Proton NMR studies of symmetrically-substituted N,N-dialkyltrifluoroacetamides:  medium effects

Suárez, Cristina; LeMaster, Clifford B.; LeMaster, Carole L.; Tafazzoli, Mohsen; True, Nancy S.

doi:10.1021/j100380a030

Cited by 28 publications

(38 citation statements)

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“…True has studied gas-phase amide rotational barriers and concludes that the rotational barrier is greater in the condensed phase than the gas phase because the bulkier transition state needs an enlarged solvent cavity, and must overcome the internal pressure of the solvent. 31 Since a rotational barrier depends on the relative stability of ground state versus transition state, perhaps the importance of ground-state effects on rotational barriers should be considered more thoroughly.…”

Section: Discussionmentioning

confidence: 99%

Solvent effects on the stability of simple secondary amides †

Morgan¹,

Kopp²

1998

J. Chem. Soc., Perkin Trans. 2

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Enthalpies of solution for N-methylpropionamide and 2-pyrrolidone in water, propan-1-ol, acetone and toluene were measured by calorimetry. Heats of vaporization were determined for the amides by ebulliometry, and enthalpies of solvation from gas phase were obtained. Enthalpies of solvation were the same for the two nearly-isomeric amides in polar, protic solvents, but in acetone and toluene dimerization of 2-pyrrolidone caused differences in enthalpies of solvation. For N-methylpropionamide, solvation enthalpy from the gas phase is highly correlated with the ability of the solvent to donate hydrogen bonds, but not well correlated with the ability of the solvent to accept hydrogen bonds, the polarity/polarizability of the solvent, or solvent relative permittivity.

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Section: Discussionmentioning

confidence: 99%

Solvent effects on the stability of simple secondary amides †

Morgan¹,

Kopp²

1998

J. Chem. Soc., Perkin Trans. 2

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“…At low temperature (down to À 938), we observed a second dynamic process that resulted in the appearance of six different signals (four Me and two CH) corresponding to two i-Pr groups in 1 H-and 13 C-NMR and splitting of all CF 2 groups to AB systems in 19 F-NMR (Fig. 2).…”

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confidence: 91%

“…8 ) In N,N-disubstituted amides, the barrier to rotation decreases upon increase of steric demand of the carbonyl substituent [1]. 9 ) For a detailed discussion of effects in N,N-dialkyl trifluoroacetamides, see [19]. 10 ) Switching to the acetyl substituent, which is sterically less-demanding than CF 3 , results in substantial increase of the barrier to rotation in MeC(O)C(O)NMe 2 (86.1 kJ ¥ mol À1 ) [20].…”

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confidence: 99%

Study of Unusually High Rotational Barriers about SN Bonds in Nonafluorobutane‐1‐sulfonamides: The Electronic Nature of the Torsional Effect

Lyapkalo

Reißig

Schäfer

et al. 2002

Helvetica Chimica Acta

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Dedicated to Professor Dieter Seebach on the occasion of his 65th birthday Slow rotation about the SÀN bond in N,N-disubstituted nonafluorobutane-1-sulfonamides 1 can easily be detected by NMR measurements at room temperature. This effect causes magnetic nonequivalence of otherwise identical geminal substituents in symmetrical staggered ground-state conformation A. The torsional barriers determined (62 ± 71 kJ ¥ mol À1 ) proved to be the highest ever observed for sulfonamide moieties. They are comparable to the values routinely measured for carboxylic acid amides or carbamates. The restricted rotation is interpreted as result (n N Àd S )-p and of n N Às* S;C interactions, which develop substantial S,N double-bond character in sulfonamides 1. The S,N binding interaction is increased by the highly electron-withdrawing effect of the perfluorobutyl group. The anticipated symmetry of the ground-state conformation A and the considerable shortening of the SÀN bond (1.59 ä) compared to the mean value in sulfonamides (1.63 ä) are confirmed by single-crystal X-ray study of N,N-dibenzylnonafluorobutane-1-sulfonamide (1c).Restricted rotation [1] is a well-known phenomenon for compounds I with single bonds YÀA that adopt considerable double-bond character due to the binding interactions of the electron-withdrawing p-system XY with the lone pair at A, as expressed by the dipolar mesomeric formula II (Scheme 1). This has been described for Y and A elements of the second row of the periodic table. Not surprisingly, the highest values of barriers to rotation are normally observed for compounds containing an amino group (A N), due to the very strong p-donating character of the N-atom. Hindered rotation of this type is well-documented, and the barriers to rotation have been measured for many classes of compounds [1] [2], including esters, nitrites, enamines, hydrazones, thio-and selenoamides, nitrosoamines, and triazenes [3]. Undoubtedly, carbamates and amides proved to be the most frequently occurring substances exhibiting atropisomerism about their CÀN bonds. Reviews on dynamic effects in amides and related compounds [2] [4] reflect the fundamental character and significance of this topic. Elements of the third (e.g., Si, P, S) and higher periods exhibit much lower tendencies to p-bond. As a consequence, the barriers to rotation in the

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“…Unfortunately, gas-phase dynamic NMR studies of primary amides are difficult for several reasons. First, the vapor pressure of primary amides is extremely low, usually less than that for the tertiary amides that have been more extensively studied. − Vaporization of enough amide for acquisition of gas-phase spectra is further impaired by the affinity of the amide hydrogens for the glass NMR tube. Also, whereas dimethyl amides have three equivalent protons at the exchanging sites, primary amides have only one.…”

Section: Introductionmentioning

confidence: 99%

Gas-Phase Nuclear Magnetic Resonance Study of (¹⁵N)Trifluoroacetamide: Comparison of Experimental and Computed Kinetic Parameters

LeMaster

True

1999

J. Am. Chem. Soc.

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Rate constants obtained from total line-shape analysis of 11 temperature-dependent gas-phase NMR spectra of (15N)trifluoroacetamide yielded the following kinetic parameters: ΔG ⧧ 298 = 15.1(0.36) kcal mol-1, ΔH ⧧ = 13.3(1.3) kcal mol-1, and ΔS ⧧ = −5.9(4.5) cal mol-1 K-1. Ab initio calculations performed at the MP2 level using the 6-311++G(d,p) basis set calculated ΔG ⧧ 298 = 15.19 kcal mol-1, ΔH ⧧ = 14.35 kcal mol-1, and ΔS ⧧ = −2.82 cal mol-1 K-1 in reasonable agreement with experiment. Hartree−Fock calculations using that basis set were also in reasonable agreement with experiment, yielding ΔG ⧧ 298 = 16.03 kcal mol-1, ΔH ⧧ = 14.35 kcal mol-1, and ΔS ⧧ = −5.62 cal mol-1 K-1. DFT calculations (B3-PW91) using the same basis set gave results that were considerably less accurate.

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Proton NMR studies of symmetrically-substituted N,N-dialkyltrifluoroacetamides: medium effects

Cited by 28 publications

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Solvent effects on the stability of simple secondary amides †

Solvent effects on the stability of simple secondary amides †

Study of Unusually High Rotational Barriers about SN Bonds in Nonafluorobutane‐1‐sulfonamides: The Electronic Nature of the Torsional Effect

Gas-Phase Nuclear Magnetic Resonance Study of (¹⁵N)Trifluoroacetamide: Comparison of Experimental and Computed Kinetic Parameters

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Proton NMR studies of symmetrically-substituted N,N-dialkyltrifluoroacetamides: medium effects

Cited by 28 publications

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Solvent effects on the stability of simple secondary amides †

Solvent effects on the stability of simple secondary amides †

Study of Unusually High Rotational Barriers about SN Bonds in Nonafluorobutane‐1‐sulfonamides: The Electronic Nature of the Torsional Effect

Gas-Phase Nuclear Magnetic Resonance Study of (15N)Trifluoroacetamide: Comparison of Experimental and Computed Kinetic Parameters

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Gas-Phase Nuclear Magnetic Resonance Study of (¹⁵N)Trifluoroacetamide: Comparison of Experimental and Computed Kinetic Parameters