From supramolecular chemistry to fundamental organic chemistry: bis-rosette nanotubes and novel molecular frameworks

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2017
İğci, Cansu
Novel supramolecular structures are inspired by many functional biological systems present in nature. The design of supramolecular components is concerned with the conceptualization of an organized state of matter that possess the steric and functional information necessary to translate complex mixture into ordered structures. The organization of matter brought about through non-covalent forces, and the structures generated are almost impossible to put together from a covalent perspective. Hydrogen bonding as a non-covalent interaction mostly used as a design tool, and the structures expressed have helices, three-dimensional structures, sheet (two-dimensional) structures or cyclic rosettes. One of the first group of structures in supramolecular chemistry area is the cyclic hexameric aggregate based on the melamine-cyanuric acid cocrystal. Bis-rosette derivatives of these structure where assembly of 6 compounds with 36 hydrogen bonding could not be obtained due to steric hindrances and high energy eclipsed conformations of cyanuric acid and melamine units. The other important hydrogen bonding aggregate type molecules includes the use of GΛC heterocycles which forms well-ordered hexameric rosettes. Design of the DNA-base hybrid molecule was done in a way that each molecule function exclusively as the corner piece of a hexagon. The combination of the A-A-D (A: acceptor, D: donor) sequence of cytosine and complementary D-D-A sequence of guanine at an angle of 120° to each other in pyrido [4,3-d] pyrimidine, leads to the assembly of independent hexagonal aggregates in the solid state. This is purely the result of the communication of programmed, 18 hydrogen-bonding information. Even though a variety of derivatives with different properties have been designed and synthesized, there is only one example of a bisrosette structure using GΛC heterocycles. This bis-rosette supramolecular cages are formed by taking advantage of hyrophobic effect. However, to date organic solvent based bis-rosette cage structures that only utilizes H-bonding has not been achieved. Here we describe the synthesis of a well-designed precursor that will form supramolecular cage structures using only programmed H-bonding information in organic solvents. The precursor structure was formed by combining two guanine-cytosine units with a rigid bridging group. Synthesis of the compound has been successfully completed in 12 steps. Bis- rosettes are stacked on top of each other to form organic nanotubes. The formation of nanotube structures has been proven by electron microscopy and supported by computational studies. Synthesis of non-planar aromatic compounds (like fullerene fragments), a class of textbook changing materials, has always attracted great theoretical interest and became important targets for synthesis. Triquinacene and its saturated derivative triquinane have an important place in research. Triquinacene was first synthesized by R. B. Woodward, one of the most prominent organic chemists of his time, and since then many other studies were performed. The main motivation behind the synthesis of triquinacene was the realization of infamous docehadrane through an electrocyclic dimerization reaction. Additionally, many discussions related homoaromaticity was based on the experimentation done on the triquinacene structure. Hundreds of triquinacene derivatives were synthesized since then, and many studies were performed addressing their properties: from rearrangement chemistry to metal complexation. Even though many interesting studies were performed on the triquinacene framework, the hetero- analogues of this system were not investigated for years. The nitrogen-containing derivative of the triquinacene was first realized by the Mascal group. Same group has also achieved the synthesis of the non-planar aromatic derivative, azacepentalinide anion and showed that it is remarkably more stable than the carbon counterpart. Theoretical calculations showed that the oxygen and sulfur containing derivatives of it are also non-planar, highly aromatic compounds. Some of the heterotriquinane derivatives that were synthesized toward realization of the non-planar aromatic compounds showed various unusual and interesting structural properties and reactivity that ultimately pushed the boundaries of our knowledge in organic chemistry. For example, oxotriquinane, an oxonium ion, shows unusual stability. The discovery that oxygen can make more than three covalent bonds was also made using an oxotriquinane derivative. The molecule that contains the world’s longest carbon-oxygen bond is an oxotriquinane derivative too. The nitrogen analogue, azatriquinane is the most basic trialkylamine known. Like triquinacene, there are various important studies on different areas of research about tribenzotriquinane. Surprisingly, synthesis of the hetero-analogues of tribenzotriquinane, which also represents the tribenzo derivatives of heterotriquinanes, has not been pursued in the literature. Here, we described the synthesis of tribenzooxotriquinane derivatives and investigation of any unusual reactivity and/or structural property that these structures might reveal. A multi-step but convergent synthetic approach was utilized towards the synthesis of target molecules. Tribenzocyclononatriene, an important core unit in our synthetic approach, has been successfully synthesized. The functionalization of the core unit was carried out towards the synthesis of target compound, tribenzooxotriquinane. The details of the approaches will be discussed in detail.
Citation Formats
C. İğci, “From supramolecular chemistry to fundamental organic chemistry: bis-rosette nanotubes and novel molecular frameworks,” M.S. - Master of Science, Middle East Technical University, 2017.