Benzyne Adds Across a Closed 5-6 Ring Fusion in C70: Evidence for Bond Delocalization in Fullerenes

Addition of benzyne to C70 results in four isomeric monoadducts (compounds 1a-d) in a 42:35: 13:10 ratio as determined by 1H NMR. These compounds were separated by repeated passes through HPLC columns. The major isomer (1a) resulted from addition to the highly pyramidalized C1-C2 bond as shown by 13C NMR and UV/vis spectroscopy. The structure of the C1-C2 adduct (1a) was confirmed by X-ray crystallography. The second isomer (1b) was assigned as an adduct to the C5-C6 bond on the basis of 13C NMR and UV/vis spectroscopy. Compound 1c exhibited a 13C spectrum consistent with an adduct to the C7 and C8 positions. The presence of sp3 13C NMR resonances proved that the C7-C8 bond is still intact, making this the first identification of direct addition to a 5-6 ring fusion in a fullerene, and the first example of an adduct to a 5-6 ring fusion where the ring fusion bond remains intact. The fourth isomer (1d) displayed spectral data consistent with a compound with C1 symmetry and is assigned as an adduct to the C7-C21 bond. A great deal has been learned about the reactivity of the fullerenes in the short period since they became available in bulk quantities. The characteristic pyramidalization of sp2 carbons of fullerenes results in high reactivity.1 One of the most general patterns of reactivity is that fullerene chemistry is largely the chemistry of addition reactions.2,3 The structure of C60 is usually drawn as a complex [5]radialene (Figure 1), with the double bonds positioned between six-membered rings (at “6-6 ring fusions”), and this is consistent with measured bond lengths4 and with the reactivity5 of C60 toward nucleophilic addition,6-11 toward reduction,12 and in a host of cycloadditions.13 Herein we report the addition of benzyne across the C7-C8 carboncarbon bond in C70, the first example of direct addition across a closed 5-6 ring fusion in a fullerene.14 The chemistry of C70 is more complicated than the chemistry of C60 due to lower symmetry and to differences in reactivity between the various double bonds. There are four different types of double bonds in C70 in the valence-bond picture of C70 shown in Figure 2. Drawing double bonds in the manner traditional for fullerenes leads to double bonds localized in a set of five-membered rings (e.g. the C22-C23 bond in Figure 2). The most pyramidalized carbons make up a set of reactive double bonds at the poles (e.g. C1-C2 in Figure 2). Pyramidalization and reactivity decrease for bonds closer to the relatively flat equator. (1) Haddon, R. C. Science 1993, 261, 1545-1550. (2) Hirsch, A. Synthesis 1995, 895-913. (3) Tetrahedron Symp. Print 60, 1996; Smith, A. B., III, Ed. (4) David, W. I. F.; Ibberson, R. M.; Matthewman, J. C.; Prassides, K.; Dennis, T. J. S.; Hare, J. P.; Kroto, H. W.; Taylor, R.; Walton, D. R. M. Nature 1991, 353, 147-149. (5) The Chemistry of Fullerenes; Taylor, R., Ed.; World Scientific: River Edge, NJ, 1995. (6) Fagan, P. J.; Krusic, P. J.; Evans, D. H.; Lerke, S. A.; Johnston, E. J. Am. Chem. Soc. 1992, 114, 9697-9699. (7) Balch, A. L.; Cullison, B.; Fawcett, W. R.; Ginwalla, A. S.; Olmstead, M. M.; Winkler, K. J. Chem. Soc., Chem. Commun. 1995, 2287-2288. (8) Keshavarz-K, M.; Knight, B.; Srdanov, G.; Wudl, F. J. Am. Chem. Soc. 1995, 117, 11371-11372. (9) Kusukawa, T.; Ando, W. Angew. Chem., Int. Ed. Engl. 1996, 35, 1315-1317. (10) Wang, G.-W.; Shu, L.-H.; Wu, H.-M.; Lao, X.-F. J. Chem. Soc., Chem. Commun. 1995, 1071-1072. (11) Schick, G.; Kampe, K.-D.; Hirsch, A. J. Chem. Soc., Chem. Commun. 1995, 2023-2024. (12) (a) Bergosh, R. G.; Meier, M. S.; Laske Cooke, J. A.; Spielmann, H. P.; Weedon, B. R. J. Org. Chem. 1997, 62, 7667-7672; (b) For an extensive review of reduced fullerenes, see: Gol’dshleger, N. F.; Moravshii, A. P. Russ. Chem. ReV. 1997, 66, 323-342. (13) Meier, M. S. In ref 5, pp 174-194. (14) An earlier report of 5-6 closed adducts was found to be in error. For details, see: Schick, G.; Grösser, T.; Hirsch, A. J. Chem. Soc., Chem. Commun. 1995, 2289-2290. Figure 1. The 6-6 and 5-6 ring fusions in C60. Figure 2. The structure and (partial) numbering of C70. Numbering from Godly and Taylor.15 2337 J. Am. Chem. Soc. 1998, 120, 2337-2342 S0002-7863(97)03957-7 CCC: $15.00 © 1998 American Chemical Society Published on Web 02/26/1998 The cycloaddition of benzyne to C70 has been reported to produce four isomers of monoadduct, although these were not characterized as purified compounds.16 We treated a solution of C70 and 2 equiv of anthranilic acid in benzene with 2 equiv of isoamyl nitrite, producing a mixture of products.16,17 The ensemble of monoadduct isomers, present in a 42:35:13: 10 ratio as determined by 1H NMR, was separated from the crude reaction mixture by GPC18 and obtained in a combined yield of 34%. The individual isomers were purified by HPLC with use of, in separate steps, Regis “Buckyclutcher” and Cosmosil “Buckyprep” columns. The absorption spectra of these four isomeric monoadducts (1a-d) are shown in Figure 1. The major isomer exhibits a 13C NMR spectrum that consists of a total of 42 lines comprising 10 single-intensity resonances (2 sp2 resonances and 2 sp3 resonances from the fullerene, plus 6 sp2 resonances from the benzene ring) and 31 sp2 resonances of double intensity and one of quadruple intensity (2 overlapping double intensity resonances). These data are consistent with 1a, resulting from addition to the C1-C2 bond. Under ideal conditions, 1a should produce a spectrum consisting of 8 single intensity sp2 resonances, 33 double intensity sp2 resonances, and 2 single intensity sp3 resonances. The absorption spectrum (Figure 3) is also consistent with a C1-C2 adduct.19-22 The C1-C2 double bond, involving two of the most pyramidal carbons in the molecule,1,23 is the most reactive site in C70. The structure of [1,2]benzeno[70]fullerene (1a) was determined by X-ray crystallography. Single crystals grown from carbon disulfide/toluene included one toluene molecule in the crystal lattice. The structure was complicated by a positional disorder in which the benzyne moiety lies on a pseudomirror plane, and two positions of the C70 portion of the molecule are related by the pseudomirror. Site occupancies for the two positions refined to 60.97% and 39.03%. Because of limited data, only the coordinates of the benzyne addend and the six C70 atoms closest to it were allowed to vary, with the equivalent bond lengths constrained to be equal with a standard deviation of 0.03 Å. The remainder of the C70 cage and the molecule of toluene of crystallization were treated as rigid groups with idealized geometries. A plot of C70C6H4•toluene is shown in Figure 4. Essential crystal data and experimental details are given in Table 1, and selected bond distances and angles are given in Table 2. Full crystallographic tables are included in the Supporting Information. The connectivity of the structure is firmly established by this structure determination. Bonding in the benzyne addend is quite delocalized, with C-C distances of about 1.38(2) Å. Fusion to C70 causes some angular distortions, with C-C-C angles in the benzyne ring ranging from 113(2)° to 124(1)°. The linking cyclobutene ring is strongly distorted, with C-C-C angles exo to the benzyne ring being about 10° larger than those exo to C70. The C70 bond to which the benzyne is fused, C01-C02, is extremely stretched with a bond length of 1.66(2) Å. The (15) Godly, E. W.; Taylor, R. Pure Appl. Chem. 1997, 69, 14111434. (16) Taylor has pointed out that several different isomers of C70 adducts should display the same number of carbon resonances, and thereby casting doubt on some published structural assignments: Darwish, A. D.; Avent, A. G.; Taylor, R.; M., W. D. R. J. Chem. Soc., Perkin Trans. 2 1996, 20792084. (17) Darwish, A. D.; Abdul-Sada, A. K.; Langley, G. J.; Kroto, H. W.; Taylor, R.; Walton, D. R. M. J. Chem. Soc., Chem. Commun. 1994, 21332134. (18) Meier, M. 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J. Am. Chem. Soc. 1997, 119, 943946. (28) Gerst, M.; Beckhaus, H.-D.; Rüchardt, C.; Campbell, E. E. B.; Tellgmann, R. Tetrahedron Lett. 1993, 34, 7729-7732. (29) Seiler, P.; Herrmann, A.; Diederich, F. HelV. Chim. Acta 1995, 78, 344-354. (30) Wilson, S. R.; Wu, Y. J. Am. Chem. Soc. 1993, 115, 10334-7. (31) Wood, J. M.; Kahr, B.; Hoke, S. H. I.; Dejarme, L.; Cooks, R. G.; Ben-Amotz, D. J. Am. Chem. Soc. 1991, 113, 5907-5908. (32) Smith, A. B. I.; Strongin, R. M.; Brard, L.; Furst, G. T.; Romanow, W. J.; Owens, K. G.; Goldschmidt, R. J.; King, R. C. J. Am. Chem. Soc. 1995, 117, 5492-5502. (33) Wang, Y.; Schuster, D. I.; Wilson, S. R.; Welch, C. J. J. Org. Chem. 1996, 61, 5198-5199. (34) Zhang, X.; Fan, A.; Foote, C. S. J. Org. Chem. 1996, 61, 54565461. (35) Bingel, C. Chem. Ber. 1993, 126, 1957-1959. Figure 3. Absorption spectra of 1a-1d.