Electron Transfer Oxidation of Benzene and Aerobic Oxidation to Phenol

Article abstract of DOI:10.1021/acscatal.6b02083

The activation of very strong C-H bonds, such as those found in benzene, is important also in the quest for new routes for its functionalization. Using the H₅PV₂Mo₁₀O₄₀ polyoxometalate as an electron transfer oxidant in >50% aqueous H₂SO₄ as solvent, the formation of a benzene radicaloid species at RT as probed by visible spectroscopy and by EPR spectroscopy recorded at X-band and W-band, including ELDOR-detected NMR, was verified. The viability of the ET oxidation of benzene is supported by DFT calculations, showing the reaction to be exergonic under these conditions. Furthermore, we show that in the presence of O₂, very selective hydroxylation to phenol took place.

Full text of DOI:10.1021/acscatal.6b02083

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Letter
Electron Transfer Oxidation of Benzene and Aerobic Oxidation to Phenol
Bidyut Bikash Sarma, Raanan Carmieli, Alberto Collauto,
Irena Efremenko, Jan M.L. Martin, and Ronny Neumann
ACS Catal., Just Accepted Manuscript • Publication Date (Web): 22 Aug 2016
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ACS Catalysis
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Electron Transfer Oxidation of Benzene and Aerobic Oxidation to
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‡
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Bidyut Bikash Sarma, Raanan Carmieli, Alberto Collauto , Irena Efremenko, Jan M. L. Martin,
†
and Ronny Neumann* .
†
‡
§
Department of Organic Chemistry, Weizmann Institute of Science, Rehovot, Israel 76100
Department for Chemical Research Support, Weizmann Institute of Science, Rehovot, Israel 76100
Department of Chemical Physics, Weizmann Institute of Science, Rehovot, Israel 76100
ABSTRACT: The activation of very strong C-H bonds such as those found in benzene is important also in the quest for
new routes for its functionalization. Using the H PV Mo O polyoxometalate as an electron transfer oxidant in > 50%
5
2
10 40
aqueous H SO as solvent the formation of a benzene radicaloid species at RT as probed by visible spectroscopy, and EPR
2
4
spectroscopy recorded at X-band and W-band including ELDOR detected NMR was verified. The viability of the ET oxi-
dation of benzene is supported by DFT calculations showing the reaction to be exergonic under these conditions. Further
we show that in the presence of O very selective hydroxylation to phenol took place.
2
KEYWORDS – Aerobic Oxidation, Benzene, Polyoxo-
metalate, EPR, Electron Transfer
oxidation of benzene with H
2
O
2
or with O
recent report, where the
H PV Mo O polyoxometalate is used together with gra-
2
and a reducing
1
1
agent. There is also
a
5
2
10 40
Introduction
phitic carbon-nitride (C N ) to oxidize benzene to phenol
3 4
1
2
with O only. The authors propose here also that the
2
The direct aerobic oxidation of benzene to phenol is a
challenging topic of considerable current interest against
the backdrop of an industrial process that has a low atom
reaction proceeds via electron transfer from C N to the
3
4
substrate. The combined, C N and H PV Mo O , cata-
3
4
5
2
10 40
1
2
1
lyst deactivates quickly apparently due to tar formation.
Over the years we have promoted the use of the
PV 40 polyoxometalate as an electron transfer
economy. The literature is abound with reports on the
use of H O as oxidant where in the majority of the cases
2
2
2
the reactive oxidizing species is the hydroxide radical.
H
5
Mo10O
2
There is also a large body of research where it is likely
that H O is formed in situ under aerobic conditions, e.g.
(ET) and electron transfer-oxygen transfer (ET-OT) cata-
lyst for the oxidation of hydrocarbons and related sub-
2
2
3
,4
using H or CO, respectively as reductants. Successful
strates, where the substrate scope was limited by the oxi-
2
5
13
hydroxylation of benzene with N O, has led to numerous
dation potential of H
5
PV
Mo10
2
O
40, which is around 0.4-
2
reports on the use of NH and O with the implication the
0.45 V versus SCE in many organic solvents. Recently, we
have reported that in >50% aqueous sulfuric acid superior
rates and increased substrate scope were found as a result
3
2
6
N O is formed as an intermediate. Despite the advances
2
in using H O , and N O as oxidants and the mechanisti-
2
2
2
cally similar H /O CO/O2 and NH /O combinations,
there is an obvious interest in the oxidation of benzene to
of the higher oxidation potential of H PV Mo10O40 in
5 2
2
2
3
2
1
4
>50% aqueous sulfuric acid. For example, toluene can be
oxidized to benzaldehyde through an ET-OT reaction
phenol with O only. One development is the use of light
2
7
in aerobic reactions. However, under thermal reaction
mechanism in aqueous H
acetic acid or other organic polar solvents.
Scheme 1. ET oxidation of benzene by H PV Mo O
10 40
2 4
SO but there is no reaction in
1
4b
conditions the most studied system in this context is the
heterogeneous oxidation over a Cu/ZSM-5 zeolite and
5
2
8
more recently other Cu catalysts. The mechanistic in-
and subsequent pathways to phenol.
formation for this reaction is scant but in one case there is
some evidence for the formation of a benzene radical cat-
ion and then the hypothesis of the formation of a perox-
9
ide species involved in the actual hydroxylation reaction.
In a different approach Pd(II) in acetic acid has been used
for activation of the C-H bond of benzene, formation of
the acetate and/or phenol, where the co-formed Pd(0)
species are reoxidized by O in the presence of a co-
2
1
0
catalyst. Vanadium catalysts have also been used for the
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This previous research has led to the presently ad-
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dressed research questions. (1) Can benzene be oxidized
to a radical cation by H PV Mo O in H SO through an
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2
10 40
2
4
ET step? and (2) can the resulting benzene radical cation
be oxidized anaerobically via oxygen transfer from the
polyoxometalate or aerobically oxygenated to phenol,
potentially setting the stage for a novel benzene to phenol
reaction pathway? These questions related to the research
theme are summarized in Scheme 1.
Figure 2. CW X-band EPR spectra of 1 µL C H /0.1 mM
6
6
1
2
H PV Mo O in 80% H SO ; 50 µL HFIP. Red- C H ; Black-
5
2
10 40
2
4
6
6
1
3
C H .
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Results and Discussion
The X-band CW spectrum, at RT gave a narrow EPR
signal at g = 2.0057 typical for an organic radical, Figure 2
Formation of a benzene radical cation. An initial indica-
tion for an electron transfer interaction between benzene
and H PV Mo O was manifested in the visible spectrum,
13
(
red plot). A second sample of C H /H PV Mo O in
6 6 5 2 10 40
5
2
10 40
8
0% H SO benzene, Figure 2 (black plot) was measured
2 4
Figure 1. In 80% aqueous H SO where the oxidation po-
2
4
as well in order to better identify the radical cation by the
expected hyperfine splitting from the interaction of the
free electron with the 13-C nuclei. Instead, however, the
broad featureless spectrum was obtained probably due to
slow tumbling rate of the benzene molecule, which is
likely ion-paired or bound to H PV Mo O . For better
tential of H PV Mo O
10 40
is very significantly in-
5
2
creased,~1.17 V versus SCE, Figure S1, two peaks emerge in
the visible spectrum that can be assigned to a benzene
related charge transfer peak at 520 nm and a peak at 680
1
5
nm associated to the reduced polyoxometalate.
5
2
10 40
resolution and confirmation, W-band (95 GHz) EPR
measurements were carried out at 20 K. While
H PV Mo O reduced by H exhibited only the expected
5
2
10 40
2
12
V(IV) EPR spectrum the and
C H /H PV Mo O gave two nicely resolved signals, one
C H /H PV Mo O
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5
2
10 40
13
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5
2
10 40
from V(IV) and the other, which was much weaker, from
an organic radicaloid species situated at g = 2.0057 as also
observed in the X-band spectrum, Figure 3.
Figure 1. Visible spectrum of a mixture of 1 µL benzene with
.1 mM H PV Mo O in 80% H SO .
0
5
2
10 40
2
4
Buoyed by this observation, we sought for direct evi-
dence for the presence of a benzene radical cation by EPR
spectroscopy. Based on a previous study where it was ob-
served that the radical cation of anthracene could be sta-
1
6
bilized by the presence of hexafluoro-i-propanol (HFIP),
HFIP was also added to the C H /H PV Mo O in 80%
6
6
5
2
10 40
H SO4 mixtures. Three samples were prepared: (i)
2
H PV Mo O which was reduced by H , (ii) natural iso-
5
2
10 40
2
1
2
tope abundance benzene, nominally
H PV Mo O and (iii) C H with H PV Mo O .
5
C H with
6 6
1
3
2
10 40
6
6
5
2
10 40
Figure 3. W-band EPR spectra. Blue H PV Mo O /H , red-
5
2
10 40
2
1
2
13
C H /H PV Mo O , black- C H /H PV Mo O .
6
6
5
2
10 40
6
6
5
2
10 40
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One can conclude, therefore, that there appears to be no
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kinetically feasible anaerobic ET-OT pathway from ben-
zene to phenol, see Scheme 1 (middle), despite the fact
that such a reaction is clearly exergonic, see below.
Table 1. Aerobic Oxidation of Phenol and Derivatives
Sub-
strate
Conver-
sion,
ArOH,
mol%
2-/4-
ratio
Others, mol%
mol%
a
PhH
65±5
95
97
80
70
60
---
PhOPh, 5
PhOPh, 3
10
11
12
13
14
15
16
17
18
19
20
21
22
23
24
25
26
27
28
29
30
31
32
33
34
35
36
37
38
39
40
41
42
43
44
45
46
47
48
49
50
51
52
53
54
55
56
57
58
59
60
b
PhH
10
---
c
PhF
78
62
68
55/45
C H FO , 20
6
3
2
PhCl
PhNO2
60/40 C H Cl , 30
6 4 2
13/87
C H (NO ) , 40
6 4 2 2
1
3
Figure 4. ELDOR detected NMR of C H /H PV Mo O .
6
6
5
2
10 40
Reaction conditions: 0.5 mmol ArH, 0.46 mmol
H PV Mo O •32H O, 5 mL 50% H SO , 10 bar O , 170 °C, 6
The color of the spectrum corresponds to the field positions
according to the arrows.
5
2
10 40
2
2
4
2
h. Analyses were done by GC-MS, GC-FID, using p-xylene as
external standard. (a) average of 5 runs; (b) 5 bar O . (c) 2-
In order to conclusively verify that the peak at g =
.0057 is indeed from benzene and not some impurity or
2
fluoro-1,4-benzoquinone.
2
other artifact, further W-band ELDOR detected NMR
spectra were recorded at different magnetic field posi-
tions within the EPR spectrum, Figure 4. The ELDOR de-
tected NMR spectra recorded on the first (red) and the
eighth (blue) line of the V(IV) species shows signals at the
Larmor frequency of hydrogen atoms from the solution
situated at 144.1 MHz and 147.3 MHz respectively. These
signals are assigned to the protons from the polyoxomet-
alate and solvent. The ELDOR detected NMR spectrum
recorded at the g = 2 peak shows a signal at 36 MHz asso-
ciable to the Larmor frequency of 13-C atoms. The peak at
In order to further probe phenol formation from ben-
zene with H PV Mo O in 50 % aqueous H SO , we
5
2
10 40
2
4
turned to aerobic reactions, Table 1. For benzene, the re-
action proceeded sluggishly at 10 bar O with a good se-
2
lectivity to phenol with some formation of diphenylether.
No sulfate esters were observed presumably because there
was sufficient water present to shift the equilibrium to
phenol. At lower O pressure the reaction was much less
2
efficient; there was no oxygenation with 10 bar air. Some
substituted substrates, PhX, where X = F, Cl, and NO2
were also reacted. The phenols were formed as an isomer-
ic mixture. Interestingly for benzene as substrate no 1,4-
benzoquinone was observed, but for fluorobenzene signif-
icant amounts of 2-fluoro-1,4-benzoquinone were found.
Chlorobenzene and nitrobenzene also reacted to yield the
corresponding phenols, but acid catalyzed disproportion-
7
2 MHz is assigned to the sum frequencies, and the signal
at 144.1 MHz is associable to hydrogen atoms. All these
easily detected strong signals are consistent with the
presence of a benzene radicaloid species. In addition, on
the blue trace there are six additional peaks ranging from
19
7
5 MHz to 180 MHz. These peaks are assigned to the 51-V
ation reactions became significant. For PhBr and PhI
1
7
transitions as reported previously. A 13-C Mims ENDOR
spectrum was also measured, Figure S2, at the field posi-
tion of the benzene species further verifying that the 13-C
signal could only originate from isotopically labeled ben-
zene and not from any other source.
(
not shown) this disproportionation was essentially the
only reaction observed. It should be noted that although
0 bar O was used in the reaction, the amount of O in
1
2
2
solution was in fact very low due to poor solubility of O2
in the reaction medium (~1 mM), leading to
PV ratio of 100:1 in the reaction (see the SI
for the relevant calculation). Since the results show that a
reaction between the reduced polyoxometalate and O is
needed for oxygenation to occur, the low concentration in
the solution phase of O is the likely reason for the slow
oxygenation reaction. It should be noted that after the
reaction H PV 40 can be recovered unchanged, alt-
hough during the reaction several forms of the catalyst
a
Oxygenation of benzene to phenol. Initial attempts at
H
5
Mo10O40/O
2 2
the anaerobic oxidation of benzene with H PV Mo O
10 40
5
2
via the electron transfer (benzene to polyoxometalate)
oxygen transfer (polyoxometalate to benzene) mecha-
2
14b,16
nism
in 80 % H SO at 100 °C yielded phenylmaleic
2
2
4
anhydride, as the major product, without discernable
formation of phenol. One can assume that this compound
is formed via the lower temperature homogeneous
H PV Mo O version of the classic Mars-van Krevelen
Mo10O
2
5
20
may be present as previously discussed.
5
2
10 40
16,18
type oxidation reaction of diphenyl,
formed in situ as an intermediate. Dilution of the reaction
mixture to 50 % aqueous H SO showed no product for-
the latter being
Computational DFT analysis. The free energy of reac-
tions in 50% H SO were analyzed by DFT calculations,
2
4
2
4
Scheme 2, show that reactions between neutral
V
mation up to 170 °C. Above this temperature, one could
observe the formation of diphenylsulfone as the major
product in increasing amounts as a function of tempera-
ture, via sulfonation reactions unrelated to oxygenation.
H PV Mo O and benzene are all significantly ender-
5
2
10 40
V
gonic. On the other hand, protonation of H PV Mo O
10 40
5
2
V
+
to yield [H PV Mo O ] , 2, is exergonic making an elec-
6
2
10 40
tron transfer process between 2 and benzene feasible
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(
ΔG298 = 0.9 kcal/mol). Protonation of 4 can further stabi- tive to delineate a specific reaction pathway. Given the
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lize the interaction by 3.5 kcal/mol. These results support
an affirmative answer to the first question addressed in
the Introduction, i.e. whether the first reaction in Scheme
strong dependence of phenol formation on the O pres-
2
sure, the reaction of O with an intermediate to yield a
2
superoxide and/or a peroxide by electron transfer(s) to
yield an intermediate viable for oxygenation would be a
reasonable assumption.
1
is possible in highly acidic media. Notably CPET reac-
tions are not realistic.
Scheme 2. ΔG298 of Outer Sphere Reactions (in blue)
Between Benzene and H PV Mo O in 50% H SO .
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10 40
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Figure 5. A free energy profile and key structures an ET-OT
anaerobic mechanism showing formation of phenyl-
polyoxometalate adducts 8T and 8S and thermodynamic
interdiction of their transformation to phenol.
To further analyze the fate of the benzene radical cati-
on we calculated an ET-OT pathway, showing intermedi-
ates and a key transition state for possible formation of
phenol in the absence of O , Figure 5 and 6. Comprehen-
2
sive figures of optimized structures can be found in Figure
S3 of the SI The initial ET induces high positive charge
and large spin densities on the two symmetric C-atoms of
benzene, Figure 5, which allows its coordination to yield
7
T, a triplet intermediate where benzene radical cation is
bound to 4. Hydrogen transfer stimulated by a charge
induced interaction of a very acidic H atom bound to the
coordination site in 7T leads to 9T, where the H atom is
bound to the unfavorable terminal O atom, Figures S4, S5.
Non-activated proton migration to the bridging sites yield
triplet 8T and open shell 8S as the most stable calculated
species. The calculated spin densities correlate well with
the intermediate observed in the EPR experiments with
only ~1% of the electron spin density on the benzene moi-
ety, Figures S5, S6. These low free energy species repre-
sent the lowest energy points on the reaction profile and
de facto thermodynamically preclude OT to yield phenol
Figure 6. Detailed geometries of the initial, transition and
final states of the benzene C-H bond breaking step.
In answer to the questions asked at the outset, we have
shown by using visible spectroscopy and X-band and W-
band EPR spectroscopy that ET oxidation of benzene is
possible even at RT and that under aerobic, but not an-
aerobic conditions, phenol can be formed.
INFORMATION
Corresponding Author
*
Email: Ronny.Neumann@weizmann.ac.il
and
the
protonated
reduced
polyoxometalate
IV
+
Present Addresses
[H PV Mo O ] . Thus, though the second reaction in
8
2
10 40
Scheme 1 is exergonic, it is blocked by formation of ad-
ducts 8T and 8S.
Author Contributions
The manuscript was written through contributions of all
authors.
Considering 2 as the most stable form of the polyoxo-
metalate in 50% sulfuric acid, benzene oxidation to phe-
nol is found to be exergonic by 14.7 kcal/mol, compared
to 43.3 kcal/mol for benzene oxidation by oxygen, C H +
ASSOCIATED CONTENT
Supporting Information
6
6
½
O → C H OH. Thus, in the presence of oxygen as a
2 6 5
Supporting Information. Experimental and computational
methods, and additional data. The Supporting Information is
available free of charge on the ACS Publications website.
final oxidant, transformation of 8T and 8S to phenol be-
comes exergonic by 34.0 and 30.5 kcal/mol, respectively.
Without experimental evidence at this time for any in-
termediates in the aerobic reaction after initial formation
of the benzene radical cation, it would be most specula-
ACKNOWLEDGMENT
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This research was supported by the Israel Science Founda-
8. (a) Hamada, R.; Shibata, Y.; Nishiyama, S.; Tsuruya, S. Phys.
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tion Grants 763/14 and 2046/14. Daniella Goldfarb is thanked
for the use of the W-band EPR and advice. R.N. is the Rebec-
ca and Israel Sieff Professor of Organic Chemistry.
Chem. Chem. Phys. 2003, 5, 956-965. (b)Ichihashi, Y.; Kamizaki,
Y.; Terai, N.; Taniya, K.; Tsuruya, S.; Nishiyama, S. Catal. Lett.
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Corberan, V. C. J. Catal. 2007, 250, 184-189. (e) Acharyya, S. S.;
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