A New Approach to the Synthesis of [n]Phenacenes

Posted May 28th, 2010 at 10:12 am.

Kirbi Krisfalusi and Leila Foroughi
Mentor: Dr. Frank Mallory

[n]Phenacenes are molecules with n hexagonal benzene rings fused together in an extended zig-zag array that is only one benzene ring wide and one benzene ring thick. Molecules of graphite, in comparison, consist of many hexagonal benzene rings that are fused together in sheets that are one benzene ring thick but have an extended width as well as an extended length. That is, [n]phenacenes are related to graphite as ribbons are related to sheets. Because graphite is known to conduct electricity, it has been speculated that due to the extensive pi-electron system in [n]phenacenes, these molecules may one day be useful as "molecular wires." In work by members of Professor Mallory's research group, the world's record for an [n]phenacene has been increased from six fused rings to eleven fused rings. The ongoing objective is to develop iterative methods of synthesis to achieve a dramatic increase in this world's record.

In previous syntheses carried out at Bryn Mawr, it has been found that phenacenes become very insoluble as the value of n increases, thereby thwarting the efforts to carry out chemical reactions that will elongate the molecules in an iterative fashion. Our new scheme employs a different solubilizing strategy using polyether substituents, which are known to increase solubility markedly when compared to the alkyl substituents that have been used in our past work. This new scheme also increases the photochemical efficiency of the steps in the iterative synthesis in which new carbon-carbon bonds in the [n]phenacenes are produced by the photocyclization of a diarylethylene derivative. We are modifiying our synthesis by incorporating the double bond of the diarylethylene into a five-membered ring, thereby locking in the cis configuration of the bouble bond that is required for the photocyclization reaction.

This summer, we will be synthesizing large quantities of 1,8-dibromophenanthrene which will be used as a centerpiece of our initial target molecule, a substituted [11]phenacene. The 1,8-dibromophenanthrene will then be subjected to a butyllithium reaction with an epoxide containing polyether solubilizing groups. The resulting alcohol will then be oxidized into a ketone using the Swern oxidation. The resulting ketone will then be added to 1,8-dibromophenanthrene in another butyllithium reaction. This will produce a stilbene derivative which upon photocyclization will form the [11]phenacene derivative with polyether solublizing substituents.

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