of these reactions. The successful
application of variational transition state theory including multidimensional
tunneling to theoretically predict isotope effects, described in this dissertation,
represents a significant advance in our research methodology.
The role of dynamic effects in aldol reactions is examined in great detail. The
study of the proline catalyzed aldol reaction has revealed an intriguing new dynamic
effect - quasiclassical corner cutting - where reactive trajectories cut the corner
between reactant and product valleys and avoid the saddle point. This phenomenon
affects the KIEs observed in this reaction in a way that is not predictable by
transition state theory. The study of the Roush allylboration of aldehydes presents an
example where recrossing affects experimental observations. The comparative study
of the allylboration of two electronically different aldehydes, which are predicted to have different amounts of recrossing, suggests a complex interplay of tunneling and
recrossing affecting the observed KIEs.
The Mukaiyama aldol reaction has been investigated and the results
unequivocally rule out the key carbon-carbon bond forming step as rate-limiting.
This raises several interesting mechanistic scenarios - an electron transfer
mechanism with two different rate-limiting steps for the two components, emerges
as the most probable possibility. Finally, labeling studies of the base catalyzed 1,3-
proton transfer reaction of fluorinated imines point to a stepwise process involving
an azomethine ylide intermediate. It is found that dynamic effects play a role in
determining the product ratio in this reaction.
| Identifer | oai:union.ndltd.org:tamu.edu/oai:repository.tamu.edu:1969.1/ETD-TAMU-2009-12-7239 |
| Date | 2009 December 1900 |
| Creators | Vetticatt, Mathew J. |
| Contributors | Singleton, Daniel A. |
| Source Sets | Texas A and M University |
| Language | en_US |
| Detected Language | English |
| Type | Book, Thesis, Electronic Dissertation, text |
| Format | application/pdf |
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