Iron(III)-catalyzed direct synthesis of diphenylmethyl esters from 2-diphenylmethoxypyridine

Article abstract of DOI:10.1080/00397911.2019.1625063

A highly efficient method has been developed for the synthesis of diphenylmethyl (DPM) esters from 2-diphenylmethoxypyridine. Various carboxylic acids readily reacted with 2-diphenylmethoxypyridine in the presence of FeCl₃ as a catalyst to provide the desired DPM esters with high yields. The procedure is facile and enables effective synthesis of a variety of esters for the protection of carboxylic acids.

Full text of DOI:10.1080/00397911.2019.1625063

Synthetic Communications
An International Journal for Rapid Communication of Synthetic Organic
Chemistry
ISSN: 0039-7911 (Print) 1532-2432 (Online) Journal homepage: https://www.tandfonline.com/loi/lsyc20
Iron(III)-catalyzed direct synthesis
of diphenylmethyl esters from 2-
diphenylmethoxypyridine
Van Hieu Tran, Minh Thanh La & Hee-Kwon Kim
To cite this article: Van Hieu Tran, Minh Thanh La & Hee-Kwon Kim (2019): Iron(III)-
catalyzed direct synthesis of diphenylmethyl esters from 2-diphenylmethoxypyridine, Synthetic
Communications, DOI: 10.1080/00397911.2019.1625063
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https://doi.org/10.1080/00397911.2019.1625063
Iron(III)-catalyzed direct synthesis of diphenylmethyl esters
from 2-diphenylmethoxypyridine
Van Hieu Trana,b, Minh Thanh Laa,b, and Hee-Kwon Kima,b
aDepartment of Nuclear Medicine, Molecular Imaging and Therapeutic Medicine Research Center,
b
Chonbuk National University Medical School and Hospital, Jeonju 54907, Republic of Korea; Research
Institute of Clinical Medicine of Chonbuk, National University-Biomedical Research Institute of Chonbuk
National University Hospital, Jeonju 54907, Republic of Korea
ABSTRACT
ARTICLE HISTORY
Received 15 April 2019
A highly efficient method has been developed for the synthesis of
diphenylmethyl (DPM) esters from 2-diphenylmethoxypyridine.
Various carboxylic acids readily reacted with 2-diphenylmethoxypyri-
dine in the presence of FeCl3 as a catalyst to provide the desired
DPM esters with high yields. The procedure is facile and enables
effective synthesis of a variety of esters for the protection of carbox-
ylic acids.
KEYWORDS
Diphenylmethyl esters;
protection; catalysts;
iron(III) chloride
GRAPHICAL ABSTRACT
Introduction
Protection and deprotection are commonly used in multistep reactions. As the total syn-
thesis of natural products or bioactive compounds possessing various reactive functional
groups has increased, many types of protecting groups have been developed. The diphe-
nylmethyl (DPM) group, first employed for thiol protection,[1] is one of the most com-
mon protecting motifs for carboxylic acids because its deprotection can be easily
performed under mild conditions. For example, the DPM group can be cleaved from
DPM esters by means of acidic treatment or hydrogenation with Pd.[2–8] Due to flexibil-
ity in the cleavage of the DPM group, many procedures have utilized DPM esters as
intermediates in multistep syntheses of biological compounds, including leukotriene
antagonists, adenosine receptor antagonists, and antibiotics.[9–11]
CONTACT Hee-Kwon Kim
hkkim717@jbnu.ac.kr
Department of Nuclear Medicine, Molecular Imaging and
Therapeutic Medicine Research Center, Chonbuk National University Medical School and Hospital, Jeonju 54907,
Republic of Korea; Research Institute of Clinical Medicine of Chonbuk, National University-Biomedical Research Institute
of Chonbuk National University Hospital, Jeonju 54907, Republic of Korea
Supplemental data for this article can be accessed on the publisher’s website.
ß 2019 Taylor & Francis Group, LLC

2
V. H. TRAN ET AL.
Previous work
Ph
Ph
O
O
Ph
O
TsOH
H
+
O
O
OH
OH
Ph
Ph
Ph
O
Ph
Ph
Ph
Ph
Ph
O
Oxone/Al2O3, I2
NH2
+
N
Ph
This work
O
Ph
O
Ph
3 mol% FeCl3
+
OH
O
Ph
Ph
Ph
Ph
O
N
Scheme 1. Synthesis of diphenylmethyl esters.
Traditionally, insertion of a DPM group into carboxylic acid to produce the corre-
sponding ester has been carried out by reactions with benzhydrol in the presence of acid
(Scheme 1).[12] Such reactions have employed harsh conditions such as the use of strong
acids or large amounts of acid, which may affect several sensitive functional groups. Tri-
diphenylmethyl phosphate, diphenylmethyl diphenyl phosphate, and diphenyldiazo
methane have also been used as mild alkylating agents for the protection of carboxylic
acids.[13–15] However, the preparation of alkyl phosphates is costly and diazo is a safety
risk, specifically having a risk of explosion and being difficult to handle. Instead, benzo-
phenone hydrazone has been utilized as a diazo precursor for the DPM protection of
esters. One-pot two-step reactions using benzophenone hydrazine have been used to pro-
tect carboxylic acids with oxidative agents such as I2, PhI(OAc)2, oxone, peracetic acid,
NaClO with TEMPO catalyst, oxalyl chloride with DMSO, or MnO2.[16–22]
In additions, diphenylmethyl esters were prepared from diphenylmethyl
trichloroacetimidate.[23]
2-O-substituted pyridines have recently been noticed for the preparation of
esters.[24,25] Dudley and colleagues employed 2-benzyloxy-1-methylpyridinium salt as
a benzyl transfer agent for syntheses of benzyl esters.[24,25] However, the protocol
required two steps and one day of reaction time. In addition, reactions to synthesize
benzyl esters required high temperatures such as 83 ꢀC. From the 2-benzyloxy-1-
methylpyridine mediated reaction, we expected that this strategy could be extended
to other reactions, including the synthesis of DPM esters. We showed DPM esterifi-
cation using BF3ꢁOEt2 in the previous study.[26] In this study, we tried to find more
efficient DPM esterification using a novel metal catalyst. We hypothesized that the
combination of 2-diphenyloxy-1-methylpyridine as a starting material and a novel
catalyst could lead to highly efficient reaction conditions to form DPM esters. To
the best of our knowledge, there is no synthetic protocol using metal-based catalysts
to prepare DPM esters from 2-diphenylmethoxypyridine. Herein, we describe a novel
facile synthesis of DPM esters from 2-diphenylmethoxypyridine using a catalytic
amount of metal-based reagents.

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SYNTHETIC COMMUNICATIONSV
3
Ph
Ph
Ph
O
N
Ph
N
OH
O
OTf
1
OH
R
3
MeOTf
4
base
(ref. 24)
Cl
O
Ph
O
Ph
OH
R
, catalyst
4
O
Ph
R
Ph
O
N
5
2
Scheme 2. Novel synthetic strategy of diphenylmethyl esters.
Table 1. Catalyst screening for DPM esterification.a
Ph
O
Catalyst
Ph
O
+
OH
O
Ph
Ph
Ph
O
N
Ph
DCE, rt, 12h
5
2
4
Entry
Catalyst
Time
Temp.
Yieldb (%)
1
2
3
4
5
6
7
8
ZnCl2
BiCl3
12
12
12
12
12
12
12
12
r.t.
r.t.
r.t.
r.t.
r.t.
r.t.
r.t.
r.t.
NRc
NRc
2
2
3
CuCl2
AlMe3
MnCl
ZrCl4
FeCl3
None
3
94
NRc
aReaction conditions: compound 2 (1.5 mmol), carboxylic acid (1.0 mmol), catalyst (0.03 mmol), DCE (2 mL), 12 h.
bIsolated yield after purification via flash column chromatography.
cNo reaction.
Result and discussion
DPM esters were synthesized from diphenylmethyl alcohol in 2-step routes, as shown in
Scheme 2. In the first reaction, 2-diphenylmethoxypyridine (compound 2) was readily
obtained by treating diphenylmethyl alcohol with 2-chloropyridine in the presence of
potassium and 18-crown-6 according to our previously reported method.[26]
Recently we noticed that the previous method using MeOTf to produce 2-benzyloxy-
1-methylpyridinium salt, followed by carboxylic acid treatment, provided DPM esters in
low yields. We assume that a catalytic reagent such as a Lewis acid could be a better
choice in DPM esterification and etherification.
In the initial study, benzoic acid was chosen as a substrate for the reaction with 2-
diphenylmethoxypyridine. Esterification reactions of benzoic acid were carried out in
the presence of 1.5 equiv. of 2-diphenylmethoxypyridine, 1.0 equiv. of benzoic acid, and
0.03 equiv. of various Lewis acids at room temperature for 12 h, and the reaction yields
of the corresponding esters were examined.
The screening study indicated that reactions with Lewis acids such as ZnCl2 and
BiCl3 could not produce the target product (Table 1), and that a series of Lewis acids,
including CuCl2, AlMe3, MnCl2, and ZrCl4, could produce only small amounts of the

4
V. H. TRAN ET AL.
Table 2. Solvent screening for in situ esterification.a
Ph
O
O
Ph
FeCl3
+
OH
O
Ph
Ph
Ph
Ph
O
N
Solvent, rt, 12h
4
5
2
Entry
Catalyst (equiv.)
Solvent
Temp.
Yieldb (%)
1
2
3
4
5
6
7
8
FeCl3 (0.03)
FeCl3 (0.03)
FeCl3 (0.03)
FeCl3 (0.03)
FeCl3 (0.03)
FeCl3 (0.03)
FeCl3 (0.2)
FeCl3 (0.1)
FeCl3 (0.01)
FeCl3 (0.03)
FeCl3 (0.03)
MeCN
THF
Toluene
1,4-dioxane
CH2Cl2
DCE
r.t.
r.t
r.t.
r.t.
r.t.
r.t.
r.t.
r.t.
26
22
61
NRc
73
94
95
94
74
95
95
DCE
DCE
DCE
DCE
9
r.t.
10d)
11d)
50 ꢀC
70 ꢀC
DCE
aReaction conditions: compound 2 (1.5 mmol), carboxylic acid (1.0 mmol), solvent (2 mL), 12 h.
bIsolated yield after purification via flash column chromatography.cNo reaction.
cReaction was conducted for 4 h.
desired DPM esters: most reaction yields were less than 10%. Finally, FeCl3 was tested
for the synthesis of DPM esters; the reaction in the presence of FeCl3 yielded the
desired esters with significantly enhanced yield (94%). This result means that FeCl3
could be used as an effective catalytic reagent for DPM esterification.
Next, the effect of the solvent upon reaction was explored to determine the optimal
reaction conditions. As shown in Table 2, reactions using FeCl3 in THF, MeCN, and
1,4-dioxane provided poor yields in the production of DPM ester. Using toluene as the
reaction solvent increased the transformation yield of carboxylic acid to the target prod-
uct to 61%, but many unreacted starting materials were still observed. The chlorinated
solvents including dichloromethane (DCM) and dichloroethane (DCE) were also tested
for DPM esterification and fortunately gave significantly enhanced reaction yields: the
target DPM ester was obtained in 73% yield from reaction in DCM and in 94% yield in
DCE. Thus, DCE was chosen as the most effective solvent for further study of FeCl3-
catalyzed DPM esterification reactions.
In addition, a series of amounts of FeCl3 (0.2 equiv., 0.1 equiv., 0.03 equiv., and 0.01
equiv.) was tested for DPM esterification. As listed in Table 2, the synthetic yield for
the desired ester increased with an increasing amount of FeCl3 to 0.03 equiv. (94% at
room temperature in DCE), though further addition of FeCl3 did not improve the reac-
tion yield. These results suggest that efficient preparation of DPM ester compounds can
be achieved using 0.03 equiv. (3 mol%) of FeCl3 catalyst.
Temperature effect on DPM esterification was also investigated. The target product
was prepared with yields of 95% at 50 ꢀC and 70 ꢀC for 4 h. But, we selected room tem-
perature for future study because mild reaction condition is more suitable.
Utilizing these optimized reaction conditions, the scope of DPM ester synthesis was
explored. These reactions of carboxylic acids with 2-diphenylmethoxypyridine were car-
ried out using 3 mol% of FeCl3 in DCE at room temperature for 12 h. The synthesis of
DPM esters was demonstrated to be tolerant to various carboxylic acid starting

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SYNTHETIC COMMUNICATIONSV
5
Table 3. Scope of DPM esterification using 2-diphenylmethoxypyridine.a
Ph
Ph
O
N
2
Ph
O
O
3 mol% FeCl3
DCE, rt, 12h
O
Ph
O
R
OH
R
4
5
O
Ph
O
Ph
CH3
Ph
O
Ph
O
Ph
O
Ph
H3CO
5a, 94%
5b, 93%
5c, 92%
O
O
Ph
O
O
Ph
O
O
Ph
Cl
Ph
Ph
Ph
O2N
Cl
5e, 92%
5f, 84%
5d, 75%
O
Ph
O
Ph
O
Ph
O
Ph
O
Ph
O
Ph
5g, 92%
5i, 93%
5h, 84%
O
O
Ph
O
Ph
O
O
Ph
Ph
O
Ph
Ph
Ph
5k, 93%
5j, 95%
5l, 98%
O
O
Ph
O
Ph
O
Ph
O
Ph
O
Ph
5o, 93%
5m, 97%
5n, 96%
O
Ph
O
Ph
O
Ph
O
O
Ph
O
Ph
O
Ph
5q, 94%
5r, 93%
5p, 95%
O
Ph
N
O
Ph
5s, 83%
aReaction conditions: carboxylic acid (1.0 mmol), compound
(0.03 mmol), DCE (2 mL), r.t. for 12 h.
2 (1.5 mmol), FeCl3

6
V. H. TRAN ET AL.
Ph
O
Ph
O
N
Ph
O
2
Ph
OH
FeCl3 (3 mol%)
DCE, r.t., 12h
O
Ph
Ph
5a
4a
100.0 mmol, 12.21 g
26.22 g, 91%
Scheme 3. Gram-scale reaction of benzoic acid (4a) with 2-diphenylmethoxypyridine (2).
materials, as listed in Table 3. Several substituted aromatic carboxylic acids with differ-
ent electronic effects were tested to synthesize DPM esters. Reactions of benzoic acids
with electron-donating groups (methoxy and methyl) and electron-withdrawing sub-
stituents (nitro and chloro) resulted in successful conversions of carboxylic acids to the
desired DPM esters in the range from 75% to 93% yield (5b–5f). This result suggests
that the synthetic yields of DPM esterification using FeCl3 were not significantly influ-
enced by the differences in electronic effects arising from these substituents of ben-
zoic acids.
Alkyl-substituted carboxylic acids were also used to prepare DPM esters. The esterifi-
cation of alkyl-substituted carboxylic acids afforded the desired products in high yields
(5g–5k). Particularly, pivalic acid which has tert-butyl group, and cyclohexane carbox-
ylic acid which is a cyclic substrate also reacted readily with 2-diphenylmethoxypyridine
to provide the corresponding esters in yields of 93% and 95%, respectively.
Alkenyl- and alkynyl-substituted carboxylic acids, which are unsaturated carboxylic
acids with reactive p bonds, were also employed in DPM esterification to assess the
scope of this method. These substrates were transformed into the desired esters in excel-
lent yields without the formation of side products (5l and 5m).
The scope of this synthetic protocol was extended to carboxylic acids with more
bulky groups that could have steric effects on the reactions. Treatment of 3,5-dimethyl-
benzoic acid and 2,4,6-trimethylbenzoic acid in the presence of catalytic FeCl3 led to
efficient synthesis of the desired product with yields of 96% and 93%, respectively (5n
and 5o). Furthermore, diphenylacetic acid, which has two phenyl groups and thus
potentially a large steric effect, provided the target product 5p in 95% yield under the
same conditions.
In addition, FeCl3 was used with heterocyclic acid compounds to examine whether it
also catalyzed these compounds. In these experiments, 2-furoic acid bearing oxygen,
and 2-picolinic acid bearing nitrogen were treated with 2-diphenylmethoxypyridine,
providing the corresponding DPM esters in yields of 93% and 83%, respectively (5r
and 5s). This indicated that FeCl3 is an effective catalytic reagent for the successful syn-
thesis of DPM esters from heterocyclic carboxylic acids under mild conditions.
Next, a scale-up of DPM esterification was conducted (Scheme 3). The DPM ester
was successfully prepared in a gram-scale reaction of benzoic acid 4a (100.0 mmol,
12.21 g) with 2-diphenylmethoxypyridine 2; the target ester product 5a was produced in
91% yield via the optimized reaction conditions. This reaction was thus demonstrated
to be effective and scalable.

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SYNTHETIC COMMUNICATIONSV
7
Conclusion
In conclusion, a novel reaction method was developed for practical DPM esterification
of various carboxylic acids. Novel synthetic methods of DPM esters are important to
develop protecting procedures for organic chemistry and to prepare bioactive com-
pounds. This study featured FeCl3-catalyzed synthesis of carboxylic acids with 2-diphe-
nylmethoxypyridine to produce DPM esters with high yield. The novel procedures are
highly efficient in yielding DPM esters, and thus have potential application to the prep-
aration of bioactive compounds such as drugs.
Experimental
Reagents and solvents were commercially available and used as received. Column chro-
matography was performed using silica gel 60 (0.040–0.063 mm) and eluted with proper
1
mixture (DCM/hexane). All of the new compounds were identified by 400 MHz H and
100 MHz 13C NMR spectra in deuterated chloroform (CDCl3) with tetramethylsilane
(TMS) as an internal reference, and high resolution mass spectroscopy (Supplementary
Data). The identity of the known compounds was established by the comparison of
1
their H and 13C NMR peaks with the authentic values.
General procedure for the preparation of DPM ester compounds (5a–5s)
To a solution of benzoic acid (0.122 g. 1.00 mmol) and 2-diphenylmethoxypyridine
(0.39 g, 1.50 mmol) in DCE (2 mL) FeCl3 (0.0048 g. 0.03 mmol) was added. The mixture
was stirred at room temperature for 12 h. The reaction mixture was extracted with ethyl
acetate (2 ꢂ 10 mL), and then washed with water (10 mL), followed by brine (10 mL).
The organic layer was dried over anhydrous sodium sulfate and concentrated under
reduced pressure. The resulting residue was then purified using flash column chroma-
tography on silica gel with hexane-CH2Cl2 as eluent to afford the desired product 5a as
a white solid (0.271 g, 94%).
Selected spectroscopic data
Benzydryl benzoate (5a)
White solid (94% yield); m.p. 90–92 ꢀC; 1H NMR (600 MHz, CDCl3) d 8.15 (d,
J ¼ 6.0 Hz, 2H), 7.57 (td, J ¼ 6.5 Hz, J ¼ 1.5 Hz, 1H), 7.50–7.44 (m, 6H), 7.36 (t,
J ¼ 6.0 Hz, 4H), 7.29 (t, J ¼ 6.0 Hz, 2H), 7.13 (s, 1H); 13C NMR (150 MHz, CDCl3) d
165.7, 140.4, 133.3, 130.3, 129.9, 128.7, 128.6 128.1 127.2, 77.5; HRMS (ESI) m/z
(M þ H)þ calcd for C20H17O2 ¼ 289.1229, found 289.1227.
Benzhydryl 4-chlorobenzoate (5d)
1
White solid (75%); m.p. 90–92 ꢀC; H NMR (400 MHz, CDCl3) d 8.10 (dt, J ¼ 8.8 Hz,
J ¼ 2,4 Hz, 2H), 8.48–7.31 (m, 12H), 7.14 (s, 1H); 13C NMR (100 MHz, CDCl3) d

8
V. H. TRAN ET AL.
164.74, 140.02, 139.63, 131.17 (2C), 128.81 (2C),128.61(4C), 128.18 (2C), 127.12 (4C);
HRMS (ESI) m/z (M þ H)þ calcd for C20H16ClO2 ¼ 323.0839, found 323.0840.
Benzhydryl heptanoate (5g)
1
Colorless oil (92%); H NMR (400 MHz, CDCl3) d 7.38–7.31 (m, 10H), 6.93 (s, 1H),
2.47 (t, J ¼ 7.6 Hz, 2H), 1.69 (m, 2H), 1.30 (m, 6H), 0.91 (m, 3H); 13C NMR (100 MHz,
CDCl3) d 172.81, 140.38 (2C), 128.48 (4C), 127.84 (2C), 127.09 (4C), 76.60, 34.61,
31.46, 28.79, 24.94, 22.49, 14.04; HRMS (ESI) m/z (M þ H)þ calcd for C20H25O2 ¼
297.1855, found 297.1856.
Benzhydryl 2-phenylacetate (5k)
1
White solid (93%); m.p. 45–47 ꢀC; H NMR (400 MHz, CDCl3) d 7.34–7.30 (m, 15H),
6.91 (s, 1H), 3.77 (s, 2H); 13C NMR (100 MHz, CDCl3) d 170.47, 140.10, 133.83, 129.40,
128.59, 128.48, 127.89, 127.15, 127.02. 77.26, 41.67; HRMS (ESI) m/z (M þ H)þ calcd
for C21H19O2 ¼ 303.1385, found 303.1387.
Benzhydryl furan-2-carboxylate (5r)
1
White solid (93%); m.p. 89–91 ꢀC; H NMR (400 MHz, CDCl3) d 7.63–7.62 (m, 1H),
7.46–7.32 (m, 11H), 7.14 (s, 1H), 6.55–6.54 (m, 1H); 13C NMR (100 MHz, CDCl3) d
157.75, 146.57 (2C), 144.64, 139.90, 128.56 (4C), 128.05 (2C), 127.21 (4C), 118.35,
111.89, 77.19; HRMS (ESI) m/z (M þ H)þ calcd for C18H15O3 ¼ 279.1021,
found 279.1024.
Disclosure statement
No potential conflict of interest was reported by the authors.
Funding
This research was supported by Basic Science Research Program through the National Research
Foundation of Korea (NRF) funded by the Ministry of Education [2018R1D1A1B07047572]. Tran
Van Hieu and La Minh Thanh were supported by the BK21 Plus program in the Chonbuk
National University Medical School.
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