Collins, Sean Christopher (2010-08). Mechanistic Investigation into the Sommelet-Hauser Rearrangement of an Allyl Ammonium Ylide Through Determination of 13C KIEs. Master's Thesis.
Thesis
The [2,3]-sigmatropic rearrangement is a pericyclic reaction of great synthetic utility to organic chemists. Within the scope of this reaction exist some cases in which the product corresponding to a [1,2] rearrangement is formed, despite the fact this is a forbidden process. Generally this is explained by a radical dissociation-recombination pathway; however, studies into the failure of transition state theory and the necessity to incorporate dynamic effects into mechanistic theory lead us to believe such products may arise from these phenomena. In particular, the possibility that many of these products result from an "unsymmetrical bifurcating surface" in the potential energy landscape is intriguing. To investigate this possibility, the Sommelet-Hauser rearrangement of N-allyl-N,N-dimethylglycine methyl ester was explored. The combined use of experimental and theoretically predicted kinetic isotope effects (KIEs) has been previously shown to deliver great mechanistic insight into reactions. The combination of these techniques, however, has found little employ in studying [2,3] rearrangements. This combination was used to study this reaction, using the Singleton method for determining small heavy-atom isotope effects. Resulting experimental KIEs suggest the reaction proceeds by an asynchronous, concerted, early transition state, and is relatively exothermic. This agrees with previous studies and Hammond's postulate. Predicted theoretical KIEs are in good agreement with experimental KIEs, and the associated transition structure confirms the results suggested by experiment. Interestingly, as calculations proceed from gas phase to solvent models, the activation barrier of the reaction increases, while its exothermicity decreases. The energy difference determined between the lowest and second lowest energy transition structures decreases to 0.81 kcal/mol in the PCM model, so we cannot exclude the contribution of this transition structure to the reaction. However, qualitative results from the associated KIEs and energetics are consistent with the lowest energy transition structure. This reaction does not seem to afford the [1,2] product, and most likely dynamic effects are insignificant in determining product distribution. However, the study has validated, with respect to this body of reactions, both the use of the Singleton method for KIE determination and the combination of these experimental and theoretical techniques.
The [2,3]-sigmatropic rearrangement is a pericyclic reaction of great synthetic
utility to organic chemists. Within the scope of this reaction exist some cases in which
the product corresponding to a [1,2] rearrangement is formed, despite the fact this is a
forbidden process. Generally this is explained by a radical dissociation-recombination
pathway; however, studies into the failure of transition state theory and the necessity to
incorporate dynamic effects into mechanistic theory lead us to believe such products
may arise from these phenomena. In particular, the possibility that many of these
products result from an "unsymmetrical bifurcating surface" in the potential energy
landscape is intriguing. To investigate this possibility, the Sommelet-Hauser
rearrangement of N-allyl-N,N-dimethylglycine methyl ester was explored. The combined
use of experimental and theoretically predicted kinetic isotope effects (KIEs) has been
previously shown to deliver great mechanistic insight into reactions. The combination of
these techniques, however, has found little employ in studying [2,3] rearrangements.
This combination was used to study this reaction, using the Singleton method for
determining small heavy-atom isotope effects.
Resulting experimental KIEs suggest the reaction proceeds by an asynchronous,
concerted, early transition state, and is relatively exothermic. This agrees with previous
studies and Hammond's postulate. Predicted theoretical KIEs are in good agreement
with experimental KIEs, and the associated transition structure confirms the results
suggested by experiment. Interestingly, as calculations proceed from gas phase to
solvent models, the activation barrier of the reaction increases, while its exothermicity
decreases. The energy difference determined between the lowest and second lowest
energy transition structures decreases to 0.81 kcal/mol in the PCM model, so we cannot
exclude the contribution of this transition structure to the reaction. However, qualitative
results from the associated KIEs and energetics are consistent with the lowest energy
transition structure. This reaction does not seem to afford the [1,2] product, and most
likely dynamic effects are insignificant in determining product distribution. However,
the study has validated, with respect to this body of reactions, both the use of the
Singleton method for KIE determination and the combination of these experimental and
