My PhD dissertation - Institut Fresnel

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24 on the leaving alkyl group during the rate determining step of alkaline decomposition. Other investigations on substituent effects were ran with o- and p-substituted-benzyltriphenyl phosphonium salts in different conditions [36, - ] 130 132 and showed ρ ranging from +2.2 to +5.6. Values of ρ ranging from +4 to +5 are in good agreement with other known base-promoted reactions in which benzylic carbanions in hydroxylic solvents are proposed as intermediates [ , ] 133 134 . It was thus believed, at that stage, that the relative ease of alkyl expulsion was governed by the stabilities of the corresponding carbanions. However, B. Siegel [131] interestingly reported a Hammett analysis for variously substituted Y-benzyltriphenyl phosphonium bromides in which σ – for substituents on the benzyl group (Y) were graphed against log kY (kY being the third order rate constant of alkaline decomposition of the salt). The plot, unfortunately, did not support a linear correlation which would be expected if a carbanionic character was developed on the leaving group, and delocalized to the entire aromatic ring, in the course on the expulsion of an alkyl carbanion. In contrast, a plot of those log kY against σ gave a very good linear correlation (r = 0.99) with a slope ρ of +3.7. In view to those results, it can be assumed that a polarization or partial negative charge is present on the leaving benzylic group in the rate determining step but no proper negative charge is delocalized to the whole aromatic ring. This assumption thus agrees with the hypothesis of a concerted expulsion-reprotonation of the alkyl group. Another argument favoring a concerted version of the alkaline decomposition of phosphonium salts was suggested by a work reported by C. Eaborn et al. [ ] 135 . Indeed, they assumed that for a similar decomposition of silicon and tin compounds, the expelled anions were not completely free but rather protonated as formed. When performing the reaction in MeOH−MeOD (1:1) they observed kinetic isotope effects of kH/kD of 1.4 to 4.6 depending on the substrates. J.R. Corfield and S. Trippett [ ] 136 later performed alkaline decompositions of tetraphenylphosphonium iodide and cumyltriphenylphosphonium iodide in H2O/D2O (1:1) and observed kinetic isotope effects kH/kD of 1.22 and 1.21 respectively. These differences in reaction rates, even if lower than those observed by C. Eaborn et al. [135] , can be explained by a mechanism of phosphonium salt decomposition in which, in the rate-determining step, little breaking of the P−R bond has occurred while there is a corresponding little transfer of a proton to the incipient carbanion. Furthermore, since isotopic effects are dependent from the reaction solvents involved, the range of kH/kD values obtained by J.R. Corfield and S. Trippett [136] can not be directly correlated to those obtained by C. Eaborn [135] . δ (1−δ) H C P O O
25 The assumption reported by these authors were based on the fact that a "free" carbanion is so reactive that it would presumably not significantly discriminate between a proton and a deuterium in the rate determining step. In contrast, a partially polarized and concerted transition state such as the one proposed above could show a more contrasted reactivity toward reprotonation with deuterated solvent or not. This hypothesis was later consolidated by a work of F.Y. Khalil and G. Aksnes [ ] 137 in which they observed kinetic isotope effects of larger magnitude, ranging from 1.55 to 2.36 depending on the temperature of decomposition for tetraphenylphosphonium chloride in dioxane/H2O or dioxane/D2O mixtures. These latter experimental observations, though not completely unequivocal, do fully support the Hammett analyses according to what the alkaline decomposition would rather proceed through a concerted mechanism. Since electron-withdrawing substituents obviously accelerate the rate of the alkaline decomposition of phosphonium salts, but no completely delocalized negative charge is apparently formed in the course of the rate determining step, it was still legitimate to believe that the ease of alkyl expulsion from the salt was somehow influenced by their ability to accommodate a negative charge. In contrast, if a concerted mechanism appeared to be favored in the light of the arguments discussed, steric factors could influence, to some extent, the departure of one alkyl substituent rather than the other. As a matter of fact, it is known that intramolecular strains play an important role on the rate of nucleophilic substitutions and particularly SN2 type reactions [ ] 138 . In this context, the results collected in this study will be discussed and compared to those reported in the literature, to evaluate the importance of front- strains (F-strains) as well as back-strains (B-strains), which could direct the course of the decomposition reaction. 3.3.2 Principle of the Method Devised For the purpose of this study, a series of phosphonium halides bearing two different types of alkyl substituents were prepared and submitted to alkaline decomposition in a butanol/water mixture. On the basis of the alkaline decomposition mechanism commonly assumed [29 - 34] , a plausible energetic profile is proposed in figure 2 to illustrate the principle of the study endeavored herein. It was, in this case, assumed that the ratio of the phosphine oxides obtained should be directly proportional to the difference in free enthalpy between ≠ both products (mentioned as G in Figure 2). As a matter of fact, the crucial elimination ∆∆ 12 1 step where one alkyl group (for example, R ) is expelled rather than the other one present on
Page 1: Various Facets of Organophosphorus
Page 5: III Remerciements Je tiens à remer
Page 9: 3.3.6 Relevance of the Data Assesse
Page 13: VII Literature References 115 Compo
Page 17: 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: 3.3.1 Mechanistic Discussion on the
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 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
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
Page 133 and 134:
111 1 H NMR: δ = 7.2 (m, 20 H), 7.
Page 135 and 136:
113 2,2’-Dilithio-6,6’-bis(meth
Page 137:
References
Page 140 and 141:
116 Conference Proceedings, Naples,
Page 142 and 143:
118 [ 70] J. Drowart, C.E. Myers, R
Page 144 and 145:
120 [117] S.T. Graul, R.R. Squires,
Page 146 and 147:
122 [170] B.H. Lipshutz, F. Kayser,
Page 148:
124 [212] M. Schlosser, in Organome
Page 152:
O P 27i: p. 30, 80 C 9H 21OP P 26b:
Page 156:
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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