An Insight into the Aromaticity of Fullerene Anions: Experimental Evidence for Diamagnetic Ring Currents in the Five-Membered Rings of C606- and C706-

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• 作者：Tamar Sternfeld ; Carlo Thilgen ; Roy E. Hoffman ; Marí ; a del Rosario Colorado Heras ; Franç ; ois Diederich ; Fred Wudl ; Lawrence T. Scott ; James Mack ; and Mordecai Rabinovitz
• 刊名：Journal of the American Chemical Society
• 出版年：2002
• 出版时间：May 22, 2002
• 年：2002
• 卷：124
• 期：20
• 页码：5734 - 5738
• 全文大小：65K
• 年卷期：v.124,no.20(May 22, 2002)
• ISSN：1520-5126

文摘

Reduction of the two "closed" [6,6] methanofullerenes, [6,6]C₆₁H₂ (1) and [6,6]C₇₁H₂ (5), to thecorresponding hexaanions with lithium metal causes the bridgehead-bridgehead bonds to open, at leastpartially, and this change gives rise to diamagnetic ring currents in the resulting homoconjugated six-membered rings (6-MRs). These new ring currents shield the overlying hydrogen atoms on the methylenebridge and induce upfield shifts of 1.60 and 0.11 ppm in their ¹H NMR resonances, respectively. Analogousreduction of the already "open" [5,6]methanofullerenes, [5,6]C₆₁H₂ (2) and [5,6]C₇₁H₂ (3 and 4), only slightlyenhances the shielding of the hydrogen atoms over the homoconjugated 6-MRs (upfield shifts of 0.13,0.68, and 0.14 ppm, respectively) but leads to exceptionally strong diamagnetic ring currents in thehomoconjugated five- membered rings (5-MRs), as evidenced by dramatic shielding of the hydrogen atomssituated over them (upfield shift of 5.01, 6.78, and 1.63 ppm, respectively). The strongest shielding is seenfor the hydrogen atom sitting over the 5-MR at the pole of C₇₁H₂^6- (fchars/delta.gif" BORDER=0 > = -0.255 ppm) indicating that theexcess charge density is concentrated at the poles.