Higher kekulenes and Polymeric Helicenes
AbstractThe main objective of this project was to synthesize, isolate and characterize polymeric helicenes. Because helicenes might have potential applications in the synthesis of ligands for asymmetric catalysis, molecular actuators, conductive and chiroptical materials, we are interested in developing them in polymeric form. Ring closing metathesis (RCM) reaction was used to furnish the helicenes from a suitably designed polymer precursor, poly{4,6-di[(1E)-(2-2H)-propenyl]}-, m</italic)-phenylene. End group analysis revealed that the polymerization reaction resulted in the formation of both acyclic and cyclic polymers, precursors for helicenes and kekulenes respectively. Large quantities of monomers were required to optimize both the polymerization and ring closing metathesis reactions. A key finding in the monomer synthesis was the use of the Wittig reaction to circumvent the low yield encountered in a dehydration route towards the synthesis of the target AB monomer for Suzuki polymerization. These reactions were optimized leading to the synthesis of 23 g of monomer in 12 weeks. The Suzuki polycondensation reaction was also optimized leading to the formation of acyclic polymers and macrocycles. RCM of the polymers afforded mixtures of polymeric helicenes and higher kekulenes. A major finding in this project was the ability to monitor the RCM reaction by observing the disappearance on an IR spectrum of a C-D label that was incorporated into the monomer. This allowed monitoring of the reaction, which in turn allowed the reaction conditions to be optimized. Purification and chromatography using open column gel permeation chromatography afforded [7]kekulene, a mixture of higher kekulenes ([8] - [14]kekulenes) and mixtures of helicenes. The [7]kekulene and helicene mixture were partially characterized using NMR, MALDI-TOF, UV-vis and IR spectroscopic data. Enough material was not available for complete characterization of [7]kekulene. [n]Helicenes were isolated and characterized as a mixture. We report in this dissertation the first synthesis of higher kekulenes by RCM. We have proof for the formation of polymeric helicenes. [7]Kekulene with a non-planar geometry, mixtures of higher kekulenes and mixtures of polymeric helicenes were synthesized and isolated. Having optimized the reactions for the synthesis of polymeric helicenes and higher kekulenes, these reactions can be carried out on a larger scale to obtain enough material for complete characterization of [7]kekulene and polymeric helicenes.
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