1. Exploring Routes to the Synthesis of Dibenzopentalenes
Thomas J. Williams, Dr. Richard P. Johnson
University of New Hampshire, Department of Chemistry, Durham, NH
April 18th, 2018
Introduction & History
Initial Experimentation
Proposed Syntheses
Dibenzo[cd,gh]pentalene is a little-studied chemical motif with an interesting electronic structure which conceptually merges the known compounds biphenyl with pentalene. According to our Density Functional Theory (DFT) calculations, dibenzopentalene is predicted to possess a triplet ground state diradical. Previous research on this these compounds was performed in the early to mid 1970s by Barry Trost and Philip Kinson at the University of Wisconsin. Their lengthy synthesis produced milligram-scale quantities of the dihydrodibenzopentalene non-aromatic analog. X-ray crystallography of the structure they synthesized suggested planarity, which is consistent with our calculations. This work explores simple and practical routes towards the syntheses of molecules with this skeleton to examine their fundamental chemical and electronic properties.
A variety of routes have been proposed for the continuation of this project. Our first was a concerted dicyclization of the diol of diphenic acid, synthesized via esterification, then reduction. This attempt was rather naïve, however because it was performed prior to calculations. Post-calculations, thallium ring contractions were explored using phenanthrene, however these were, again, unsuccessful - likely because breaking any kind of aromaticity is a highly unfavorable process.
Two specific methods are being followed to complete the synthesis of dihydrodibenzopentalene. The first route passes through a carbene. This will be completed by oxidization to an aldehyde, then using tosylhydrazine chemistry to prepare a diazo precursor to the carbene. A method based on C-H insertion will also be explored. This process substitutes the alcohol with a trifluoroacetate group, followed by a palladium catalyzed cyclization to afford the product. This is a known high-yield method for the production of fluorene.
Current Chemistry
One of the most plausible routes to the dicyclized final product involves the synthesis of a carbene. This method requires the production of 2-fluorenylmethanol, which has been successfully completed. This was made possible through the single cyclization of diphenic acid into the ketone with sulfuric acid, reduction of that ketone with sodium hydroxide and hydrazine, esterification of the carboxylic acid using thionyl chloride then methanol and triethylamine, and finally reducing the carboxylic acid with lithium borohydride. The following steps of this synthesis involve the oxidation of the alcohol to an aldehyde, then the aforementioned carbene synthesis and the final ring closure.
Moving Forward
We are looking to expand the pentalene homolog across larger, more conjugated systems. Specifically, we are interested in applications to dibenzochrysene and 6-circulene.
Calculations
We used quantum mechanical calculations to analyze the energies and plausibility of key molecules and reactions we wish to perform. We calculated the energy of the electronic states of our desired products, both as closed and open shelled configurations. Further, we carried out isodesmic strain estimates of the second cyclization step. Finally, we compared the energy barriers in the formation of fluorene and dihydrodibenzopentalene. These computations predict substantial strain. One likely synthetic route passes through the cyclization of a carbene
Acknowledgements
Dr. Johnson and the Johnson Research Group.
The Chemistry Department at the University of New Hampshire.
My fellow Senior Chemistry Majors.
The National Science Foundation (CHE – ).
Predicted Energies:
Closed Shell (Singlet): 6.09 kcal/mol
Open Shell Diradical (Singlet): 3.06 kcal/mol
Diradical (Triplet): 0.00 kcal/mol
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