Inverse electron demand Diels-Alder (IEDDA)-driven domino reactions of chromone-fused electron deficient dienes - new methodology for synthesis of xanthones and xanthone-based polycyclic heteroaromatic molecules

• Main Author: Dang, Anh-Thu T., 1975-
• Other Authors: Memorial University of Newfoundland. Dept. of Chemistry
• Format: Thesis
• Language: English
• Published: 2009
• Subjects: Diels-Alder reaction; Diolefins; Xanthone > Synthesis; Newfoundland studies; University of Newfoundland
• Online Access: http://collections.mun.ca/cdm/ref/collection/theses4/id/61546

Thesis (Ph.D.)--Memorial University of Newfoundland, 2010. Chemistry Includes bibliographical references. The research described in this thesis mainly dealt with methodology development, design and synthesis, as well as the study of the physical properties of new xanthones and xanthone-based molecules. An offshoot of this work was concerned with the development of a synthetic approach to novel cyclic oligophenylenes. The inverse electron demand Diels-Alder (IEDDA)-based methodology was employed in all of the projects. -- The first objective was to employ the IEDDA reaction in a new approach to xanthones. Specifically, nine new 2-substituted-4-methoxyxanthones 1.228a-i and eight new 2-substituted-3,4-dimethoxyxanthones 1.229a-h were synthesized using IEDDA-driven domino reactions between chromone-fused dienes 1.15 and the corresponding electron-rich dienophiles 1.225 and 1.226, respectively. This work is described in Chapter 2. This methodology was also employed for the synthesis of arylxanthones in Chapter 3. A series of 2-aryl-4-methoxyxanthones 3.9a-c and 2-substituted-4-phenylxanthone, such as 3.18, was obtained from the reactions of the corresponding dienes 3.8a-c or 1.15b with dienophile 1.227. However, the desired 2,4-diarylxanthones 3.10 could not be synthesized by this method. -- The second objective was to synthesize two different types of hetero[5]acenes from appropriate xanthone frameworks, i.e. 3,4-dimethoxyxanthones and 4-methoxyxanthones, which were successfully obtained by the established IEDDA methodology. For example, Chapter 4 describes the synthesis of xanthonoid hetero[5]acenes 4.22 and 4.12-4.14 from the key intermediate 3,4-dimethoxyxanthone 4.23 via a selective demethylation, followed by an intramolecular nucleophilic aromatic substitution. Among these newly-synthesized compounds, hetero[5]acene-based crown ether 4.14 was designed for future complexation studies with ions toward chemosensor applications. The attempted synthesis of hetero[n]acenes (n = 5, 9) using double IEDDA reactions suffered from difficulties in the isolation of bis(diene) intermediate 4.26. At best, trace amounts only of hetero[5]acene 4.25b (mass spectroscopic analysis) were obtained. -- In Chapter 5, another class of hetero[5]acenes, referred to as iwdonor-acceptor xanthone-carbazole hybrid systems", was constructed from key intermediate 2-o-nitrophenyl-4-methoxyxanthones 5.31a-f and 5.38, via microwave-assisted Cadogan reactions, followed by zV-alkylation. A series of xanthone-carbazoles with or without substituents (OMe, Br) was synthesized, in which the "angular" isomers are the major product and the "linear" isomers are the minor ones in all cases. Suzuki couplings were effective in the synthesis of aryl-substituted xanthone-carbazole compounds. Physical studies on newly-synthesized xanthone-carbazoles showed some interesting features. For instance, the angular isomers show green fluorescence and quasi-reversible redox behaviour, while the linear ones show blue fluorescence in most cases and irreversible redox behaviour. Most of the xanthone-carbazoles exhibit moderate fluorescence quantum yields, narrow HOMO-LUMO energy gaps, low HOMO energies, and some of them have close interplanar distances in solid-state packing (X-ray analysis), which make them very promising candidates in OLED or OFET applications. -- Finally, in Chapter 6, the synthesis of a cyclic oligophenylene was attempted using a six-fold IEDDA reaction. Although this project was unsuccessful, investigations aimed at the improvement of the reaction conditions as well as the design of a more reactive bis(diene) should be informative to future work.