Anion−π Catalysis

Abstract

The introduction of new noncovalent interactions to build functional systems is of fundamental importance. We here report experimental and theoretical evidence that anion−π interactions can contribute to catalysis. The Kemp elimination is used as a classical tool to discover conceptually innovative catalysts for reactions with anionic transition states. For anion−π catalysis, a carboxylate base and a solubilizer are covalently attached to the π-acidic surface of naphthalenediimides. On these π-acidic surfaces, transition-state stabilizations up to ΔΔGTS = 31.8 ± 0.4 kJ mol–1 are found. This value corresponds to a transition-state recognition of KTS = 2.7 ± 0.5 μM and a catalytic proficiency of 3.8 × 105 M–1. Significantly increasing transition-state stabilization with increasing π-acidity of the catalyst, observed for two separate series, demonstrates the existence of “anion−π catalysis.” In sharp contrast, increasing π-acidity of the best naphthalenediimide catalysts does not influence the more than 12 000-times weaker substrate recognition (KM = 34.5 ± 1.6 μM). Together with the disappearance of Michaelis–Menten kinetics on the expanded π-surfaces of perylenediimides, this finding supports that contributions from π–π interactions are not very important for anion−π catalysis. The linker between the π-acidic surface and the carboxylate base strongly influences activity. Insufficient length and flexibility cause incompatibility with saturation kinetics. Moreover, preorganizing linkers do not improve catalysis much, suggesting that the ideal positioning of the carboxylate base on the π-acidic surface is achieved by intramolecular anion−π interactions rather than by an optimized structure of the linker. Computational simulations are in excellent agreement with experimental results. They confirm, inter alia, that the stabilization of the anionic transition states (but not the neutral ground states) increases with the π-acidity of the catalysts, i.e., the existence of anion−π catalysis. Preliminary results on the general significance of anion−π catalysis beyond the Kemp elimination are briefly discussed

Controlling the crystalline three-dimensional order in bulk materials by single-wall carbon nanotubes

“…The presence of carboxylates and Na þ -from the basic soft conditions-complexes might influence the highest occupied molecular orbital/lowest unoccupied molecular orbita levels of the corresponding electroactive species, namely, exTTF and SWCNTs, or might even interact with them through anion-p or cation-p interactions 55,56 . However, our investigations reveal that on photoexcitation, localized exTTF-and SWCNT-excited states evolve and transform rapidly into well-characterized electron transfer products ruling out any major impact.…”

Highly efficient catalysis of the Kemp elimination in the cavity of a cubic coordination cage

“…29,30 The Kemp elimination has been studied in particular detail as an exemplar E2 elimination that can be adapted to a wide range of reaction rates, and as a sensitive probe for catalytic systems, both biological and artificial. [31][32][33][34][35][36] The reaction is first order in OH -under basic conditions and reaches a minimum rate at around pH 6 when water, rather than hydroxide, acts as the base. The observed rate constants for the base catalysed (dashed black line) and spontaneous (dashed blue line) reactions are shown in Fig.…”

Enolate chemistry with anion–π interactions

“…Pertinent anion transport experiments have identified anion-p interactions 1,4 and halogen bonds 5,6 as promising candidates to contribute to the acceleration of transformations with anionic transition states. Anion-p interactions [7][8][9][10][11][12][13][14][15][16][17][18][19][20][21][22][23][24] are particularly intriguing because the complementary cation-p interactions are essential in biocatalysis 25,26 and are increasingly used in organocatalysis 25,27,28 . In sharp contrast, anion-p interactions are essentially unexplored in catalysis 23,24 .…”

“…To attract anions, the negative quadrupole moment of p-basic aromatics has to be inverted. This is possible with the introduction of strongly electron-withdrawing substituents 1,4,[7][8][9][10][11][12][13][14][15][16][17][18][19][20][21][22][23][24] . Popular examples are hexafluorobenzene or 2,4,6-trinitrotoluene.…”

“…Popular examples are hexafluorobenzene or 2,4,6-trinitrotoluene. Naphthalenediimides (NDIs) have been identified as privileged platforms to explore anion-p interactions because their intrinsic quadrupole moment is very high, and the introduction of withdrawing substituents provides access to the strongest organic p-acids known today 1,23,24 .…”