An alternative route for boron phenoxide preparation from arylboronic acid and its application for C[sbnd]O bond formation

Article abstract of DOI:10.1016/j.tetlet.2020.152197

An efficient synthetic route to benzyl phenyl ether preparation has been successfully developed via a one-pot synthetic protocol utilizing a combination of arylboronic acids, hydrogen peroxide (H₂O₂), and benzyl halides. The whole procedure consists of two consecutive reactions, formation of boron phenoxide from arylboronic acids and its nucleophilic attack. A simple operation under mild conditions such as room-temperature ionic liquid (choline hydroxide), aerobic environment, and absence of metal- and base-catalysts has been employed. Expansion to utilize benzyl surrogates was also successfully accomplished.

Full text of DOI:10.1016/j.tetlet.2020.152197

Journal Pre-proofs
An alternative route for boron phenoxide preparation from arylboronic acid
and its application for C – O bond formation
Seong-Ryu Joo, In-Kyun Lim, Seung-Hoi Kim
PII:
S0040-4039(20)30662-6
DOI:
Reference:
https://doi.org/10.1016/j.tetlet.2020.152197
TETL 152197
To appear in:
Tetrahedron Letters
Received Date:
Revised Date:
Accepted Date:
14 May 2020
16 June 2020
23 June 2020
Please cite this article as: Joo, S-R., Lim, I-K., Kim, S-H., An alternative route for boron phenoxide preparation
from arylboronic acid and its application for C – O bond formation, Tetrahedron Letters (2020), doi: https://
doi.org/10.1016/j.tetlet.2020.152197
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1
An alternative route for boron phenoxide preparation from arylboronic acid and its
application for C – O bond formation
Seong-Ryu Joo, In-Kyun Lim, Seung-Hoi Kim*
Department of Chemistry, Dankook University
119 Dandaero Cheonan Korea 31116
kimsemail@dankook.ac.kr
Highlights:
 Efficient synthetic route to benzyl phenyl ether preparation
 Development of two C–O bonds formation in a one-pot system
 Versatile combination of arylboronic acids, hydrogen peroxide, and benzyl halides in
choline hydroxide
 Performance under a metal- and base-catalyst-free in an aerobic environment
 Further applicability to various electrophiles
Graphical Abstract
To create your abstract, type over the instructions in the template box below.
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An alternative route for boron phenoxide preparation from
arylboronic acid and its application for C – O bond formation
Leave this area blank for abstract info.
Seong-Ryu Joo, In-Kyun Lim and Seung-Hoi Kim
B(OH)2
O
O
E+
O2
H2
B(OH)2
E+
E
R
R
R
rt, air
27 examples
(up to 99%)
: benzyl halides
acid chlorides
R = H, alkyl, carbonyl
Choline
hydroxide
◆ one-pot synthesis ◆ green solvent ◆ base-free ◆ open air condition

1
Tetrahedron Letters
journal homepage: www.elsevier.com
An alternative route for boron phenoxide preparation from arylboronic acid and its
application for C – O bond formation
Seong-Ryu Joo, In-Kyun Lim and Seung-Hoi Kim
Department of Chemistry, Dankook University 119 Dandaero Cheonan Republic of Korea 31116
ARTICLE INFO
ABSTRACT
Article history:
Received
Received in revised form
Accepted
An efficient synthetic route to benzyl phenyl ether preparation has been successfully developed
via a one-pot synthetic protocol utilizing a combination of arylboronic acids, hydrogen peroxide
(H2O2), and benzyl halides. The whole procedure consists of two consecutive reactions,
formation of boron phenoxide from arylboronic acids and its nucleophilic attack. A simple
operation under mild conditions such as room-temperature ionic liquid (choline hydroxide),
aerobic environment, and absence of metal- and base-catalysts has been employed. Expansion
to utilize benzyl surrogates was also successfully accomplished.
Available online
Keywords:
arylboronic acids
hydrogen peroxide
choline hydroxide
benzyl phenyl ether
one-pot synthesis
2020 Elsevier Ltd. All rights reserved.
 Corresponding author. Tel.: +82-41-550-3434; fax: +82-41-559-7860; e-mail: kimsemail@dankook.ac.kr

2
Functional group transformation and carbon-heteroatom
bond forming of organic compounds are considered as the
fundamentals in construction of numerous organic compounds.1
Accordingly, these strategies have been frequently utilized to
obtain a variety of functional materials in both academic and
industrial areas. Along with the facile functional group
transformations, one-pot synthesis has also gained considerable
and broad range of interests in synthetic organic chemistry owing
to the simplicity of their process compared with step-by-step
production. One-pot synthesis has special benefits, such as no
intermediate isolation, no change of reaction conditions, and no
other additives, among the others.
pioneering performance has been explored by Yang et al.
assigned as Route [C] in Scheme 1.9 In their study, utilizing
arylboronic acids and H2O2 in the presence of Pd-catalyst has
been performed for the preparation of various benzyl phenyl
ethers. Inspired by this outcome and our previous results,2 we
postulated that if the reaction media were able to keep the
intermediate alive, then it could act as a readily available
nucleophile for subsequent nucleophilic attack to benzyl
substrates, furnishing benzyl phenyl ethers in a one-pot system.
To verify the efficiency of the proposed strategy, the whole
process should consist of a simple catalyst-free, one-pot
operation in an aerobic environment.
Given the aspects of sustainable chemistry, bio- and eco-
friendly reaction platforms could be suitable approaches to
reducing the economic and environmental burdens of various
chemical processes. As part of our continuous endeavor of
searching for more versatile synthetic routes, 2 we were able to
accomplish oxidative hydroxylation of arylboronic acids to the
corresponding phenolic derivatives using a greener conditions:
such as combination of hydrogen peroxide and choline, a room-
temperature ionic liquid (RTIL).
Table 1. Optimization for one-pot synthesis of benzyl phenyl
ether (2a)
B(OH)2
1. 30% aq. H2O2/ChOH/rt
O
2. Benzyl bromide/rt/24 h
1a
2a
With the promising outcomes obtained from the
aforementioned our study, we focused intensively on expanding
the scope of the reaction system. One of the possibilities is to
1a
(mmol)
BnBr
ChOH
(mL)
Entry
H2O2 (mL)
Yield (%)
(mmol)b
conduct
a consecutive coupling reaction of the reaction
1
2
3
4
5
2.0
2.0
2.0
2.0
2.0
0.2
0.2
0.4
0.4
0.4
1.7
1.7
1.7
2.4
1.0
0.5
1.0
1.0
1.0
1.0
42
66
67
71
99
intermediate, supposedly a boron phenoxide (IA in Scheme 2),
with several suitable electrophiles. Among the electrophiles,
benzyl halides would be good candidates for benzyl phenyl ether
preparation under the base- and metal-catalyst-free conditions.
Furthermore, the whole process could be carried out in a one-pot
environment.
To date, as depicted in Scheme 1, several synthetic routes for
the preparation of benzyl phenyl ethers have been widely used.
a Isolated yield (based on benzyl bromide)
b BnBr: benzyl bromide
OH
Br
+
To elucidate the protocol, a model platform consisting of
phenylboronic acid (1a) and benzyl bromide was selected. To our
delight, formation of two carbon-oxygen bonds resulting in
ethereal linkage via a consecutive coupling reaction was
successfully achieved in satisfactory manner. Optimization of the
reaction conditions for achieving best yield of 2a was carried out,
and the results are described in Table 1.
Y
X
A 3
[
]
Various conditions/Base
C 9
B(OH)2
+
B 8
BF3K
+
[
]
[
]
Br
OH
O
Y
X
X
Y
Y
Pd(OAc)2/H2O2/NaOH/80 o
C
X
Cu(OAc)2/DMAP/4A MS/rt
ChOH/H2O2/rt
This study
The first attempt was carried out with 2.0 mmol of
phenylboronic acid (1a) with H2O2 (0.20 mL) in choline hydroxide
(0.50 mL) at room temperature (Table 1, entry 1). After the
mixture of 1a and H2O2 was stirred at room temperature for 5 min,
benzyl bromide (1.7 mmol) was subsequently added into the
reaction mixture. The GC–MS analysis of the reaction aliquot
showed the formation of the corresponding product 2a (Figure 1).
Building on the initial result, the subsequent experiment was
focused on the effect of the amount of the solvent (ChOH). A
moderately improved yield of 2a was obtained using 1.0 mL of
ChOH (Table 1, Entry 2). Under the same conditions, increment of
the amount of the H2O2 also gave 2a in a similar manner (Table 1,
Entry 3). No significant improvement (71%) was observed using
increased amount of benzyl bromide up to 2.4 mmol (Table 1,
Entry 4). In contrast, when the reaction was performed with 0.5
equivalent of benzyl bromide (1.0 mmol), outstanding outcome
(99%) was achieved (Table 1, Entry 5).10
B(OH)2
X
+
X
Y
Scheme 1. Representative approaches for benzyl phenyl ether
preparation
Phenol benzylation using benzylating reagents, mainly benzyl
bromide, under the various conditions is one of the readily
available pathways that is assigned as Route [A] in Scheme 1.3
Since the first report by McKillop,4 although some drawbacks limit
the application of this approach, it has been frequently used in
benzyl phenyl ether preparation. For most cases, the presence of
an appropriate base was mandatory for the successful carbon-
oxygen bond formation. In a related development, several
improved conditions such as use of deep eutectic solvent,5 phase
transfer catalyst,6 and mild base,7 have been recently revealed. In
addition to this process, alternative interesting approaches
utilizing organoboron derivatives have been explored. As shown
in Scheme 1 Route [B], Batey et al. reported the copper-catalyzed
alcohol etherification with potassium aryltrifluoroborates in the
presence of 4Å molecular sieves.8 More recently, a very

3
phenyl ether preparation. As the optimum conditions were
established in Table 1, we further examined the utility of the new
protocol using the various arylboronic acids and benzyl bromides.
The results are presented in Table 2.
Equivalent to IA
Benzyl bromide
Product (2a)
Table 3. Reaction with benzyl chlorides
Arylboronic
acid
Benzyl
chloride
Yield
(%)
Entry
Product
F
B(OH)2
B(OH)2
Cl
Cl
2l
(91%)
O
1
F
Cl
O
Cl
Cl
2m
(42%)
2
3
Cl
F
F
B(OH)2
Figure 1. Reaction progress monitored by GC
2n
(51%)
O
Cl
H3C
Cl
H3C
Cl
Cl
Consequently, this condition was regarded as a standard
condition throughout the present study.
F
F
B(OH)2
CHO
2o
(50%)
O
Cl
4
Cl
CHO
Table 2. Benzylation using benzyl bromides
a Conditions: 2.0 mmol of boronic acid, 1.0 mmol of benzyl chloride, 0.4 mL
of H2O2, 1.0 mL of ChOH at room temperature
Br
B(OH)2
Y
H2O2
O
b Isolated yield based on benzyl chloride
Y
ChOH
rt
(1.0 eq)
X
X
2a 2m
-
(2.0 eq)
From the reactions with arylboronic acids bearing functional
groups, the ethereal linkage formation was also successful,
resulting in the moderate to high yields of the corresponding
ethereal compounds (2a – 2i). Notably, the present system
displayed a considerable tolerance toward carbonyl and bromine
functionalities (2j – 2m). The expected product formation using
naphthalene-2-boronic acid also occurred. However,
unfortunately, the obtained product was not analytically pure
owing to several inseparable by-products.
O
O
CH3
O
2a (99%)a
2b (64%)
2c (79%)
Br
O
H3C
O
O
H3C
For better understanding of the reactivity of our approach, we
further investigated the substrate scope of the benzylation using
benzyl chlorides, and the results are described in Table 3. To
address this, 4-fluorobenzyl chloride was selected as a substrate
and subjected to react with 1a under the one-pot preparation
conditions used before. Significantly, the corresponding 4-
fluorobenzyl phenyl ether (2n) was obtained in an excellent yield.
Slightly disappointing results (2o – 2q) were observed using
sterically demanding benzyl chlorides in which it could be
attributed to a steric effect.
2d (73%)
2e (96%)
2f (69%)
CH3
CH3
OCH3
O
O
O
H3C
2g (76%)
2h (58%)
2i (84%)
CHO
Taking into account the reactivity obtained thus far, C–O bond
formation of IA using somewhat different substrates in place of
the benzyl halides would be a more challenging subject. To
elucidate the diversity, we have now undertaken substitution
O
OHC
O
O
H3C
O
2j (70%)
2k (87%)
2l (71%)
reaction
with
benzyl
tosylate,
ally
bromide,
2-
bromoacetophenone, and quinoxaline as depicted in Scheme 1.
Unfortunately, employing benzyl tosylate led to 2a with severely
diminished yield (17%) along with a mixture of unidentified
products. It is of interest that both allyl bromide and 2-
bromoacetophenone turned out to be the suitable substrates
yielding the expected products (2r and 2s, respectively) in
excellent yield.
Br
O
2m (70%)
a Isolated yield based on benzyl bromide.
Based on these observations, we could become assured that
the combination of arylboronic acids, H2O2, and benzyl bromides
in ChOH would be an alternative efficient route for the benzyl
Table 4. Coupling reaction of IA with benzyl surrogates

4
Entry
1
Surrogate
OTs
Product
Yield(%)a
An ester moiety is a characteristic subunit in a large number
in natural and synthetic products showing a wide range of
biological activities, which is utilized as a building block in the
synthesis of fine organic compounds. Owing to its versatility, a
variety of synthetic methodologies have been reported in which
both transesterification and acylation of alcohols have been
considered the most accessible protocols. To accomplish these
strategies, acid- or base-catalyzed reaction of alcohols with
carboxylic acids, anhydrides or acid chlorides has been frequently
employed.11 In addition, a metal complex-mediated acylation of
alcohols has also been frequently utilized, although some
disadvantages such as harsh reaction conditions, toxicity, and
tedious workup are still presented.12
Considering of these aspects, we, next, focused on the
preparation of ester derivatives utilizing the same strategy used
above by simply changing the electrophile to acid chlorides under
similar conditions. As shown in Table 5, a wide range of acid
chlorides were employed, providing the desired esters in
moderate to satisfactory manners. Not only aryl acid chlorides
but also cyclohexanecarbonyl chloride was suitable electrophiles
to produce the corresponding products (3a – 3g). Moreover,
heteroaryl acid chlorides were also selected as coupling
substrates, yielding 3h and 3i with moderate yields.
O
2a (17%)
O
O
O
Br
2
2p (88%)
O
3
2q (95%)
2r (76%)
Br
N
N
OPh
OPh
Br
Br
4b
N
N
Cl
OPh
5c
2s (23%)
N
N
N
N
Cl
N
Cl
PhO
N
OPh
a Isolated yield based on benzyl surrogate
b 0.5 eq. of quinoxaline employed
c 0.3 eq. of TCT employed
OH
HO
O
H
B
O
Quinoxaline possessing sterically demanding reaction sites
was employed in our novel protocol under the standard
conditions used above. Spectroscopic analyses indicated that
disubstituted product (2t) was obtained as major one. To
demonstrate the practicality and robustness of this approach, we
applied it toward the reaction with 2,4,6-trichloro-1,3,5-triazine
(TCT) which is an efficient reagent for further transformation in
organic synthesis. Under the conditions presented in Table 4, the
whole procedure underwent smoothly, and product (2u) was
exclusively obtained with 23% isolated yield.
H
ChOH
OH
ChOH
HO
OH
HO
O
H
B
B
O
H
H
O
O
H
-H2O
OB(OH)2
Benzyl halides
Benzyl phenyl
ethers
Acid chlorides
Esters
Table 5. Coupling reaction with acid chlorides: Preparation of
IA
esters
Scheme 2. Plausible mechanism for the present protocol
B(OH)2
1. 30% aq. H2O2/ChOH/rt
Esters
Although there is a paucity of crucial study of reaction
mechanism, consecutive reactions, formation of IA via ipso-
hydroxylation of arylboronic acids followed by its nucleophilic
reaction, could be attributed to the formation of two C– O bonds
of the final products. One-pot system containing a basic RTIL
(ChOH) was sufficient to provide a basic environment, leading to
an enhanced nucleophilicity of IA. The formation of intermediate
(IA) was indirectly confirmed by the isolation of phenol which is
the equivalent of IA.13 Based on these assumptions and
X
2. Acid chlorides/rt/24 h
(1.0 eq)
3a 3i)
(
-
(2.0 eq)
Cl
O
O
O
O
O
O
3a (97%)a
3b (97%)
3c (92%)
observations mentioned above, we proposed
a following
Br
mechanistic pathway for the present protocol (Scheme 2). The
initial step involves the activation of H2O2 by ChOH via hydrogen
bonding, followed by attack on boronic acid. Migration of phenyl
group generates boron phenoxide (IA), which then undergoes
coupling reaction to afford the corresponding product.
O
O
O
O
O
OMe
O
3d (90%)
3e (90%)
3f (50%)
In conclusion, we have developed a simple and green protocol
for producing benzyl phenyl ethers and ester derivatives, which
were accomplished using two C– O bond formation in a one-pot
system under mild reaction conditions.14 The combination of eco-
friendly oxidant (H2O2), sustainable reaction media (choline
hydroxide), and readily available reaction substrates was
efficiently cooperated in an aerobic environment. In addition, the
use of base- and metal-catalyst-free conditions should be
highlighted. Further investigations on the scope and limitations
S
O
O
OMe
O
O
O
O
O
3g (87%)
3h (79%)
3i (44%)
a Numbers in parenthesis are isolated yield (based on acid chlorides)

5
including related applications are currently undertaken in our
laboratory.
Synthesis 2011, 1621; (f) Yadav, P.; Lagarkha, R.;
Zahoor, A. Asian J. Chem. 2010, 22, 5155.
13. Under the standard conditions, the reaction mixture
using 1a was quenched with 3 M HCl solution before the
addition of benzyl bromide. After an appropriate work-
up and purification, we were able to isolate the product,
which is identified as phenol by the spectroscopic
analyses (1H & 13C NMR and FT-IR).
References and notes
1. Katritzky, A.; Taylor, R. Comprehensive Organic
Functional Group Transformations II, 2nd Ed. 2005,
Elsevier Ltd.
2. (a) Shin, E. -J.; Kwon, G. -T.; Kim, S. -H. Synlett 2019, 30,
1815; (b) Shin, E. -J.; Kim, H. -S.; Joo, S. -R.; Shin, U. S.;
Kim, S.-H. Cat. Lett. 2019, 149, 1560; (c) Joo, S. -R.; Kwon,
G. -T.; Park, S. -Y.; Kim, S. -H. Bull. Korean Chem. Soc.
2019, 40, 465; (d) Shin, E. -J.; Joo, S. -R.; Kim, S. -H.
Tetrahedron Lett. 2019, 60, 1509.
3. Selected recent examples, see; (a) Lourenco, N. M. T.;
Afonso, C. A. M. Tetrahedron 2003, 59, 789; (b) Godfrey,
J. D.; Mueller, R. H.; Sedergran, T. C.; Soundararajan, N.;
Colandrea, V. J. Tetrahedron Lett. 1994, 35, 6405; (c)
Zhang, M.; Flynn, D. L.; Hanson, P. R. J. Org. Chem. 2007,
72, 3194; (d) Shah, S. T. A.; Khan, K. M.; Hussain, H.;
Anwar, M. U.; Fecker, M.; Voelter, W. Tetrahedron 2005,
61, 6652; (e) Shah, S. T. A.; Khan, K. M.; Heinrich, A. M.;
Choudhary, M. I.; Voelter, W. Tetrahedron Lett. 2002, 43,
8603; (f) Brown Ripin, D. H.; Vetelino, M. Synlett 2003,
2353; (g) Bu, X. L.; Jing, H. W.; Wang, L.; Chang, T.; Jin, L.
L.; Liang, Y. M. J. Mol. Catal. A: Chem. 2006, 259, 121; (h)
Gathirwa, J. W.; Maki, T. Tetrahedron 2012, 68, 370.
4. McKillop, A.; Fiaud, J. C.; Hug, R. P. Tetrahedron 1974, 30,
1379.
14. Representative one-pot procedure; A flask was charged
with phenylboronic acid (4.0 mmol), choline hydroxide
(aq. 40-50 wt%, 2.0 mL), and H2O2 (aq. 30 wt%, 0.8 mL).
Then, the mixture was stirred at room temperature in
open air for 1 h. Next, 0.34 g of benzyl bromide (2.0
mmol) was added into the flask at room temperature,
then the resulting mixture was allowed to stir at room
temperature for 2 h. Quenched with 3 M HCl solution,
then extracted with extracted with diethyl ether (3 X 10
mL). The combined organic layers were washed with
brine, dried with anhydrous Na2SO4, and the volatile
solvent was evaporated under reduced pressure. The
crude mixture was purified by column chromatography
on silica gel (hexanes only). Benzyl phenyl ether (2a);
colorless oily liquid. 1H NMR (400 MHz, CDCl3): δ (ppm)
7.48 – 7.40 (m, 4H), 7.37 – 7.28 (m, 3H), 7.03 – 6.97 (m,
3H), 5.10 (s, 2H); 13C NMR (100 MHz, CDCl3): δ (ppm)
158.8, 137.1, 129.5, 128.6, 127.9, 127.5, 120.9, 114.9,
69.9.
Supplementary Material
5. Singh, A. S.; Shendage, S. S.; Nagarkar, J. M. Tetrahedron
Lett. 2014, 55, 7243.
Supplementary material that may be helpful in the review
process should be prepared and provided as a separate electronic
file. That file can then be transformed into PDF format and
submitted along with the manuscript and graphic files to the
appropriate editorial office.
6. (a) Wang, H.; Ma, Y.; Tian, H.; Yu, A.; Chang, J.; Wu, Y.
Tetrahedron 2014, 70, 2669; (b) Srivastava, P.;
Srivastava, R. Tetrahedron Lett. 2007, 48, 4489; (c)
Benaglia, M.; Cinquini, M.; Cozzi, F.; Tocco, G.
Tetrahedron Lett. 2002, 43, 3391; (d) Denmark, S. D.;
Weintraub, R. C.; Gould, N. D. J. Am. Chem. Soc. 2012,
134, 13415.
7. Gathirwa, J. W.; Maki, T. Tetrahedron 2012, 68, 370.
8. Quach, T. D.; Batey, R. A. Org. Lett. 2003, 5, 1381
9. Chen, Q. -Q.; Zhang, H. -X.; Yang, J. -X. Catal. Commun.
2019, 119, 115.
Click here to remove instruction text...
10. When the reaction was carried out with 1.7 mmol of
benzyl bromide, formation of unidentified products was
increasingly observed in a prolonged reaction time
(after 24 h). It could be inferred that the whole amounts
of intermediate (IA) were not fully participated in the
following substitution reaction.
11. For selected recent reports, see: (a) Prajapti, S. K.;
Nagarsenkar, A.; Babu, B. N. Tetrahedron Lett. 2014, 55,
910; (b) Liu, Z.; Ma, Q.; Liu, Y.; Wang, Q. Org. Lett. 2014,
16, 236; (c) Lu, N.; Chang, W.-H.; Tu, W.-H.; Li, C.-K.
Chem. Commun. 2011, 47, 727; (d) Vuluga, D.; Legros, J.;
Crousse, B.; Bonnet-Delpon, D. Chem. Eur. J. 2010, 16,
1776.
12. For selected recent reports, see: (a) Kumar, U. N.; Reddy,
B. S.; Reddy, V. P.; Bandichhor, R. Tetrahedron Lett. 2014,
55, 910; (b) Baldwin, J. N.; Nord, N. A.; O’Donnell, D. B.;
Mohan, S. R. Tetrahedron Lett. 2012, 53, 6946; (c) Taylor,
E. J.; Williams, M. J. J.; Bull, D. S. Tetrahedron Lett. 2012,
53, 4074; (d) Zarei, A.; Hajipour, A. R.; Khazdooz, L. Synth.
Commun. 2011, 41, 1772; (e) Das, R.; Chakraborty, D.

6
Graphical Abstract
To create your abstract, type over the instructions in the template box below.
Fonts or abstract dimensions should not be changed or altered.
An alternative route for boron phenoxide preparation from
arylboronic acid and its application for C – O bond formation
Leave this area blank for abstract info.
Seong-Ryu Joo, In-Kyun Lim and Seung-Hoi Kim
B(OH)2
O
O
E+
O2
H2
B(OH)2
E+
E
R
R
R
rt, air
27 examples
(up to 99%)
: benzyl halides
acid chlorides
R = H, alkyl, carbonyl
Choline
hydroxide
◆ one-pot synthesis ◆ green solvent ◆ base-free ◆ open air condition

7
Declaration of interests
☒ The authors declare that they have no known competing financial interests or personal relationships
that could have appeared to influence the work reported in this paper.
☐The authors declare the following financial interests/personal relationships which may be considered
as potential competing interests: