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, is a major project. 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 unknown.
We determined the mechanism using computational and experimental chemistry. It is straightforward – an arenium cation engages in electrophilic aromatic substitution with another aryl moiety. A radical cation mechanism was also investigated but exhibits much higher activation barriers.
We also addressed a deeper mechanistic question: “What enables this reaction to form many bonds in a single step?” By examining the full reaction coordinate for the conversion of hexaphenylbenzene to hexa-peri-hexabenzocoronone, we found the answer: the reaction becomes increasingly exergonic with decreasing reaction barriers [2. “A Slippery Slope: Mechanistic Analysis of the Intramolecular Scholl Reaction of Hexaphenylbenzene”, Rempala, P.; Kroulík, J.; King, B. T. J. Am. Chem. Soc., 2004, 126, 15002-15003 and 5. “Investigation of the Mechanism of the Intramolecular Scholl Reaction of Contiguous Phenyl Benzenes”, Rempala, P.; Kroulík, J.; King, B. T. J. Org. Chem. 2006, 71, 5067-5081].
Our next task was to determine the scope of this reaction. We observed that, although effective for large systems, the Scholl reaction failed miserably for small arenes, e.g. the conversion of o-terphenyl to triphenylene. We demonstrated that this was due to rapid oligomerization of the products. We also demonstrated that classic directing group effects are manifest, which is consistent with an arenium cation mechanism [8. “Controlling the Scholl Reaction”, King, B. T.; Kroulík, J; Robertson, C. R.; Rempala, P.; Hilton, C. L.; Korinek, J. D.; Gortari, L. M. J. Org. Chem., in press].
We are also studying new organometallic routes to PAHs. The serendipitous discovery of the square antiprismatic Zr(2,2’-biphenyldiyl)4−4, from the reaction of 2,2’-dilithiobiphenyl with zirconocene dichloride, led to an novel family of organozirconium reagents (Fig. 2) [4. “Four Homologous Zirconium 2,2′-Biphenyldiyls: Synthesis, Structure, and Reactivity”, Hilton, C. L.; King, B. T., Organometallics 2006, 25, 4058-4061].
The zirconate bearing three biphenyldiyl ligands, Zr(biphe)3−2 exhibits escpecially 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 [7. “Uncatalyzed Zirconium-Mediated Biphenylation of o-Dihalobenzenes to Form Triphenylenes”, Hilton, C. L.; Jamison, C. R.; King, B. T. J. Am. Chem. Soc. 2006, 126, 1284-1284].
Our interests extend to the electronic nature of PAHs. We have developed rules for valence-bond 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 [1. “Clar Valence Bond Representation of p-Bonding in Carbon Nanotubes”, Ormsby, J. O.; King, B. T. J. Org. Chem. 2004, 69, 4287 - 4291. Cover feature].
We have applied this VB model to understand patterns of addition reactions to CNTs, a reaction that underlies nearly all CNT chemistry. Simple models, based on sterics, had been proposed but failed to predict long-range patterns. Our model predicts reactivity [9. “The Regioselectivity of Addition to Carbon Nanotube Segments”, Ormsby, J. O; King, B. T. J. Org. Chem. accepted for publication].
The synthesis of polymeric helicenes is a long-term project. Polymeric helicenes could serve as surrogates for CNTs, with similar dimensions and electronic properties. The mechanical properties will, however, be complimentary – the helicenes will be elastic, not stiff.
Demonstration of potential utility of the helicenes was also important. We envisioned using helicenes as actuators that would respond to a perturbation to peripheral functionality. A molecular dynamics study supported this idea. The extension (180%) was exceptional (cf., piezoelectrics, 104%) and the effect was reversible [6. “Simulation of Actuation by Polymeric Polyelectrolyte Helicenes”, Rempala, P.; King, B. T., J. Chem. Comp. Theor. 2006, 2, 1112-1118].
