A Molecular Seesaw Balance: Evaluation of Solvent and Counteranion Effects on Pyridinium−π Interactions

Mimicking enzymatic cation-π interactions in hydrazide catalyst design: access to trans-decalin frameworks

Chiral bicyclic hydrazide organocatalysts have previously been shown to catalyze the cyclization of (Z)-polyene substrates with high enantioselectivity, but with poor selectivity for the corresponding (E)-polyenes. Here we demonstrate that diazapane carboxylates bearing terphenyl groups efficiently catalyze (E)-polyene bicyclization with enantioselectivities up to 94 : 6 er and with high diastereoselectivity for trans-decalin formation. The catalysts function by simultaneously initiating the cyclization via iminium ion formation and stabilizing intermediates/transition states by cation-π interactions.

Unveiling the role of pyrylium frameworks on π-stacking interactions: a combined ab initio and experimental study

A multidisciplinary study is presented to shed light on how pyrylium frameworks, as π-hole donors, establish π-π interactions. The combination of CSD analysis, computational modelling (ab intitio, DFT and MD simulations) and experimental NMR spectroscopy data provides essential information on the key parameters that characterize these intereactions, opening new avenues for further applications of this versatile heterocycle.

Benchmarking London dispersion corrected density functional theory for noncovalent ion-π interactions

The strongly attractive noncovalent interactions of charged atoms or molecules with π-systems are important binding motifs in many chemical and biological systems. These so-called ion-π interactions play a major role in enzymes, molecular recognition, and for the structure of proteins. In this work, a molecular test set termed IONPI19 is compiled for inter- and intramolecular ion-π interactions, which is well balanced between anionic and cationic systems. The IONPI19 set includes interaction energies of significantly larger molecules (up to 133 atoms) than in other ion-π test sets and covers a broad range of binding motifs. Accurate (local) coupled cluster values are provided as reference. Overall, 19 density functional approximations, including seven (meta-)GGAs, eight hybrid functionals, and four double-hybrid functionals combined with three different London dispersion corrections, are benchmarked for interaction energies. DFT results are further compared to wave function based methods such as MP2 and dispersion corrected Hartree-Fock. Also, the performance of semiempirical QM methods such as the GFNn-xTB and PMx family of methods is tested. It is shown that dispersion-uncorrected DFT underestimates ion-π interactions significantly, even though electrostatic interactions dominate the overall binding. Accordingly, the new charge dependent D4 dispersion model is found to be consistently better than the standard D3 correction. Furthermore, the functional performance trend along Jacob's ladder is generally obeyed and the reduction of the self-interaction error leads to an improvement of (double) hybrid functionals over (meta-)GGAs, even though the effect of the SIE is smaller than expected. Overall, the double-hybrids PWPB95-D4/QZ and revDSD-PBEP86-D4/QZ turned out to be the most reliable among all assessed methods for the description of ion-π interactions, which opens up new perspectives for systems where coupled cluster calculations are no longer computationally feasible.

Shedding Light on the Interactions of Hydrocarbon Ester Substituents upon Formation of Dimeric Titanium(IV) Triscatecholates in DMSO Solution

The dissociation of hierarchically formed dimeric triple lithium bridged triscatecholate titanium(IV) helicates with hydrocarbyl esters as side groups is systematically investigated in DMSO. Primary alkyl, alkenyl, alkynyl as well as benzyl esters are studied in order to minimize steric effects close to the helicate core. The 1 H NMR dimerization constants for the monomer-dimer equilibrium show some solvent dependent influence of the side chains on the dimer stability. In the dimer, the ability of the hydrocarbyl ester groups to aggregate minimizes their contacts with the solvent molecules. Due to this, most solvophobic alkyl groups show the highest dimerization tendency followed by alkenyls, alkynyls and finally benzyls. Furthermore, trends within the different groups of compounds can be observed. For example, the dimer is destabilized by internal double or triple bonds due to π-π repulsion. A strong indication for solvent supported London dispersion interaction between the ester side groups is found by observation of an even/odd alternation of dimerization constants within the series of n-alkyls, n-Ω-alkenyls or n-Ω-alkynyls. This corresponds to the interaction of the parent hydrocarbons, as documented by an even/odd melting point alternation.

Chasing Weak Forces: Hierarchically Assembled Helicates as a Probe for the Evaluation of the Energetics of Weak Interactions

London dispersion forces are the weakest interactions between molecules. Because of this, their influence on chemical processes is often low, but can definitely not be ignored, and even becomes important in cases of molecules with large contact surfaces. Hierarchically assembled dinuclear titanium(IV) helicates represent a rare example in which the direct observation of London dispersion forces is possible in solution even in the presence of strong cohesive solvent effects. Hereby, the dispersion forces do not unlimitedly support the formation of the dimeric complexes. Although they have some favorable enthalpic contribution to the dimerization of the monomeric complex units, large flexible substituents become conformationally restricted by the interactions leading to an entropic disadvantage. The dimeric helicates are entropically destabilized.

Regioselectivity and Mechanism of Synthesizing N-Substituted 2-Pyridones and 2-Substituted Pyridines via Metal-Free C-O and C-N Bond-Cleaving of Oxazoline[3,2-a]pyridiniums

Novel intermediate oxazoline[3,2-a]pyridiniums were facilely prepared from 2-(2,2-dimethoxyethoxy)-pyridines via acid promoted intramolecular cyclization. Sequentially, the quaternary ammonium salts were treated with different nucleophiles for performing regioselective metal-free C-O and C-N bond-cleaving to afford prevalent heterocyclic structures of N-substituted pyridones and 2-substituted pyridines. The reaction mechanism and regioselectivity were then systematically explored by quantum chemistry calculations at B3LYP/6-31 g(d) level. The calculated free energy barrier of the reactions revealed that aniline and aliphatic amines (e.g., methylamine) prefer to attack C8 of intermediate 4a, affording N-substituted pyridones, while phenylmethanamine, 2-phenylethan-1-amine and 3-phenylpropan-1-amine favor to attack C2 of the intermediate to form 2-substituted pyridines. With the optimized geometries of the transition states, we found that the aromatic ring of the phenyl aliphatic amines may form cation-π interaction with the pyridinium of the intermediates, which could stabilize the transition states and facilitate the formation of 2-substituted pyridines.