UVM Theses and Dissertations
Format:
Print
Author:
Bowser, Amy M.
Dept./Program:
Chemistry
Degree:
PhD
Abstract:
This work imparts a novel organic transformation that provides entry into highly hnctionalized moieties through a 1,3-Diaza-Claisen rearrangement. A selection of N-Alkyl-N'-tosyl thioureas were activated by EDCI and the resulting carbodiimides were allowed to react with a representative assembly of azanorbornenes at room temperature. Fused, bicyclic guanidines resulted from this transformation, in moderate to good yields. It was believed that the products were obtained through a 3,3-sigmatropic rearrangement and crossover experiments ruled out the possibility of a retro Diels-Alder mechanistic pathway. Design and development of the method was then assumed. Due to the fact that isocyanates and isothiocyanates are isoelectronic with carbodiimides, it was conceivable that they might react similarly with azanorbornenes, affording a zwitterionic intermediate that would undergo a 1,3-Diaza-Claisen rearrangement, affording the corresponding ureas and thioureas. It was found that iso(thio)cyanates not activated by an electron withdrawing group reacted with azanorbornenes, at reflux in benzene, to give the expected fused, bicyclic ureas and thioureas. At higher a temperature, triazinone was observed, indicating the possibility of an alternate mechanistic pathway. Iso(thio)cyanates that were activated by an electron withdrawing group were found to undergo the same 1,3-Diaza-Claisen rearrangement, at room temperature in chloroform, affording their corresponding fused, bicyclic ureas and thioureas. Again, crossover experiments ruled out an alternate mechanistic pathway. Competition experiments were also organized to study the reactivities of the iso(thio)cyanates and gain insight regarding the rate determining step.
During the course of this work, it was noted that the possibility for an ionic, rather than a sigmatropic, mechanism existed. Thus, a challenge to the proposed pathway was made. It was necessary to examine this finding experimentally and probe the scope of the reaction as a whole. To this end, it was necessary to employ a reactive species other than azanorbornenes. A review of the rearrangement using aziridines, vinyl pyrrolidines, and azabicyclooctenes rather than azanorbornenes was undertaken. Though rearrangement was observed in the absence of a catalyst, H¹-NMR studies did show an increase in the percent conversion when a palladium catalyst was used. However, the overall yields were comparable.
During the course of this work, it was noted that the possibility for an ionic, rather than a sigmatropic, mechanism existed. Thus, a challenge to the proposed pathway was made. It was necessary to examine this finding experimentally and probe the scope of the reaction as a whole. To this end, it was necessary to employ a reactive species other than azanorbornenes. A review of the rearrangement using aziridines, vinyl pyrrolidines, and azabicyclooctenes rather than azanorbornenes was undertaken. Though rearrangement was observed in the absence of a catalyst, H¹-NMR studies did show an increase in the percent conversion when a palladium catalyst was used. However, the overall yields were comparable.