RUI: Aromaticity, Tautomerization and Metalation of Carbaporphyrinoid Systems

Board Of Trustees Of Illinois State University, Normal IL

Investigators

Abstract

With this award, the Chemical Synthesis program of the NSF Division of Chemistry is supporting the research of Professor Timothy Lash at Illinois State University to explore the synthesis, characterization, and reactivity of compounds that resemble a class of molecules called "porphyrins". Porphyrins possess many valuable properties and have applications in material science, catalysis and medicine. They also have an impressive array of functions in nature that range from oxygen transportation to photosynthesis. Porphyrins contain 4 nitrogen atoms, and related systems, called carbaporphyrins, have a carbon atom in place of one of the nitrogens. These molecules often have properties that complement those of naturally occurring porphyrins. The supported work is leading to efficient routes to novel carbaporphyrins and related systems that have unique reactivity and spectroscopic properties. Access to new classes of porphyrin analogues are improving our understanding of fundamental chemistry concepts such as aromaticity, a fundamental concept in chemistry which can lead to the enhanced stability of molecules. Furthermore, these types of structures commonly form stable complexes with palladium, rhodium and other important metal catalysts. Professor Lash and his students are studying applications of these molecules in the preparation of fine chemicals, as these derivatives modify the catalytic activity of bound metals. In addition, related compounds are being explored for use as photosensitizers in photodynamic therapy. These projects are not only providing an excellent environment for training undergraduates and Master's level graduate students in the field of organic synthesis, they are also exposing these students to the multidisciplinary nature of modern scientific research. In this research, synthetic routes to new porphyrin-like systems, including phenaliporphyrins, quiniporphyrins and carbachlorins, are being developed. Further application of the "3 + 1" variant of the MacDonald reaction is providing structures that incorporate phenalene, quinoline and numerous related heterocyclic units. The competition between aromatic hydroxyporphyrinoids and nonaromatic keto-tautomers can be particularly insightful, and these processes are being probed using spectroscopic techniques. True chlorins are widely studied, in part because they show the potential for applications as photosensitizers. Carbachlorins, which contain a carbocyclic ring in place of one of the pyrrole units, are generally more stable than the parent tetrapyrrolic structures and may have equally useful properties. However, carbachlorins have been little studied to date, and the development of new routes to these dihydroporphyrinoids will be of significant value. The introduction of fused aromatic rings onto carbaporphyrin structures offers the opportunity to further modify the properties of these compounds, and a new methodology to prepare carbaporphyrins with fused naphthalene, anthracene and related structural units is being developed. This approach is also being applied to be synthesis of larger dimeric systems that are expected to exhibit strong electronic interactions upon protonation. Hence, these investigations are revealing important new porphyrinoid systems and will have a major impact in the field of Porphyrin Chemistry. This award reflects NSF's statutory mission and has been deemed worthy of support through evaluation using the Foundation's intellectual merit and broader impacts review criteria.