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

Abstract

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.

Conformational properties of 1-cyano-1-silacyclohexane, C5H10SiHCN: Gas electron diffraction, low-temperature NMR and quantum chemical calculations

“…Whereas a solvation effect, possibly related to the polarity of the cyano group, we note that continuum solvation calculations, by using both the COSMO 43 and SMD 44 solvation models (implicit solvation models), were carried out separately at the B3LYP give positive A values both in gas-phase and in solutions. In this context, however, it is notable that some 1-methyl-3-hetero(X)-1-silacyclohexanes (X = NMe, 56 O, 57 and S 58,59 ) also show Me-axial preferences in the gas-phase (GED) but Me-equatorial preferences by DNMR in solutions.…”

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

“…As expected, due to small conformational energy A Si (Me) of 0.23 kcal/mol the Me ax :Me eq ratio was 35:65. The activation barrier was very small, 4.6 kcal/mol [71]. In gas phase the Me ax conformer slightly predominates (54:46).…”

“…They include disubstituted at silicon compounds with (Me, F), (Me, OPr-i) and (Me, Ph) pairs of substituents. The simplest Si-chiral compound of this series, 3-methyl-3-silatetrahydropyran, was synthesized by the base-catalyzed cyclization followed by dephenylation and reduction [71] as shown in Figure 17. The problem was that it is the most volatile compound as compared to other analogues having boiling point of 62 °C at 104 mm Hg and is easily lost with the solvent during isolation.…”

“…They include disubstituted at silicon compounds with (Me, F), (Me, OPr-i) and (Me, Ph) pairs of substituents. The simplest Si-chiral compound of this series, 3-methyl-3-silatetrahydropyran, was synthesized by the base-catalyzed cyclization followed by dephenylation and reduction [71] as shown in Figure 17 Such a flexibility might be suggestive of the cis-fused structure of the two rings in solution, because in decalin the ring inversion is possible only in the cis-isomer, whereas the structure of the trans isomer is rigid. This seeming discrepancy can be rationalized by the fact that in N-fused compounds, such as quinolizidine, inversion at nitrogen atom is possible, as shown in Figure 15.…”

Very low‐temperature dynamic 29Si NMR study of the conformational equilibrium of (1,1′‐phenyl‐1,1′‐silacyclohex‐1‐yl)disiloxane

“…In continuation of our ongoing research on the conformational analysis of silacyclohexanes and sila(hetero)cyclohexanes, we synthesized (1,1′‐phenyl‐1,1′‐silacyclohex‐1‐yl)disiloxane 1 and examined its low‐temperature 1 H and 13 C NMR spectra; unfortunately, neither were informative as far as the conformational composition of compound 1 is concerned. Therefore, we estimated the relative energies of the three possible conformers (Figure ) theoretically at the M062X/6‐311G(d,p) and MP2/6‐311G(d,p) levels of theory and showed the ax – ax conformer to be the most stable and the eq – eq conformer the least stable one (cf.…”