Graphene oxide-phenalenyl composite: transition metal-free recyclable and catalytic C-H functionalization

Article abstract of DOI:10.1039/C8CC05941A

An efficient route towards a heterogeneous transition metal-free catalytic C-H functionalization using a covalently linked graphene oxide-phenalenyl conjugate is described herein (28 examples, which include a core of some biologically relevant biaryl and hetero-biaryls). It is an environmentally benign, economical and heterogeneous platform, whose catalytic activity can easily be regenerated through a simple washing-drying technique and the catalytic activity can be retained even after 10 cycles.

Full text of DOI:10.1039/C8CC05941A

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Graphene oxide–phenalenyl composite:
transition metal-free recyclable and
catalytic C–H functionalization†
Cite this: Chem. Commun., 2018,
54, 13220
Received 23rd July 2018,
Accepted 22nd October 2018
a
a
Bhagat Singh,
Rupankar Paira, *b Goutam Biswas,a Bikash Kumar Shaw
a
and Swadhin K. Mandal
*
DOI: 10.1039/c8cc05941a
rsc.li/chemcomm
An efficient route towards a heterogeneous transition metal-free transition metals have paved the ground for transition metal-
catalytic C–H functionalization using a covalently linked graphene free catalyts.20–23 Towards this objective,24–28 many organic
oxide–phenalenyl conjugate is described herein (28 examples, which catalysts as single electron transfer (SET) initiators have been
include a core of some biologically relevant biaryl and hetero-biaryls). reported in the literature, but none in a recyclable way. In this
It is an environmentally benign, economical and heterogeneous regard, our group has explored the phenalenyl (PLY) ligand as a
platform, whose catalytic activity can easily be regenerated through SET initiator under homogeneous reaction conditions with
a simple washing–drying technique and the catalytic activity can be low catalyst loading (2–5 mol%), however without achieving
retained even after 10 cycles.
recyclability.29,30 In this context, it may be worth mentioning
that phenalenyl radicals have been well-known for almost six
With increasing population and demand for numerous chemical decades,31–35 yet their utility in designing catalysts has been
products, the chemical waste of present-day ton-scale industries is emerging recently36,37 and exploration of a heterogeneous plat-
on the rise and has become a global concern in the past century,1 form has been missing from the current literature. This study
necessitating use of toxic and precious transition metal-free recycl- addresses the chemical tethering of GO with phenalenyl units
able heterogeneous catalysts to minimize chemical scrap.2 Along to carry out catalytic C–H functionalization under transition
these lines, the role of graphene oxide (GO) as a catalytic support metal free conditions in a recyclable fashion. We began with
has been noteworthy during the last decade.3–5 However, its the aim of grafting the PLY unit through an amide linkage, due
application in transition metal-free C–H arylation has remained to the high abundance of –COOH groups on GO.38,39 Towards
virtually unexplored (except for a report with limited substrate this goal, a PLY-based trifluoroacetic acid salt was prepared
scope and poor recyclability, which requires reoxidation of the and attached to acyl-chloride functionalized GO, affording the
catalyst).4a It may be noteworthy that many modern-day pharma- phenalenyl linked GO (GO–PLY, see ESI,† Scheme S1). In FT-IR
ceuticals including agrochemicals6–8 and also numerous organic studies (Fig. 1a), –CO2H groups appeared at 1732 cmÀ1 in free
ligands and functional materials are derived from biaryl motifs,9–12 GO5 (red), which vanished in the GO–PLY spectrum (black) with the
e.g. many non-steroidal anti-inflammatory drugs (anticancer,13 appearance of new amide bands at 1633 cmÀ1 and 1561 cmÀ1 40
,
anti-HIV,14 antiarthritic,15 and anti-proliferative16), antagonists along with C–H stretching peaks at 2951, 2921 and 2853 cmÀ1
and inhibitors (mGluR5,17 serine protease and caspase 318), (from the methylene groups attached to phenalenyl). Additionally,
including etoricoxib19 (one of the top-selling anti-arthritic drugs) the peak at 281 nm due to p–p* transitions,5 in the UV-Vis spectrum
have been found to contain biaryl units. Therefore, construction of of GO (Fig. 1b), was shifted to 276 nm in GO–PLY (black) (due to
such cores has been widely explored, mainly using transition disruption of electronic conjugation upon functionalization5),
metal-based catalysts.20 However, several shortcomings, including along with the appearance of a PLY backbone (p–n and p–p*
the sensitive reaction conditions, high expense and toxicity of transitions in free PLY (blue)) at 503 nm and 354 nm. However,
the absorption at 264 nm is merged with the p–p* transitions of the
GO backbone. Further, the solid state 13C NMR (CP/MAS) of GO
(red) reveals hydroxyl (13C–OH) and epoxide (O–13C–O) groups, at
71 and 61 ppm respectively,40,41 which become smaller in GO–PLY
(black, 1588 cmÀ1 and 1327 cmÀ1), on deoxygenation of the GO
sheets. Also, new signals at 25–45 ppm and 67 ppm and
broadening of the signal near 116 ppm (Fig. 1c) indicate the
presence of N-alkyl (N-CH2 and N-CH3) and O-alkyl (O-CH2)
a Department of Chemical Sciences, Indian Institute of Science Education and
Research, Kolkata, Mohanpur-741246, India.
E-mail: swadhin.mandal@iiserkol.ac.in
b Department of Chemistry, Maharaja Manindra Chandra College, 20 Ramkanto
Bose Street, Kolkata-700003, India. E-mail: rupankarpaira85@gmail.com
† Electronic supplementary information (ESI) available: General experimental
procedures, characterization data of the heterogeneous catalyst and spectroscopic
data for all new compounds (PDF). See DOI: 10.1039/c8cc05941a
13220 | Chem. Commun., 2018, 54, 13220--13223
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The inter-layer separation of 0.35 nm, found in the HR-TEM
image (Fig. 2c), correlates very well to the Bragg’s angle of 25.431
(2y), observed in powder XRD.45 A sharp, hexagonal ring
appeared in the selected area electron diffraction (SAED) pattern
of the marked region (red circle, Fig. 2b), indicating the presence
of the crystalline morphology of GO–PLY (Fig. 2d), also sup-
ported by the fast Fourier transform spectrum (FFT, see ESI,†
Fig. S2). Finally, thermogravimetric analysis revealed that
GO–PLY remains more stable compared to GO, as expected from
the TEM and PXRD studies (a decrease in inter-planar spacing
amplifies the thermal stability46), and confirms that it remains
stable at our optimized catalytic reaction temperature (130 1C),
as there was only 3.5% degradation up to 200 1C for GO–PLY (see
ESI,† Fig. S3). With the GO–PLY in hand, we employed it for
the C–H functionalization of benzene (1a) with 4-iodoanisole
(2a). By tuning several reaction parameters, the desired biaryl
Fig. 1 (a) FT-IR spectra of GO (red) and GO–PLY (black). (b) UV-Vis
absorption spectra of GO (red), free phenalenyl compound (blue) and
GO–PLY (black) in DMF. (c) Solid state 13C CP/MAS NMR spectrum of GO
(red) and GO–PLY (black) with 1H decoupling. (d) Raman spectra of GO product 4aa was obtained in 95% yield, when 0.21 mmol
(red) and GO–PLY (blue, black) at 687 nm.
4-iodoanisole and 4 mL benzene were reacted in a sealed tube
using 12 equiv. of KOtBu (3) base and 50 mg of the GO–PLY at
groups and enrichment of the sp2 carbons from the phenalenyl
130 1C for 12 hours (see ESI,† Table S1). The requirement of
excess base can be attributed to the presence of acidic hydroxyl
backbone, respectively. Further, the G and D bands at 1594 cmÀ1
groups in GO,38,39 which consumes the incoming KOtBu, during
the arylation reaction. A 10-fold increase in the amount of 2a was
also well-tolerated, yielding 84% 4aa. However, a control experi-
ment without GO–PLY, but with 12 equiv. KOtBu, yielded only
20% of the desired product (Table S1, entry 7, ESI†). Also, only
traces of 4aa were formed (o2%, Table S1, entry 8, ESI†) when
free GO was used, attesting the essentiality of GO–PLY in this
protocol. Next, studies on the reusability of GO–PLY revealed
that it can sustain its activity for ten successive catalytic cycles,
where the catalyst can be regenerated using a simple washing
and drying method (Fig. 3). Interestingly, its characterization
after ten cycles using Raman spectroscopy revealed a blue shift
of the G and D bands to 1604 cmÀ1 and 1335 cmÀ1, indicating an
increase in the inter-planar spacing (Fig. 1d).
The superiority of the chemically linked phenalenyl–GO
composite over a simple physical mixture was established
by performing a recyclability experiment, using a physical
mixture of GO and PLY, leading to only 28% conversion from
the 2nd cycle onwards. Next, to explore the substrate scope, we
employed 3-iodoanisole (2b) to produce 3-methoxy biphenyl (4ba)
with 79% yield (Table 1), a notably higher yield as compared to
the earlier literature report on GO-based catalysis (29.4%).4a
Also a wide variety of iodoarenes and halogenated heterocycles,
and 1354 cmÀ1, due to the E2g phonon mode and the so-called
disorder-induced in-plane A1g zone-edge mode, respectively, in
the Raman spectrum of GO,42 were red shifted in GO–PLY (black),
indicating a decrease in the interplanar spacing between the GO
sheets owing to edge functionalization. Interestingly, the ratio
of peak intensity ID/IG, through which one can characterize the
level of disorder due to functionalization,43 from GO (0.93) to
GO–PLY (1.03) was as expected.44 Next, the powder XRD
(Fig. 2a) of GO (red) revealed an intense and sharp peak at 2y
= 11.82 ([001] plane with an interplanar spacing of 0.74 nm),45
which was broadened and shifted to 2y = 25.43 in GO–PLY
(black) (interplanar spacing of 0.35 nm), showing a decrease in
the interlayer electrostatic repulsion leading to a lowering in
d-spacing.45 All these observations indicate the chemical grafting
of PLY onto the GO sheets. Also, the HR-TEM images in Fig. 2b
clearly show the presence of few layer stacks in GO–PLY.
Fig. 2 (a) Powder XRD patterns of GO (red) and GO–PLY (black). (b) Multilayer
stacks, shown in dotted lines (black). (c) d spacing perfectly matches with
XRD results. (d) HR-TEM of GO–PLY, the SAED patterns indicate the
presence of crystalline phases in GO–PLY.
Fig. 3 Reusability of the GO–PLY. GO–PLY was recovered by filtration
and washing with water and organic solvents, dried in vacuum and utilized
in the next cycle. The isolated yields are reported and reproduced twice.
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conditions.23,29,30 Also, a competition reaction between electron-
deficient 1n and electron-rich 1a (see ESI,† Scheme S2) produced
the biaryls in comparable amounts, suggesting a radical mecha-
nism, in agreement with the literature reports of homogeneous
conditions.28–30 Further, a low KH/KD ratio (1.17) (see ESI,†
Scheme S3) suggested the non-intervention of the C–H bond
rupture in the rate limiting step, a similar trend to that observed
under homogeneous conditions.28,30
Table 1 Substrate scope of GO–PLY catalyzed C–H arylation
Based on the above findings and the present literature on
the mechanistic pathway of transition metal-free arylation in
homogeneous media,23,28–30 we propose a probable mecha-
nistic cycle, as shown in Scheme 1, where the formation of
potassium ion-coordinated complex I was rationalized through
the interaction of KOtBu and the phenalenyl moiety of GO–PLY,
creating an empty NBMO on the phenalenyl.37 Successively
complex I accepts an electron from KOtBu and generates a
phenalenyl radical anion complex II, along with a tert-butoxy
radical, which eventually undergoes decomposition.50 A con-
comitant SET from radical II to the aryl halide antibonding
MO regenerates complex I and an aryl halide radical anion
intermediate III, which subsequently generates an aryl radical,
IV (Scheme 1). The arene present in the medium traps this aryl
radical to produce V, which transfers another electron to I and
regenerates the phenalenyl based radical initiator II and inter-
mediate VI. Finally, the cation VI undergoes a base-assisted
deprotonation, followed by aromatization to produce the biaryl/
hetero-biaryl product.
In summary, a novel phenalenyl functionalized GO-based
catalytic system was developed and characterized using UV-Vis,
IR, solid state NMR, Raman, XRD, SEM, TEM and TGA analysis
and employed under transition metal-free conditions for the
catalytic C–H arylation of unactivated arenes. The catalytic
system is heterogeneous in nature, and works in a recyclable
fashion (ten cycles) resulting in high yields and wide substrate
scopes, including biologically important biaryl and hetero-
biaryl drug intermediates and pharmacophores.
Reaction conditions: aryl iodide (0.21 mmol), KOtBu (12 equiv.),
GO–PLY (50 mg), and arene (4 mL) were heated in a sealed tube at
130 1C for 12 h.
efficiently produced the corresponding biphenyls/terphenyls/
hetero-biaryls with good to excellent yields (53–96%, Table 1).
Moreover, many arenes and heteroarenes, such as naphthalene,
pyridine, N-methylpyrrole and pyrazine, efficiently participated
in the present methodology to produce various hetero-biaryls
with high yields (64–79%, Table 1). It’s worth mentioning that
some of our synthesized biaryls and heterobiaryls belong to a
number of biologically important pharmacophores.7,47–49 For
example, fumac gel, a well-known commercial pain reliever
composed of felbinac (3% w/w), can be obtained easily from
4-methylbiphenyl (4da, Fig. 4).47 Another important biaryl based
drug core is solifenacin, one of the best-selling drugs used as
medication for an overactive bladder, which can be constructed
from 1-phenylisoquinoline (4sa) in a couple of steps (Fig. 4).48,49
Also, 1-phenyl-tetrahydroisoquinoline, an efficient NMDA receptor,
is achievable from 4sa in a single step.49 Apart from these drugs,
pharmacophores of many bioactive moieties were also found to
contain different biaryl systems synthesized by this protocol, as
summarized in Fig. 4. For example, 3-phenylpyridine (4ra), 2-phenyl-
thiophene (4oa) and 1-phenylisoquinoline (4sa) constitute the basic
pharmacophores of etoricoxib (antiarthritic drug),19 antiproliferative
drugs16 and non-peptidic caspase-3 inhibitors,18 respectively.
SKM thanks CSIR, India (Grant No. 01(2779/14/EMR-II)) and
RP thanks SERB (DST), India (Grant No. YSS/2014/000877)
for financial support. BKS is thankful to SERB (DST) for the
NPDF-Fellowship (PDF/2016/000213) and BS is thankful to
IISER-Kolkata for the research fellowship.
Finally, the underlying mechanism was investigated, employing
a radical scavenger, TEMPO, which immobilized the formation
of biaryl product (see ESI,† Table S3), indicating the presence
of radical intermediates, as also reported in previous similar
transition metal-free catalytic reactions under homogeneous
Fig. 4 Some representative drug intermediates and pharmacophores
achievable through the present methodology.
Scheme 1 Plausible mechanism for direct C–H functionalization.
13222 | Chem. Commun., 2018, 54, 13220--13223
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27 H. Q. Zhao, J. Shen, J. J. Guo, R. J. Ye and H. Q. Zeng, Chem.
Commun., 2013, 49, 2323–2325.
Conflicts of interest
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J. A. Murphy, Angew. Chem., Int. Ed., 2016, 55, 4492–4496; (b) S. Zhou,
E. Doni, G. M. Anderson, R. G. Kane, S. W. MacDougall, V. M.
Ironmonger, T. Tuttle and J. A. Murphy, J. Am. Chem. Soc., 2014,
136, 17818–17826; (c) W. Liu, H. Cao, H. Zhang, H. Zhang,
K. H. Chung, C. He, H. Wang, F. Y. Kwong and A. Lei, J. Am. Chem.
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29 (a) J. Ahmed, P. Sreejyothi, G. Vijaykumar, A. Jose, M. Raj and
S. K. Mandal, Chem. Sci., 2017, 8, 7798–7806; (b) J. Ahmed,
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There are no conflicts to declare.
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