Silyl Groups Are Strong Dispersion Energy Donors

Abstract

We present an experimental and computational study to investigate noncovalent interactions between silyl groups that are often employed as “innocent” protecting groups. We chose an extended cyclooctatetraene (COT)-based molecular balance comprising unfolded (1,4-disubstituted) and folded (1,6-disubstituted) valance bond isomers that typically display remote and close silyl group contacts, respectively. The thermodynamic equilibria were determined using nuclear magnetic resonance measurements. Additionally, we utilized Boltzmann weighted symmetry-adapted perturbation theory (SAPT) at the sSAPT0/aug-cc-pVDZ level of theory to dissect and quantify noncovalent interactions. Apart from the extremely bulky tris­(trimethylsilyl)­silyl “supersilyl” group, there is a preference for the folded 1,6-COT valence isomer, with London dispersion interactions being the main stabilizing factor. This makes silyl groups excellent dispersion energy donors, a finding that needs to be taken into account in synthesis planning.

Context-Dependent Significance of London Dispersion

“…Moreover, the syn – syn preference was greatest for those bearing the largest alkyl groups. These results contrast with the behavior of silyl substituted COT where steric hindrance was not compensated by LD forces . However, it is imperative to underscore that this study was limited to THF, and solvents with different properties might have exerted a significant influence on the outcomes.…”

“…However, the use of intramolecular systems brings other context-dependent caveats, such as steric strain. 20,25 Meanwhile, the geometries of specific functional group interactions in intramolecular contexts may differ from those of intermolecular complexes, which may be closer to the isolated equilibrium separation and geometry. In addition, electron delocalization via induction/polarization makes larger contributions to molecular interactions at short separations, which may further cloud the view of LD.…”

“…However, the use of intramolecular systems brings other context-dependent caveats, such as steric strain. 20,25 Meanwhile, the geometries of specific functional group interactions in intramolecular contexts may differ from those of intermolecular complexes, which may be closer to the isolated equilibrium separation and geometry. In addition, Due to the experimental challenge of discerning the significance of LD interactions against multiple background contributions, it is increasingly common for studies of LD to compare experimental results with computationally determined geometries and energies.…”

“…1,13,16 In a follow-up study, Schreiner and co-workers expanded their investigation of COT balances by incorporating silyl groups. Despite the established reputation of these groups as good LD energy donors, 20 the equilibrium consistently exhibited a shift toward the unfolded 1,4 conformation at varying temperatures, with the trimethyl silane variant standing as the sole exception. The general trend was explained by the voluminous nature of the silyl groups, which were found to strain the core structure of the COT in the 1,6-conformation.…”

“…These results contrast with the behavior of silyl substituted COT where steric hindrance was not compensated by LD forces. 20 However, it is imperative to underscore that this study was limited to THF, and solvents with different properties might have exerted a significant influence on the outcomes. Additionally, the exploration primarily focused on relatively small alkyl groups.…”