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Interaction of PhB(OH)3– with complex 1
As a comparatively strong Lewis acid, PhB(OH)2 is quantitatively converted into PhB(OH)3– in the presence of a stoichiometric amount of TBAOH. The reaction between PhB(OH)2 and the anionic boronate form PhB(OH)3– is known to be quantitatively displaced in favour of the latter in the presence of OH– in water or in DMF.129 The pKa of PhB(OH)2 in water is 8.9.130 To the best of our knowledge, there was no data in dioxane. PhB(OH)2 was characterized by a broad peak at 28.9 ppm in 11B NMR. Upon addition of 1.0 equiv of TBAOH in water, a new broad peak appeared at 3.6 ppm corresponding to PhB(OH)3–. Addition of more TBAOH did not affect the chemical shift, which is consistent with this reaction being quantitative. Competition for hydroxide binding between PhB(OH)2 and complex 1 was then investigated by CV (Figure 10, a). As stated above, an equimolar mixture of complex 1 and TBAOH in dioxane in the presence of 2 equiv of added PPh3, is characterized by a reduction peak at E = –2.2 V vs SCE (R2). When PhB(OH)2 (20 equiv) was added to the mixture, the reduction peak R1 went back, suggesting that 1 is regenerated from 3-4 via a rapid equilibrium (Figure 10, a). After a few minutes at 45 °C, the solution turned red (characteristic color of PPh3-ligated Ni(0)), and two oxidation peaks Ox1 and Ox2 appeared at about –0.4 V and –0.2 V (Figure 10, b). The latter oxidation peaks could be attributed to PPh3-ligated Ni(0), as confirmed by addition of an authentic sample of [Ni(cod)2] (cod = cis,cis-cycloocta-1,5-diene) and PPh3.
Effect of the OH–/PhB(OH)2 ratio on the kinetics of TM and RE
With this model system in hand –which reproduces under stoichiometric conditions the process relevant for the catalytic cycle– we studied in detail the kinetics of the TM reaction. A premixed solution of PhB(OH)2 (10 equiv) and TBAOH (4 equiv) in dioxane was added to complex 1.
19F{1H} NMR analysis showed complete conversion of 1 into the coupling product 12 within 12 h. No intermediate product was observed, which is in agreement with RE being faster than TM.131 Under these conditions, integration of the peak at –117.5 ppm, corresponding to the coupling product 12 allowed to follow the kinetics of the reaction (Figure 12). Apparent first order with respect to complex 1 was obtained, suggesting that polynuclear species are likely not involved in the rate-determining TM step.
Electronic Effects of TM
Electronic effects due to the substituents on both the electrophile and the organoboron derivatives are among the most important parameters that usually influence the outcome of S-M reactions. Moreover, for the construction of a non-symmetrical biaryl Ar-Ar’, two pairs of electrophile/boron derivative partners are possible in principle (either ArB(OH)2 and Ar’X or Ar’B(OH)2 and ArX) and it is not obvious how to make the most appropriate choice without prior experiments. Since TM is the rate-determining step of the catalytic cycle, electronic effects due to the boronic acid partner are likely more important than those on the electrophile (except for substrates for which OA is actually rate-determining, such as chloroarenes).138 Unfortunately, literature reports are not unanimous on this point. Positive, negative, or zero Hammett ρ values have been observed for various Pd-catalyzed S-M couplings.139 Interestingly, for some Ni-catalyzed S-M reactions, electron-poor arylboronic acids react faster than electron-rich ones, which is counterintuitive if we consider that the organo-boron derivative is playing the role of a nucleophile in the TM step.140 To assess the effect of substituents on the arylboronic acid, two Hammett plots were constructed using para-substituted boronic acids at two different initial boronic acid/TBAOH ratios (Figure 16). For a TBAOH / PhB(OH)2 molar ratio of 0.4, the trend we found is in agreement with the literature:35 electron-poor boronic acids react faster than electron-rich ones (Figure 16, a).
Effects of the nature of the halide and of the phosphine
As already stated in the introduction, a wide variety of electrophiles can be useful substrates for the nickel-catalyzed S-M reaction. In particular, readily available and cheap chloroarenes can be activated under mild conditions without using expensive ligands, which is a major advantage compared to palladium.In this perspective, we investigated whether substituting the Br– ligand with Cl– in the model complex 1 would have significant impact on reactivity. Complex trans- [ArNi(PPh3)2Cl] (1-Cl, Ar = 4-F-2-Me-C6H4), which is representative of the product deriving from the OA of a chloroarene with Ni(0), was thus prepared.144
Table of contents :
TABLE OF CONTENTS
ABBREVIATIONS
GENERAL INTRODUCTION
CHAPTER I – MECHANISTIC STUDIES TOWARD GREENER PROCESSES: CHALLENGES AND TOOLS
1.What is a chemical mechanism?
2.Experimental techniques for mechanistic investigation
2.1. Kinetics and reactivity
2.2. Electrochemistry
2.3. Nuclear Magnetic Resonance spectroscopy of heteronuclei
2.4. Electron Paramagnetic Resonnance (EPR) spectroscopy
3.Theoretical Tools
3.1. Theoritical foundations
3.2. The Born-Oppenheimer approximation and nuclear energy
3.3. Electronic energy and Slater approximation
3.4. How to evaluate the electronic energy?
3.5. The Density Functional Theory (DFT)
3.6. Choice of the basis set and functional
3.7. Empirical dispersion corrections
3.8. Modelization of solvent effects
3.9. Calculation of thermodynamic parameters
3.10. Charge and bond analyses
3.11. Indicators of chemical reactivity: electronegativity, global and
local hardness, Fukui functions and global electrophilicity index
3.12. Conclusion
CHAPTER II – BORON-TO-TRANSITION-METALS TRANSMETALLATION: MECHANISTIC STUDIES
1.Context of the Study
1.1. Transition-metal catalyzed coupling reactions
1.2. Generalities on the mechanism of the Suzuki-Miyaura crosscoupling reaction
1.3. The Palladium-to-Boron Transmetallation Step
2.Transmetallation from Boron to Nickel
2.1. Previous works
2.2. Choice of the model reaction
2.3. Formation of hydroxo-bridged dinuclear complexes
2.4. Decomposition of the complex with excess base
2.5. Interaction of PhB(OH)3– with complex 1
2.6. Effect of the OH–/PhB(OH)2 ratio on the kinetics of TM and RE
2.7. Influence of Br– and PPh3 on the rate of TM and RE
2.8. Mechanism of the TM step
2.9. Mechanism of the RE step
2.10. Electronic Effects of TM
2.11. Effects of the conter-ion
2.12. Effects of the nature of the halide and of the phosphine
2.13. Conclusions
CHAPTER III – LEWIS ACID AND REDOX CATALYTIC PROPERTIES OF TRIFLATE AND TRIFLIMIDE SALTS
1.Triflate and triflimide salts in catalysis
1.1. Lewis acidity: definition and quantification
1.2. Triflate and triflimide salts: history, preparation and charaterization
1.3. Application of triflate and triflimides in synthesis
2.Mechanistic study of a model reaction: Al(OTf)3-catalyzed amination of alcohol
2.1. Description of the model reaction and solvent effects
2.2. Nature of Al(OTf)3 in nitromethane and coordination of BnOH
2.3. Deactivation of Al(OTf)3 by aniline in nitromethane
2.4. Competition between BnOH and aniline for Al in toluene .
2.5. Validation of the DFT methodology
2.6. Determination of the structure of the catalyst by DFT calculations
2.7. Amination Mechanism
2.8. Conclusions
3.Rational design of Lewis acids for the direct amination of alcohols
3.1. Experimental trends for amination within a series of Lewis acids
3.2. Experimental descriptors of Lewis acidity
3.3. Structures of metal triflate and triflimides salts
3.4. Theoretical descriptors of Lewis acidity
3.5. Fukui functions and local hardness
3.6. “In silico Child’s method”
3.7. Synthesis and characterization of titanium triflimide
3.8. Catalytic activity of 4
3.9. Mechanistic insights
3.10. Conclusions
4.Iron triflate salts for oxidation of cyclohexane
4.1. Industrial synthesis of cyclohexanone and cyclohexol .
4.2. Iron catalyzed oxidation
4.3. Cyclohexane oxidation using TBHP as oxidizing agent .
4.4. EPR investigation
GENERAL CONCLUSIONS AND PERSPECTIVES
EXPERIMENTAL SECTION
