RESUMO
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 Phax â Pheq 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 Pheq 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 Phax conformer in the gas phase and of Pheq 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 Phax conformer and additional destabilization of 3-Pheq conformer by repulsion of unidirectional dipoles of the endocyclic oxygen lone pair and of the highly polar axial Si-O bond.
RESUMO
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.
RESUMO
The first Si-H-containing azasilaheterocycle, 1,3-dimethyl-3-silapiperidine 1, was synthesized, and its molecular structure and conformational properties were studied by gas-phase electron diffraction (GED), low temperature NMR, IR and Raman spectroscopy and quantum chemical calculations. The compound exists as a mixture of two conformers possessing the chair conformation with the equatorial NMe group and differing by axial or equatorial position of the SiMe group. In the gas phase, the SiMe(ax) conformer predominates (GED: ax/eq = 65(7):35(7)%, ΔG = 0.36(18) kcal/mol; IR: ax/eq = 62(5):38(5)%, ΔG = 0.16(7) kcal/mol). In solution, at 143 K the SiMe(eq) conformer predominates in the frozen equilibrium (NMR: ax/eq = 31.5(1.5):68.5(1.5)%, ΔG = -0.22(2) kcal/mol). Thermodynamic parameters of the ring inversion are determined (ΔG() = 8.9-9.0 kcal/mol, ΔH() = 9.6 kcal/mol, ΔS() = 2.1 eu). High-level quantum chemical calculations (MP2, G2, CCSD(T)) nicely reproduce the experimental geometry and the predominance of the axial conformer in the gas phase.
RESUMO
Structure and the conformational properties of 1,3,3-trimethyl-1,3-azasilinane have been studied. According to gas electron diffraction (GED), the molecule exists in a slightly distorted chair conformation with the N-Me group in equatorial position. High-level quantum chemical calculations excellently reproduce the experimental geometry. Employing variable temperature (1)H and (13)C NMR spectroscopy down to 103 K, the conformational equilibrium could be frozen and the barrier to ring inversion determined.
RESUMO
The sila-Pummerer rearrangement of organosilicon cyclic sulfoxides proceeds via the two transition states, the first one with pentacoordinated Si that connects the reagent and the intermediate ylide and the second one that connects the ylide and the product of rearrangement.
