NMR Study of Rotational Barriers and Conformational Preferences. I. Cyclohexyl Fluoride

The 1H and 19F n.m.r. spectra of the title compound and its 3,3,6,6-tetradeutero analog have been measured in a variety of solvents and over a wide range of temperatures. These measurements have been used as the basis of calculations of the thermodynamic and kinetic parameters for the chair–chair conformational equilibrium of this substance. The poor agreement between the different methods of evaluating ΔG is discussed.

Section: Discussionmentioning

confidence: 99%

Section: Rate Calculationsmentioning

confidence: 99%

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Conformational Analysis of trans-1-Fluoro-2-iodocyclohexane: a Nuclear Magnetic Resonance Study

Hall¹,

Jones²

1973

“…The activation energies, AGfA+, and AG'E+A, were calculated from the coalescence temperature (T,) for the carbon 6 signal using two different methods developed for unequal proportions of conformers (44,45). Both methods gave practically the same values of activation energies, the largest difference observed being 0.2 kcal mol-I.…”

Section: -Acetoxytetrahydropyran (3)mentioning

confidence: 99%

The conformational behaviour of 2-aryloxytetrahydropyrans and 2-acetoxytetrahydropyran, and barrier to ring inversion

Ouedraogo¹,

Lessard²

1991

CHFCl? solvents show that the axial preference increases with electron withdrawal in the aryloxy group: from 79% (AGoE +A = -0.4 kcal mol-I) (4-OCH,) to 90% (AGoE+A = -0.7 kcal mol-I) (4-NO2) in CF2Br2. The axial preference (anomeric effect) is smaller in the more polar CHFC12 solvent, as expected, and the substituent effect is smaller also: change in AGoE+A from -0.3 (4-OCH,) to -0.5 (4-NO2) kcal mol-I. However, the axial preference of 2-acetoxytetrahydropyran is shown to be smaller than that of 2-phenoxytetrahydropyran in CF2Br2 solvent although the acetoxy group is expected to be more electronegative than the phenoxy group. Furthermore, the polarity of the solvent has no effect on the conformational equilibrium of 2-acetoxytetrahydropyran. The results are interpreted in terms of both orbital and electrostatic interactions. The ring-inversion barrier of 2-alkoxy and 2-aryloxytetrahydropyrans, determined by the temperature-coalescence method, is lower than that of tetrahydropyran by about 1.5 kcal rnol-I .Key words: 2-substituted tetrahydropyrans, conformational analysis, anomeric effect, ring inversion.ADAMA OUEDRAOGO et JEAN LESSARD. Can. J. Chem. 69.474 (1991).Les donntes de la resonance magnktique nuclCaire du I3C a 156 K, dans les solvants CF2Br2 et CHFCl,, montrent que la prCfCrence axiale d'une serie de tetrahydropyranes portant, en position 2, un groupement phCnoxy substitut a la position par0 croit avec le pouvoir Clectroattracteur du substituant : de 79% (AGOE-,~ = -0.4 kcal mol-') pour un groupement 0CH3 a 90% (AGoE +A = -0.7 kcal mol-I) pour un groupement nitro dans CF2Br2. La prCfCrence axiale (l'effet anomere) est moindre dans CHFCl?, solvant plus polaire, tel qu'attendu, et l'effet de substituant est tgalement plus faible : variation de AGoE + A de -0.3 (OCH,) a -0.5 (NO2) kcal mol-I. Toutefois, dans CF2Br,, la proportion de conformere axial est moins ClevCe pour le 2-acCtoxytCtrahydropyrane que pour le 2-phtnoxytetrahydropyrane m&me si le groupement acCtoxy est plus Clectrontgatif que le groupement phCnoxy. De plus, la polarit6 du solvant n'a pas d'influence sur l'tquilibre conformationnel du 2-acCtoxytttrahydropyrane. Les rCsultats sont interprttCs en termes d'interactions orbitalaires et Clectrostatiques. La dktermination de la barriere d'inversion de cycle d'alcoxy-et d'aryloxytCtrahydropyranes par la mCthode de coalescence de tempkrature montre que la barritre est plus faible que celle du tttrahydropyrane d'environ 1,5 kcal par mole.Mots elks : tCtrahydropyranes substituts en position 2, analyse conformationnelle, effet anomere, inversion de cycle.

“…The transition state for this process is thought to involve a half-chair structure with boat and twist-boat forms existing as intermediates along the reaction path ( Fig. 1) (3,(8)(9)(10)(11)(12). Typical barriers for ring inversion are 40-45 kJ mol-' while the energy differences between the equatorial and axial conformers are generally 0-4 kJ mol-' (3,7,8,13).…”

mentioning

confidence: 99%

Ring inversion in solid fluorocyclohexane

Wasylishen¹

1986

Inversion of the cyclohexane ring system in the solid state has been directly observed in a ' 9~ nmr study of solid fluorocyclohexane. The barrier to ring inversion in the orientationally disordered solid phase, AG+,,, = 44.1 k 0.9 kJ molpl at 263 K is found to be only slightly larger than the value previously reported for a CFC13 solution of fluorocyclohexane. The population of the equatorial conformation is slightly favored over the axial conformation with K = P,/Pa = 1.56 at 281 K; similar values have been obtained in CFCI3 and in the gas phase. RODERICK E. WASYLISHEN. Can. J. Chem. 64, 2094Chem. 64, (1986.Une Ctude par rmn du 19F du fluorocyclohexane B l'ttat solide a permis d'observer directement l'inversion du noyau cyclohexane ii 1'Ctat solide. On a trouvC que, a 263 K, la barrikre a I'inversion de cycle, dans le phase solide dCsordonnCe du point de vue de I'orientation, AG*,,, = 44,l L 0,9 k.J mol-', n'est que 1Cgerement supCrieure B la valeur rapportee antkrieurement pour une solution de fluorocyclohexane dans le CFC13. La population de la conformation Cquatoriale est lCgerement favorisCe par rapport a la conformation axiale et K = P,/Pa = 1,56, ii 281 K; des valeurs semblables ont Ct C obtenues pour des solutions dans le CFC13 ainsi qu'en phase gazeuse.[Traduit par la revue]In the gas phase (1, 2) and in solution (3-8) the cyclohexane ring of monosubstituted cyclohexanes is known to undergo rapid ring inversion (I). The transition state for this process is thought to involve a half-chair structure with boat and twist-boat forms existing as intermediates along the reaction path ( Fig. 1) (3,8-12). Typical barriers for ring inversion are 40-45 kJ mol-' while the energy differences between the equatorial and axial conformers are generally 0-4 kJ mol-' (3,7,8,13). Although some infrared and Raman evidence exists for ring inversion in solid cyclohexane (1 l), fluorocyclohexane, and chlorocyclohexane (14, 15), the rates of inversion have not been determined. Here we present preliminary results of a nmr study of solid fluorocyclohexane, which clearly demonstrate that the inversion process takes place in the solid state at rates comparable to those reported in CFC13 solutions (3).Representative variable temperature 19F nmr spectra of solid fluorocyclohexane are shown in Fig. 2. The spectra were obtained on a commercial sample (Cationics, Inc.) at 339.6736 MHz with 'H decoupling ( y B z = 2 kHz) using a Nicolet 360 nrnr spectrometer. The sample (mp = 285 K) was degrlssed and sealed under vacuum at 77 K in a 5-mm nmr tube. The relatively narrow nmr lines observed for solid fluorocyclohexane imply that it exists as an orientationally disordered solid between the mp and 183 ir 3 K. Below 183 +-3 K we were unable to detect a 1 9~ resonance because of dipolar broadening. A similar orientationally disordered solid phase is known to exist for cyclohexane between the mp (279.8 K) and 186.0 (16).The spectra in Fig. 2