Carbocatalytic Oxidative Dehydrogenative Couplings of (Hetero)Aryls by Oxidized Multi-Walled Carbon Nanotubes in Liquid Phase

Article abstract of DOI:10.1002/chem.201903054

HNO₃-oxidized carbon nanotubes catalyze oxidative dehydrogenative (ODH) carbon–carbon bond formation between electron-rich (hetero)aryls with O₂ as a terminal oxidant. The recyclable carbocatalytic method provides a convenient and an operationally easy synthetic protocol for accessing various benzofused homodimers, biaryls, triphenylenes, and related benzofused heteroaryls that are highly useful frameworks for material chemistry applications. Carbonyls/quinones are the catalytically active site of the carbocatalyst as indicated by model compounds and titration experiments. Further investigations of the reaction mechanism with a combination of experimental and DFT methods support the competing nature of acid-catalyzed and radical cationic ODHs, and indicate that both mechanisms operate with the current material.

Full text of DOI:10.1002/chem.201903054

A Journal of
Accepted Article
Title: Carbocatalytic Oxidative Dehydrogenative Couplings of
(Hetero)Aryls by Oxidized Multi-Walled Carbon Nanotubes in
Liquid Phase
Authors: Tom Wirtanen, Santeri Aikonen, Mikko Muuronen, Michele
Melchionna, Marianne Kemell, Fatemeh Davodi, Tanja Kallio,
Tao Hu, and Juho Helaja
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To be cited as: Chem. Eur. J. 10.1002/chem.201903054
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Carbocatalytic Oxidative Dehydrogenative Couplings of
(Hetero)Aryls by Oxidized Multi-Walled Carbon Nanotubes in
Liquid Phase
Tom Wirtanen,[a]* Santeri Aikonen,[a] Mikko Muuronen,[a] Michele Melchionna,[b] Marianna Kemell,[a]
Fatemeh Davodi,[c] Tanja Kallio,[c] Tao Hu,[d] and Juho Helaja[a]*
Abstract: HNO3-oxidized carbon nanotubes catalyze oxidative
dehydrogenative (ODH) carbon-carbon bond formation between
electron-rich (hetero)aryls with O2 as a terminal oxidant. The
recyclable carbocatalytic method provides a convenient and an
operationally easy synthetic protocol for accessing various
benzofused homodimers, biaryls, triphenylenes and related
benzofused heteroaryls that are highly useful frameworks for material
chemistry applications. Carbonyls/quinones are the catalytically
active site of the carbocatalyst as indicated by model compounds and
titration experiments. Further investigations of the reaction
mechanism with a combination of experimental and DFT methods
support the competing nature of acid-catalyzed and radical cationic
ODHs, and indicate that both mechanisms operate with the current
material.
At present, carbocatalytic gas-phase oxidative dehydrogenative
(ODH) reactions of hydrocarbons are well established and
numerous studies have identified carbonyl/quinoidic groups as
active sites in various carbon materials.[1,2] Likewise,
carbocatalyzed liquid-phase reactions have been recently
undergoing fast developments.[3] Graphene oxide (GO),[4] one of
the most highly oxidized forms of carbon, has been a popular
choice as a carbocatalyst or in many cases as a stoichiometric
oxidant (henceforth we call the latter as carbon-mediated
reactions).[5] The GO contains carboxylic, anhydride, phenolic,
and lactone groups mainly on edges, while epoxy and hydroxyl
groups lie on the basal plane of the material. These and other
oxygen containing functional groups, together with edges and
defects on the carbon framework, can contribute to both the
Scheme 1. Previous and current carbocatalyzed ODH C-C bond forming
couplings
stoichiometric and the catalytic activity of GO and other carbon
materials such as carbon nanotubes (CNT) or active carbons (AC).
Most of the published liquid-phase carbocatalytic or carbon-
mediated reactions are functional group modifications: oxidations,
reductions, or conversions based on acid-base chemistry.
substrates are still rare. Notably, Friedel-Crafts (FC) alkylations
Meanwhile, reports on direct carbocatalytic (or carbon-mediated)
carbon-carbon bond formations from non-functionalized
of styrenes and benzyl alcohols,[6] and xanthanes (Scheme 1b)[7]
by GO in presence of air have been recently reported. From a
strict mechanistic view, both reactions operate in a cascade
manner: the GO catalyzed C-H oxygenation to C-OH or C-OOH
is followed by an acid catalyzed carbocation formation leading
eventually to FC-coupling with the aryl substrate. Moreover,
analogic FC-protocol has been developed to activate allylic
alcohols towards thiophene nucleophiles.[8] Also oxidative GO-
mediated homocouplings of electron-rich aryls[9] and GO-
catalyzed couplings of 2-naphtol derivates (Scheme 1c) have
been described.[10]
[a]
Dr. T. Wirtanen, M.Sc. S. Aikonen, Dr. M. Muuronen, Dr. M. Kemell,
Dr. J. Helaja, Department of Chemistry, University of Helsinki, A. I.
Virtasen aukio 1, P.O. Box 55, University of Helsinki, 00014 Finland
E-mail: juho.helaja@helsinki.fi
[b]
[c]
Dr. M. Melchionna, Department of Chemical and Pharmaceutical
Sciences, University of Trieste, Via L. Giorgieri 1, 34127 Trieste.
Prof. T. Kallio, M.Sc. F. Davodi
Department of Chemistry and Materials Science, Aalto University,
P.O Box 16100, 00076 Aalto, Finland
[d]
Dr. T. Hu, Research Unit of Sustainable Chemistry, Faculty of
Technology, University of Oulu, FI-90014 Oulu, Finland
We have previously reported that oxidized active carbon
(oAC) can catalyze the C-C bond-forming ODH reactions of
indoles and analogues in the presence of O2,[11] where
experimental evidences point the reaction to proceed via radical
Supporting information for this article is given via a link at the end of
the document.
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mechanism by quinoidic oxygen centers.[12,13] Based on this, we
envisioned that carbocatalysis could be used as a heterogeneous
replacement for oxidative processes that are catalyzed or
mediated by molecular quinones,[13,14] transition metals,[15]
hypervalent iodines,[16] or nitrosoniums.[17] To that end, oxidized
carbon nanotubes (oCNTs) would be the most suitable
candidates as they offer significant advantages over oAC such as
the high mechanical strength, better stability towards oxidation by
O2,[18] and fewer micropores susceptible to catalyst
deactivation.[19] Still, we found only a few examples in the
literature of liquid-phase carbocatalysis that utilize oCNTs such
as the catalytic nitrobenzene reductions by Su and co-workers.[20]
Here we report for the first time that oCNTs are highly efficient
catalysts for forming C-C bonds oxidatively between two
unfunctionalized positions of electron-rich aryls. Furthermore,
easy setup, workup, and recyclability of oCNTs allow a facile
replacement of traditional methods for these reactions with
abundant carbon catalyst and oxygen, which is of high urgency
with relation to current guidelines on sustainability of chemical
processes and endangered elements.
We selected HNO3-oxidized multi-walled (MW)CNTs out of
the several reported materials[21] for catalysis testing as the
oxidation procedure is well-studied including the spectroscopic
and thermogravimetric analysis of produced functional groups on
the MWCNTs.[22,23] For the test reaction of catalysis we selected
dimerization of 1a (Table 1) to 2a as it is one of the prototypical
ODH-reactions that has been studied with different oxidative
systems such as DDQ/H+,[24] PIFA,[25] MoCl5,[26] NaNO2/H+,[27]
electrolysis,[28] FeCl3/SiO2,[29] and MnO2/BF3•OEt2.[30]
Table 1. Deviation to the standard reaction conditions
Entry:
Deviation from standard conditions
Yield [%]
1
None
83 (82)[a]
2
no MsOH
0
3
no oCNTs
0
4
Ar-atmosphere
16
66
27
63
39
53
24
36
9
5
CNTs instead of oCNTs
HCl-treated CNTs instead of oCNTs
Active carbon (AC) instead of oCNTs
Air oxidized AC instead of oCNTs
HNO3-oxidized AC instead of oCNTs
GO instead of oCNTs
5 h reaction time
6
7
8
9
10
11
12
Anisole as solvent
[a] Isolated yield
Using toluene as solvent, we screened several variables
such as temperature, molarity, reaction time, loading of catalyst
and the amount of MsOH additive (SI), which is known to increase
oxidation potential of molecular quinones.[31] The temperature and
the amount of catalyst affected the activity most from these
variables. Finally, 2a was obtained with 82% isolated yield.
Additional experiments with different conditions or catalysts
were carried out to investigate the nature of the catalysis (Table
1). In the absence of MsOH or oCNT, reaction does not proceed
(entries 2 and 3). The yield also drops to 16% when reaction is
performed under Ar (entry 4). Similarly, reaction time reduction to
5 h lowers the yield to 36% (entry 11). In comparison with oCNTs,
different oxidatively treated AC materials perform modestly with
yields of 39% and 53% (entries 8 and 9) while a slightly better
yield of 63% was received with untreated AC (entry 7). Also GO
gave a relatively poor yield (entry 10). The related oxidation ability
of AC has been previously associated to absorbed oxygen in AC
that enables some stoichiometric activity in ODH couplings.[32]
The HCl-treated CNTs provide a poor yield of 27% (entry 6). It
has been shown that this treatment lowers overall oxygen content
of MWCNTs.[22]
Scheme 2. Intermolecular coupling scope (V vs. Fc).
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Next, we explored different intermolecular couplings of
(hetero)arenes (Scheme 2). The coupling product 2c was
obtained in a high 86% yield after 15 h at 60 °C, and 2b was
obtained in a 62% yield after 24 h at 70 °C. These yields
correlated well with the oxidation potentials of the substrates. We
found out the oxidation potential limit for substrate to be reactive
is ca. 1.1 V (Fc): this is best seen by comparing two borderline
substrates, 1d and 1e, with oxidation potentials of 1.11 V and 1.05
V, respectively. 1d was not reactive, while 5% yield of coupling
product of 1e was obtained. The lower oxidation potentials of
arenes 1a-c (0.67 – 0.83 V) seem to favor the couplings. In
accordance, the low oxidation potentials of heteroarenes 1f and
1g (0.84 V and 0.78 V, respectively) are in agreement with their
facile conversion to 2f and 2g (without MsOH) with 88% and 72%
yields, respectively.
Likewise, oCNTs can catalyze several intramolecular
cyclodehydrogenations with slightly modified reaction conditions
(Scheme 3). Cyclization of 3a to 4a proceeds with an excellent
91% yield, while 3b was converted to 4b with an 88% yield.
Interestingly, 3c required a very long reaction time and after 6
days, 29% yield was obtained together with 60% of unreacted
starting material. To our pleasure, 4d, which has been indicated
as potential substructure for p-type organic semiconductors[33]
and promising framework for optoelectonic applications,[34] was
obtained in an 84% yield. Similarly, 4f, substructure found in
molecules with interesting chiroptical properties for e.g. spintronic
devices,[35] was obtained in 82% yield. Also, 4e could be obtained
in 21% yield. This framework has been used in a host-material for
C60-fullerenes.[36]
whether oCNTs behave similarly, the catalyst was treated with
phenyl hydrazine (PH) to selectively block the carbonyl/quinone
groups of oCNTs.[37] When PH-oCNTs were used as catalyst,
yields of 2a and 4a dropped to 0%. Furthermore, when oCNTs
were replaced with tetracene, anthraquinone or 9,10-
phenanthrenequinone, yields received were 0%, 3%, and 21% of
2a, respectively (Figure 1, top). Recently, these compounds have
been used as model compounds for zig-zag edges and
carbonyls/quinones.[7,38] These experiments indicate that
carbonyls/quinones are the catalytically active sites. The
presence of C=O groups (0.74% at) as well as O=C-O (5.55% at)
and C-OH groups (3.84% at) were confirmed with XPS surface
analysis of oCNT (Figure 1, bottom). Also no metals were
detected on the surface in the same XPS survey. The EDX-
analysis, however, showed that CNT-materials contain ca. 1 wt%
of embedded Co and Mo that could not be removed by HCl or
HNO3 treatments (SI). Similar abundance of these metal
impurities were found in ICP-OES analysis of untreated CNTs,
HCl-washed CNTs and oCNTs. Importantly, both HCl and HNO3
treatments decrease the amount of the metallic impurities
whereas HCl treatment decreases and HNO3 treatment increases
the yield of the test reaction. With ICP-MS, we also detected low
quantities of potentially catalytic metals such as Ni (<1000 ppm),
Cu (<200 ppm), Pd (<0.4 ppm), while the lowest amounts were
found for oCNT (Table S2, SI). Notably, the strong correlation
between the catalytic activity and quinoidic groups is clear based
on several experiments, i.e., XPS, model compound study, C=O
blocking experiment, catalytic studies with other carbon materials,
and electrochemistry.
Figure 1. Top: Yields of 2a received with model compounds and PH-titrated
oCNTs. Bottom: O1s XPS of oCNT
Two previously discussed intramolecular mechanisms were
studied computationally for o-terphenyls 3a–c in Scheme
3:[14,31,39] acid-catalyzed mechanism operating via arenium ion (B-
D, Figure 2), and mechanism operating via radical cation (E-F,
Figure 2). We do not consider the widely accepted gas-phase
quinoidic hydrogen atom abstraction ODH-mechanism[40] to be
important in our acidic conditions. To understand how different
quinoidic groups with varying electronic properties affect the
mechanism, we studied two catalysts that were experimentally
Scheme 3. Intramolecular coupling scope of the reaction (V vs. Fc).
Concerning the catalytic active site, we have previously
observed a correlation between the catalytic activity of the oAC
and the amount of carbonyl groups in it.[12] In order to trace
tested
with
the
intermolecular
reaction:
9,10-
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phenanthrenequinone (PhQ, Figures 1 and 2) and anthraquinone
(AQ, Figures 1 and S4). Protonated catalyst, PhQ, oxidizes the
substrates exergonically (E) and reversibly, and the radical cation
(E) present the free energy minimum for the reactions, see Figure
2. The formed radical cation can cyclize directly to form the C-C
bond with activation free energies of 24.7, 25.5, and 38.8 kcal/mol
in Figure 2, for 3a, 3b, and 3c, respectively. While the barriers for
3a and 3b are in accordance with the experimental yields: 91%
and 88%, respectively, the activation free energy for radical
cationic C-C bond formation for 3c is considered too high. The
acid-catalysed C-C bond formation, however, is lower in energy
and thus obtained activation free energy for 3c is 34.1 kcal/mol,
while the radical cationic mechanism is still operative with 3a (24.7
kcal/mol) and 3b (25.5 kcal/mol). Therefore, we consider that
substrates with activated nucleophilic bond-forming carbon, i.e.,
3a and 3b, react via radical cationic mechanism, while for 3c the
arenium cation mechanism is prevalent. Furthermore, depending
on the proton affinity and redox potentials of the oxidant, the
oxidation (E) can become endergonic (AQ in Figure S4) and
favour arenium cation mechanism for all 3a–c, or even more
exergonic and drive the reaction via radical cationic C-C bond
formation also with 3c, see ortho-benzoquinone in Figure S4. The
shift in operative mechanism is thus not only dependent on the
substrate but also on the nature of the quinoidic groups of the
oCNT. Moreover, we observed aromatic proton-deuterium
exchange during the coupling of 1a to 2a under acidic conditions
in d12-toluene (SI), which is in accordance with the reversibly
nature of the oxidation and the protonation.
time had to be increased to 1 d for a full conversion. The 3rd, 4th
and 5th cycle gave 72%, 77% and 75% yields after 1 d,
respectively, confirming the good recyclability of the catalyst. The
XPS analysis of catalyst after five cycles shows an increase of
oxygen content, specifically in the abundance of C-O groups
(3.8% to 12.2%), but also in reduced amounts of C=O groups
(0.74% to 0.18%, SI). This could be interpreted as wearing of
active sites of oCNT. However, for a great part, the C-O content
increase can be explained by the absorption of some starting
material / product (3a/4a) on oCNTs that is also observed by a
mass increase of the recycled catalyst. Correspondingly, some C-
O
group can arise from reduction of C=O (quinone to
hydroquinone conversion), which is expected to be reversible
under reaction conditions.
To further elucidate the acid additive effects on the redox
behaviour of oCNTs, we measured cyclic voltammograms (CV)
from differently functionalized CNTs in various H2SO4
concentrations (Figure 3 and SI). The CV recorded for untreated
CNT is very close to those previously reported in the literature.[41]
The oxidative treatment increases the overall current densities.
Electrochemical response in 100 – 400 mV (vs. SCE) potential
region has been associated to the o-quinones in graphite
electrode materials.[42] Notably, upon an increase of the acid
concentration, a gradual shift of reduction peak potentials are
observed in both regions i.e. protons participate in the oCNT
redox process (See further discussion in SI).
Figure 3. CV of HCl treated CNT and oCNT measured in various H2SO4
concentrations. Scan rate: 50 mV/s.
.
In conclusion, we have devised oCNT catalysis for a diverse
set of electron rich substrates to complete ODH C-C coupling
reactions utilizing O2 as a terminal oxidant. To our knowledge, this
is the first example of a C-C coupling by CNT catalysis. The
catalyst can be used in the synthesis of several important
frameworks for materials chemistry and it displays recyclability.
Catalytic activity is associated to the presence of quinoidic
oxygens and is improved by MsOH additive. We anticipate that
the development of oCNT-materials with better oxidative powers
facilitate the discovery of new carbocatalytic reactions.
Figure 2. Energy profiles with phenanthrenequinonium cation for oxidation
induced (to left) and acid-catalyzed (to right) C-C bond formation for 3a (black
square), 3b (green circle), and 3c (red triangle). Favored pathway is presented
with solid line. See SI for full details.
Recyclability of the carbocatalyst was tested on
a
cyclodehydrogenation of 3a to 4a with ~230 mg of catalyst. In the
first step, yield of 81% of 4a was obtained after 13 h. In the second
cycle, yield of 73% was recorded. However, in this case, reaction
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Acknowledgements
Financial support from Academy of Finland [project no. 129062
(J.H.)] is acknowledged. Ilkka Vesavaara is acknowledged for
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Entry for the Table of Contents
Catalysis with carbon material: HNO3-oxidized carbon nanotubes catalyze oxidative
dehydrogenative (ODH) carbon-carbon bond formation between electron-rich (hetero)aryls
with O2 as a terminal oxidant.
T. Wirtanen,* S. Aikonen, M. Muuronen,
M. Melchionna, M. Kemell, F. Davodi, T.
Kallio, T. Hu, J. Helaja*
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Carbocatalytic Oxidative
Dehydrogenative Couplings of
(Hetero)Aryls by Oxidized Multi-
Walled Carbon Nanotubes in Liquid
Phase
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