My PhD dissertation - Institut Fresnel

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58 ≠ -1 torsional barrier at that temperature was found to amount G = 11.7 ± 0.3 kcal·mol . A full lineshape analysis was eventually achieved ∆ coal [ ] 211 to evaluate the variation of the rotation rate constant as a function of the temperature. An Eyring plot of ln(k/T) as a function of 1000/T afforded a reasonable correlation (R² = 0.972) and gave a calculated enthalpy ∆ = ≠ H 12.5 kcal·mol -1 and entropy ∆ = - 0.5 cal·mol ≠ -1 -1 S ·K . Evaluation of the reaction order for the exchange process observed would probably help to get a better idea about the aggregation state which possibly facilitates, or on the contrary, lowers the rotation about the biphenyl bond. To achieve this aim, several coalescence experiments were donein the same conditions as previously described but varying various parameters. Thus the stoichiometry of the lithiating was first lowered to two equivalents, leaving two equivalents of tert-butyl bromide in presence in the reaction mixture. Although the 1 H- NMR spectra of the lithiated biphenyl generated were presented traces of impurities in the high field zone, the coalescence temperature observed in the same experimental conditions (concentration, solvent) was sensibly the same, indicating no major influence of the tert-butyllithium concentration on the dilithiobiphenyl rotation. Attempts to perform the same experiment with more than four equivalents of tert-butyllithium (excess of lithiating reagent remaining in the reaction mixture) resulted in unexploitable spectra. This fact is presumably due to the low stability of tert-butyllithium towards tetrahydrofuran attack [ ] 212 . Butyllithium was also used instead of tert-butyllithium to generate the expected dilithiobiphenyl, in order to check the implication of the lithiation reagent in a possible aggregate. Unfortunately, this reagent did not allow complete lithium/bromine permutational exchange at the usual reaction temperature, giving a mixture of brominated and lithiated biphenyl. This mixture of compounds did not allow accurate determination of the coalescence temperature since the four doublets of diastereotopic protons were partially overlapping. Finally, the experiment was tried once more, increasing the concentration from 0.09 M initially chosen to 0.19 M. The coalescence temperature was determined to be of Tc = -10 ± 1 °C which did not constitute a significant difference in torsional barrier compared to what was calculated previously. Increasing concentration resulted in the precipitation of the lithiated species formed, thus preventing any NMR experiments. These modifications of the reaction parmeters, showing no significant deviation compared to the initial experiments performed, suggested that the rotational barrier calculated by means of this NMR study is independent from aggregation factors. The free rotation of the biphenyl motif about the pivot single bond seemingly occurs in a monomolecular process. However, since the energy required for the "aggregation/disaggregation" process usually falls in the range of few kcal.mol -1 , it is probable that NMR technique do not reveal such difference in energy. On the other hand,
the steric bulk and charge delocalization are accepted to disfavor aggregation 59 [ ] 213 and could thus provide an explaination for the apparent unsensitivity of the dilithiobiphenyl toward reaction parameters' variation. Furthermore, the entropy value calculated thanks to the full lineshape analysis performed, is somewhat close to 0 and suggested that the rotational process studied is happening as a monomolecular internal rotation. This deduced fact tends to confirm the data collected experimentally by changing concentration or stoichiometry of the lithiating reagent. Accuracy of the results obtained by variable temperature "dynamic" NMR experiments (DNMR) has been extensively discussed in many text books [206b, 214, 215] and stems from two main factors. First of all, the equation used for the determination of is derived from approximations, which obviously induce some deviations in the calculated parameter. On the other hand, the determination of the coalescence temperature is the major experimental source of error ≠ ∆Gcoal [ 216 ] . Indeed, M.L. Martin, J.J Delpuech and G.J. Martin [215] evaluated that overestimating Tc by 2 °C at - 73 °C and for ∆ν = 20 Hz, ≠ -1 resulted in an increase of G of 0.1 kcal·mol . This error could be minimized ∆ coal experimentally by calibrating the NMR temperature probe by means of a platinum thermocouple, and by slowly increasing the temperature in the region around coalescence. Although some thermodynamical data were available for o,o'-dilithiobiphenyl derivatives [202, 59] , it was not yet possible to compare the value calculated with pertinent examples since no literature precedents were reported for similar substrates. However, O. Desponds and M. Schlosser [59] found the torsional barrier of 1,11-dilithio-5,5,7,7- tetramethyl-5,7-dihydrodibenz[c,e]oxepin (48) to fall in the range of 12.0 - 12.5 kcal·mol -1 . Li O 48 Li The fact that ortho- and ortho'- oxygen atoms should cause less steric repulsion than isopropylidene groups at the same positions, and the presence of a rigid bridged structure would bring to the conclusion that the oxepin derivative should have a much higher energy barrier that biphenyl 47. Furthermore, despite the important steric congestion, the bridge in the biphenyl structure forces the gem-dimethyl groups to face
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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
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1 Introduction 1 The present thesis
Page 23 and 24:
3 research groups in the field are
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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 and 74: 53 Single-crystal X-ray analysis of
Page 75 and 76: 55 To convert atropisomer Ia into I
Page 77: 57 This finally allows one to use t
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
Page 125 and 126: 103 extraction protocol with ethyl
Page 127 and 128: 105 extracted with ethyl acetate (2
Page 129 and 130:
107 (+)-(M)-Dimethyl-6,6’-dibromo
Page 131 and 132:
109 (+)-(M)-2,2’-Dibromo-6,6’-b
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111 1 H NMR: δ = 7.2 (m, 20 H), 7.
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113 2,2’-Dilithio-6,6’-bis(meth
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References
Page 140 and 141:
116 Conference Proceedings, Naples,
Page 142 and 143:
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:
Page 160:
Nom Maurin Prénom Michaël 129 Cur
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