Electrochemical Hydroxylation of Arenes Catalyzed by a Keggin Polyoxometalate with a Cobalt(IV) Heteroatom

Article abstract of DOI:10.1002/anie.201801372

The sustainable, selective direct hydroxylation of arenes, such as benzene to phenol, is an important research challenge. An electrocatalytic transformation using formic acid to oxidize benzene and its halogenated derivatives to selectively yield aryl formates, which are easily hydrolyzed by water to yield the corresponding phenols, is presented. The formylation reaction occurs on a Pt anode in the presence of [Co^IIIW₁₂O₄₀]^5? as a catalyst and lithium formate as an electrolyte via formation of a formyloxyl radical as the reactive species, which was trapped by a BMPO spin trap and identified by EPR. Hydrogen was formed at the Pt cathode. The sum transformation is ArH+H₂O→ArOH+H₂. Non-optimized reaction conditions showed a Faradaic efficiency of 75 % and selective formation of the mono-oxidized product in a 35 % yield. Decomposition of formic acid into CO₂ and H₂ is a side-reaction.

Full text of DOI:10.1002/anie.201801372

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Accepted Article
Title: Electrochemical Hydroxylation of Arenes Catalyzed by a Keggin
Polyoxometalate with a Co(IV) Heteroatom
Authors: Ronny Neumann, Alexander M Khenkin, Miriam Somekh, and
Raanan Carmieli
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Angewandte Chemie International Edition
10.1002/anie.201801372
COMMUNICATION
Electrochemical Hydroxylation of Arenes Catalyzed by a Keggin
Polyoxometalate with a Co(IV) Heteroatom
[
a,‡]
[a,‡]
[b]
[a]
Alexander M. Khenkin,
Miriam Somekh,
Raanan Carmieli, and Ronny Neumann*
Abstract: The sustainable, selective direct hydroxylation of arenes,
such as benzene to phenol is an important research challenge. Mostly
thermochemical approaches have been studied, but these still have
various drawbacks. An electrocatalytic transformation using formic
acid to oxidize benzene and its halogenated derivatives to selectively
yield aryl formates that are easily hydrolyzed by water yielding the
corresponding phenols is presented. The formylation reaction occurs
heteroatom, could be considered a “soluble anode”, that is a one-
electron outer sphere oxidant. This anion was active only for the
oxidation at the benzylic position of reactive, i.e. alkylarenes with
electron donating groups, which have relatively low oxidation
[
12]
potentials, such as 4-methoxy toluene. The mechanism of this
reaction was suggested to include an initial reversible electron
transfer followed by a slow chemical step of nucleophilic capture
of the intermediate radical cation. Somewhat later on, the same
5
–
on a Pt anode in the presence of [Co(III)W12
O
40
]
as catalyst and Li
4
–
formate as electrolyte via formation of a formyloxyl radical as the
reactive species, which was trapped by a BMPO spin trap and
identified by EPR. Hydrogen was formed at the Pt cathode. The sum
group showed data that the analogous [Co(IV)W12
40
O ] appeared
to be electrochemically accessible, but carried out no further
[
13]
studies.
We have now found that the stronger oxidant,
4
–
transformation is ArH + H
conditions showed a Faradaic efficiency of 75 % and selective
formation of the mono-oxidized product in 35 yield.
Decomposition of formic acid to CO and H is the side-reaction. This
2
O ® ArOH + H
2
. Non-optimized reaction
[Co(IV)W12O40] , formed in acetonitrile, can oxidatively
dehydrogenate alkylarenes at the benzylic position presumably
a
%
through the formation of a radical cation and then a radical by a
5
–
2
2
proton coupled electron transfer (PCET), while [Co(III)W12
40
O ]
electrocatalytic reaction may lead to new processes for arene
hydroxylation.
is not an oxidant for these reactions. Benzene was not directly
4
–
oxidized by [Co(IV)W12O40] . However, in formic acid as
solvent/reagent with lithium formate as electrolyte, benzene and
its halogenated derivatives reacted by oxidation of an aromatic C-
H bond leading to the formation of aryl formates likely via
The sustainable oxidation of aromatic compounds such as the
hydroxylation of benzene and its derivatives remains a significant
formation of oxygen centered formyloxyl radicals, HC(O)O•. This
[
1]
research goal. Most sustainable strategies use hydrogen
peroxide directly or formed within a reaction mixture by various
4–
reaction is catalyzed by [Co(IV)W12O40] . Phenol derivatives are
then easily formed by hydrolysis. Thus, although formic acid is a
reagent in this electrocatalytic transformation it is recovered
through hydrolysis. In sum, the reaction is an indirect
[
2]
strategies, although the use of molecular oxygen only has also
[
3]
been reported. The reaction may occur by oxygen transfer via
[
4]
metal-oxo complexes, or via radicals, predominately hydroxy
radicals. Recently, there has been much renewed interest and a
flurry of activity in the long known area of electrocatalytic
hydroxylation of benzene with H
product, Scheme 1.
2
2
O to yield phenol with H as co-
[
5]
transformations, related to synthetic organic chemistry.
Specifically as related to this research a few recent reports have
appeared on anodic oxidations of alkylarenes and aliphatic
[Co(IV)W12O40]4–
H O
2
Ph-H +HCOOH
H + Ph-OOCH
Ph-OH + HCOOH
2
[
6]
[7]
Sum
[Co(IV)W12O40]4–
HCOOH
hydrocarbons, and coupling of arenes. In the context of the
research herein, there is also a report on the gas phase V
PhH + H O
PhOH + H
2
2
2
O
5
anodic oxidation of benzene where hydroxy radicals were formed
from water vapor, however benzene and water are immiscible,
Scheme 1. Pathway for the electrochemical hydroxylation of benzene to phenol.
[
8]
very much complicating the reaction.
Cathodic aerobic
[
9]
oxidations have also been reported. Aerobic hydroxylation
through formation of a benzene radical cation has also been
The cyclic voltammetry measurement of a 1 mM solution of
[
10]
[11]
K
5
Co(III)W12
M LiClO , showed two quasi-reversible redox couples at 0.75 and
.65 V that are assigned as Co(III)/Co(II) and Co(IV)/Co(III)
transitions, respectively, Figure 1. Electrolysis of such a yellow
solution (1 mM K Co(III)W12 40 and 0.1 M LiClO ) in acetonitrile
O40 in acetonitrile, solubilized in the presence of 0.1
considered, as has use of N
more than 30 years ago Eberson reported that a polyoxometalate
2
O as a “green” oxidant. Already
4
5
–
1
anion, [Co(III)W12
O40] , Figure 1, which has a Keggin structure
with coordinatively and sterically inaccessible Co(III)
a
5
O
4
at 1.8 V yielded after one equivalent of electrons a greyish
solution. The UV-vis spectrum of this solution showed a maximum
[
a]
Dr. Alexander M. Khenkin, Ms. Miraim Somekh and Professor
Ronny Neumann
Department of Organic Chemistry
-1
-1
at 575 nm, e = 182 M cm , Figure S1. Iodometric titration of the
grey compound showed that two electrons were needed to reduce
6
– [14]
Weizmann Institute of Science, Rehovot, Israel 76100
E-mail: Ronny.Neumann@weizmann.ac.il
Dr. Raanan Carmieli
Department of Chemical Research Support
Weizmann Institute of Science, Rehovot, Israel 76100
These authors contributed equally to the research.
this solution to the known emerald green [Co(II)W12
reasonably we can assign the grey compound as being
Co(IV)W12 This compound is not stable at room
temperature and in the absence of a substrate decomposed to the
O40] . Thus,
[
b]
‡]
K
4
40
O .
5
–
[
initial yellow [Co(III)W12
the other hand, a 1 mM solution of [Co(IV)W12
was active for the fast (within a minute) oxidative dehydrogenation
O
40
]
anion; t1/2 = 832 s (Figure S2). On
4
–
40
O ] in acetonitrile
Supporting information for this article is given via a link at the end of
the document.
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Angewandte Chemie International Edition
10.1002/anie.201801372
COMMUNICATION
of 1-phenylcyclohexene, 1,2,3,4-tetrahydronaphthalene and 4-
phenyl-ethylbenzene to biphenyl, naphthalene and 4-phenyl
styrene, respectively, with an average yield relative to
Benzene did not react to yield any products using acetic acid
or acetonitrile as solvent under such electrocatalytic conditions.
This can be attributed to a lack of a feasible pathway for low
temperature oxidation directly from a phenyl radical cation, even
4
–
[
Co(IV)W12
O
40
]
of 10%.
[
10a]
if formed as previously noted.
After surveying by cyclic
voltammetry, the oxidation of benzene in formic acid/lithium
formate, Figure S4, an electrolysis reaction in formic acid as
solvent and lithium formate as electrolyte, showed the rather
efficient and novel oxidation of benzene and its halogenated
derivatives to yield phenyl formate and halophenyl formates. The
aryl formates can be easily hydrolyzed by acid catalysis to yield
the corresponding phenols, Table 1.
[
a]
Table 1. Electrochemical Oxidation of Benzene and Halogenated Benzenes.
[
b]
Substrate
PhH
Product (R=OCH)
PhOR
FE
58
0
Yield, mol%
14
0
[
c]
PhH
PhH
PhH
PhH
PhH
PhH
PhF
PhOR
[
d]
Figure 1. Figure 1. CV scan of 1 mM K
5
Co(III)W12
O
40 (0.1 M LiClO
4
, 100 mV/sec,
PhOR
46
26
75
7
11
9
acetonitrile). Insert – ball and stick model of the anion (Co-blue; O-red; W-black).
[
[
e]
PhOR
f]
PhOR
35
7
Furthermore, cyclic voltammetry scans under the conditions
described in Figure 1 showed catalysis with an approximately 10-
fold increase in current in the presence of substrates such as
ethylbenzene, Figure S3. In Scheme 2 one can see the results for
controlled potential electrocatalytic reactions of two alkylated
arenes in acetonitrile and acetic acid. Depending on the
conditions, dehydrogenation, oxidation and acetoxylation in the
presence of acetic acid was observed. In acetonitrile for ethyl and
i-propyl substituents dehydrogenation was the major pathway and
in acetic acid nucleophilic substitution predominated. For methyl
substituents, there was selective formation of aldehyde, whereas
as in acetic acid acetoxylation was predominant. As proposed in
the past for the oxidation of 4-methoxytoluene with
[
[
e,f]
g]
PhOR
PhOR
2
17
10
13
25
8
F-PhOR (o:m:p – 17:6:77)
Cl-PhOR (o:m:p – 28:4:68)
Br-PhOR (o:m:p – 25:5:70)
I-PhOR (o:m:p – 30:4:66)
40
78
95
16
25
PhCl
PhBr
PhI
1
1
,2-Cl
,2-Cl
2
2
Ph
Ph
3,4-Cl
3,4-Cl
2
2
PhOR (75) 2,3-Cl
PhOR (75) 2,3-Cl
2
2
PhOR (25)
PhOR (25)
28
d]
4
8
5
–
[
Co(III)W12O40] , the intermediacy of a radical intermediate and a
[
e]
PCET mechanism is supported a kinetic isotope effect (KIE) in the
1,2-Cl Ph
3,4-Cl PhOR (72) 2,3-Cl PhOR (28)
76
28
37
38
2
2
2
competitive oxidation of 1:1 ethylbenzene:ethylbenzene-d10
[
15]
1
,3-Cl
2
Ph
2,4-Cl
2
PhOR (88) 2,6-Cl
2
PhOR (12)
where k
H
/k
D
= 1.71
[
a] Reaction conditions: 10 µmol K
5
Co(III)W12O40, 1 mmol substrate, 0.5 mmol
O
+
OAc
AcOH
MeCN
FE-21%
Conv-20% 67%
O
LiOOCH, in 3 mL HCOOH. Potential 1.8 V versus Pt. Anode – Pt gauze,
Cathode - Pt wire in a single cell configuration; t – 3 h, 25 °C. [b] Typically the
formate ester was the only product, the exception being the reaction of PhBr
and PhI where ~40% of the ester was hydrolyzed to the phenol derivative. It is
possible that a small amount of HX is formed during the reaction leading to
catalysis of the hydrolysis reaction. [c] no LiOOCH [d] Reaction using 10 µmol
+
+
FE-15%
Conv-26%
2
4%
4%
72%
69%
33%
CHO
+
CH2OAc
CHO
AcOH
MeCN
FE-19%
FE-15%
3
1%
Conv-31%
Conv-18%
100%
5 5
H Co(III)W12O40, [e] No K Co(III)W12O40 [f] 1.2 mmol LiOOCH. [g] using a glassy
Scheme 2. Electrochemical Oxidation of Alkylarenes Catalyzed by
40
Co(IV)W12O ] in Acetonitrile and Acetic Acid Showing Reaction Selectivity.
carbon anode. FE- Faradaic efficiency for formation of ArOOCH. ArOOCH were
easily hydrolyzed to ArOH by addition of small amounts of acid see SI.
4
–
[
Reaction Conditions: 10 µmol polyoxometalate, 40 µmol substrate, 0.5 mmol
LiClO , 3 mL solvent in air. K Co(III)W12 40 in MeCN and H Co(III)W12
AcOH. Potential 1.8 V versus Ag/AgNO . Anode – Pt gauze, Cathode - Pt wire;
4
5
O
5
O40 in
Various points should be emphasized (a) the reactions were
selective to the formation of monoxidation products. (b) The ratio
of ortho:meta:para isomers formed in the reactions of
halobenzenes are indicative of a radical reaction, which was (c)
supported by a KIE in the competitive oxidation of 1:1 benzene:
3
undivided cell configuration; t – 3 h; 25 °C. FE- Faradaic efficiency for oxidation
of the organic substrate. Conversion and selectivity was determined by GC-FID.
The source of oxygen atom in the aldehyde/ketone products is O
2
. This was
demonstrated by carrying out the oxidation of ethylbenzene in acetonitrile in the
1
8
presence of 96.2 %
8-O. It should be noted that H
by separately reacting 16-O acetophenone with H
O
2
instead of air. Acetophenone was obtained with 73.2%
[
16]
6 H D
benzene-d where KIE; k /k = 1.07 was measured and (d) a
1
2
O exchanges slowly with the ketone as shown
1
8
2
O.
product ratio of 1.1:1 ClPhOOCH:PhOOCH in a competition
experiment using equimolar concentrations of benzene and
chlorobenzene. (e) The presence of formate is required. (f) There
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Angewandte Chemie International Edition
10.1002/anie.201801372
COMMUNICATION
is some reaction in the absence of K
5
Co(III)W12
O
40, but its
step of formate adsorption, a one-electron oxidation would yield
an oxygen centered formyloxyl radical, HC(O)O•, which we have
identified by EPR using BMPO (5-tert-butoxycarbonyl-5-methyl-1-
pyrroline-N-oxide) as a spin trap, Figure 2. From the simulation,
hyperfine splitting constants of A = 15.5 G and A = 22 G
presence significantly increases the efficiency both in terms of
yield and Faradaic efficiency for the formation of aryl formates. (g)
Further significant improvement in the reaction efficiency was
observed by the addition of additional amounts of the lithium
formate electrolyte, leading to Faradaic efficiencies of up to 75%
and yields of aryl formates of 35 mol% at ~100 % selectivity. (h)
No isotope effect was observed using DCOOD as solvent. (i) The
1
4
1
associated with N and H atoms were obtained. One observes
two magnetically equivalent hydrogen atoms due to the presence
of two diastereomers in the spin adduct product. It should
additionally be noted that the presence of formate anion is indeed
needed for the formation of the formyloxy radical and that in
reaction in the presence of air or under N
2
yielded the same result
within experimental error. Thus, O
reaction.
2
does not participate in the
3 3
CH COOH/CH COOLi no similar acetyloxy radical is formed. See
In addition, as shown in Table 2, a complete analysis of the
reaction products including the formation of CO and H shows (j)
the supporting information including cyclic voltammetry and
additional EPR measurements for more details on this topic.
2
2
that the reaction of benzene for 45 min showed a Faradaic
efficiency for all products of >97% and yielded 39 µmol PhOOCH,
4
7 µmol CO
oxidation of arenes is the decomposition of formic acid to CO
. (k) In fact, under the same reaction conditions in the absence
of benzene, equimolar amounts of H and CO (79 ± 5 µmol) were
formed. (l) K Co(III)W12 40 catalyzes the oxidation of formic acid
since in its absence only 18 ± 2 µmol H and CO each were found.
m) The use of lithium acetate as electrolyte in the presence of
2
and 88 µmol H
2
. Thus, the additional reaction in this
2
and
H
2
2
2
5
O
2
2
(
formic acid as solvent yielded significantly less products. (n)
Finally, the stability of the catalyst was assessed. The IR
spectrum after the reaction after removal of all volatiles showed a
5 40
spectrum that was the sum of the IR spectra of K Co(III)W12O
and LiOOCH, the nonvolatile components of the reaction mixture,
Figure S7. The UV-vis spectrum of the green reduced
polyoxometalate,
6
K Co(II)W12O40 shows no change in the
spectrum before and after the reaction. Both these measurements
indicate good catalyst stability with no formation of additional
species, Figure S8. However, there is a slow decrease in current
over time, Figure S5, that may be associated with slow catalyst
decomposition.
[
a]
Figure 2. EPR Spectrum of the Spin Adduct of BMPO and the Formyloxy
Radical. Experimental spectrum (black) and simulated spectrum (red). The
hyperfine splitting is due to the N and H atoms shown in red.
2 2
Table 2. Formation of H and CO during Electrolysis of Formic Acid.
Conditions
All components
No PhH
H
2
, µmol
CO
2
, µmol
PhOOCH, µmol
88±3
82±3
18±2
34±2
47±3
76±3
18±2
20±2
39±3
Therefore, in the presence of an arene substrate and preferably
5–
4–
in the presence of [Co(III)W12
O
40] /[Co(IV)W12O40] , one can
suggest the following reaction pathway, Scheme 3.
No Co(III)W12
O40 /No PhH
LiOAc electrolyte
11±2
Anode:
Pt + HCOOH
Pt- -HCOO–
Pt- -HCOO–
+ H+
ads
[
5
a] Reaction conditions: 10 µmol K Co(III)W12O40, 1 mmol substrate, 0.5 mmol
Pt + e– + HCOO
LiOOCH, in 3 mL HCOOH. Potential 1.8 V versus Pt. Anode – Pt gauze,
Cathode - Pt wire in an undivided cell configuration; t – 45 min, 25 °C.
ads
H+ + M [Co(IV)W12O40] + e– (M = H, Li, K)
HM [Co(III)W12O40
]
4
4
Catalysis:
Based on the experiments described above, it is worthwhile to
propose a reaction mechanism for the oxidation of benzene (PhH)
to phenyl formate (PhOOCH). It should be recognized that this is
Pt- -HCOO– + M [Co(IV)W12O ] + H+
ads
4
40
HCOO + HM [Co(III)W12O40]
4
H
OOCH
+ H+ + e–
OOCH
an anaerobic process since the results were identical under N
2
+ HCOO
and in the presence of air. Furthermore, the electrochemical
decomposition of formic acid has been rather intensively studied
Cathode:
H+ + 2 e–
2
H2
[
17]
in the context of the possibility of using formic acid in fuel cells.
Although in the past there was uncertainty concerning the reaction
Scheme 3. Suggested Reaction Pathway for Benzene Oxidation
[
18]
mechanism at the anode, recent research suggested the initial
adsorption of a formate anion on the platinum electrode that is
After the first step of formate adsorption preferably on Pt but to
some degree on glassy carbon, a one-electron oxidation will yield
[
19]
maximized by addition of formate to formic acid. After the first
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Angewandte Chemie International Edition
10.1002/anie.201801372
COMMUNICATION
an oxygen centered formyloxyl radical, HC(O)O•. From the results Acknowledgements
4
–
40
in Tables 1 and 2, this reaction is catalyzed by [Co(IV)W12O ]
but also occurs to some degree in its absence. By calculations it
has been shown that the formyloxyl radical is decarboxylated
This research was supported by the Israel Science Foundation
grant # 763/14 and the Israel Ministry of Science and Technology.
R.N. is the Rebecca and Israel Sieff Professor of Organic
Chemistry.
[
20]
more slowly than the analogous acetyloxyl radical. Thus, the
formyloxyl radical has a sufficient lifetime in the presence of
arenes to react to form a cyclohexadienyl formate radical
intermediate species followed by formation of aryl formates. The
radical nature of this reaction is supported by trapping of the
formyloxy radical and is also supported by the KIE observed for
the oxidation of benzene, the product ratio formed in the
competitive oxidation of PhCl:PhH, and the product distribution
observed in the oxidation of halo-benzenes.
Keywords: Polyoxometalate • Electrocatalysis • Anodic
oxidation • C-H bond activation • Formic acid
[
[
1]
2]
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An additional advantage of this reaction system is that aryl
[
21]
formates are very sensitive to hydrolysis. Thus, after extraction
of the aryl formates from the reaction solution they were treated
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with 0.5 mol % 60 HClO ; the corresponding phenol derivatives
were formed within 10 min at room temperature at quantitative
conversions. Therefore, it is easy to form phenol and its
derivatives by mild acid catalysis from the initially formed esters
and in essence to recover the formic acid that was reacted leading
in sum to the following electrochemical transformation where
formic acid is a sacrificial but recoverable reagent:
7
1
7
782; b) K. Hirose, K. Ohkubo, S. Fukuzumi, Chem. – Eur. J. 2016, 22,
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ArH + H
2 2
O ® ArOH + H
[
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Although the electronic structure of the putative formyloxyl radical
[
22]
has been investigated computationally, and its formation from
[
23]
formic acid on Pt(111) has also been studied computationally,
as far we could determine this is the first example that explicitly
indicates the in situ formation of a formyloxy radical from formic
acid in an oxidative electrochemical transformation and further its
reactivity - in this case the C-H bond activation of arenes to yield
[5]
a
formate ester. The new electrochemical transformation
described herein may provide new opportunities for the
sustainable preparation of phenol and some of its derivatives in
order to move away from the complicated cumene hydroperoxide
process that has a low per pass conversion of cumene and
2
299; J. B. Sperry, D. L. Wright, Chem. Soc. Rev. 2006, 35, 605–621.
[
[
6]
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An electrocatalytic transformation
uses formic acid as reactant for the
formation of phenyl formate from
benzene. The anodic reaction is
Alexander M. Khenkin, MiriamSomekh,
Raanan Carmieli, and Ronny Neumann*
Page No. – Page No.
5
–
catalyzed by [Co(III)W12
O
40
]
as pre-
Electrochemical Hydroxylation of
Arenes Catalyzed by a Keggin
Polyoxometalate with a Co(IV)
Heteroatom
catalyst with formation of the
formyloxyl radical as the reactive
intermediate. The facile hydrolysis of
phenyl formate yields phenol. In sum,
the transformation is ArH + H
ArOH + H
2
O ®
2
.
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