Exploring the Limits of Intramolecular London Dispersion Stabilization with Bulky Dispersion Energy Donors in Alkane Solution.

We present an experimental study of a cyclooctatetraene-based molecular balance disubstituted with increasingly bulky tert-butyl (tBu), adamantyl (Ad), and diamantyl (Dia) substituents in the 1,4-/1,6-positions for which we determined the valence-bond shift equilibrium in n-hexane (hex), n-octane (oct), and n-dodecane (dod). Computations including implicit and explicit solvation support our temperature-dependent NMR equilibrium measurements indicating that the more sterically crowded 1,6-isomer is always favored, irrespective of solvent, and that the free energy is quite insensitive to substituent size.

Gauging the Steric Effects of Silyl Groups with a Molecular Balance.

We present an experimental and computational study of a cyclooctatetraene (COT)-based molecular balance disubstituted with commonly used silyl groups. Such groups often serve as protecting groups and are typically considered innocent bystanders. Our motivation here is to determine the actual steric effects of such groups by employing a molecular balance. While in the unfolded 1,4-valence isomer the silyl groups are far apart (dσ-σ ≥ 5.15 Å), the folded 1,6-isomer is affected greatly by noncovalent interactions due to close σ-σ contacts (dσ-σ ≤ 2.58 Å). In order to investigate the thermodynamic equilibrium between the 1,6- and 1,4-valence isomers, we employed temperature-dependent nuclear magnetic resonance measurements. Additionally, we assessed the nature of attractive and repulsive interactions in 1,6-disilyl-COT derivatives via a combination of local energy decomposition analysis (LED) and symmetry-adapted perturbation theory (SAPT) at the DLPNO-CCSD(T)/def2-TZVP and sSAPT0/aug-cc-pVDZ levels of theory. We identified London dispersion interactions as the main contributor to the molecular stability of the folded states, whereas Pauli exchange repulsion and a resulting internal strain favor the unfolded diastereomer.

Intramolecular London Dispersion Interactions Do Not Cancel in Solution.

We present a comprehensive experimental study of a di-t-butyl-substituted cyclooctatetraene-based molecular balance to measure the effect of 16 different solvents on the equilibrium of folded versus unfolded isomers. In the folded 1,6-isomer, the two t-butyl groups are in close proximity (H···H distance ≈ 2.5 Å), but they are far apart in the unfolded 1,4-isomer (H···H distance ≈ 7 Å). We determined the relative strengths of these noncovalent intramolecular σ-σ interactions via temperature-dependent nuclear magnetic resonance measurements. The origins of the interactions were elucidated with energy decomposition analysis at the density functional and ab initio levels of theory, pinpointing the predominance of London dispersion interactions enthalpically favoring the folded state in any solvent measured.

Hexaphenylditetrels - When Longer Bonds Provide Higher Stability.

We present a computational analysis of hexaphenylethane derivatives with heavier tetrels comprising the central bond. In stark contrast to parent hexaphenylethane, the heavier tetrel derivatives can readily be prepared. In order to determine the origin of their apparent thermodynamic stability against dissociation as compared to the carbon case, we employed local energy decomposition analysis (LED) and symmetry-adapted perturbation theory (SAPT) at the DLPNO-CCSD(T)/def2-TZVP and sSAPT0/def2-TZVP levels of theory. We identified London dispersion (LD) interactions as the decisive factor for the molecular stability of heavier tetrel derivatives. This stability is made possible owing to the longer (than C-C) central bonds that move the phenyl groups out of the heavily repulsive regime so they can optimally benefit from LD interactions.

Alcohol cross-coupling for the kinetic resolution of diols via oxidative esterification.

We present an organocatalytic C-O-bond cross-coupling strategy to kinetically resolve racemic diols with aromatic and aliphatic alcohols, yielding enantioenriched esters. This one-pot protocol utilizes an oligopeptide multicatalyst, m-CPBA as the oxidant, and N,N'-diisopropylcarbodiimide as the activating agent. Racemic acyclic diols as well as trans-cycloalkane-1,2-diols were kinetically resolved, achieving high selectivities and good yields for the products and recovered diols.