Study and Development of Synthetic Methodology Towards Pyrrolic Frameworks
Beh, Michael H. R.
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This thesis focuses on the study and development of synthetic methodology towards pyrrolic frameworks. Three projects are discussed: the development of open-air conditions for the synthesis of F-BODIPYs, mechanistic studies in the synthesis of symmetrical dipyrrins, and the development of new reactions for the synthesis of sulfur-bridged pyrroles. The atmospheric effects of water on the synthesis of F-BODIPYs were explored. A methodology was developed for the high-yielding synthesis of F-BODIPYs involving non-anhydrous reagents and requiring no precautions to exclude moisture. This simple and robust strategy requires the addition of two aliquots of NEt3 and BF3OEt2. The ratio and amounts of NEt3 and BF3OEt2 used in each aliquot are critical to success (6 equivalents of NEt3 and 9 equivalents of BF3OEt2 per each aliquot). Most important is that the protocol can be completed using air-dried bench-top apparatus, without the need to either purchase anhydrous solvents or achieve and maintain anhydrous solvents and conditions. The synthesis of symmetric α-free meso-H-dipyrrin hydrobromides from 5-unsubstituted 2-formylpyrroles (2-formyl-5-H-pyrroles) was investigated and it was found that the self-condensation produces regioisomeric dipyrrins through adoption of two mechanistic pathways. Through a systematic study involving variously substituted and isotopically labelled 2-formyl-5-H-pyrroles, evidence was provided to suggest that not only does there exist two mechanistic pathways, but that the steric bulk of the substituent adjacent to the 5-unsubstituted position influences which pathway dominates. There are few synthetic routes to preparing sulfur-bridged dipyrrolyl sulfides, most of which require use of sulfur dichloride, a restricted chemical unavailable within Canada. Towards the preparation of sulfur-bridged di-, tri-, and tetrapyrroles, the first example of electrophilic aromatic substitution of pyrrole using thionyl chloride to produce dipyrrolyl sulfides was developed. The use of thionyl chloride results in an interesting auto-reduction of the sulfoxide centre to produce a sulfide-bridged dipyrrole. The new synthesis was investigated and a mechanism for the auto-reduction reaction was proposed.