Conformational analysis of 4,4‐dimethyl‐1‐(trifluoromethylsulfonyl)‐1,4‐azasilinane and 2,2,6,6‐tetramethyl‐4‐(trifluoromethylsulfonyl)‐1,4,2,6‐oxazadisilinane

Abstract

4,4‐Dimethyl‐1‐(trifluoromethylsulfonyl)‐1,4‐azasilinane 1 and 2,2,6,6‐tetramethyl‐4‐(trifluoromethylsulfonyl)‐1,4,2,6‐oxazadisilinane 2 were studied by variable temperature dynamic 1H, 13C, 19F NMR spectroscopy and theoretical calculations at the DFT (density functional theory) and MP2 (Møller‐Plesset 2) levels of theory. Both kinetic (barriers to ring inversion) and thermodynamic data (frozen conformational equilibria) could be obtained for the two compounds. The computations revealed two minima on the potential energy surface for molecules 1 and 2 corresponding to the rotamers with the CF3SO2 group directed ‘inward’ and ‘outward’ the ring, the latter being 0.2–0.4 kcal/mol (for 1) and 1.1 kcal/mol (for 2) more stable than the former. The vibrational calculations at the DFT and MP2 levels of theory give the values of the free energy difference ΔGo for the ‘inward’ ‘outward’ equilibrium consistent with those determined from the experimentally measured ratio of the rotamers. The structure of crystalline compound 2 was ascertained by X‐ray diffraction analysis. Copyright © 2011 John Wiley & Sons, Ltd.

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

“…Since the total reaction rate cannot be larger than the rate of its limiting step, the overall barrier must be equal For N-triflyl-4-silapiperidine, the MP2 calculated ∆E is 0.6 kcal/mol in favor of the 'outward' rotamer, corresponding to the ratio of 95:5. The LT 13 C-NMR spectroscopy showed the presence of two conformers in the ratio 98:2, which excellently coincides with calculations [57].…”

“…The strongly electron acceptor triflyl group at the nitrogen atom makes it planar, but still two rotamers with the 'inward' or 'outward' CF3 group differing in energy by 1.1 kcal/mol in favor of the latter may exist. According to the X-ray analysis, in the crystal the compound exists as the single 'outward' rotamer [57] (Figure 12). For N-triflyl-4-silapiperidine, the MP2 calculated ΔE is 0.6 kcal/mol in favor of the 'outward' rotamer, corresponding to the ratio of 95:5.…”

“…The strongly electron acceptor triflyl group at the nitrogen atom makes it planar, but still two rotamers with the 'inward' or 'outward' CF 3 group differing in energy by 1.1 kcal/mol in favor of the latter may exist. According to the X-ray analysis, in the crystal the compound exists as the single 'outward' rotamer [57] (Figure 12). It also deserves mentioning that the value of AMe in 3-methyl-3-silathiane is 0.35 kcal/mol [35], that is, larger than in the absence of sulfur atom in the ring (0.23 kcal/mol, Table 1).…”

Conformational equilibrium and dynamic behavior of bis‐N‐triflyl substituted 3,8‐diazabicyclo[3.2.1]octane

“…In a series of papers, we have studied the conformational behavior of N‐trifluoromethylsulfonyl (N‐triflyl) heterocyclohexanes with different heteroatoms in the ring . In all these compounds, two dynamic processes occur: inversion of the six‐membered ring and restricted rotation about the N–S bond.…”

Unusual conformational preferences of 1,3‐dimethyl‐3‐isopropoxy‐3‐silapiperidine

“…The 4‐silapiperidine and 4‐silapiperidinium rings were shown to adopt a chair conformation with equatorial N‐organyl group and a flattened “Si‐part” of the ring, as is the case for other silacyclohexanes and silaheterocyclohexanes . No conformational studies were reported for 4‐silapiperidines except for the recent work on 1‐triflyl‐4,4‐dimethyl‐4‐silapiperidine for which one dynamic process with the barrier of 13.1 kcal/mol was measured (actually, it is a barrier to rotation about the N–S bond) . NMR studies performed for ammonium salts of 4‐hydroxy‐4‐aryl‐4‐silapiperidines showed the presence of two isomers differing in the orientation of the R group at nitrogen and having the OH group in axial and aryl substituents in equatorial position at the silicon atom .…”