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Electrochemical Organic Synthesis Ole Hammerich

Electrochemical Organic Synthesis Ole Hammerich. Electrochemical Organic Synthesis , 2013. What is ’ organic electrochemistry ’ ?.

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Electrochemical Organic Synthesis Ole Hammerich

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  1. Department of Chemistry, Organic Chmeistry ElectrochemicalOrganicSynthesisOle Hammerich ElectrochemicalOrganicSynthesis, 2013

  2. Department of Chemistry, Organic Chmeistry What is ’organicelectrochemistry’ ? • Organic electrochemistry is concerned with the exchange of electrons between a substrate and an electrode and the chemical reactions associated with such processes. • Organic electrochemical processes are conceptually related to other organic reactions that include one or more electron transfer steps, such as oxidation by metal ions (e.g., Fe3+ and Ce4+) and reduction by metals (e.g. Na, K, Zn, Sn). • At the borderline of organic chemistry, electron transfer processes play an important role in many reactions that involve organometallic compounds and in biological processes such as, e.g., photosynthesis. ElectrochemicalOrganicSynthesis, 2013

  3. Department of Chemistry, Organic Chmeistry Organic redox reactions vis-à-viselectrochemical reactions In the electrochemical process, the oxidation agent is replaced by the anode (+) and the reduction agent by the cathode (-)here illustrated by functional group conversion. ElectrochemicalOrganicSynthesis, 2013

  4. Department of Chemistry, Organic Chmeistry Organicelectrochemicalconversions • Additions: • R-CH=CH-R + 2Nu- R-CHNu-CHNu-R + 2e- • R-CH=CH-R + 2e-+ 2H+ R-CH2-CH2-R • Substitutions: • R-CH3 + Nu-  R-CH2Nu + 2e-+ H+ • R-Cl + CO2 + 2e- R-COO- + Cl- • Eliminations: • R-CH2-CH2-R  R-CH=CH-R + 2e-+ 2H+ • R-CHNu-CHNu-R + 2e-  R-CH=CH-R + 2Nu- • Cleavages: • RS-SR  2RS+ + 2e-  furtherreaction of RS+ • RS-SR + 2e-  2RS- • Couplings: • 2R-H  R-R + 2e- + 2H+ • 2R-CH=CH-EWG + 2e- + 2H+  (Hydrodimerization) ElectrochemicalOrganicSynthesis, 2013

  5. Department of Chemistry, Organic Chmeistry Additions, examples ElectrochemicalOrganicSynthesis, 2013

  6. Department of Chemistry, Organic Chmeistry Substitutions, examples ElectrochemicalOrganicSynthesis, 2013

  7. Department of Chemistry, Organic Chmeistry Eliminations, examples ElectrochemicalOrganicSynthesis, 2013

  8. Department of Chemistry, Organic Chmeistry Cleavages, examples ElectrochemicalOrganicSynthesis, 2013

  9. Department of Chemistry, Organic Chmeistry Couplings, examples Kolbe-reaction Hydrodimerization ElectrochemicalOrganicSynthesis, 2013

  10. Department of Chemistry, Organic Chmeistry Coupling/condensation reactions, example Hydrodimerization Dieckmann condensation Fussing, I., Güllü, M., Hammerich, O., Hussain, A., Nielsen, M.F., Utley, J.H.P. J. Chem. Soc. Perkin Trans. II, 1996, 649-658. ElectrochemicalOrganicSynthesis, 2013

  11. Department of Chemistry, Organic Chmeistry Electron transfer induced (catalyzed) chainreactions [2+2] cycloaddition SRN1 ElectrochemicalOrganicSynthesis, 2013

  12. Department of Chemistry, Organic Chmeistry Organicchemistry is usually ’two-electronchemistry’ • Most persistentorganiccompounds have an evennumber of electrons • G.N. Lewis (1916): A covalentbond is the result of two atoms or groupssharingan electron-pair • Most organicredoxreactionsarecomprised of one or more ’two-electronconversions’ • Examples of reductions: • Ar-NO2 Ar-NO  Ar-NHOH  Ar-NH2 • R-COOH  R-CHO  R-CH2OH  R-CH3 • R-SO2-R  R-SO-R  R-S-R • R-CN  R-CH=NH  R-CH2NH2 ElectrochemicalOrganicSynthesis, 2013

  13. Department of Chemistry, Organic Chmeistry Organicelectrochemistryis usually ’one-electronchemistry’ Electrochemistry is ’electron transfer chemistry’ and electrons are transferred one-by-one driven by the electrode potential • and so are protons ! • Thus, the electrochemical reduction of a –CH=CH- system • R-CH=CH-R + 2e- + 2H+ R-CH2-CH2-R • is a four-step process including the transfer of 2 electrons and 2 protons • The order of the four steps depends on the substrate and the conditions ElectrochemicalOrganicSynthesis, 2013

  14. Department of Chemistry, Organic Chmeistry The mechanism of electrochemicalhydrogenation rds ElectrochemicalOrganicSynthesis, 2013

  15. Department of Chemistry, Organic Chmeistry Organicelectrochemistryis usually ’one-electronchemistry’ Electrochemistry is ’electron transfer chemistry’ and electrons are transferred one-by-one driven by the electrode potential. • For neutral π-systems the primary intermediates are radical cations and radical anions, that is, the intermediates are radicals and ions at the same time and it is not easy to predict whether the radical character or the ion character predominates for a given radical ion. • For charged π-systems the primary intermediates are radicals that may dimerize. ElectrochemicalOrganicSynthesis, 2013

  16. Department of Chemistry, Organic Chmeistry Organicelectrochemistryis usually ’one-electronchemistry’ Electrochemistry is ’electron transfer chemistry’ and electrons are transferred one-by-one driven by the electrode potential • For neutral σ-systems electron • transfer is dissociative resulting in • radicalsand cations or anions • For charged σ-systems dissociative • electron transfer results in neutral • fragments and radicals ElectrochemicalOrganicSynthesis, 2013

  17. Department of Chemistry Radical ions and neutral radicalsarereactive species • Electron transfer reactions • Someorganic solvents maybeoxidized or reduced • Cleavagereactions • Inherent - owing to bondweakening • Couplings • Inherent - owing to the radicalcharacter • Reactions of radicalcations with nucleophiles and of radical anions with electrophiles (electrochemical ‘umpolung’) • Mostly non-inherent - owing to the ioniccharacter • Most organic solvents arenucleophiles and/or electrophiles • Most organic solvents are bases and somearealsoBrønstedacids - the kinetics of proton transfer processes are solvent dependent • Atom (hydrogen) abstractions • Inherent - owing to the radicalcharacter • Someorganic solvents are hydrogen-atom donors

  18. Department of Chemistry, Organic Chmeistry The number of experimental parameters that may be manipulated in electrosynthesisis large including the electrode potential (driving force, rate of the ET process) current density (conversion speed) electrode material (overpotential - catalysis) solvent (often the reagent) and the supporting electrolyte (conductivity) mass transfer to/from the electrodes (stirring/pumping rate) cell design (electrode surface area, separation of anolyte and catholyte) in additionto, e.g., the temperature, the pressure etcetc Importantexperimental parameters in electrochemistry Any of these parameters may affect which products are formed and/or yields Take-home-message: Do as told in the recipe ! ElectrochemicalOrganicSynthesis, 2013

  19. Department of Chemistry, Organic Chmeistry The electrode potential – the driving force • The Nernst equation • The standard potential, Eo and the formal potential, Eo' • n is the number of electrons (for organic compounds, typically, n = 1) • R is the gas constant • T is the absolute temperature • F is the Faraday constant • Parentheses, (), are used for activities and brackets, [], for concentrations • fO and fR are the activity coefficients of O and R, respectively. The heterogenous electron transfer rate constants, ksred and ksox ksred ksox Most organic compounds are oxidized or reduced in the potential range +3 to -3 V ElectrochemicalOrganicSynthesis, 2013

  20. Department of Chemistry, Organic Chmeistry The current – conversion speed • The heterogenouselectron transfer rate constants, ks • ksred = ko exp[–αnF (E – Eo) /(RT)] • ksox = ko exp[(1 – α)nF (E – Eo) /(RT)] • TheButler-Volmer equation • i = nFA(ksred[O]x=0 – ksox[R]x=0) • = nFAko {[O]x=0 exp[–αnF (E – Eo) /(RT)] – [R]x=0 exp[(1 – α)nF (E – Eo) /(RT)]} • kois the standard heterogeneous electron transfer rate constant • αis theelectrochemical transfer coefficient • (corresponds in electrochemistry tothe Brønsted coefficient in organic chemistry) • A is the electrodearea • [O]x=0 and [R]x=0 are the surface concentrations of O and R, respectively • (governed by the Nernst equation) The current (the conversion speed) is potential dependent Mass transport (stirring, pumping) is important ElectrochemicalOrganicSynthesis, 2013

  21. Department of Chemistry - Organic Chemistry - Ole Hammerich Constant potential or constantcurrentelectrolysis ? Requires a setup with a reference electrode The potential is essentiallyconstantduringconstantcurrentelectrolysis; thus a reference electrode is not needed Constantcurrentelectrolysis is most simple and preferredwheneverpossible

  22. Department of Chemistry, Organic Chmeistry The current flow through the solution • is caused by the transport of ions A highconcentration of the supportingelectrolyte is important (to lower the solution resistance) ElectrochemicalOrganicSynthesis, 2013

  23. Department of Chemistry, Organic Chmeistry The electrode material • The potential limiting processes (in aqueous solution or watercontainingorganic solvents) • 2 H2O + 2e-→ H2 + 2 OH- • 2 H2O → O2 + 4 H+ + 4e- • Overpotential for hydrogen evolution • Pd < Au < Fe < Pt < Ag < Ni < Cu < Cd < Sn < Pb < Zn < Hg • Overpotential for oxygen evolution • Ni < Fe < Pb < Ag < Cd < Pt < Au • Special electrodematerials • Glassycarbon, carbon rods, boron-dopeddiamond (BDD), • Dimensionally stable anodes (DSA, Ti covered with metal oxides) --- • Cave: The electrodemaydissolveduring oxidations (M  Mn+) ElectrochemicalOrganicSynthesis, 2013

  24. Department of Chemistry, Organic Chmeistry Solvent and supporting electrolyte Solvents for oxidation: MeCN, CH2Cl2, MeOH (methoxylations) Solvents for reduction: MeCN, DMF, DMSO, THF Supportingelectrolytes for aproticconditions: R4NBF4, R4NPF6 typically Bu4NPF6 Substitutions/additions: MNu or R4NNu Alkoxylations: KOH • The Solvent: • In addition to the usual solvent properties: • Applicable in the potential range +3V to -3V • Medium to high dielectric constants • The supporting electrolyte • Applicable in the potential range +3V to -3V • Well dissociated • Both: • Easy to remove during work-up • Preferably non-toxic • ---------------- • Aprotic • Non-nucleophilic and/or non-electrophilic • Recyclable • ---------------- ElectrochemicalOrganicSynthesis, 2013

  25. Department of Chemistry, Organic Chmeistry Components of a simple, undividedcell for laboratoryscaleconstantcurrentelectrolysis C cathode Pt anode 3 cm ElectrochemicalOrganicSynthesis, 2013

  26. Department of Chemistry, Organic Chmeistry Undivided ? Divided ? • Two processes aregoing on in the electrochemicalcell, always ! • An oxidation at the anode • A reduction at the cathode • Potential problem: • The productformed by oxidation at the anode mayundergoreduction(e.g., back to the startingmaterial) at the cathode • In such a case a dividedcell is needed ElectrochemicalOrganicSynthesis, 2013

  27. Department of Chemistry, Organic Chmeistry The classical, dividedlaboratoryscalecell (H-cell) workingelectrode compartment counterelectrode compartment cooling ElectrochemicalOrganicSynthesis, 2013

  28. Department of Chemistry, Organic Chmeistry Small, large and very large (divided) flow cells Notice the small distance between the electrodes Electrochemicalsynthesesareeasilyscalable (expandablereactionvessels) ElectrochemicalOrganicSynthesis, 2013

  29. Monsanto (Solutia), BASF, Asahi Chemical Department of Chemistry, Organic Chmeistry Somecommercialprocesses ElectrochemicalOrganicSynthesis, 2013

  30. Department of Chemistry, Organic Chmeistry Voltage difference vs. potential difference • Two-electrode system for electrochemicalsynthesis in an undividedcell • The voltage difference, V,between the twoelectrodes is NOT the same as the potential difference, E • V = E + iRs • Rs: the solution resistance • iRs: the ohmic drop (Ohm’s law) • Rsmayamount to several hundred ohms ifspecial • precautionsare not taken • => practical implications ElectrochemicalOrganicSynthesis, 2013

  31. Department of Chemistry, Organic Chmeistry The power supply – constantcurrent source Max 100 V Max 1A • If i=1A and Rs=100ΩthenΔV = 100V + ΔE ≈ 100V • 100V∙1A = 100 W (= heat, need for cooling) • (ii) Waste of energy (= money) • (ii) 100V maybedangerous ElectrochemicalOrganicSynthesis, 2013

  32. Department of Chemistry, Organic Chmeistry The undividedcell put together coolingbath (ice/water) ElectrochemicalOrganicSynthesis, 2013

  33. Department of Chemistry, Organic Chmeistry The advantage of electrolysis in a boiling solvent –the electrochemicalPummererreaction (substitution) Almdal, K., Hammerich, O. SulfurLett. 1984, 2, 1-6.

  34. Department of Chemistry, Organic Chmeistry Pros 1. Replacement of inorganic redox reagents with electrode processes often reduces the number of steps in the overall reaction 2. Electrode reactions are often selective and present direct routes to products otherwise difficult to make (via electro-chemical ‘umpolung’) Electrons are cheap and are easy to transport. Electricity can be made from many different natural resources Green technology; no toxic wastes, no fire or explosion hazards, no storage and handling of aggressive reagents, mostly room temperature chemistry Electrochemical synthesis is easily scalable to the industrial level Cons Organic electrochemistry is (still) considered a specialists topic and is usually not a part of the chemistry curriculum. Reaction mechanisms are often complex and require insight into radical ion (and radical) chemistry Requires equipment (electrodes, cells, current sources and potentiostats) that is often not available in the traditional laboratory Electron transfer is heterogeneous and for that reason electrochemical reactions take time. (1 Mole of e- = 1 F = 96485 C = 96485 A·s = 26.8 A·h) Organicelectrochemicalsynthesis in summary ElectrochemicalOrganicSynthesis, 2013

  35. Department of Chemistry, Organic Chmeistry Literature • Lund/Hammerich eds.: • OrganicElectrochemistry, 4th ed., • Dekker, 2001. Shono: Electroorganicsynthesis, Academic Press, 1991. Recipe book Pletcher/Walsh: Industrial Electrochemistry, Chapman & Hall, 1990. ElectrochemicalOrganicSynthesis, 2013

  36. Department of Chemistry, Organic Chmeistry Recipe no 1 • To a magnetically stirred solution of 1 g of KOH in 150 mL of methanol at ~0°C (ice-bath) is added 4.6 g (0.033 mol) of 1,4-dimethoxybenzene. • The solution is electrolyzed at a constant current of 1 A for 2 h in an undivided cell using a Pt gauze anode and a C cathode. • After oxidation, the solution is concentrated under reduced pressure. To the residue is added 100 mL of water that is extracted with three 50 mL portions of ether. After removal of solvent, the residue is recrystallized from light petroleum to give ~5 g of the product (m.p. 40-41°C). ElectrochemicalOrganicSynthesis, 2013

  37. Department of Chemistry, Organic Chmeistry Recipe no 2 • Into a cell equipped with a Pt anode and a C cathode is added a solution of furan (2 g) in a mixture of AcOH (120 mL) and MeCN (30 mL) containing AcONa (6 g). • The mixture is cooled to 3 ~ 7°C during the oxidation. • After 2.5 F (~1 A for 2h) of charge has passed, the reaction mixture is poured into water and extracted with CH2Cl2. The extracts are dried with MgSO4 and distilled to give the product. ElectrochemicalOrganicSynthesis, 2013

  38. Department of Chemistry, Organic Chmeistry Recipe no 3 • A solution of tetrahydrofuran (7.4 mmol = 0.53 g) and Et4NOTs (2 mmol = 0.6 g) in a mixed solvent of acetic acid (10 mL) and methanol (120 mL) is put into an undivided cell equipped with a platinum anode and a graphite rod cathode. • After 10 F (~1A for 4 h) of charge is passed, the product is obtained by distillation. ElectrochemicalOrganicSynthesis, 2013

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