Probing regioselectivity in the Diels -Alder cycloadditions between *[60]fullerene and aryl substituted acenes

The formation and reactivity of previously unknown 4,7-diphenylisobenzofuran, 5, is reported. The Diels-Alder reaction between 5 and p-benzoquinone in boiling glacial acetic acid yields an unprecedented exo,exo anti dual cycloaddition product, 16b, in excellent yield and with 100% diastereoselectivity. Differences between the reactivity of 5 and the more common 1,3-diphenylisobenzofuran are highlighted. Reactive 5 is utilized to form the anthracene 11, tetracene 15, and pentacene 22. The latter two compounds are reacted with [60]fullerene to explore the potential role of CH/pi interactions in the cycloaddition of [60]fullerene across phenyl substituted acenes. [60]Fullerene shows an unusual propensity for cycloaddition across the 5,12 position of 15 and the 5,14 and 7,12 positions of 22. This reactivity is compared and contrasted to the reactivity of other dienophiles including dimethyl acetylenedicarboxylate, ethylene, singlet oxygen, maleic anhydride, tetrachloroquinone, and 2,3-dichloro-5,6-dicyanobenzoquinone. NOESY (nuclear Overhauser effect spectroscopy) methods are especially useful for the discrimination of 5,12 and 6,11 cycloaddition regioisomers of 15 and 41-45, as well as select Diels-Alder cycloaddition regioisomers of 22. The Diels-Alder reactivities of [60]fullerene across various aryl substituted derivatives of 15, compounds 41-45, are also studied. It is observed that electron donating substituents enhance regioselective [60]fullerene addition across the 5,12 position while electron withdrawing groups reduce 5,12 regioselectivity. Direct evidence for aryl CH-fullerene pi interactions comes from a careful examination of 1H NMR chemical shifts for the ortho phenyl protons in fullerene-acene adducts. A careful review of the Cambridge Crystallographic Database reveals numerous instances of aryl CH/fullerene contacts within 3 A. The combined results suggest that aryl CH-fullerene pi interactions play a significant role in fullerene-acene chemistries. The tantalizing possibility that this interaction involves pi* orbitals on [60]fullerene and sigma orbitals on the aryl CH donors, an inverse electron demand CH/pi interaction, is suggested, but more study is required