Stacked but not Stuck: Unveiling the Role of π→π* Interactions with the Help of the Benzofuran-Formaldehyde Complex

Highly Acidic Electron-Rich Brønsted Acids Accelerate Asymmetric Pictet-Spengler Reactions by Virtue of Stabilizing Cation-π Interactions

Electron-rich heteroaromatic imidodiphosphorimidates (IDPis) catalyze the asymmetric Pictet-Spengler reaction of N-carbamoyl-β-arylethylamines with high stereochemical precision. This particular class of catalysts furthermore provides a vital rate enhancement compared to related Brønsted acids. Here we present experimental studies on the underlying reaction kinetics that shed light on the specific origins of rate acceleration. Analysis of Hammett plots, kinetic isotope effects, reaction orders, Eyring plots, and isotopic scrambling experiments, allowed us to gather insights into the molecular interactions between the chiral Brønsted acid and catalytically formed intermediates. Based on rigorously determined pKa values as well as the experimental evidence, we propose that attractive intermolecular forces offered by electron-rich π-surfaces of the chiral counteranion enthalpically stabilize cationic intermediates and transition states by way of cation-π interactions. This view is furthermore supported by in-depth density functional theory calculations. Our deepened understanding of the reaction mechanism allowed us to develop a method for accessing 1-aryltetrahydroisoquinolines from aromatic dimethyl acetals, a substrate class that was thus far inaccessible via catalytic asymmetric Pictet-Spengler reactions.

The Semiexperimental Approach at Work: Equilibrium Structure of Radical Species

The so-called semiexperimental (SE) approach is a powerful technique for obtaining highly accurate equilibrium structures for isolated systems. This Featured Article describes its extension to open-shell species, thus providing the first systematic investigation on radical equilibrium geometries to be used for benchmarking purposes. The small yet significant database obtained demonstrates that there is no reduction in accuracy when moving from closed-shell species to radicals. We also provide an extension of the applicability of the SE approach to medium-/large-sized radicals by exploiting the so-called "Lego-brick" approach, which is based on the assumption that a molecular system can be seen as formed by smaller fragments for which the SE equilibrium structure is available. In this Featured Article we show that this model can be successfully applied also to open-shell species.

Unveiling the Noncovalent Interactions between Formamide and Heteroaromatics: Microwave Spectroscopy of the Formamide Complexes with Furan and Thiophene

The noncovalent interactions between formamide (FM) and the heteroaromatic compounds (furan and thiophene) were investigated through microwave spectroscopy and theoretical calculations. Each of the investigated complexes exhibits a single rotational spectrum corresponding to the lowest energy structure predicted theoretically. In the detected structures, N-H···O and C-H···O hydrogen bonds dominate the complexation between FM and furan, resulting in a planar configuration. Conversely, a superposed configuration linked by a N-H···π hydrogen bond and C═O···π contact is observed for the FM-thiophene complex. In both cases, hydrogen bonding interactions with N-H as proton donor rank as the dominant forces, and the interaction energy of N-H···O is larger than that of N-H···π. It was found that the electrostatic component is the largest contributor to the attraction between FM and furan, while the dispersion component is the most significant attractive factor in the FM-thiophene complex. These findings highlight the distinct features of hydrogen bonding interactions of amides with heteroaromatics in the studied complexes.

Engineering π-Conjugation of Phenylalanine Derivatives for Controllable Chiral Folding and Self-Assemblies

π-π stacking interaction is an attractive interaction that involves aromatic groups containing π-conjugated domains. It is a promising strategy for stabilizing folded structures with interesting chiroptical properties and manipulating the supramolecular chiral self-assembly process. In this study, we report the engineering of π-conjugated amino acids that utilize π-π stacking interactions to manipulate chiral folding as well as self-assembly evolution. Stepwise conjugation of phenyl, naphthyl, and pyrenyl to N-terminal phenylalanine derivatives witnessed the folding through intramolecular π-interactions in solution phase, which facilitated the formation of chiral geometry and the emergence of chiral optics. Introduction of aromatic domains efficiently lowers the critical aggregation concentration in the aqueous media. Molecular folding enables a special concentration-dependent self-assembly, whereby the supramolecular chirality accomplished inversion with the evolution of helical nanoarchitectures. This work develops a strategy to engineer π-conjugated amino acids with controllable folding behaviors, which also offers implications for the rational design of functional chiral materials.

Conformational equilibria and interaction preference in the complex of isoprene-maleic anhydride

The rotational spectrum of the isoprene-maleic anhydride complex has been investigated by pulsed jet Fourier transform microwave spectroscopy and interpreted with complementary quantum chemical calculations. Theoretical predictions have yielded four plausible isomers, all residing within an energy window of 12 kJ mol-1. However, two distinct isomers characterized by a π-π stacked configuration have been experimentally observed in pulsed jets, which have differed in the orientation of isoprene over maleic anhydride. The relative population ratio of the two detected isomers has been estimated to be NI/NII ≈ 3/1 from rigorous measurements of the relative intensity on a set of μc-type transitions. Remarkably, this study underscores the pivotal role played by the interaction between the CC bonding orbital (π) of isoprene and the CC antibonding orbital (π*) of maleic anhydride in stabilizing the target complex.

Noncovalent interactions of aromatic heterocycles: rotational spectroscopy and theoretical calculations of the thiazole-CF4 and thiazole-SF6 complexes

The complexes of thiazole with CF4 and SF6 have been investigated by Fourier transform microwave spectroscopy and quantum chemical calculations. One rotational spectrum was observed for the thiazole-CF4 complex. Experiments and theoretical computations confirmed that the observed structure of thiazole-CF4 is primarily formed due to N⋯CCF4 interaction with the C atom of CF4 located in the plane of the thiazole ring. The rotational transitions of thiazole-CF4 exhibit A/E torsional splitting induced by the internal rotation of the -CF3 top. The potential barrier of the -CF3 internal rotation is 0.2411(1) kJ mol-1, consistent with the calculated value (∼0.3 kJ mol-1). For the thiazole-SF6 complex, one conformer with SF6 located above the thiazole ring is detected. The observed structure of thiazole-SF6 is mainly stabilized by van der Waals interactions. The energy decomposition analysis reveals that the electrostatics and dispersion are the dominant attractive contributions to the formation of thiazole-CF4 and thiazole-SF6 dimers, whereas the weight of the dispersion term becomes more significant in the thiazole-SF6 complex compared to that of the thiazole-CF4 complex.

Noncovalent Interactions between Aromatic Heterocycles and Carboxylic Acids: Rotational Spectroscopy of the Furan-Formic Acid and Thiophene-Formic Acid Complexes

The binary molecular complexes formed between the aromatic heterocycles furan and thiophene with formic acid were investigated using pulsed-jet Fourier transform microwave spectroscopy and quantum chemical computations. For both of the complexes, rotational spectra of the lowest energy isomer were detected and assigned. Rotational spectroscopic results and density functional theory calculations support that the preferred conformation of the furan-formic acid complex is characterized by a relatively strong O-H···O and a weak C-H···O hydrogen bonds while the O-H···π and C-H···O hydrogen bonds stabilize the thiophene-formic acid complex. Natural bond orbital analysis further proves the experimental observation, suggesting that the strength of the O-H···O(furan) interaction is about two times stronger than that of O-H···π(thiophene). The symmetry adapted perturbation theory analysis reveals that electrostatic interaction is dominant in stabilizing the two complexes and that dispersion becomes significant in the thiophene-formic acid complex compared to furan-formic acid.