Electrochemical Reactivity and Electrosynthesis of Heteroelement Compounds


 

Electrochemical Reactivity
Electrochemical Methodology
Electrogenerated Reagents
Redox Chemistry of Metallatranes
Silyl Radicals
Electrosynthesis



 
 
 
 
 
 
 
 
 
 
 
 


Electrochemical Reactivity

In two words, our work deals with the electrochemical reactivity, mostly of organosilicon compounds.
     What is this reactivity, is seen from the following.  All electrochemical reactions begin at the electrode when a substrate, say A, produces via electron transfer various primary intermediates, say A*. Obviously, the reactivity at this step (first bifurcation) and the primary reactions of thus produced transient species A* (bifurcation 2) are crucial for the unique pathway the process adopts leading to stable final products. That is why of all steps of an electrochemical process (mass transfer of substrates/intermediates/products, electrodics, reactions in the bulk of solution..) we mainly focus on this primary reactivity, the electrochemical reactivity.

     Different approaches exist to treat this topic; we follow and develop the one considering electroactive species and electrogenerated intermediates through the prism of their structure, and orbital, electronic and stereoelectronic effects in these species determining the reactivity in electrochemical context.



   Thus, stepwise and dissociative electron transfer mechanisms in anodic and cathodic reactions of organosilicon compounds were demonstrated.  Large variety of electrochemical reactions has been shown to intervene in oxidation of hexaorgano disilanes, - from totally reversible electron transfer resulting in persistent cation radicals up to dissociative Si-Si bond cleavage occurring at the same elementary step as electron transfer.



     Organo chlorosilanes can also follow both electron transfer mechanisms. Here, the stereoelectronic interactions are of extreme importance: in contrast to carbon-derived systems which form stable p-system based anion-radicals, specific s-orbital interactions of the bonds formed by silicon allow anion radicals formation from some totally aliphatic silicon derivatives.

     Another remarkable example, - we have shown that double bonds formed by Si and Ge revealed quite unexpected behavior in redox reactions, totally unknown in carbon electrochemistry.

     The most surprising result was that these bonds can consecutively give away or accommodate two electrons and this without the cleavage of the M-M linkage! Diode-like behavior of these systems due to mixing the electrons of M=M double bonds with n-electrons of S was demonstrated.     

     Depending on donor-acceptor interactions through space, easily modulated by polarity of the reaction media, products of different nature were obtained from the oxidation of arylalkylselenides. So simple change of proportions of the components of a binary solvent directs the process to either allylic or a-Nu-substituted products.

return

Electrogenerated Reagents in Synthesis

     Electrochemical activation of originally non-reactive molecules allows to produce various very reactive species or else to switch the native reactivity of substrates to its opposite (electrochemical umpolung); thus on-demand in situ prepared species eagerly react with diverse substrates leading to target products. Same exemples realized in this way are given below.

  1. Electrogeneration of silanones, very potent silicon transferring agents enabling introduction of various functional groups into permethylsiloxanes.

  2. Silyl-anions, produced by cathodic Si-Cl bond cleavage, formed cyclic siloxodisilanes, shown to be efficient modifying additives to silicone resins.

  3. Silyl radicals by electrochemistry is a currently running new project, soon we shall tell more about that.

  4. Electrogenerated carbanions enable an easy and elegant synthesis of cyclic carbosilanes including spirosilanes; another example is the electrosynthesis of disila cyclobutanes by consecutive inter- and intramolecular substitutions at Si-Cl bond by electrogenerated carbanions.

  5. Oxidative cleavage of C-S bonds in several diorganyl disulfides provides carbocations which, stabilized in liquid SO2, can further react with aromatic substrates allowing efficient t-butylation and benzylation of aromatic ring.

  6. Thiolate and selenolate anions, produced by cathodic cleavage of S-S or Se-Se bonds, were used in a smooth synthesis of thio- ans selenosilanes sought for generation of silyl radicals, soft silylation and simultaneous S(Se) and Si groups transfer.

  7. Thionyl- and especially selenyl cations, produced at the anode, allow a palette of interesting functionalizations of olefines, dienes, aromatics, acetylene and enol compounds.

     Quite often, due to ease of selenenylation and deselenenylation, selenium-based electrogenerated reagents provide very mild and convenient ways for functionalizing organic compounds, especially when the transformations of unsaturated sites are concerned.




P.S. Looking nice on paper, we like that all above sounds as an easy and clean chemistry. However, a great deal of purely synthetic organic work is always involved - from making starting compounds to separating and identifying the electrochemically formed products.

 

return


Electrochemical Methodology

Doing physical organic chemistry by means of electrochemistry implies theoretical and experimental methodology which is rather straightforward:

  • Molecular modeling of the systems to study, physico-chemical consideration, kinetics and mechanism simulation.

  • Cyclic voltammetry, RDE and related methods.

  • Spin trapping and direct detection of electrochemically
    produced radical species.

  • Controlled multielectrode EPR-spectroelectrochemistry.

  • Low temperature CV-EPR.

  • UV-Vis spectroelectrochemistry.

  • Oxygen-free temperature dependent conductivity measurements.

  • Reverse electrochemistry, flow-through processes, low-temperature electrolyses.

  • Electrosynthesis (direct and electrocatalyzed) in liquefied gases and ionic liquids.

   

return

Redox Chemistry of Metallatranes

   One of the most interesting features of metallatranes, first prepared back in 60's, is the intramolecular dative coordination N->M (M = Si, Ge, Sn) which accounts for their specific properties in various contexts. We undertook first study on electrochemistry of silatranes and germatranes with 5- and 6-membered lateral chains, either purely aliphatic or containing fused aromatic rings.

 

Though the lone pair of nitrogen is largely involved in the 3c-4e N->M coordination, electrochemical electron transfer (ET) is reversible for all metallatranes studies, unlike tertiary amines; the UV spectra of their cation radicals show new absorbance band due to SOMO-LUMO transition.

   When lateral conjugation of the M-C part of the N-M-C 3c-4e bond is impossible because of orthogonal orientation of the p-type orbitals of the substituent R (Ar, Npth, vinyl etc), distonic cation radicals are formed, with the spin remaining at nitrogen and the positive charge distributed over M and the lateral branches of the atrane cage.

   EPR-spectroelectrochemistry reveals large (end-to-end width D ~ 160 G) well resolved spectra organized in 9 groups with relative intensities as 1:1:4:3:6:3:4:1:1, as e.g. for Ph-silatrane cation radical.

 There follows that N atom is practically planar (hfc aN = 18-19 G, NBO electronic configuration of N is 2s(0.69) 2p(2.4) and N(pz) NAO occupancy is 0.985) and three a-C-H bonds are axially oriented showing most conjugation with the spin carrying N(pz) orbital. Both ESR data and Fermi contact couplings from DFT B3LYP/LANL2DZ show no contribution from M in the spin delocalization.

   The geometry changes around M in these cation radicals show opposite trends: Si becomes closer to trigonal bipyramid geometry, Ge atom can go into or out of Ge(-O)3 plane preferring trigonal bipyramid or tetrahedral configuration, respectively, while Sn mostly flattens as follows from ESR data and DFT B3LYP/LANL2DZ calculations.

 

   Even when R has lower own IP than N of the atrane (Alk2NC6H4-, thienyl etc) and the atrane moiety is not immediately affected by ET, thus induced positive charge on M markedly changes its geometry.

 

   In contrast to aryl- and vinyl-substituted metallatranes, hyperconjugation of s(Si-Csp3) bond in the cation radicals of benzyl germatranes enables lateral "spin leakage" from N to the benzylic aromatic ring of the substituent. The formation of distonic cation radicals of metallatranes with the R groups "ending" electron transmission and electrochemically induced geometry changes around M atom will provide a new insight into the electronic interactions in these compounds and open possibilities for designing molecular electronic devices with specific electronic, optical and electromechanical properties. 

   ESR-spectroelectrochemistry also enabled us to make evident the process of penta- to hexa-coordination changes in the cation radicals of arylgermatranes. Primary cation radicals of o-Br and o-F-Ph germatranes are formed in the geometry close to that of the starting molecule and then slowly evolve to adopt 6-coordinated configuration with additional coordination from ortho-halogen substituent. Kinetics and activation parameters of this process were measured as well.

 

return

Silyl Radicals by Reversed Electrochemistry

   Most of electrochemical processes of neutral silicon compounds have 2-electron character, the situation very typical in organic electrochemistry. It means that if first electron transfer (ET) results in cleavage of a chemical bond and leads to radical species, the applied potential needed to break this bond is too high compared the Eo of this radical so an immediate second ET transforms it into a non-paramagnetic species.

   To study these apparently 2-electron systems, we used the principle of "reversed electrochemistry" in a two-compartment flow-through electrochemical cell – when first realizing the overall 2e process in one part of this cell and then making one step back in the second compartment by applying the potential of the opposite polarity. Now, same silyl radicals R3Si can be prepared staring from both sides – either oxidation or reduction of neutral organosilicon molecules, provided that they are electrochemically active.

   This method clearly reveals the fundamental link between silylium-type cations, silyl radicals and silyl anions which are just one electron away from each other.

   For studying thus generated paramagnetic transients, four-electrode flow-through ESR-spectroelectrochemical cell was specially designed. This cell also turned out to be very useful for operating at different temperatures and with different solvents, including liquefied gazes like liq-SO2.

 

   The use of this ESR-EC-cell allowed studying "hidden" radical intermediates directly or using spin-trapping. Here is an example of a trialkyl silyl radical formation in a +2e/-e process when all three radical species – primary Si-radical and resulted from intramolecular addition across the double bond secondary C-radicals, - were detected by spin-trapping with a-phenyl-N-tert-butyl nitrone (PBN).

return

Electrochemical synthesis

Here are several examples of electrosyntheses developed based on the study of reaction mechanisms. Some of these processes are realized in gram quantity, some are in a  half-kilogram per day... Several processes, especially those with isolated yields above 80-85%, provide an efficient complement or even an alternative to the methods of traditional organic synthesis.

    SN2-like processes involving electro generated anionic species allowed an efficient synthesis of various carbosilanes, - linear, cyclic and spirocyclic. Electrochemical silylation of phenylacetylene provides access to diverse products used in the production of biotransplants, polymers and as precursors for Si-C ceramics.



     The formation of Si-Si bonds via 2-electron reduction of chlorosilanes is a main process in chlorosilane electrochemistry. This way, several disila disiloxo penta- and hexanes were prepared to serve as additives to silicone resins boosting their mechanophysical properties.

     Transient silanones, with their extremely strong affinity to insert into polar bonds, e.g. Si-O, allowed to introduce a large number of substituents and functional groups into permethyl siloxanes, both linear and cyclic. 

     The insertion of silanones into Si-H bonds provided a unique possibility to obtain products with four different groups at the Si atom, and this is just in one step. The use of ionic liquids in this process turned out to be very beneficial for its chemoselectivity.









     Selenium-assisted anodic reactions, using a minimal amount of Ph2Se2, offer a palette of possibilities for functionalization of unsaturated substrates, - with the same olefin, the products of vicinal additions or formally of allylic substitution can be obtained. The PhSe group, easily removable afterwards, provides good possibilities for mild protection-deprotection:

                

     Electrooxidation of several heteroatom-substituted substrates allows activation of carbon at a specific position affected by electronic or stereoelectronic effects of the heteroatom(s); the attack of an external nucleophile on this reaction center is thus being rendered more chemoselective.

 

     Anodic oxidation easily produces cationic and cation-like species that, being good electrophiles, can induce a number of useful reactions. Here anodic aromatic alkylation (R = t-Bu) and byproduct-free aromatic dimerization are illustrated. Last process is particularly efficient for the electrosynthesis in ionic liquids in an undivided cell since the coupled cathodic process is simple and harmless hydrogen evolution.

     Cathodic Ni(II) cathalysed formation of biaryls and bibenzyls from the corresponding organohalides was shown to be easily and selectively realizable in neat ionic liquids, with no co-solvent added at all.  
     Addition of small amount of liquid sulfur dioxide to BMIM NTf2 ionic liquid dramatically decreases the viscosity of this media and allows combining its low nucleophilicity with a good processability and recycling use, the latter is rather easy in this case because the SO2 is a gas at room temperature and all is needed is just to let it evaporate.

    Cathodic decarbonylation of phthalimide is a very convenient way for preparing isoindoline, the added value of this process is more that 1000 times. The tricky thing here is to master the interplay of hydrogen evolution and that of the target process.

return

Universite de Rennes I UMR 6510 Research Teaching Misc