Reaction of Cobaloxime
with Hydrogen Evolution
Literature Seminar
B4 Yuki Nishioka
2019/01/31(Thu)1

Table of Contents• Introduction• Electrochemical System• Thermodynamic Analysis• Synthesis using cobaloxime• Summary2

Table of Contents• Introduction• Electrochemical System• Thermodynamic Analysis• Synthesis using cobaloxime• Summary3Introduction

Aiwen Lei et al. J. Am. Chem. Soc. 2016, 138, 12037

・Anti-Markovnikov regioselectivity oxygenation of alkenes
・Reaction without hydrogen acceptor
4Introduction

Zhiping Li and Chao-Jun Li, J. Am. Chem. Soc. 2004, 126, 11810

Chao-Jun Li et al. Proc. Natl. Acad. Sci. USA, 2006, 103, 8928.

In the conventional dehydrogenative cross coupling reaction,hydrogen acceptor is required.
→Not clean reaction
5

Introduction

G.N. Schrauzer, K. Kohle, Chem. Ber., 1964, 97, 3056

Cyanocobalamin,
https://ja.wikipedia.org/wiki/シアノコバラミン

6Table of Contents• Introduction• Electrochemical System• Thermodynamic Analysis• Synthesis using cobaloxime• Summary7Cyclic Voltammetry

https://ja.wikipedia.org/wiki/サイクリックボルタンメトリー

E₀ = (Epc + Epa)/2◀酸化還元電位8Hydrogen Evolution by Cobaloxime9Cyclic Voltammogram of 1・-0.55 V vs SCE in the absence ofacid was assigned to the CoII/I .
CV of 1(0.3 mM) in CH₃CN containing 0.1 M
II/I[nBu₄N][ClO₄] in the presence of tosic acid: ・ The potential (Co ) did notBlack; no acidchange as the acid concentration
Red; 1.5 mM TsOH・H₂O
was varied.
Blue; 4.5 mM TsOH・H₂O
Green; 9 mM TsOH・H₂O

Jonas C. Peters et al. J. Am. Chem. Soc. 2007, 129, 8988

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Cyclic Voltammogram of 2・CV of 2 was similar to that of 1.
・The current at -0.6 V vs SCE isdue to the existence of Co(III)-H
CV of 2(0.6 mM) in CH₃CN containing 0.1 M
intermediate.
[nBu₄N][ClO₄] in the presence of tosic acid:・The direct reduction of acid
Black; no acid
Red; 9 mM TsOH・H₂O
containing tosic acid took place at-1.0 V vs SCE.
Jonas C. Peters et al. J. Am. Chem. Soc. 2007, 129, 8988

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Cyclic Voltammogram of 4a (TsOH)

・Reversible reductions wereobserved for the CoIII/II at -0.06 Vvs SCE and for the CoII/I at -0.38 V
CV of 4a(0.15 mM) in CH₃CN containing 0.1 M
vs SCE.
[nBu₄N][ClO₄] in the presence of tosic acid:
Black; no acid・ No catalytic waves wereRed; 1.5 mM TsOH・H₂O
observed at potentials near theBlue; 9 mM TsOH・H₂O
E(CoIII/II).
Jonas C. Peters et al. J. Am. Chem. Soc. 2007, 129, 8988

12

Cyclic Voltammogram of 4a (HBF₄)

CV of 4a(0.5 mM) in CH₃CN containing 0.1 M[nBu₄N][ClO₄] in the presence of HBF₄・Et₂O:・CV of 4a in the presence ofHBF₄・Et₂O was similar to that of4a in the presence of tosic asid.

Jonas C. Peters et al. J. Am. Chem. Soc. 2007, 129, 8988

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Cyclic Voltammogram of 4b・Reversible reductions wereobserved for the CoII/I at -0.35 Vvs SCE.
Jonas C. Peters et al. J. Am. Chem. Soc. 2007, 129, 8988

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Quasi-reversible Redox Wave for 1

CV of 1(0.25 mM) in CH₃CN in thepresence of tosic acid:
Black; 1 mM TsOH
Red; 1.6 mM TsOH
Blue; 2.6 mM TsOH

・In the absence of acid, no thiswave was observed.
・Authors tentatively assign thiswave to the “Co(III)-H”/“Co(II)-H”
redox couple.

Jonas C. Peters et al. J. Am. Chem. Soc. 2007, 129, 8988

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Reaction Mechanism of H₂ Evolution

Jonas C. Peters et al. J. Am. Chem. Soc. 2007, 129, 8988

Is hydrogen evolution due to monometallic
pathway or bimetallic pathway?
16Equilibrium Constant of Monometallic Pathway(ΔG = −RTlnK∆G = −nFE)

Jonas C. Peters et al. J. Am. Chem. Soc. 2007, 129, 8988

17Equilibrium Constant of Bimetallic Pathway

Jonas C. Peters et al. J. Am. Chem. Soc. 2007, 129, 8988

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Calculation Result of pKmono and pKbiJonas C. Peters et al. J. Am. Chem. Soc. 2007, 129, 8988

For complex 1 and 2 the monometallic reduction of acid,
Kmono is very unfavorable, except in the presence of thestrongest acid HBF₄・Et₂O.
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Short Summary
・By using cobaloxime, hydrogen can be evoluatedwith energy saving.
・Reagents with high reducing power become
unnecessary.
・For complex 1 and 2 the monometallic reduction ofacid, Kmono is very unfavorable, except in the presenceof the strongest acid HBF₄・Et₂O.
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Table of Contents• Introduction• Electrochemical System• Thermodynamic Analysis• Synthesis using cobaloxime• Summary21Thermodynamic Analysis of Reaction Mechanism

Harry B. Gray et al. J. Am. Chem. Soc. 2010, 132, 1060

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Thermodynamic Analysis of Reaction Mechanism

Harry B. Gray et al. J. Am. Chem. Soc. 2010, 132, 1060
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Thermodynamic Analysis of Reaction Mechanism

Harry B. Gray et al. J. Am. Chem. Soc. 2010, 132, 1060

ΔG = −RTlnK∆G = −2.302RTlogK∆G = 2.302RTpKa

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Table of Contents• Introduction• Electrochemical System• Thermodynamic Analysis• Synthesis using cobaloxime• Summary25

Reaction Accompanied by Hydrogen Evolution

Aiwen Lei et al. J. Am. Chem. Soc. 2016, 138, 12037

・Anti-Markovnikov regioselectivity oxygenation of alkenes
・Reaction without hydrogen acceptor
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Conventional Wacker Oxidation

・It can only oxidize terminal alkene.
・Anti-Markovnikov regioselectivity
oxygenation of alkenes

27

Reaction Mechanism

Aiwen Lei et al. J. Am. Chem. Soc. 2016, 138, 12037

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Isotope Labelling Experiments

・The epoxidation−isomerization
pathway can be ruled out.
・O atom of carbonyl compounds
originates from water
Aiwen Lei et al. J. Am. Chem. Soc. 2016, 138, 12037

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Kinetic Isotope Effect Experiments

・C−H bond breaking of the olefin is
not involved in the rate-determining
step of the reaction.
・The rate-determining step of thetransformation might involve theO−H bond cleavage of water.
Aiwen Lei et al. J. Am. Chem. Soc. 2016, 138, 12037
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Alternative Reaction Mechanism

Aiwen Lei et al. J. Am. Chem. Soc. 2016, 138, 12037
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Proposal of Reaction Mechanism

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Table of Contents• Introduction• Electrochemical System• Thermodynamic Analysis• Synthesis using cobaloxime• Summary33

Summary

・Cobaloxime can achieve reaction
without hydrogen acceptor
・The reaction catalyzed bycobaloxime proceed with bimetallic
pathway34