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The 3,3-disubstitued 3-silaheterocyclohexane with an electronegative substituent at silicon, 3-fluoro-3-methyl-3-silatetrahydropyran 1, was synthesized, and its molecular structure and conformational properties studied by gas-phase electron diffraction (GED) and low temperature C-13 and F-19 NMR spectroscopy. Quantum-chemical calculations were carried out both for the isolated species and Hcomplexes in gas and in polar medium. The predominance of the 1-FeqMeax conformer (1-F-eq:1-F-ax ratio of 65:35, Delta G degrees = 0.37 kcal/mol) determined from GED is close to the theoretically estimated conformational equilibrium, especially at the DFT level. In solution, low temperature NMR spectroscopy showed no decoalescence of the signals in C-13 (down to 95 K) and F-19 NMR spectra (down to 123 K). However, the calculated F-19 chemical shift of -173.6 ppm for the 1-FeqMeax conformer practically coincides with the experimentally observed value (-173 to -175 ppm) as distinct from that for the 1-FaxMeeq conformer (-188.8 ppm), suggesting compound 1 to be anancomeric in solution, in compliance with its theoretical and experimental preference in the gas phase.
The first conformational analysis of 3-silathiane and its C-substituted derivatives, namely, 3,3-dimethyl-3- silathiane 1, 2,3,3-trimethyl-3-silathiane 2, and 2-trimethylsilyl-3,3-dimethyl-3-silathiane 3 was performed by using dynamic NMR spectroscopy and B3LYP/6-311G(d,p) quantum chemical calculations. From coalescence temperatures, ring inversion barriers ;G; for 1 and 2 were estimated to be 6.3 and 6.8;kcal/mol, respectively. These values are considerably lower than that of thiacyclohexane (9.4;kcal/mol) but slightly higher than the one of 1,1- dimethylsilacyclohexane (5.5;kcal/mol). The conformational free energy for the methyl group in 2 (;;G°;=;0.35;kcal/mol) derived from low-temperature 13C NMR data is fairly consistent with the calculated value. For compound 2, theoretical calculations give ;E value close to zero for the equilibrium between the 2-Meax and 2-Meeq conformers. The calculated equatorial preference of the trimethylsilyl group in 3 is much more pronounced (;;G°;=;1.8;kcal/mol) and the predominance of the 3-SiMe3 eq conformer at room temperature was confirmed by the simulated 1H NMR and 2D NOESY spectra. The effect of the 2-substituent on the structural parameters of 2 and 3 is discussed.
The conformational equilibria of 3-methyl-3-silathiane 5, 3-fluoro-3-methyl-3-silathiane 6 and 1-fluoro-1-methyl-1- silacyclohexane 7 have been studied using low temperature C-13 NMR spectroscopy and theoretical calculations. The conformer ratio at 103 K was measured to be about 5(ax):5(eq) - 15:85, 6(ax):6(eq)-50:50 and 7(ax):7(eq)-25:75. The equatorial preference of the methyl group in 5 (0.35 kcal mol(-1)) is much less than in 3-methylthiane 9 (1.40 kcal mol(-1)) but somewhat greater than in 1-methyl-1-silacyclohexane 1 (0.23 kcal mol(-1)). Compounds 5-7 have low barriers to ring inversion: 5.65 (ax -> eq) and 6.0 kcal mol(-1) (eq -> ax) (5), 4.6 kcal mol(-1) (6), 5.1 kcal mol(-1) (Me-ax -> Me-eq), and 5.4 kcal mol(-1) (Me-eq -> Me-ax) (7). Steric effects cannot explain the observed conformational preferences, like equal population of the two conformers of 6, or different conformer ratio for 5 and 7. Actually, by employing the NBO analysis, in particular, considering the second order perturbation energies, vicinal stereoelectronic interactions between the Si-X and adjacent C-H, C-S, and C-C bonds proved responsible.
The conformational analysis of the first representative of the Si-alkoxy substituted six-membered Si,N-heterocycles, 1,3-dimethyl-3-isopropoxy-3-silapiperidine, was performed by low-temperature 1H and 13C NMR spectroscopy and DFT theoretical calculations. In contrast to the expectations from the conformational energies of methyl and alkoxy substituents, the Meaxi-PrOeq conformer was found to predominate in the conformational equilibrium in the ratio Meaxi-PrOeq : Meeqi-PrOax of ca. 2 : 1 as from the 1H and 13C NMR study. The thermodynamic parameters obtained by the complete line shape analysis showed that the main contribution to the barrier to ring inversion originates from the entropy term of the free energy of activation.
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-Bueq conformer. The barrier to interconversion has been measured (4.2-4.6 kcal/mol) and the conformational equilibrium [Delta nu = 1990.64 ppm (Si-29), 618.9 ppm (C-13), 1-Ph-ax:1-Pheq = (95.6-96.6%):(3.4-4.4%), K = 25 +/- 3, Delta G degrees = -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.
The conformational equilibria of 3-methyl-3-silathiane 5, 3-fluoro-3-methyl-3-silathiane 6 and 1-fluoro-1- methyl-1-silacyclohexane 7 have been studied using low temperature 13C NMR spectroscopy and theoretical calculations. The conformer ratio at 103;K was measured to be about 5ax:5eq;=;15:85, 6ax:6eq;=;50:50 and 7ax:7eq;=;25:75. The equatorial preference of the methyl group in 5 (0.35;kcal;mol;1) is much less than in 3-methylthiane 9 (1.40;kcal;mol;1) but somewhat greater than in 1-methyl-1- silacyclohexane 1 (0.23;kcal;mol;1). Compounds 5-7 have low barriers to ring inversion: 5.65 (ax;;;eq) and 6.0 (eq;;;ax) kcal mol;1 (5), 4.6 (6), 5.1 (Meax;;;Meeq) and 5.4 (Meeq;;;Meax) kcal;mol;1 (7). Steric effects cannot explain the observed conformational preferences, like equal population of the two conformers of 6, or different conformer ratio for 5 and 7. Actually, by employing the NBO analysis, in particular, considering the second order perturbation energies, vicinal stereoelectronic interactions between the Si-X and adjacent C-H, C-S, and C-C bonds proved responsible.
The spatial magnetic properties (Through Space NMR Shieldings - TSNMRS) of two cyclobutadiene derivatives (2 and 5) and of a number of cyclobutadiene dianion derivatives (3, 4 and 6-8) have been calculated by the GIAO perturbation method employing the Nucleus-Independent Chemical Shift (NICS) concept of P. v. Ragué Schleyer, and visualized as Iso-Chemical-Shielding Surfaces (ICSS) of various size and direction. TSNMRS values can be successfully employed to quantify and visualize the (anti)aromaticity of the compounds studied and to discuss the influence of Li+ complexation to cyclobutadiene dianion (4a, 7 and 8) on planar 4c,6e or three-dimensional 6c,6e aromaticity.
A number of N-substituted 2,2-dimethyl-1,4,2-oxazasilinanes 1 were synthesized and studied by variable temperature dynamic H-1 and C-13 NMR spectroscopy, room temperature N-15 NMR spectroscopy and theoretical calculations at the DFT and MP2 levels of theory. Both the preferred conformers were assigned and the barrier to the ring inversion of the saturated six-membered ring determined. From 1 the corresponding methyl iodide salts were produced, their structure studied by X-ray analysis and found to be in excellent agreement with the results of the theoretical calculations.
4-Alkyl-2,2,6,6-tetramethyl-1,4,2,6-oxaazadisilinanes RN[CH2Si(Me)2]2O [R = Me (1), i-Pr (2)] were synthesized by two methods which provided good yields up to 84%. Low temperature NMR study of compounds (1) and (2) revealed a frozen ring inversion with the energy barriers of 8.5 and 7.7 kcal/mol at 163 and 143 K, respectively, which is substantially lower than that for their carbon analog, N-methylmorpholine. DFT calculations performed on the example of molecule (1) showed that N-Meax conformer to exist in the sofa conformation with the coplanar fragment C-Si-O-Si-C, and its N-Meeq conformer in a flattened chair conformation.
A silicon analog of quinolizidine 3,3,7,7-tetramethylhexahydro-1H-[1,4,2]oxazasilino[4,5-d][1,4,2]oxazasilin-9a-yl)methanol 3 was synthesized. X-ray diffraction analysis confirmed the trans configuration and low temperature NMR spectroscopy both the flexibility (barrier of interconversion 5.8 kcal mol(-1)) and the conformational equilibrium (chair-chair and chair-twist conformers) of the compound. The relative stability of the different isomers/conformers of 3 was calculated also at the MP2/6-311G(d,p) level of theory. Intra- and intermolecular hydrogen bonding in 3 and the appropriate equilibrium between free and self-associated molecules was studied in solvents of different polarity. Both the N-methyl quaternary ammonium salt and the O-trimethylsilyl derivative of 3 could be obtained and their structure determined.
Acid-catalyzed reaction of trifluoromethanesulfonamide with paraformaldehyde in ethyl acetate led to the formation of oxymethylated products that did not form in the reaction carried out in sulfuric acid. Following products were obtained: 5-trifluoromethylsulfonyl-1,3-dioxazinane, 3,7-bis-(trifluoromethylsulfonyl)-1,5,3,7-dioxadiazocane, and a complex of trifluoromethanesulfonamide with 2,4,8,10-tetraoxospiro[5,5]undecene, 1:1. The spiroring resulted from the cyclization of pentaerythritol under the action of formaldehyde. The pentaerythritol formed in its turn by oxymethylation of the methyl group of ethyl acetate with paraformaldehyde followed by the reduction of the COOEt group into CH2 OH by the formaldehyde.
Reactions of trifluoromethanesulfonamide with alpha-methylstyrene, 2-methylpent-1-ene, and cycloocta-1,5-diene in the system t-BuOCl-NaI were studied. In the reaction with alpha-methylstyrene 1-iodo-2-phenylpropan-2-ol was the only isolated product. The reaction with 2-methylpent-1-ene gave a mixture of N,N'-(2-methylpentane-1,2-diyl)bis(trifluoromethanesulfonamide), trifluoro-N-(2-hydroxy-2-methylpentyl)-methanesulfonamide, and N,N'-[oxybis(2-methylpentan-2,1-diyl)]bis(trifluoromethanesulfonamide). Trifluoromethanesulfonamide reacted with cycloocta-1,5-diene to produce a mixture of 2,5-diiodo-9-(trifluoromethylsulfonyl)-9-azabicyclo[4.2.1]nonane and 2,5-diiodo-9-oxabicyclo[4.2.1]nonane; this reaction may be regarded as the first example of direct assembly of bicyclononane skeleton.
In the oxidative system (t-BuOCl+NaI) trifluoromethanesulfonamide is regio- and stereoselectively added to only one double bond of cyclopentadiene and 1,3-cyclohexadiene giving rise to 1,1,1-trifluoro-N-(5-iodocyclopent-2-en-1-yl)methanesulfonamide 7 and trans-N,N'-cyclohex-3-en-1,2-diylbis(1,1,1-trifluoromethanesulfonamide) 8. The structure of 7 and 8 was determined by X-ray, NMR, and MS. With 1,4-cyclohexadiene, addition to both double bonds occurs with the formation of N,N'-(4-chloro-5-iodocyclohexan-1,2-diyl)bis(1,1,1-trifluoromethanesulfonamide) 9. Under the action of sodium iodide in acetone, the latter product undergoes halogenophilic attack with the reduction of the CHI group and elimination of HCl to give trans-N,N'-cyclohex-4-en-1,2-diylbis(1,1,1-trifluoromethanesulfonamide) 10, whose structure was also determined by X-ray analysis. 1,3,5-Cycloheptatriene under these conditions is oxidized to benzaldehyde and does not react with trifluoromethanesulfonamide.