9. Synthesis of 18π annulenic fluorofullerenes from tertiary carbanions: size matters!

A range of tertiary carbanions XCH(CO₂Et)₂ of differing sizes have been reacted with C₆₀F₁₈ to assess the steric effect of X on the position of nucleophilic substitution. For X = CO₂Et, NO₂, P(O)(OMe)₂, SO₂CH₂Ph, the all trans annulenes (trannulenes) were obtained as a result of extended S_N2′ (i.e. S_N2″) substitution; in the case of the phosphorus compound, with reduced amounts of base (DBU) dephosphonylation of one or more P(O)(OMe)₂ groups by hydrogen occurred. Trannulene formation did not occur for X = F, CN due to the smaller size of the nucleophile, and in the latter case substitution was shown to take place by an S_N2′ mechanism, resulting in the addend being adjacent to a fluorine addend. Trannulenes (X = CO₂Et, Br, Cl) exhibited reversible one-electron reductions at potentials (−0.02 to −0.09 V) significantly more positive than for [60]fullerene. Trannulene (X = NO₂) exhibited an irreversible one-electron reduction (0.08 V); the irreversibility may be associated with fluorine loss. Conformational isomerism at temperatures below 298 K was observed for all trannulene derivatives as a result of eclipsing addend–addend interactions. Minimum energy conformations with a rotational energy barrier of 12–15 kcal mol⁻¹ were observed when these interactions are calculated using molecular mechanics.