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44077-G1
Exploring the Chemical Potential of 2-Iminoimidazolines in Small Molecule and Macromolecular Synthesis
The overarching goal of this research project to develop new and efficient synthetic routes to N-heterocycles and other structurally-advanced compounds rich in nitrogen. Recently, we discovered that the reaction between imidazolylidenes, benzimidazolylidenes, and other N-heterocyclic carbenes (NHCs) with alkyl, aryl, or acyl azides affords 1,3-disubstituted triazenes in excellent yields. Considering the unusual structure of these compounds and the efficient method of preparation, initial efforts were focused on exploring the potential of his novel class of imidazole-derived substrates in functional macromolecular materials and biological applications.
To evalute the electronic properties of these materials, a systematic series of 1,3-disubstituted-triazenes were synthesized by coupling functionalized benzimidazol-2-ylidenes, as their free NHCs or generated in situ from their respective benzimidazolium precursors, to various aryl azides in modest to excellent isolated yields (36 – 99%). Electron delocalization between the two coupled components was studied using UV-Vis spectroscopy, NMR spectroscopy, and X-ray crystallography. Depending on the complementarity of the functional groups on the NHCs and the organic azides, the respective triazenes were found to exhibit lamba max values ranging between 364 and 450 nm. Additional support for efficient electronic communication across the triazeno linkage was obtained using X-ray crystallography, which revealed bond alteration patterns in a series of triazenes characteristic of donor-acceptor compounds.
The thermal stabilities of these triazenes were studied using thermogravimetric analysis and found to be strongly dependent on the sterics of the benzimidazol-2-ylidene component as well as the electronics of the azide component. For example, triazenes possessing bulky N-substituents (e.g., neo-pentyl, tert-butyl, etc.) were stable in the solid-state to temperatures exceeding 150 degrees C, whereas analogues with small N-substituents (e.g., methyl) were found to slowly decompose at room temperature. Triazenes featuring electron-rich phenyl azide components decomposed at higher temperatures than their electron-deficient analogues. The products of the thermally-induced triazene decomposition reaction were found to be molecular nitrogen and the respective guanidine. To determine the mechanism of decomposition, an isotopically-labeled triazene was prepared and exposed to elevated temperatures. The products of this decomposition reaction were consistent with the Staudinger reaction.
Founded on these preliminary results, attention shifted toward exploring the potential of this NHC/azide coupling reaction in accessing unique macromolecular materials. In particular, to demonstrate the potential of this reaction as a convenient method of installing new or altering existing functional groups on polymeric materials, poly(p-azidomethylstyrene) was first prepared using standard methods. Subsequent addition of a commercially-available NHC afforded a polymer with pendant triazeno moieties, as determined by NMR spectroscopy, IR spectroscopy, and size exclusion chromatography. The carbene-azide coupling reaction was found to be high-yielding and the extent of functionalization linearly correlated with the amount of free NHC added. The versatility of this reaction as a method for post-polymerization modification was further elaborated by combination with Cu(I) catalyzed “click” chemistry. Thus, after poly(p-azidomethylstyrene) was partially modified with phenylacetylene, residual azide moieties were reacted with free NHCs. This reaction sequence effectively enabled the synthesis of a random copolymer bearing triazenes and triazoles. Thermogravimetric analysis revealed that the NHC-modified poly(p-azidomethylstyrene)s cleanly extruded molecular nitrogen at 135 degrees C. Thus, thermal treatment proved to be an effective method for transforming polymers with pendant triazenes to their respective guanidines.
As noted above, 1,3-disubstituted triazenes obtained from coupling NHCs to azides cleanly decompose to their key respective guanidines at elevated temperatures. The 2-imino groups in these guanidines are significantly polarized and therefore may be exploited to facilitate nitrogen transfer to a wide range of substrates. As such, future efforts will be directed toward exploring the reactivity of 2-iminoimidazolines, including chiral variants, with epoxides to generate a new class of aziridines. Recent calculations have suggested that 2-iminoimidazolines may dissociate into nitrenes and stable carbenes at elevate temperatures. Thus, efforts toward finding a 2-iminoimidazoline with the right combination of electronic and steric parameters to facilitate this transformation. The potential impact of 2-iminoimidazolines on organometallic chemistry and metal-mediated catalysis will also be explored through the synthesis and study of chelating 2-iminoimidazoline based ligands for azophilic transition metals.
This research project has positively impacted the careers of involved researchers (three students and one PI) in three unique ways: (1) Thus far, efforts related to this project have resulted in seven publications. (2) The students involved have learned new synthetic skills and characterization techniques in organic, organometallic, and polymer chemistry. (3) Data generated during this project have been used as preliminary results for successful proposals sent to the NSF, the DoD, and a number of private funding organizations.
