CH/piinteractions have garnered attention recently although those involving fullerenes are still largely unstudied. An exhaustive search of the Cambridge Structural Database by Nishio[1] revealed numerous fullerene containing crystals in which CH/fullerene-pi contacts fell within the van der Waals distance. Crystals reported by Olmstead et al show strong evidence for CH/fullerene-pi interactions involving ethyl-substituted porphyrins[2]. In this investigation of CH/fullerene-pi interactions, we synthesized 1,4-diphenyltetracene and studied the regioselectivity of its Diels-Alder reactivity with [60]fullerene. The target 1,4-diphenyltetracene is synthesized in seven steps from commercially available starting material. It is observed that [60]fullerene undergoes preferential cycloaddition across the 5,12 position rather than 6,11 position of 1,4-diphenyltetracene under both kinetic and thermodynamic conditions. It is proposed that the increased reactivity of the diene moiety closest to the phenyl substituents in 1,4-diphenyltetracene results from favorable CH/fullerene-pi interactions. Dimethyl acetylenedicarboxylate (DMAD) and maleic anhydride were also reacted with 1,4-diphenyltetracene for comparison purposes. DMAD adds selectively to 1,4-diphenyltetracene across the 5,12 position under kinetic conditions. Maleic anhydride, in contrast, shows little regioselectivity or endo/exo-selectivity upon reaction with 1,4-diphenyltetracene. A NOESY (nuclear Overhauser effect spectroscopy) methodology for the discrimination of 5,12 and 6,11 cycloaddition regioisomers is reported. In the case of maleic anhydride addition, NOESY is also used to differentiate endo from exo stereoisomers.

[1] Nishio, M.; Suezawa, H.; Yoshida, Y.; Ishihara, S.; Umezawa, Y. Eng. Comm. 2003, 5, 514.

[2] Olmstead, M.; Costa, D.; Maitra, K.; Noll, B.; Phillips, S.; Van Clacar, P.; Blach, A. J. Am. Chem. Soc. 1999, 121, 7090.