Dissertation Defenses

Doctoral students who have an upcoming dissertation oral defense are posted here. So why not take this opportunity to learn about the research that our graduate students are doing!

Dissertation Defense for Sarah Joiner

Program: Chemistry: PHD

Department Contact Email: cindi.rohwer@unh.edu

Defense Title: REARRANGEMENTS, CYCLIZATIONS, AND COUPLING REACTIONS

Defense Date and Time: 05/28/15 11:00 am

Defense Location: Parsons Hall, Room W131

Defense Advisor: Professor Richard Johnson


Defense Abstract: REARRANGEMENTS, CYCLIZATIONS, AND COUPLING REACTIONS
OF POLYCYCLIC AROMATIC HYDROCARBONS
by
Sarah L. Skraba Joiner
University of New Hampshire, September 2015
ABSTRACT
We have studied series of rearrangements in polycyclic aromatic hydrocarbons in a solution of trifluoromethane sulfonic acid (TfOH) in 1,2-dichloroethane (DCE). Ipso protonation of these arenes allows for 1,2-aryl group migration. In phenyl-substituted benzenes, meta- type products are generally formed as the major isomer at equilibrium. Potential energy surfaces for these rearrangements were calculated using density functional theory (DFT); major product formation is consistent with the formation of the lowest energy carbocation. Similar rearrangements were also observed in the binaphthyl system, which undergoes 1,2-naphthyl shifts in superacid media, forming 2,2’-binaphthyl as the major rearranged product. Prolonged heating in these cases results in Scholl cyclization. We have also observed skeletal rearrangements of acenes in similar reaction media. From anthracene, phenanthrene is formed as the major product, along with reduced isomers. Mechanisms for isomerization were studied using DFT, in which protonation at a ring juncture allows for formation of a spirocyclic carbocation intermediate that can isomerize or automerize. In the case of anthracene/phenanthrene, we have found the likely mechanism involves reduction, followed by isomerization, and finally re-oxidation. Similar rearrangements are observed in chrysene isomers; experiments and computational analysis suggest a direct isomerization mechanism. We have also studied acyl group migrations of aromatic ketones. 1-Acetonaphthone rearranged to 2-acetonaphthone upon heating in a TfOH/DCE solution. DFT analysis of the potential energy surface shows the formation of an intermediate π-complex, bridging the 1- and 2- isomers. The major isomer formed is dictated by formation of the lowest energy carbocation. In these reactions, deacylation also occurs; the amount increases as the stability of the acylium ion formed increases. In addition to rearrangements, we have also studied intermolecular Scholl coupling reactions of polycyclic aromatic hydrocarbons. We have used TfOH with 2,3-dichloro-5,6-dicyano-1,4-benzoquinone (DDQ) as an added oxidant to synthesize rylenes and hexabenzocoronene derivatives from naphthalene and phenanthrene, respectively. While further work needs to be completed on the optimization of these syntheses and the purification of these compounds, these reactions show promise as a simple route to these materials.


 

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