Anion-π Catalysis on Carbon Nanotubes

Abstract

Induced π acidity from polarizability is currently emerging as most effective to stabilize anionic transition states on aromatic π surfaces, that is anion-π catalysis. To access extreme polarizability, we here propose a shift of attention from homogeneous toward heterogeneous anion-π catalysis on higher carbon allotropes. According to benchmark enolate addition chemistry, multi-walled carbon nanotubes equipped with tertiary amine bases outperform single-walled carbon nanotubes clearly. This difference is consistent with polarizability of the former not only along but also between the tubes. Inactivation by π-basic aromatics and saturation with increasing catalyst concentration support that catalysis occurs on the π surface of the tubes. Increasing rate and selectivity of existing anion-π catalysts (naphthalenediimides > fullerenes) on the Surface of unmodified nanotubes is consistent with transition-state stabilization by electron sharing into the tubes, i.e., induced anion-π interactions. On pristine tubes, anion-π catalysis is realized by noncovalent interfacing with π-basic pyrenes.