Silacyclohexanes and silaheterocyclohexanes—why are they so different from other heterocyclohexanes?

At the Experimental Limit of the NMR Conformational Analysis: 29Si and 13C NMR Study of the Conformational Equilibrium of 1-Phenyl-1-tert-butylsilacyclohexane

The low temperature (95 K) NMR study of 1-Ph-1-t-Bu-silacyclohexane (1) showed the conformational equilibrium to be extremely one-sided toward thePh_ax,t-Bu_eq conformer. The barrier to interconversion has been measured (4.2-4.6 kcal/mol) and the conformational equilibrium [Δν = 1990.64 ppm (²⁹Si), 618.9 ppm (¹³C), 1-Ph_ax:1-Ph_eq = (95.6-96.6%):(3.4-4.4%), K = 25 ± 3, ΔG° = -RT ln K = 0.58-0.63 kcal/mol] analyzed. The assignment and quantification of the NMR signals is supported by MP2 and DFT calculations.

Stereochemistry of Simple Molecules inside Nanotubes and Fullerenes: Unusual Behavior of Usual Systems

Over the past three decades, carbon nanotubes and fullerenes have become remarkable objects for starting the implementation of new models and technologies in different branches of science. To a great extent, this is defined by the unique electronic and spatial properties of nanocavities due to the ramified π-electron systems. This provides an opportunity for the formation of endohedral complexes containing non-covalently bonded atoms or molecules inside fullerenes and nanotubes. The guest species are exposed to the force field of the nanocavity, which can be described as a combination of electronic and steric requirements. Its action significantly changes conformational properties of even relatively simple molecules, including ethane and its analogs, as well as compounds with C-O, C-S, B-B, B-O, B-N, N-N, Al-Al, Si-Si and Ge-Ge bonds. Besides that, the cavity of the host molecule dramatically alters the stereochemical characteristics of cyclic and heterocyclic systems, affects the energy of pyramidal nitrogen inversion in amines, changes the relative stability of cis and trans isomers and, in the case of chiral nanotubes, strongly influences the properties of R- and S- enantiomers. The present review aims at primary compilation of such unusual stereochemical effects and initial evaluation of the nature of the force field inside nanotubes and fullerenes.

Silacyclohexanes, Sila(hetero)cyclohexanes and Related Compounds: Structure and Conformational Analysis

Conformational analysis of Si-mono- and Si,Si-disubstituted silacyclohexanes as well as their analogues with a heteroatom(s) in the ring is reviewed with the focus on the recent results. Experimental measurements in the gas phase (gas electron diffraction, GED) and low temperature NMR spectroscopy (LT NMR) on ¹H, ¹³C and ²⁹Si nuclei are described along with theoretical calculations at the DFT and MP2 levels of theory. Structural and conformational specific features are shown to be principally different from those of the carbon predecessors—the corresponding cyclohexanes, oxanes, thianes and piperidines. The role of various effects (steric, hyperconjugation, stereoelectronic, electrostatic) is demonstrated.

Conformational Preferences of the Phenyl Group in 1-Phenyl-1-X-1-silacyclo-hexanes (X = MeO, HO) and 3-Phenyl-3-X-3-silatetrahydropyrans (X = HO, H) by Low Temperature 13C NMR Spectroscopy and Theoretical Calculations

New Si-phenyl-substituted silacyclohexanes and 3-silatetrahydropyrans have been synthesized and studied with respect to the conformational equilibria of the heterosix-membered ring by low temperature ¹³C NMR spectroscopy and quantum chemical calculations. For 1-methoxy-1-phenylsilacyclohexane 1 and 3-phenyl-3-silatetrahydropyran 4 the conformational equilibria could be frozen and assigned. The Ph_ax ⇆ Ph_eq equilibrium constants at 103 K are 2.21 for 1 and 4.59 for 4. In complete agreement with former studies of similar silicon compounds, molecules 1 and 4 prefer to adopt the Ph_eq conformation. The conformational equilibria of 1-hydroxy-1-phenylsilacyclohexane 2 and 3-hydroxy-3-phenyl-3-silatetrahydropyran 3 could not be frozen at 100 K and proved to be heavily one-sided (if not anancomeric). Obviously, there is a general trend of predominance of Ph_ax conformer in the gas phase and of Ph_eq in solution. For the isolated molecules of silanols 2 and 3, calculations allowed to explain the axial predominance of the phenyl group by a larger polarization of the Si-Ph than of the Si-O bond in the Ph_ax conformer and additional destabilization of 3-Ph_eq conformer by repulsion of unidirectional dipoles of the endocyclic oxygen lone pair and of the highly polar axial Si-O bond.

Molecular Structure and Conformational Analysis of 1-Phenyl-1-X-1-Silacyclohexanes (X = F, Cl) by Electron Diffraction, Low-Temperature NMR, and Quantum Chemical Calculations

The molecular structure and conformational preferences of 1-phenyl-1-X-1-silacyclohexanes C₅H₁₀Si(Ph,X) (X = F (3), Cl (4)) were studied by gas-phase electron diffraction, low-temperature NMR spectroscopy, and high-level quantum chemical calculations. In the gas phase only three (3) and two (4) stable conformers differing in the axial or equatorial location of the phenyl group and the angle of rotation about the Si-C_Ph bond (axi and axo denote the Ph group lying in or out of the X-Si-C_Ph plane) contribute to the equilibrium. In 3 the ratio Ph_eq:Ph_axo:Ph_axi is 40(12):55(24):5 and 64:20:16 by experiment and theory, respectively. In 4 the ratio Ph_eq:Ph_axo is 79(15):21(15) and 71:29 by experiment and theory (M06-2X calculations), respectively. The gas-phase electron diffraction parameters are in good agreement with those obtained from theory at the M06-2X/aug-ccPVTZ and MP2/aug-cc-pVTZ levels. Unlike the case for M06-2X, MP2 calculations indicate that 3-Ph_eq conformer lies 0.5 kcal/mol higher than the 3-Ph_axo conformer. As follows from QTAIM analysis, the phenyl group is more stable when it is located in the axial position but produces destabilization of the silacyclohexane ring. By low-temperature NMR spectroscopy the six-membered-ring interconversion could be frozen at 103 K and the present conformational equilibria of 3 and 4 could be determined. The ratio of the conformers is 3-Ph_eq:3-Ph_ax = (75-77):(23-25) and 4-Ph_eq:4-Ph_ax = 82:18.

One-pot enyne metathesis/Diels–Alder/oxidation to six-membered silacycles with a multi-ring core: discovery of novel fluorophores

One-pot enyne metathesis/Diels–Alder/oxidation methodology was developed to give fluorescent six-membered silacycle with multi-ring core.

Synthesis and Conformational Analysis of 3-Methyl-3-silatetrahydropyran by GED, FTIR, NMR, and Theoretical Calculations: Comparative Analysis of 1-Hetero-3-methyl-3-silacyclohexanes

3-Methyl-3-silatetrahydropyran 1 was synthesized and its molecular structure and conformational behavior was studied by gas-phase electron diffraction (GED), FTIR, low temperature (1)H and (13)C NMR spectroscopy, and by theoretical calculations (DFT, MP2). Two conformers, 1-ax and 1-eq, were located on the potential energy surface. In the gas phase, a slight predominance of the axial conformer was determined, with the ratio 1-ax:1-eq = 54(9):46(9) (from GED) or 53:47 or 61:39 (from IR). In solution, LT NMR spectroscopy at 103 K gives the ratio 1-ax:1-eq = 35:65 (-ΔG°103 = 0.13 kcal/mol). Simulation of solvent effects using the PCM continuum model or by calculation of the corresponding solvent-solute complexes allowed us to rationalize the experimentally observed opposite conformational predominance of the conformers of 3-methyl-3-silatetrahydropyran in the gas phase and in solution. Comparative analysis of the effect of heteroatom in 1-hetero-3-methyl-3-silacyclohexanes on the structure, stereoelectronic interactions, and relative energies of the conformers is done.

Structural studies of 1,1-dimethyl-2-oxy-1-silacyclohexane by means of matrix isolation infrared absorption spectroscopy

Infrared spectra of gaseous, liquid, and matrix-isolated samples of newly synthesized 1,1-dimethyl-2-oxy-1-silacyclohexane were recorded. Raman spectra of 1,1-dimethyl-2-oxy-1-silacyclohexane in liquid and solid states were obtained in the temperature range from 170 to 340 K. Ab initio HF and DFT B3LYP calculations were performed in order to determine the possible conformations of 1,1-dimethyl-2-oxy-1-silacyclohexane and to make accurate assignment of the vibrational spectral bands. The study confirms the existence of only one chair-type conformer of 1,1-dimethyl-2-oxy-1-silacyclohexane.