1,3-Dimethyl-3-silapiperidine: Synthesis, Molecular Structure, and Conformational Analysis by Gas-Phase Electron Diffraction, Low Temperature NMR, IR and Raman Spectroscopy, and Quantum Chemical Calculations

Abstract

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.

Molecular structure and conformational analysis of 3-methyl-3-silathiane by gas phase electron diffraction, FTIR spectroscopy and quantum chemical calculations

“…6-1). According to calculations, its low-frequency component with maximum at 2122 cm À1 belongs to the axial conformer 1-ax and the high-frequency component at 2144 cm À1 e to the equatorial conformer 1-eq, as was found earlier for 1,3-dimethyl-3-silapiperidine [8].…”

“…The endocyclic CSiC angle 104.9(13) in 1 and 105.8(5) in 3,3-dimethyl-3-silathiane [14] are very close to that in 1,3-dimethyl-3-silapiperidine, 103.1(7) [8] and 1-methylsilacyclohexane 102.8 (20) 5 and 104.1(2) [21]. The exocyclic CSiC angles are close to the ideal tetrahedral values, 110.8(7) in 1, 109.5(4) and 110.8(4) in 3,3-dimethyl-3-silathiane [14] and 112.6(3) 21 and 112.5(24) 5 in 1-methylsilacyclohexane.…”

“…Note, that the CeC(S) bond is about 0.01 Å shorter than the CeC(Si) bond, as follows both from the experiment and calculations. The same is true for other silaheterocyclohexanes irrespective of the nature of heteroatom in the ring e nitrogen [8,18] or oxygen [19].…”

“…The only monosubstituted on the silicon silaheterocyclohexane, for which the conformational equilibrium was studied in gas and in solution, was 1,3-dimethyl-3-silapiperidine, which showed the predominance of the Me ax conformer at ambient temperature in gas phase and of the Me eq conformer at low temperature in solution [8]. Another silaheterocycle, 3-methyl-3-silathiane 1, was studied by low-temperature NMR [9] and showed a larger predominance of the equatorial conformer (15:85) than in 1-methylsilacyclohexane (26:74).…”

Molecular structure and conformational properties of N-cyclohexylpiperidine as studied by gas-phase electron diffraction, mass spectrometry, IR spectroscopy and quantum chemical calculations

“…Similar situation occurred in case of 1,3,3-trimethyl-1,3-azasilinane [31] and 1,3-dimethyl-3-silapiperidine [32], for which the equatorial position of the methyl groups is absolutely dominating. Thus, introducing a substituent such With basis set 6-311G** Dr = r h1 -r a , The vibrational corrections Dr were calculated by the SHRINK program [25,26] using the so-called second approximation, in which a harmonic approach with nonlinear relation between Cartesian and internal coordinates was applied on the base of the force field estimated in the QC calculations at MP2/ 6-311G** level a A group number in which the amplitude was refined (see ''GED analysis'' section) as cyclohexyl, in our case, did not affect the conformational preferences of piperidine.…”