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 uri icon

abstract

  • 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

    theoretical techniques.

publication date

  • August 2010