Rational Synthesis of Buckminsterfullerene Tom Mitzel, Trinity College, Connecticut

Introduction In 1985, a third allotropic form of carbon was discovered,1 and added to the family initially consisting of graphite and diamond. The new discovery was named Buckminsterfullerene (C60), owing to the structural similarity of C60 to geodesic domes designed by Buckminster Fuller. Its soccer ball structure was proven beyond doubt in 1990,1 and, along with its fascinating chemistry, has held chemists' attention ever since.

In its short lifetime, C60 has been utilized in such diverse areas as batteries, superconducting materials, lubricants,1 and HIV inhibitors.2 Enough interest has been generated in this short period of time to warrant reviews, articles, books and one Nobel Prize dealing fullerene chemistry and structure.1,2

Endohedral complexes of fullerenes have become and area of interest due to enhanced reactivity with respect to the non-complexed molecules.1,2 High energies needed, and low yields presently obtained in guest complexation, however, has slowed research in this area,3 and an alternative method in formation of endohedral complexes is needed for progress to be made in this exciting area of chemistry.

Objectives and Goals Formation of the fullerene molecule itself is generally accomplished via high energy vaporization of carbon rods,1,2 which precludes placement of endohedral complexes within the fullerene fragment in desirable yields. A rational synthesis of C60 using a caged precursor would offer a better mechanistic alternative,4 allowing for precursors with cavities large enough to incorporate "guests" much more readily. Although there has been some interest in this area recently, rational syntheses of Buckminsterfullerene have remained largely unexplored.7

One possible route to fullerene formation is shown in Scheme 1.5 An advantage of this synthesis is that electrophilic "guests" may be trapped within the framework of D, which has a large opening lined with p-orbitals. After the "guest" is captured, fullerene formation by heating, or catalytic dehydrogenation, can be realized.

Scheme 1: Rational Synthesis of Buckminsterfullerene

Much of this chemistry, up through "joining" fragments A and B, giving (C) has been accomplished in our laboratories.5 Formation of D is envisioned via oxidative coupling under high dilution techniques,5 leaving only a few remaining steps to Buckminsterfullerene.

Rational syntheses must be versatile and an alternative route to C60 is shown in Scheme 2. As shown by Wennerström in 1977, molecules such as [26] (1,4)3(1,3,5)2 bicyclophanehexane (E) may be formed in good yield.6 The olefin units incorporated into the skeletal structure allow for good flexibility of the cage molecule, aiding in solubility and complexation of "guests", and offering another tantalizing route to fullerene formation. Efforts in our laboratory have lead to formation of E under various conditions including indium promoted aqueous coupling of the trialdehyde with p-benzylbromide, revealing the promise of this mechanistic scheme.5

Scheme 2: Carbon Cage by Wennerström

Although E does not have the correct number of carbons to form Buckminsterfullerene, this sequence could be easily altered as shown in Scheme 3. Two plausible routes are outlined, each forming a cage precursor large enough to house endohedral guests, and constructed to allow easy entry into fullerene formation.

Scheme 3: Routes to C60 Cages Enroute to Fullerenes

Significance of Work and Feasibility The rational synthesis of fullerene molecules is needed to help overcome the pitfalls that now encumber scientists. Earlier work in this area by myself and others has shown that chemistry of this nature is easily attainable using techniques often introduced at the undergraduate level.

The proposed chemistry readily involves undergraduate students. The synthetic steps outlined above will allow the student to learn many concepts, as well as practice techniques learned in the teaching laboratory. Although each step taken individually is simple, the final goal is a very worthy fullerene molecule, the synthesis of which would be widely recognized and utilized by chemists worldwide.

References (1) a) Baum, Rudy. Chemical and Engineering News, 1997, 75(1), p. 29. b) Kratschmer, W.; Lamb, L.D.; Fostiropoulos, K.; Huffman, D.R. Nature 1990, 347, 354. c) Billups, E. W.; Ciufolini, M. A. Buckminsterfullerenes, VCH Publishers, NY, NY, 1993. (2) a) Yamakoshi, Y; Yagami, T. Sueyoshi, S.; Miyata, N. J. Org. Chem. 1996, 61, 7236. b) Friedman, S.; DeCamp, D.; Sijbesma, R.; Srdanov, G.; Wudl, F.; Kenyon, G. J. Am. Chem. Soc. 1993, 115, 6506. c) Sijbesma, R.; Srdanov, G.; Wudl, F.; Castoro, J.; Wilkins, C.; Friedman, S.; DeCamp, D.; Kenyon, G. J. Am. Chem. Soc. 1993, 115, 6510. (3) a) Aldersey-Williams, H. The Most Beautiful Molecule: The Discovery of Buckyball. John Wiley and Sons, NY, NY, 1995. b) Perfect Symmetry: The Accidental Discovery of a New Form of Carbon. Oxford University Press, 1994. c) Weiske, T.; Bohme, D.K.; Hrusak, J.; Kratschmer, W.; Schwarz, H. Angew. Chem. Int. Ed. Engl. 1992, 31, 1101. (4) Thiel, W.; Patchkovskii, S. J. Am. Chem. Soc. 1996, 118, 7164. (5) a) Scott, L.T.; Bratcher, M.; Hagen, S. J. Am. Chem. Soc. 1996, 118, 8743. b) Gross, J.; Gabriele, H.; Vögtle, F.; Holger, S.; Gloe, K. Angew. Chem. Int. Ed. Engl. 1995, 34, 481. c) Cram, D.J.; Tanner, M.E.; Keipert, S.J.; Knober, C.B. J. Am. Chem. Soc. 1991, 113, 8909. d) Mitzel, T., unpublished results. (6) Höberg, H-E.; Thulin, b.; Wennerström, O. Tetrahedron Lett. 1977, 931.