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
Scheme 6. Diastereoselective Additions of Me 3 SiCN (1) to N-Protected Amino Aldehydes 84 Scheme 7. Enantioselective Conjugate Additions of Me 3 SiCN (1) 85 Scheme 8. Regioselective Additions of Me 3 SiCN (1) to Enone 32 86 Scheme 9. Three-Component Silylative Strecker Reaction (a) and Strecker-Type Cyanosilylation of Imines (b) as the Routes to Silylated r-Aminonitriles Scheme 10. Unexpected Course of Addition of Me 3 SiCN (1) to N-Sulfinylimine 35 105
Chiral macrocyclic tetra-and hexamine macrocycles derived from trans-1,2-diaminocyclohexane (DACH) in complexes with diethylzinc efficiently catalyze the asymmetric hydrosilylation of aryl alkyl ketones with enantiomeric excess of the product up to 89%. The cyclic structure of the trianglamine ligand increases the enantioselectivity of the reaction, in comparison to the catalysis with the acyclic N,N'-dibenzyl-DACH ligand. Density functional theory (DFT) computations on the structures of ligand-zinc complexes and on the structures of these complexes with a coordinated acetophenone molecule allow us to rationalize the direction of the asymmetric induction of the hydrosilylation reaction as well as the superiority of the cyclic ligand compared to the acyclic one. This is the first example of asymmetric catalysis for the hydrosilylation reaction of ketones with the use of a readily available, inexpensive, and reusable macrocyclic trianglamine ligand.
Chiral hexamine macrocycle derived from trans-1,2-diaminocyclohexane (DACH) in a complex with diethylzinc efficiently catalyzes the asymmetric hydrosilylation of N-phosphorylated aryl-alkyl or aryl-aryl ketimines in protic media with enantiomeric excess of the product approaching 100%. The cyclic structure of the trianglamine ligand increases the enantioselectivity and/or the yield of the reaction, in comparison to the catalysis by acyclic N,N'-dibenzyl-DACH ligands. Density functional theory (DFT) computations on the structure of the model ligand-zinc complex and on the structures of the pre-organized reactants together with the calculations of possible transition states allow rationalization of the direction of the asymmetric induction of the hydrosilylation reaction. This is the first example of asymmetric catalysis of the hydrosilylation reaction of ketimines with the use of a readily available and inexpensive macrocyclic trianglamine ligand.
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