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Limitation by charge or mass transfer
The electrochemical current obtained may be easily related with the kinetics of the redox reaction by derivation of the Faraday law: with q the charge transferred, F the faraday constant (96485 C mol−1), n the amount of the substance, z its charge, I the electronic current and v the reaction rate. Thus, the measure of the current obtained for an applied potential can give us information about the redox reaction rate.
The redox reaction may be limited either by i) the kinetics of the electron transfer: a redox couple may be fast or slow depending on the intrinsic activation energy for the reaction: Red + Ox = Ox + Red.12 In the case of simple electron transfer (no fragmentation or condensation coupled with the electron transfer), the solvent reorganization is generally the limiting process.
ii) the kinetics of the mass transfer: as the redox reaction must take place at the electrode (i.e. on a surface) the kinetics of the reaction is influenced by the transportation phenomenon in solution, namely, convection (macroscopic motion of the solvent), migration (charge migration under an electrochemical field) and diffusion. In the presence of a supporting electrolyte, migration of redox active species is negligible with respect to the one of the electrolytes (in our case roughly more than one hundred times more concentrated). Thanks to the solvent viscosity, the convection may also be insignificant around the electrode, in the diffusion layer (approximately 10 µm).
Description of Electrochemical technics used in the present work
Chronoamperometry. The simplest electrochemical sequence is to apply a potential and measure the current at the working electrode as a function of time, this is called chronoamperometry. When this experiment is performed on a macroscopic disk electrode (more than 100 µm, vide infra definition of micro electrodes) the diffusion limits the current, which decreases as (Scheme 3):7.
Electrochemistry to study reaction mechanisms
Electrochemistry is a powerful tool for mechanistic studies. It can be used as a traditional analytical technic, providing information about the concentration of redox active species (both CV and chronoamperometry may yield currents proportionate to the concentration under appropriate conditions). It is thus possible to estimate thermodynamic (equilibrium constants) as well as kinetic data. Electrochemistry has a high sensitivity (100 µM concentration is enough to obtain correct data) and high time-resolution (1 ms on UME). More specifically, electrochemistry can provide information about the redox state of the system it is thus particularly interesting to study metal-catalyzed processes.
Finally, electrochemistry is a dynamic analytical technic, i.e. the electron transfer can be used to locally modify the chemical mixture and to study the answer of a system. For instance, it is possible to shift an equilibrium by oxidation/reduction of one of the molecules involved in the equilibrium (allowing the measure of kinetics of rapid equilibrium) or to generate an active species (metal complex with a low or high oxidation state or organic radical) and to study its reactivity.
Nuclear magnetic resonance spectroscopy of heteronuclei
Nuclear Magnetic Resonance (NMR) has been developed in the 40’s by E. M. Purcell and F. Block. This technique uses the interaction of a nuclear spin ( ) with a magnetic field ( ) to obtain structural data on organic and inorganic compounds. A nuclear magnetic moment ( = ) interacts with a magnetic field following:16 ‹’Œ:= − ⋅ =− ⋅
This interaction leads to the splitting of the | ⟩ electronic sublevels proportionate to the extent of the magnetic field and of the gyromagnetic ratio (γ). All non-zero spin nuclei may a priori be detected using NMR. The sensitivity of a nucleus is related to the gyroscopic ratio (the sensitivity increases with the energetic gap between the spin levels) and to the natural abundance. In this work, the following nuclei .
19F and 31P NMR
19F NMR has a sensitivity nearly as good as the proton one due to its high gyroscopic ratio and high abundance, it also has a very large range of chemical shift (Scheme 8). Due to these points, fluorine NMR may be used as an atomic probe to monitor any structural change in fluorine-tagged complexes as well as to follow kinetics of reactions (using a 10 mM sample only 16 s are required to get a spectrum with a correct signal to noise ratio allowing to study reactions with t1/2 as low as 100 s).
Quantitative NMR
NMR spectrum can be used to determine thermodynamic as well as kinetic data, this requires however the signal to be proportionate to the concentration of the sample and to determine the proportionality coefficient.
18 P. S. Pregosin, R. W. Kunz, 31P and 13C NMR of Transition Metal Phosphine Complexes, 1979, Springer; O. Kühl, Phosphorus-31 NMR Spectroscopy: A Concise Introduction for the Synthetic Organic and Organometallic Chemist, 2008, Springer.
It is convenient to use proton-decoupled sequences noted, 19F{1H} and 31P{1H}. These sequences, however, produces an increase of the signal of heteronuclei, through the so-called Nuclear Overhauser Effect (NOE), roughly proportionate to the number of spatially close protons. The use of inverse-gated decoupling allows to prevent this problem.
Another problem is the relaxation delay (generally written d1), the d1 must be at least five times the spin lattice relaxation time (T1). T1 can be measured using a sequence called inversion-recovery.23 In our case a 19F nucleus in an organometallic complex relaxes nearly 10 times faster than one in an organic molecule and a d1 of 10 s was required.
Finally, an internal reference is needed, in our case fluorobenzene and/or phosphoric acid were added in a glass capillary filled with deuterated DMSO, allowing to lock and shim (without waste of deuterated solvent) as well as to have a reference.
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
