Mechanism of the Scholl Reaction
The development of the Scholl reaction, which is the acid-catalyzed oxidation of aryl CH vertices to generate new aryl-aryl bonds, has been a major project in the BTK group. Although this reaction holds enormous potential for the synthesis of new PAHs, as demonstrated by its use by Müllen to prepare large graphitic disks, its scope and mechanism had been unexplored. We study its mechanism using computational and experimental chemistry. Our calculations suggest that the mechanism involves an arenium cation intermediate that engages in electrophilic aromatic substitution with another aryl moiety. A radical cation mechanism was also investigated but exhibits much higher activation barriers. The possibility of inner-sphere electron transfer has not yet been addressed.
Reaction coordinate diagram for the arenium cation, cascade Scholl reaction.
The Smallest Known Circulene: Quadrannulene
We recently reported the preparation of the first circulene, for which we proposed the name quadrannulene. Our product, tetrabenzocirculene, is deeply bowl shaped and contains the most distorted double bonded C atom that has been structurally characterized. Our unoptimized, five-step synthesis provides tetrabenzocirculene in low yield.
Organometallic Methodology for PAHs: Zirconium-Mediated Biphenylation
We have developed new methodologies for preparing PAHs. The serendipitous discovery of the square antiprismatic complex Zr(2,2'-biphenyldiyl)4-4, from the reaction of 2,2'-dilithiobiphenyl with zirconocene dichloride, led to a novel family of organozirconium compounds. The zirconate bearing three biphenyldiyl ligands, Zr(biphe)3-2 exhibits interesting reactivity - it reacts with o-dihalobenzenes to give triphenylenes. This reaction is a fundamentally new disconnection for PAH syntheses and is capable of introducing significant molecular strain, as evidenced by the one-step preparation of 9,18-dimethyltetrabenzoanthracene.
We then applied this Zr-based methodology to the synthesis of two unusual molecules. This is first a dimer of tetrabenzanthracene linked through the gulf positions. The bond that constitutes the linkage is perhaps the most hindered single C-C bonds in chemistry. Interestingly, while this bond is highly strained, having a homolytic bond dissociation energy only 55% of that in similar compounds, it has a perfectly normal length of 1.49Å. The leads to perhaps the most important conclusions of the paper: forces, not energies, determine bond lengths.
We also used this Zr-based methodology to prepare triphenylene-based triptycenes. The development of a convenient preparation of hexabromotriptycene followed by its treatment with the Zr biphenylation reagent afforded the extended triptycene. This molecule possesses an enormous free volume, defined as the unfilled volume of a prism containing the molecule, of 700 Å3, which is about ten times greater than that of the parent triptycene. Its tripodal structure and large free volume suggest applications as a stator for molecular motors.
Theory of Carbon Nanotubes
Our interests include the electronic nature of polycyclic aromatic hydrocarbons (PAHs). We have developed rules for valence-bond (VB) descriptions of π-bonding in carbon nanotubes (CNTs). A particularly interesting finding was that conductive CNTs require only aromatic sextets for their complete valence representation. We have applied this VB model to understand patterns of addition reactions to CNTs. Simple models, based on sterics, had been proposed but failed to predict long-range patterns. Our model predicts reactivity.
- Postdoctoral Research Associate (2000-2002), University of California, Berkeley (Robert G. Bergman)
- Ph.D. (2000), University of Colorado (Josef Michl)
- B.S. (1992), Northeastern University