C70 Fullerene-Catalyzed Metal-Free Photocatalytic ipso-Hydroxylation of Aryl Boronic Acids: Synthesis of Phenols

Article abstract of DOI:10.1002/adsc.201701573

A metal-free C₇₀ fullerene-catalyzed method has been developed for the ipso-hydroxylation of aryl and heteroaryl boronic acids to corresponding phenols under photocatalytic conditions. The reaction proceeds under oxygen atmosphere and the mechanistic study revealed that C₇₀ plays a critical role in the generation of reactive oxygen species in the presence of blue light. Reactions in the presence of ¹⁸O-labelled water and oxygen confirmed the generation of reactive oxygen species from oxygen molecule. Amine used as a reductant could be recovered in the form of imine. The current method is also applicable to the synthesis of aryl ethers in one-pot two-step process. (Figure presented.).

Full text of DOI:10.1002/adsc.201701573

Title: C70 Fullerene-Catalyzed Metal-Free Photocatalytic ipso-
Hydroxylation of Aryl Boronic Acids: Synthesis of Phenols
Authors: Inder Kumar, Ritika Sharma, Rakesh Kumar, Rakesh Kumar,
and Upendra Sharma
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To be cited as: Adv. Synth. Catal. 10.1002/adsc.201701573
Link to VoR: http://dx.doi.org/10.1002/adsc.201701573

10.1002/adsc.201701573
Advanced Synthesis & Catalysis
FULL PAPER
DOI: 10.1002/adsc.201((will be filled in by the editorial staff))
C70 Fullerene-Catalyzed Metal-Free Photocatalytic ipso-
Hydroxylation of Aryl Boronic Acids: Synthesis of Phenols
Inder Kumar,a,b Ritika Sharma,a,b Rakesh Kumar,a,b Rakesh Kumar,a and Upendra
Sharma*a,b
aDepartment of Natural Product Chemistry & Process Development, CSIR-Institute of Himalayan Bioresource Technology
Palampur, Himachal Pradesh 176061, India.
b Academy of Scientific and Innovative Research, Anusandhan Bhawan, 2 Rafi Marg, New Delhi-110001, India.
E-mail: upendra@ihbt.res.in; upendraihbt@gmail.com (US)
Received: ((will be filled in by the editorial staff))
Supporting information for this article is available on the WWW under http://dx.doi.org/10.1002/adsc.201######.
Abstract. A metal-free C70 fullerene-catalyzed method has
been developed for the ipso-hydroxylation of aryl and
heteroaryl boronic acids to corresponding phenols under
photocatalytic conditions. The reaction proceeds under
oxygen atmosphere and the mechanistic study revealed that
C70 plays a critical role in the generation of reactive oxygen
species in the presence of blue light. Reactions in the
presence of 18O-labelled water and oxygen confirmed the
generation of reactive oxygen species from oxygen
molecule.
Amine used as a reductant could be recovered in the form
of imine. The current method is also applicable to the
synthesis of aryl ethers in one-pot two-step process.
Keywords: C70 fullerene; boronic acids; phenols;
photocatalysis; metal-free
furnished the corresponding phenols.[8] Leading
Introduction
examples
include
electromediated
ipso-
hydroxylation,[9]
flavin-catalyzed
biomimetic
Phenol represents an important class of
compounds that find applications in pharmaceuticals
and polymers.[1] Phenols also constitute an integral
part of natural products such as flavonoids, phenolic
acids, coumarins and lignans and are biologically
most active antioxidants, anti-cancerous, and anti-
allergic agents.[2] Adhering to the importance of
phenols, a number of methods have been developed
for their synthesis including non-oxidative
electrophilic substitution,[3] hydroxylation of aryl
halides using hydroxide salts,[4] ligand-assisted,
water-mediated hydroxylation of aryl halides,[5]
hydroxylation,[10] UV-Vis-light-induced[11] and
metal-catalyzed[12] synthesis of phenols from boronic
acids (Scheme 1).
Scheme 1. Selected methods for the synthesis of aryl
alcohols from aryl halides and aryl boronic acids.
pyrolysis of sodium salt of benzene sulfonic acids,
air oxidation of cumene, Dow process,
[6]
and
hydrolysis of diazonium salts.[7] Although efficient,
these traditional methods require harsh reaction
conditions which limit their practical use in the
organic synthesis. In addition, there is always a need
for new methodologies with better efficiency, high
selectivity, and least waste generation.
Recently, boronic acids have been explored as a
viable substrate for the synthesis of phenols where
oxidative hydroxylation of aryl boronic acids
During the last decade, photocatalysis has
emerged as a powerful tool for the organic synthesis
due to mild reaction conditions, high selectivity, and
simple reaction setup.[13] Currently, metal-based
photocatalysts are being used extensively, and
hydroxylation of aryl boronic acids to aryl alcohols
using metal photocatalysts has also been
achieved.[14] Beside these metal-catalysts, some
1
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10.1002/adsc.201701573
Advanced Synthesis & Catalysis
metal organic framework (MOF),[15] porous organic
framework (POF)[16] and organophotocatalyst[17]
have also been used for the synthesis of aryl alcohols
from aryl boronic acids/esters. Regardless of high
efficiency, the low recyclability, longer reaction
time, high cost and toxicity poses a limit on their use
in large-scale synthesis. Consequently, there is a
need for the development of a new strategy that can
overcome all these limitations.
(entry 2). Low efficiency of C60 fullerene can be
attributed to its very less absorption of light at this
much low concentration.[26] Changing the light source
to white/green LED, decreased the yield to 85%/49%,
respectively (entries 3, 4). To confirm the role of each
component, control experiments were carried out. In
the absence of photocatalyst or light, no product was
detected, thus explaining the photocatalytic nature of
this transformation (entries 5, 6).
Fullerenes are well-known photosensitizer with
very high quantum yields (Ф≈1) for the generation
of reactive oxygen species.[18] C60 and C70 fullerenes
are well known to produce reactive oxygen species
in the presence of light[18,19] and find use in many
oxidative processes including photodynamic
therapy[20] in cancer treatment. However, there are
limited reports on the use of fullerenes in organic
synthesis which includes photooxygenation of
olefins and dienes,[21a] oxidation of phenol to
quinone[21b] and sulfides to sulfoxides.[22] In a recent
report, the C70 photocatalyzed oxidation of
secondary benzylic amines to imines was also
reported, where the reactions required very low
catalyst loading and afforded high yields of products.
Additionally, products and catalyst could be
separated by a simple chromatography without
workup.[23]
Table 1. Optimization studya
Entry Variation from standard condition
2a yield
>99 (97)[c]
1
None
C60 fullerene catalyst (0.05 mol%)
White light instead of blue light
Green light instead of blue light
Without catalyst
2
3
4
5
15
85
49
n.d.[d]
Without light
Under N2
Without DIPEA[e]
6
7
n.d.
trace
trace
61
8
Air instead of O2
9
Combining the property of fullerenes to produce
reactive oxygen species[23] and boronic acids to
undergo oxidative hydroxylation in the presence of
superoxide radical anion,[14] we envisioned fullerenes
as an efficient photocatalyst for the conversion of aryl
boronic acids to aryl alcohols. C60 fullerene-based
photocatalyst was reported to catalyze the
hydroxylation of phenylboronic acid. This strategy
involves the use of complex C60 triad and tetrad
photosensitizer, which require multistep synthesis,
high catalyst loading and excessive amount of
solvent.[24] Herein, we report the environmentally
benign pristine C70 fullerene[25] as a photocatalyst for
aerobic oxidative hydroxylation of aryl boronic acids
to corresponding phenols (Scheme 1).
10
11
12
Acetone instead of CHCl3
0.1 M CHCl3
82
54
1 equiv. Of DIPEA
88
0.025 mol% of C70
9h instead of 12h
13
14
90
89
aReagents and conditions: 1a (0.1 mmol), C70 fullerene (0.05 mol%),
DIPEA (2 equiv.), O2 balloon, CHCl3 (0.05 M), Blue LED light, 12 h.
bYield determined by NMR analysis of crude reaction mixture using
1,1,2,2-tetrachloroethane as internal standard. cIsolated yield in
parenthesis. dn.d.: not detected, eDIPEA = diisopropylethylamine.
Only traces of product were detected when reaction
was carried out either under nitrogen atmosphere or
in the absence of amine donor (entries 7, 8). When
the reaction was carried out under air, only 61% of
product was obtained (entry 9). Chloroform was
found to be the best solvent under optimized reaction
conditions (entry 10).[23,26] Decreasing the amount of
solvent, amine, catalyst or time proved detrimental to
the reaction, providing reduced yield of the desired
product (entries 11, 12, 13 & 14).
Results and Discussion
Reaction conditions were screened by the
photoirradiation of phenylboronic acid in the
presence of fullerenes
C60 and C70 using white (λ =
With the best-optimized reaction conditions in
hand, substrate scope was explored with different aryl
boronic acids (Table 2). Aryl boronic acids with
electron withdrawing or donating substituent at para-
400-800 nm), blue (λ = 450-475 nm) and green (λ =
510-530 nm) lights in different solvents and
N,N-diisopropylethylamine (DIPEA) as an electron
donor. The best reaction conditions were obtained
with C70 as a photocatalyst; blue LED as a light
source in chloroform as solvent and DIPEA as an
electron donor (Table 1, entry 1). Phenol was
obtained in only 15% when C70 was replaced with C60
position
were
successfully
converted
into
corresponding alcohols with excellent yields (2b-m).
Aryl boronic acid with 4-ethyl and 4-amyl substituent
afforded corresponding phenols 2b and 2c in 88%
and 94% isolated yield, respectively. Substituents
2
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10.1002/adsc.201701573
Advanced Synthesis & Catalysis
such as phenyl, methoxy, trimethylsilyl, acetoxy were
also well tolerated providing the desired product in
excellent yields (2d-g). Boronic acid with nitrile
substituent at ortho-, meta- and para-position
afforded corresponding phenols in 67-93% yield (2r,
2n, and 2h). Halogen substituted phenols 2l and 2m
were synthesized in good to excellent yields. Para-
substituted acetyl and methyl ester boronic acid also
reacted well under present reaction conditions (2j,
2k).
Boronic acid derivative of vitamin E was successfully
converted into corresponding phenol (2aa).
To expand the generality of the developed
catalytic method, heterocyclic boronic acids were
also explored for the synthesis of corresponding
phenols. Heterocycles such as 3- and 8-quinoline
boronic acid gave the corresponding hydroxy-
quinoline in good yields (4a-b). Other heterocyclic
boronic acids such as 6-indolyl (3c), 2,6-dimethoxy-
3-pyridine (3d) and 2-methoxy-5-pyrimidine (3e)
boronic acids were also converted to the
corresponding phenols in 37-55% yields (4c-e).
Table 2. Scope with aryl boronic acidsa
Table 3. Scope with hetero-arylboronic acids.a
aReagents and conditions: 3 (0.2 mmol), C70 fullerene (0.05 mol%),
DIPEA (2 equiv.), O2 balloon, CHCl3 (0.05 M), Blue LED light, 12h.
Further, 4-methoxyphenol was obtained in 63%
yield,
when
potassium
4-
methoxyphenyltrifluoroborate was reacted under
standard reaction conditions (Scheme 2).
Scheme 2. ipso-Hydroxylation of potassium 4-
methoxyphenyltrifluoroborate using C70 catalyst.
To test the applicability of the developed
photocatalytic approach for a preparative synthesis, a
gram scale reaction for the hydroxylation of 4-
biphenylboronic acid was performed. However, in
this case, the conversion was low and therefore, the
reaction time was increased to 48h. But even after
48h only 62% yield of the desired product was
observed due to less conversion. Use of two 6W blue
LED bulbs provided 78% and 91% isolated yield of
the desired product after 24h and 48h, respectively
(Scheme 3).
aReagents and conditions: 1 (0.2 mmol), C70 fullerene (0.05 mol%),
DIPEA (2 equiv.), O2 balloon, CHCl3 (0.05 M), Blue LED light, 12h.
Phenols with a carboxylic acid substituent at meta-
and para-position were also synthesized in good to
excellent yields, respectively (2o and 2i). 3,5-
dimethyl substituted phenol (2t) was obtained in 82%
yield. In case of 3-acetamido phenylboronic acid, the
low yield of the corresponding phenol was observed
(2p). Phenylboronic acid with vinyl ether substituent
at ortho-position provided 77% yield of
corresponding phenol (2u). Further, polyaromatic
boronic acids were studied under the developed
reaction conditions. Polycyclic phenols such as
α-naphthol (2w), anthracene-2-ol (2x) and pyrene-1-
ol (2y) were obtained in 68%, 46% and 82% yields,
respectively. In the case of 2-phenylvinylboronic acid,
acetophenone was observed in 60% yield (2z).
Scheme 3. Scale-up reaction
A one-pot, two-step synthesis of ether from
boronic acid was also attempted, and for our delight,
4-cyanoboronic acid was successfully converted to 4-
3
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10.1002/adsc.201701573
Advanced Synthesis & Catalysis
allyloxybenzonitrile (5) in one-pot, two-step process
(Scheme 4). In the first step phenol was prepared
under standard reaction condition, and after
evaporating the solvent, 4-cyanophenole was refluxed
with 2 equiv. of allyl bromide in the presence of 2.5
equiv. of K2CO3 in CH3CN for 12h.
Further to confirm the role of amine, 1,2,3,4-
tetrahydroisoquinoline was used instead of DIPEA
(Scheme 5, equation i).
In this experiment, oxidation of benzylic amine to
imine (6b) was observed along with the desired
product. Imine (6b) was isolated in 75% yield,
providing an alternate pathway for the synthesis of
3,4-dihydroisoquinolines. Also, we analyzed the
crude reaction mixture with TOF-MALDI mass
analyzer, but no amine C70 adduct was detected
(Figure SI4), thus ruling out the direct interaction of
amine with fullerene under current reaction
conditions. Also, the fluorescence quenching
experiments showed that the DIPEA was not
functioning as a quencher of excited C70 fullerene
catalyst as the decrease in fluorescence intensity of
C70* by DIPEA was negligible (Figure SI5). In the
Scheme 4. One pot, 2-step synthesis of ether from
aryl boronic acid.
To get an insight into the reaction pathway of
current metal-free photocatalytic ipso-hydroxylation,
we carried out different control experiments. UV-
visible absorption analysis of different components of
the reaction showed that it is only C70 fullerene,
which shows absorption in the visible region at 468
nm (Figure 1).
presence
of
2,2,6,6-tetramethylpiperidinooxy
(TEMPO) as a radical quencher, no product was
observed, which confirms the radical mechanism of
reaction. (Scheme 5, equation ii)
1.6
C70
Boronic acid
DIPEA
Reaction Mixture
0.8
Photocatalyzed oxidative hydroxylation of boronic
acids has been proposed to involve the superoxide
radical anion (O2 ) as reactive oxygen species.[14]
When the reaction was carried out in the presence of
1 equiv. of each DABCO and 1,4-benzoquinone,
product yield decreased to 34% and 26%,
respectively. This reduction in the product yield
confirms the presence of O2 and O2 as DABCO
and 1,4-benzoquinone are known to quench these
ROS respectively [28] (Scheme 6).
0.0
200
400
600
Wavelength (nm)
Figure 1. UV-Visible spectrum of different
components used in the reaction.
1
Quantum yield of the catalyst in this system was
found to be 0.94 (≈1). The crude reaction mixture
analysis with LC-MS resulted in detection of
diisopropylamine, thus confirms the involvement of
amine donor in the course of the reaction.[26]
Although acetaldehyde was not detected in LC-MS
analysis of crude reaction mixture, diisopropylamine
might be formed through hydrolysis of corresponding
imine generated after proton abstraction from
diisopropylethylamine radical cation by intermediate
B (Scheme 8).[27]
Scheme 6. Quenching experiments
100
80
60
40
20
0
0
2
4
6
8
10
12
14
Time (h)
Figure 2. Progress of reaction with time.
Scheme 5. Mechanistic study.
4
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10.1002/adsc.201701573
Advanced Synthesis & Catalysis
Later, when we analyzed the progress of the
Scheme 8. Probable reaction pathway for the
synthesis of aryl alcohols from aryl boronic acids.
After the abstraction of proton from amine free
radical cation A generated in SET step, B leads to
formation of intermediate C. Removal of hydroxyl
anion from C followed by aryl migration generates
intermediate D, which on hydrolysis provides the
desired product.
reaction with time, it showed a linear relationship as
the yield of the product increases linearly with time
(Figure 2).
To predict the oxygen source in phenol, we carried
out the reaction in the presence of
18O–labelled
water and oxygen (Scheme 7). The GC-MS analysis
of crude reaction mixture showed no incorporation of
labelled oxygen in phenol when the reaction is
carried out in the presence of 18O–labelled water.
However, 18O incorporation was found in the
presence of 18O–labelled oxygen gas.[26] The result
of these reactions indicates the generation of reactive
oxygen species from oxygen molecule by
photoactivated C70 fullerene catalyst.
Conclusions
In conclusion, we have disclosed an efficient
photocatalytic method for the synthesis of aryl
alcohols using nascent C70 fullerene as photocatalyst
under mild conditions. This developed method is
applicable for the synthesis of a variety of phenols as
well as heterocycle phenols. The preliminary
mechanistic study revealed the critical role of C70
fullerene in generating ROS in the presence of blue
LED.
Experimental Section
General Methods
Reagent Information: Unless otherwise stated, all
reactions were carried out under oxygen atmosphere in
screw cap reaction vials. All solvents were bought from
Sigma Aldrich in a sure-seal bottle and used as such.
Chemicals were bought from Sigma Aldrich, Alfa-aesar
and TCI and used as such. For column chromatography,
silica gel (230-400 mesh) from Merck was used. A
gradient elution using n-hexane and ethyl acetate was
performed based on Merck aluminum TLC sheets (silica
gel 60F254) for the purification of products.
Scheme 7. Labelled oxygen experiments.
Based on these preliminary studies and literature
reports,[14a,17c,23] the probable mechanism was
proposed (Scheme 8), involving photocatalytic
oxidative hydroxylation of aryl (hetero-aryl) boronic
acids to corresponding phenols. C70 fullerene-catalyst
on irradiation with blue LED light comes to an
excited state, where it has greater tendency to
1
3
generate singlet oxygen O2 from triplet oxygen O2
via energy transfer process. Superoxide free radical
(O2 ), thus generated from 1O2 through single
electron transfer (SET) process from DIPEA, attacks
the electron deficient boron centre to form
intermediate B.
Analytical Information: The melting points were
recorded on a Bronsted Electro Thermal 9100. All isolated
1
compounds were characterized by H NMR, 13C NMR,
LC-MS. Also, all the compounds were further
characterized by HRMS. Mass spectra were recorded on
1
Water Q-ToF-Micro Micromass. Copies of H, 13C NMR
can be found in the NMR supporting information.
Nuclear magnetic resonance spectra were recorded
either on a Bruker-Avance 300 or 600 MHz instrument.
1
All H NMR experiments are reported in units, parts per
million (ppm) and were measured relative to the signals for
residual chloroform (7.26), acetone (2.05) and methanol
(3.31) in the deuterated solvents. All 13C NMR spectra
were reported in ppm relative to deuterated chloroform
(77.23), acetone (29.84) and methanol (49.00) and all were
1
obtained with H decoupling. Optimization studies were
5
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10.1002/adsc.201701573
Advanced Synthesis & Catalysis
done by NMR and NMR yield were calculated by using
tetrachloroethane as an internal standard.
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Clohessy, C. Irving, N. A. Anderson, G. C. Lloyd-
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General Procedure for Synthesis of aryl alcohols from
corresponding boronic acids: To an oven-dried screw cap
reaction vial charged with a spin vane magnetic stir-bar,
168 µL of C70 solution (1 mg/ 2 mL in toluene) (0.05
mol%) was added and dried over rotary evaporator. To this
phenyl boronic acid (24.2 mg, 0.2 mmol) and chloroform
(4 mL) were added followed by the addition of
diisopropylamine (69.6 µL, 2 equiv). The reaction vial was
closed with screw cap, purged with oxygen and kept for
vigorous stirring under blue LED light (strip or single
bulb) at room temperature for 12 h. For experimental set-
up, see supporting information. After completion, reaction
mixture was dried over rotary evaporator and purified by
flash chromatography using silica gel (230-400 mesh size)
as stationary phase and n-hexane: EtOAc as an eluent.
[9] H. Jiang, L. Lykke, S. U. Pedersen, W. –J. Xiaoand, K.
A. Jørgensen, Chem. Commun. 2012, 48, 7203-7205.
Acknowledgements
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The authors are thankful to the Director, CSIR-IHBT. IK and RS
acknowledge UGC and CSIR, New Delhi for SRF. This activity is
supported by SERB-DST (EMR/2014/001023) and CSIR
(MLP0066). CSIR-IHBT communication no. is 4176.
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