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Electroorganic Chemistry: Choice of Electrodes 1 Yu Kawamata Electroorganic Chemistry: Choice of Electrodes Baran Group Meeting 12/10/2016 1 Also see: Electrochemistry in Synthesis (Ambhaikar, 2005) Electroorganic Chemistry (Rosen, 2014) Unique parameters in electrochemistry: Electrochemical cell ••••• devided or undivided Current and potential ••••• constant current (c.c.) or constant potential (c.p.) Electrolyte ••••• various ammonium or alkali metal salts Electrodes ••••• anode (oxidation), cathode (reduction) Divided cell Undivided cell • Anode compartment is always • Net reduction proceeds if a sacrificial Topics not discussed: oxidation. anode is employed. • Chemically modified electrodes • Cathode compartment is always • In other cases, reaction type (net oxidation • Electrodes for inorganic and analytical electrochemistry reduction. or reduction) depends on the balance of redox potentials of compounds in the cell. 1. Electrode material Review: Pletcher, Chem. Rev. 1990, 90, 837. 1-3. Overpotential Overpotential is the potential 1-1. General Consideration difference between a half- i) Physical stability v) cost and lifetime reaction's thermodynamically determined potential and the ii) chemical stability vi) suitable physical form potential at which the redox event iii) overpotential vii) electrical conductivity is experimentally observed. iv) rate and product selecticity Thermodinamically defined potential: 1-2. Modes of action of electrodes Oxygen O + 4H+ + 4e- 2H O º = 1.23 (V) Electrodes can: transfer electrons 2 2 E red (vs SHE) absorb organic compounds Hydrogen + - º 2H + 2e H2 E = 0.00 (V) Reactivity and product selectivity might be red https://en.wikipedia.org/wiki/overpotential affected by electrode materials. act as reagents Generally, material with large O2 overpotential is used for anode and material with large H2 ovarpotential is used for cathode. e.g. • Sacrificial anode (source of electron for cathodic reduction) 1-4. Anode material • NiOOH, PbO anode 2 Sacrificial anode ••••• Al, Zn, Mg, steel etc. • Ni and Pt cathode for hydrogenation anode Non-sacrificial ••••• Pt, carbon based material (graphite, glassy carbon, RVC, BDD) Yu Kawamata Electroorganic Chemistry: Choice of Electrodes Baran Group Meeting 2 12/10/2016 Sacrificial anode Source of electron for cathodic reduction Boron-Doped Diamond (BDD) Panizza, Electrochimica Acta, 2005, 51, 191. Macpherson, Phys. Chem. Chem. Phys. 2015, 17, 2935. Top. Curr. Chem. 2012, 320, 1. Mg Al Zn Fe Ni Sn Pb Cu Hg Ag Au Pt • Highly durable under oxidative conditions Dissolve when used as anode material • Large O2 overpotential (1.1 V) and H2 overpotential (-1.1 V) Platinum • Due to its large O2 overpotential, OH radical and ozone can be generated under aqueous conditions. • Highly durable under oxidative conditions • Used for waste-water treatment (complete mineralization of organic • Wide potential range due to the large O2 overpotential molecule using OH radical) and ozone generation. • Most common anode material • Recently introduced to electroorganic synthesis Graphite • Highly stable, but can be eroded under highly oxidative conditions. • Wide potential range due to the large O2 overpotential • Absorption or intercalation of organic material may occur. • Most common anode material BDD disk SEM image of BDD Glassy carbon (Vitreous carbon) Other anode material • Amorphous form of carbon • Very hard (as hard as quartz) Lead dioxide (PbO2) Review: Hampson, Chem. Rev. 1972, 72, 680. • Good chemical stability and wide potential range • Large O2 overpotential, wide potential range. • Prepared by electrochemical deposition of PbO2 onto conductive materials • Used for production of inorganic oxidant such as perchlorate, periodate and ozone • Electroorganic application is oxidation of alkenes, arenes and alcohols Proposed structure of glassy carbon. Glassy carbon rods All carbon atoms are sp2 carbon. Ni(O)OH anode Review: Petrosyan, Russ. J. Electrochem. 2010, 46, 1199. Prepared by forming Ni(OH) on Ni plate Reticulated Vitreous Carbon (RVC) • 2 • Anodically stable only under basic conditions Review: Ponce-de-León, J. Electroanal. Chem. 2004. 561. 203. • Oxidation of alcohol and amine proceeds by anodically generated Ni(III) • Glassy carbon produced as a form on the electrode. • Same chemical property as glassy carbon Carbon sulfide electrode Review: Guillanton, Sulfur Reports. 1992, 12, 405. • Large surface area • Sacrificial anode . SEM image of RVC • Used for thiolation of organic compounds Yu Kawamata Electroorganic Chemistry: Choice of Electrodes Baran Group Meeting 12/10/2016 3 1-5. Cathode material 3. Electrode Degradation Platinum anode Littauer, Corrosion Science, 1970, 10, 29. High H2 overpotential ••••• Carbon-based material, Pb, Sn, Hg, Cd, Zn Useful for cathodic reduction of various Pt is known to be corroded slowly in the presence of halide ion. organic molecules (corrosion rate: 5mgA-1h-1 in the electrolysis of 8 M HCl for 80 h) Low H2 overpotential ••••• Pt, Ni, Cu, Ag, Fe(steinless steel) Graphite anode Hudson, J. Mater. Chem. 1997, 7, 301. These electrodes are used when H2 evolution Srivastava, J. Nanosci. Nanotechnol. 2015, 15, 1984. at cathode is desireble. Generally, graphite is not tolerated under strongly oxidizing conditions or high voltage due to the formation of graphene oxide. Glassy carbon anode Isse, Mussini, Electrochem. Comm. 2010, 12, 1531. OH radical can cause erosion of glassy carbon Conditions: H2O2, Fe(NH4)2(SO4)2 and EDTA in 0.01 M acetate buffer Example of reaction set-up with platinum anode and mercury pool cathode Wright, Chem. Soc. Rev. 2006, 35, 605 2. Price of Electrodes Micrographs of GC surface: before treatment (left), after 60 min (right) Material Supplier Quantity Price BDD anode Comninellis, J. Appl. Electrochem. 2004, 34, 935. Aldrich 1.5 g $ 480 Pt foil BDD is also known to be eroded under harsh electrchemical conditions. Alfa Aesar 0.025 mm × 25 mm × 25 mm $ 107 Pt coated Ti ebay 30 mm × 40 mm $ 12.8 Graphite many 1 lb < $ 10 Glassy carbon rod Alfa Aesar 3 mm × 25 mm × 25 mm $ 98 RVC KRRaynolds 40 mm × 40 mm $15 BDD windsor scientific 3 mm diameter $ 305 Hg Aldrich 1 kg $ 422 Al, Mg, Ni, Cu, many 1 lb < $ 10 Before electrolysis After severe anodic polarization at Pb, Sn, Fe, Zn -2 1Acm in 1 M HClO4 for 576 h at 40 °C Yu Kawamata Electroorganic Chemistry: Choice of Electrodes Baran Group Meeting 12/10/2016 4 4. Selected Electroorganic Reactions and Suitable Electrodes Oxidation of amides and carbamates (Shono oxidation) 4-1. Anodic oxidation O O Oxidation of carboxylic acids (Kolbe reaction) Review: Schäfer, Chemistry and O Physics of Lipids, 1979, 24, 321. anodic oxidation R MeOH R R N N anodic oxidation N RCOO R R R OMe -CO2 Anode: Pt (most common), Glassy carbon Cathode (not critical): Pt, carbon, Fe, Hg etc. Anode: Pt, graphite Cathode (not critical): Graphite, stainless steel, Pt, Hetero coupling Boland, Steckhan, Tetrahedron Lett. 1996, 37, 8715. Pt anode O O steel cathode Application to toal synthesis Braekman, Daloze, Eur. J. Org. Chem. 2000, 2057. MeONa + HOOC COOEt CO Me 5 MeOH, c.c. 2 graphite anode CO2Me O COOH COOEt O 4.2 eq undivided cell 5 R N graphite cathode R N OMe 43% R N Steel cathode was chosen to avoid catalytic hydrogenetion of the double bond. Et4NOTs, MeOH R = nC H c.c., undivided cell 5 11 78% Cascade cyclization Schäfer, Synthesis, 1995, 1432. (+)-calvine Regioselectivity of methoxylation is usually kinetic control, i.e., less substituted O Me O O α-position is functionalized preferentially. Pt anode H Me steel cathode + MeCO2H + Late-stage functionalization of polycyclyc lactams MeOH, KOH, c.c. 5 eq Aubé, Angew. Chem. 2015, 127, 10701. undivided cell CO2H 42% 15% O graphite anode O OMe graphite cathode Effects of anode material Bretle, J. Chem. Soc. Perkin Trans. I, 1973, 257. N N anode anode LiClO4, MeOH CO2Et c.c. undivided cell CO Et Pt cathode Pt 62% 2 COOH Graphite 5-9% 91% (dr = 4:1) Et N, EtOH, c.c. 4 4 4 3 10 other examples undivided cell RVC 45-53% Major products with graphite anode A mobile phone charger was used as a power supply. O O O O O O 4 4 4 4 Platinum anode gives radical intermediates, whereas graphite anode gives cationic intermediates. Yu Kawamata Electroorganic Chemistry: Choice of Electrodes Baran Group Meeting 12/10/2016 5 Oxidation of peptides Papadopoulos, Steckhan, Tetrahedron, 1991, 47, 563. Oxidation of electron-rich π-system Pt anode Me O Anode: Pt, graphite, RVC, BDD O Me O H Pt cathode Cathode: Pt, graphite N Ph N OMe Bz N NH H Et NCl, MeCN/MeOH Application to a natural product synthesis Moeller, J. Am. Chem. Soc. 2004, 126, 9106. O Me 4 Me CO Me c.c, undivided cell 2 84% (1.5:1 dr) TBSO Me RVC anode O anodically graphite cathode + generated Cl 2,6-lutidine, LiClO4 O Me Cl O O Me O TBSO TBSO O MeOH/CH2Cl2 O N N c.c. undivided cell Ph N OMe Ph N OMe H MeO H O Me O Me most electron-rich O O Unusual oxidation Shono, J. Am. Chem. Soc. 1990, 112 , 2369. TsOH O OH Pt anode HO Pt cathode OMe O halide additive O NHTs + OMe 88% (+)-Alliacol A N MeONa, MeOH C-H arylation Yoshida, Angew. Chem. Int. Ed. 2012, 51, 7259. c.c., undivided cell NHTs Ts 1 2 OMe carbon felt anode additive: KBr 30% 22% OMe Pt cathode naphthalene KI 82% - Bu4NB(C6F5)4 (1 eq.) radical cation Br OMe CH Cl , -90 ºC, Proposed mechanism: Br OMe 2 2 OMe c.c., divided cell OMe 6 eq. + R Br R X R R N NHTs NHTs 73% MeOH NTs Ts Br 9 other examples MeOH N-Br bond C-H amination Yoshida, J. Am. Chem. Soc. 2013, 135, 5000. homolysis 1 carbon felt anode R Hofmann-Loffler-Freytag Pt cathode N 2 NH2 pyridine N piperidine Ts Bu4NBF4, MeCN MeCN, 80 ºC c.c., divided cell 99% 12 other examples Yu Kawamata Electroorganic Chemistry: Choice of Electrodes Baran Group Meeting 12/10/2016 6 Organocatalyzed oxidation Boydston, J.
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