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Of central importance in chemistry and biology, enolate chemistry is an attractive topic to elaborate on possible contributions of anion-π interactions to catalysis. To demonstrate the existence of such contributions, experimental evidence for the stabilization of not only anions but also anionic intermediates and transition states on π-acidic aromatic surfaces is decisive. To tackle this challenge for enolate chemistry with maximal precision and minimal uncertainty, malonate dilactones are covalently positioned on the π-acidic surface of naphthalenediimides (NDIs). Their presence is directly visible in the upfield shifts of the α-protons in the 1H NMR spectra. The reactivity of these protons on π-acidic surfaces is measured by hydrogen-deuterium (H-D) exchange for 11 different examples, excluding controls. The velocity of H-D exchange increases with π acidity (NDI core substituents: SO2R>SOR>H>OR>OR/NR2>SR>NR2). The H-D exchange kinetics vary with the structure of the enolate (malonates>methylmalonates, dilactones>dithiolactones). Moreover, they depend on the distance to the π surface (bridge length: 11-13 atoms). Most importantly, H-D exchange depends strongly on the chirality of the π surface (chiral sulfoxides as core substituents; the crystal structure of the enantiopure ( R, R, P)-macrocycle is reported). For maximal π acidity, transition-state stabilizations up to −18.8 kJ mol−1 are obtained for H-D exchange. The Brønsted acidity of the enols increases strongly with π acidity of the aromatic surface, the lowest measured p Ka=10.9 calculates to a Δp Ka=−5.5. Corresponding to the deprotonation of arginine residues in neutral water, considered as 'impossible' in biology, the found enolate-π interactions are very important. The strong dependence of enolate stabilization on the unprecedented seven-component π-acidity gradient over almost 1 eV demonstrates quantitatively that such important anion-π activities can be expected only from strong enough π acids. [ABSTRACT FROM AUTHOR]

Irradiation of 2-phenyl-1-naphthol ( 6) in CH3CN/D2O (3:1) leads to very efficient incorporation of deuterium at the ortho-positions of the adjacent phenyl ring (overall Φ=0.73±0.07), along with minor incorporation at the naphthalene positions 5 and 8. These finding are explained by excited state intramolecular proton transfer (ESIPT) from the phenolic OH group to the corresponding carbon atoms, the main pathway giving rise to quinone methide (QM) 7, which has been characterized by LFP ( τ≈20 ns; 460 nm). The ESIPT reaction paths have been explored with the second order approximate coupled cluster (CC2) method. In nonprotic solvents the ESIPT from the naphthol O-H to the ortho-position of the phenyl ring proceeds in a barrierless manner along the 1La energy surface via a conical intersection with the S0 state, delivering 7. In aqueous solvent, clusters with H2O are formed wherein proton transfer (PT) to solvent and a H2O-mediated relay mechanism gives rise to naphtholates and QMs. The results are compared with 2-phenylphenol ( 3) that also undergoes barrierless ESIPT giving a QM via a conical intersection. However, due to an unfavorable conformation in the ground state, the quantum efficiency for ESIPT of 3 is significantly lower ( Φ for D-exchange=0.041). These results show that ESIPT from phenol to carbon need not be an intrinsically inefficient process. [ABSTRACT FROM AUTHOR]

The crystal structure of a malonate dilactone, placed on the π‐acidic surface of a naphthalenediimide with chiral sulfoxide acceptors in the core, is depicted on the cover. Such macrodilactones are essential to directly follow enolate–π interactions by NMR spectroscopy, also during deuterium exchange. This least interfering method is used to extract all there is to learn about the stabilization of enolate intermediates and transition states on π‐acidic aromatic surfaces. More information can be found in the Full Paper by S. Matile et al. on page 2648 ff. [ABSTRACT FROM AUTHOR]