My PhD dissertation - Institut Fresnel

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4.4.1 The Use of Variable-Temperature NMR to Study Intramolecular Mobility 54 Intramolecular movements in rotational isomers can in principle be studied by various spectroscopic methods e.g. Raman, infrared, microwave or NMR spectroscopy enriched materials [ , ] 203 204 or by kinetic measurements of the racemization process for enantio- [ ] 205 . However, each one of these techniques remains limited, to a certain extent, either by the rate of the rotational process considered, or by the structural complexity of the substrate investigated or even by the ease to access optically active substrates. Vibration spectroscopy techniques can be used for the determination of rotamer populations in case of very fast exchange processes with activation energy lower than 5 kcal·mol -1 [204] . Furthermore, these techniques are solely applicable to relatively simple structures since absorption bands for both rotamers should be clearly distinct in the spectrum to afford accurate data. NMR spectroscopy has an advantage over vibration spectroscopies in the study of rotational isomers owing to the fact that the intensities of signals are directly proportional to the number of nuclei at the magnetic site. In addition, NMR technique, due to its "time scale", afford the study of slower rotational process with activation energies of 5 to -1 [ a, ] 25 kcal·mol 206 207 , which constitute the usual range of value for sterically little encumbered biphenyls. In the rotation about a bond in a molecule, there are certain energetically favoured conformations. For instance, in biphenyl derivatives where free rotation about the pivot bond is hindered by ortho- substituents, a coplanar arrangement of the aromatic rings corresponds to an energy maximum. As a consequence, such kind of substrate are twisted in the ground state to minimize their energy level C D [ ] 208 . A B k r C B Ia Ib A D
55 To convert atropisomer Ia into Ib, it is necessary to supply the required free enthalpy of activation ∆G ≠ (Ia-Ib). The magnitude of this free enthalpy determines the rate of the thermally induced isomerization and is evidently depending on the temperature of study. The rate constant kr of this process is related to ∆G ≠ (Ia-Ib) in accordance with the Eyring equation, where T is the absolute temperature, R is the universal gas constant, kb is the Boltzmann's constant, h is Planck's constant and κ is the transmission coefficient: or k r ∆ = κ ⋅ k b ⋅T ⋅ h −1 ⎛ − ∆ exp ⎜ ⎝ RT ≠ G( Ia - Ib) ⎞ ⎟ ⎠ ⎡ ⎛ T ⎞⎤ ⋅T ⎢10. 321+ log ⎜ ⎟ ⎟⎥ ⎢⎣ ⎝ kr ⎠⎥⎦ ≠ −3 G (Ia - Ib) = 4. 57 ⋅10 (kcal·mol -1 ) Whenever rotation is too fast to allow isolation of isomers on laboratory time scale (i.e. ∆G ≠ (Ia-Ib) < 22 kcal·mol -1 ), rapid racemization processes in chiral biphenyls can be detected by NMR spectroscopy provided that the investigated substrate bears a diastereotopic pair of NMR-active nuclei (e.g. A = B = -CH2R with A ≠ C and B ≠ D). Under this condition, two types of magnetically non-equivalent hydrogens do exist, resulting from the hampered rotation about the biphenyl single bond. The intramolecular rotation can thus be revealed by the temperature dependence of the magnetic environment for the diastereotopic probes. Hence, at low temperature, "slow" isomerization leads to two separate NMR signals for both diastereotopic hydrogens whereas at higher temperature, "fast" rotation gives only one signal with an average chemical shift. W. L. Meyer and R.B. Meyer [ ] 209 reported the first example of temperature- dependent 1 H-NMR spectrum for "methylene-bearing" biphenyls. For instance, 2,2'- bis(acetoxymethyl)biphenyl (46) exhibited an AB quartet signal for the methylene hydrogens at ambient temperature, whereas the pattern collapse into a broad singlet at Tc = 94 °C (Figure 7).
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Various Facets of Organophosphorus
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III Remerciements Je tiens à remer
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3.3.6 Relevance of the Data Assesse
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VII Literature References 115 Compo
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IX Version Abrégée Les composés
Page 21 and 22:
1 Introduction 1 The present thesis
Page 23 and 24: 3 research groups in the field are
Page 25 and 26: 5 However, this mechanism suffers f
Page 27: 7 At first, a double bromine/lithiu
Page 30 and 31: 10 Although the most frequently emp
Page 32 and 33: 12 The preparation of the latter ph
Page 34 and 35: protected to its dioxolane derivati
Page 36 and 37: N (S)-14 1) NBS, THF, - 75 °C 2) -
Page 39 and 40: 3 Alkaline Decomposition of Phospho
Page 41 and 42: 3.2 Alkyl Carbanions Stabilities in
Page 43 and 44: 3.3.1 Mechanistic Discussion on the
Page 45 and 46: 25 The assumption reported by these
Page 47 and 48: 27 However, in the eventuality of a
Page 49 and 50: 29 To prepare tertiary phosphines b
Page 51 and 52: 31 Table 4. Mixed tertiary alkylpho
Page 53 and 54: 33 were comparable to those obtaine
Page 55 and 56: 35 led to the formation of di-tert-
Page 57 and 58: 37 depending on the alkyl substitue
Page 59 and 60: 39 4 A Novel Access to Atropisomeri
Page 61 and 62: 41 Since the homocoupling of the ha
Page 63 and 64: 43 Few other attempts to selectivel
Page 65 and 66: 45 The second phenol ether 36 was f
Page 67 and 68: 47 The enantiomeric purity was chec
Page 69 and 70: 49 A second purpose of this derivat
Page 71 and 72: 51 substrates considered. In conseq
Page 73: 53 Single-crystal X-ray analysis of
Page 77 and 78: 57 This finally allows one to use t
Page 79 and 80: the steric bulk and charge delocali
Page 81 and 82: 61 To check the reproducibility of
Page 83: 63 Figure 9. Full lineshape analysi
Page 86 and 87: 66 suitably substituted dilithiobip
Page 89: Experimental Part
Page 92 and 93: 70 deuterochloroform (CDCl3) unless
Page 94 and 95: 72 1 H NMR: δ = 7.32 (dddd, J = 9,
Page 96 and 97: 1 H NMR: δ = 6.96 (s, 3 H), 6.94 (
Page 98 and 99: 76 C21H21O9P (448.36) calcd. C 56.2
Page 100 and 101: 78 C12H12BrN (250.13) calcd. C 57.6
Page 102 and 103: 80 C13H13NO2 (215.25) calcd. C 72.5
Page 104 and 105: 82 3.1.1 Dialkyl-N,N-diethylaminoph
Page 106 and 107: 84 apparatus. The solution was adde
Page 108 and 109: 86 Di-tert-butylisopropylphosphine
Page 110 and 111: 88 13 C NMR: δ = 32.9 (d, J = 12 H
Page 112 and 113: 90 13 C NMR: δ = 41.5 (d, J = 61 H
Page 114 and 115: 31 P NMR: δ = 53.4 ppm. 92 C16H31O
Page 116 and 117: 94 13 C NMR: δ = 40.8 (d, J = 3 Hz
Page 118 and 119: 96 31 P NMR settings: It is known t
Page 120 and 121: 98 Theoretical ratio: (96.1:3.9) Ex
Page 123 and 124: 101 4 The Preparation and Racemate
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103 extraction protocol with ethyl
Page 127 and 128:
105 extracted with ethyl acetate (2
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107 (+)-(M)-Dimethyl-6,6’-dibromo
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109 (+)-(M)-2,2’-Dibromo-6,6’-b
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111 1 H NMR: δ = 7.2 (m, 20 H), 7.
Page 135 and 136:
113 2,2’-Dilithio-6,6’-bis(meth
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References
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116 Conference Proceedings, Naples,
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118 [ 70] J. Drowart, C.E. Myers, R
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120 [117] S.T. Graul, R.R. Squires,
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122 [170] B.H. Lipshutz, F. Kayser,
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124 [212] M. Schlosser, in Organome
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O P 27i: p. 30, 80 C 9H 21OP P 26b:
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P P P O 2: p. 11, 59 3 P O O O 11:
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Nom Maurin Prénom Michaël 129 Cur
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