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Reports: ND152167-ND1: Chemical and Conformationally Driven Switches in Covalent Gas Separation Materials

Brandon L. Ashfeld, PhD, University of Notre Dame

I.      A Formal Au/Ag-Catalyzed [3+2] Cycloaddition Toward Substituted Imidazoles.

         During the previous funding period, our efforts focused on the development of new methods in heterocycle construction that would allow a high degree of flexibility in the substitution pattern around the ring, and do so in a synthetically efficient fashion. This funding period saw a continuation of these efforts that has led to a better understanding of the benefits and limitations of our methods, and yielded significant insight into the mechanism of heterocycle formation. A major challenge in identifying the optimal material for selective and reversible carbon capture is designing a scalable synthetic approach that allows access to multiple structural motifs with the modular flexibility necessary for point modifications. Our preliminary studies led us to focus on imidazolium-based materials and the need for a universal, flexible, and scalable synthetic strategy for their assembly. The imidazole ring constitutes one of the most versatile N-heteroaromatics in organic synthesis, is a well-established precursors to stable N-heterocyclic carbenes (NHCs), and are exceptionally useful in the construction of ionic liquids. Our objective has been to develop a flexible approach toward imidazole construction that would enable the maximum amount of structural diversity at C4 and C5. To that end, we have successfully designed and implemented a Au/Ag-catalyzed [3+2] cycloaddition approach involving an amidine and propargyl bromide. A continuation of our earlier efforts have resulted in a streamlined method involving the alkylation of an amidine with a propargyl halide followed by a Au(I)/Ag(I)-catalyzed 5-endo-dig cyclization to directly assemble imidazole (eq 1). This study constitutes a new approach toward imidazole construction. Most notably, the reaction tolerates formamidines as substrates to directly construct C2-unsubstituted imidazoles, which are N-heterocyclic carbene precursors. Recent work has focused on studies aimed at elucidating a possible mechanism for imidazole formation. A series of deuterium labeling and competition experiments have led to the formulation of the catalytic cycle depicted in Scheme 1 below. A manuscript detailing these efforts is in the advanced stages of preparation, and will be submitted in the near future.

II.    Photo-Induced Conformational Switches for Low Energy Carbon Separation.

         A second area of emphasis during this funding period was the continuation of our previous work toward the design and synthesis of properly functionalized heterocyclic frameworks that undergo a conformational change upon exposure to light. The installation of a conformationally labile functionality on a heterocyclic scaffold would minimize the energetic penalty for capture and release of CO2. An imidazolide IL bearing a 2,5-dimethylthienyl ethene group will undergo a UV light initiated 6¹ electrocyclization following a covalent bonding event to CO2. This conformational change will weaken the bond the C–C bond, and thereby decrease the energy required for carbon desorption. The bis(thienyl)imidazole was synthesized in two steps from commercially available 2,5-dimethylthiophene, and converted into the corresponding IL. Exposure to 1 atm of CO2 yielded the the carboxylated adduct in near quantitative yield. Unfortunately, efforts to evaluate carbon capture and release were complicated by the rapid electro

cyclic ring opening to the starting open form of the IL under extremely facile conditions. However, after significant effort we were able to determine that the carbon-complexed imidazolide IL would undergo cyclization when exposed to UV light (Figure 1). Current efforts are aimed at providing an accurate structural characterization of the carbon-complexed and uncomplexed versions of both the open and closed forms of the IL. We anticipate that these studies will be completed before the calendar year and a manuscript will be submitted for publication shortly thereafter.

         A second material design that has been the focus of study, as an alternative to the bis(thienyl)imidazole systems, are imidazolide ILs that bear a photo labile aryl diazene functionality. Presumably the cis/trans forms of these diazenes would differ significantly in conjugation with the aromatic rings, thereby leading to changes in the exothermicity with which each form would react with CO2. The construction of our first generation designs is depicted below in Scheme 2. With ILs in hand, we conducted initial CO2 uptake experiments at 1 atm and were pleased to find that both trans-ILs reacted in high yields to provide the corresponding ammonium carboxylates (eq. 2). Subsequent exposure of either IL to light at 350-355 nm initiated an isomerization of the diazene functionality to the corresponding cis-isomers. Current efforts are focused on determining the optimal conditions for the thermally induced reversion back to the starting trans-diazenes and performing an isothermal evaluation of all four adducts.

         A final area of study enacted during the current funding period was the utility of these diazenes in the development of a new umpolung synthetic approach toward the construction of N-acyl hydrazones. Hydrazones are organic, nitrogen-containing compounds that possess similar properties to ketones and aldehydes, and are intermediates for the formation of heterocyclic compounds. These efforts have led to the recent discovery of a method for hydrazone synthesis by the treatment of a diazene with a 1,2-dicarbonyl compound in the presence of phosphine. In a relatively short amount of time, we have been able to show that the phosphorus-mediated addition to 1,2-dicarbonyls is an effective and efficient strategy for the formation of hydrazones. A manuscript on this work is currently in preparation and will be submitted in the near future.

         In summary, the work performed over the previous funding period has had a significant impact on the training of students in materials design and synthesis, and in the establishment of multiple new areas of research for our program. Students have obtained additional skills associated with small molecule synthesis and their subsequent evaluation as new carbon capture agents. These skill sets will likely improve their marketability of in an ever increasingly competitive field. At least three publications will have resulted from these efforts, which will undoubtably help to provide opportunities for these students in the future.