Palladium-catalyzed carbonylative transformation of phenols via in-situ triflyl exchangement

Article abstract of DOI:10.1016/j.jcat.2020.06.034

Phenols are attractive starting materials due to their ready availability. Herein, we developed a novel method on palladium-catalyzed alkoxycarbonylation of phenols. By using commercially available Pd(OAc)₂ and PtBu₃·HBF₄ as the catalyst system and aryl triflates as triflyl source to activate the other phenol, various carboxylic acid esters were prepared in moderate to good yields via Tf exchange and then O-Tf bond cleavage. Notably, phenols generated from aryl triflates after Tf transfer or other additional aliphatic alcohols can all be employed as nucleophiles to synthesize the corresponding esters.

Full text of DOI:10.1016/j.jcat.2020.06.034

Journal Pre-proofs
Priority Communication
Palladium-Catalyzed Carbonylative Transformation of Phenols via In-Situ
Triflyl Exchangement
Hai Wang, Youcan Zhang, Chong-Liang Li, Xiao-Feng Wu
PII:
S0021-9517(20)30267-0
https://doi.org/10.1016/j.jcat.2020.06.034
YJCAT 13805
DOI:
Reference:
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Journal of Catalysis
Received Date:
Revised Date:
Accepted Date:
12 March 2020
24 June 2020
26 June 2020
Please cite this article as: H. Wang, Y. Zhang, C-L. Li, X-F. Wu, Palladium-Catalyzed Carbonylative
Transformation of Phenols via In-Situ Triflyl Exchangement, Journal of Catalysis (2020), doi: https://doi.org/
10.1016/j.jcat.2020.06.034
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Palladium-Catalyzed Carbonylative Transformation of Phenols via In-
Situ Triflyl Exchangement
Hai Wang, Youcan Zhang, Chong-Liang Li, and Xiao-Feng Wu*
Leibniz-Institut für Katalyse e.V. an der Universität Rostock Albert-Einstein-Straße 29a, 18059 Rostock (Germany), E-
mail: Xiao-Feng.Wu@catalysis.de
ABSTRACT: Phenols are attractive starting materials due to their ready availability. Herein, we developed a novel method on
palladium-catalyzed alkoxycarbonylation of phenols. By using commercially available Pd(OAc)2 and PtBu3·HBF4 as the
catalyst system and aryl triflates as triflyl source to activate the other phenol, various carboxylic acid esters were prepared
in moderate to good yields via Tf exchange and then O-Tf bond cleavage. Notably, phenols generated from aryl triflates after
Tf transfer or other additional aliphatic alcohols can all be employed as nucleophiles to synthesize the corresponding esters.
Keywords: aryl triflates • carbonylation • palladium catalyst • Tf transfer • carboxylic acid esters
Esters are present in a range of pharmaceuticals,
polymers, agrochemicals, natural products, and biological
systems and they have also been applied as versatile
building blocks in organic synthesis.[1] Due to their
important applications, many efficient methodologies have
been developed for the construction of carboxylic acid
esters, such as esterification of carboxylic acids or activated
analogues with alcohols.[2] On the other hand, carbonylation
is one of the most powerful and straightforward method for
the direct synthesis of carbonyl-containing compounds
from the point of view of atom and step economy. Since the
seminal work by Heck and co-workers in 1974,[3,4]
palladium-catalyzed carbonylation of aryl halides with
alcohols by using carbon monoxide (CO) as an inexpensive
and readily available C1 source has become an efficient and
straightforward method for the synthesis of carboxylic acid
esters.
phenols with trifluoromethanesulfonic anhydride (Tf2O) to
give aryl triflates (ArOTf) which are outstanding
electrophiles in various organic transformations.[6] In
comparation, aryl triflates are also good alternatives to aryl
halides in alkoxycarbonylation for the synthesize of esters
(Scheme 1a).[7] The advantages are obvious, including 1)
readily and wide availabilities of phenols; 2) aryl triflates
are stable and easy to storage; 3) aryl triflates are easy to be
activated in reactions and so on. However, the aryl triflates
used usually need to be pre-prepared from
trifluoromethanesulfonic anhydride (Tf2O) and phenols
which certainly increased the number of manipulation
steps and costs and also limited certain functional groups.
Herein, we developed a novel method for the palladium-
catalyzed alkoxycarbonylation of phenols to esters with in-
situ triflate exchanging as one of the key steps (Scheme
1b).[8] More specifically, triflate (Tf-) is transferred from aryl
triflates to other phenols to form a new aryl triflates, which
undergo palladium-catalyzed carbonylation to produce the
desired products. The features of this new procedure
including: 1) using commercially available Pd(OAc)2 and
PtBu3·HBF4 as a convenient catalyst system; 2) stable aryl
triflates severing as Tf sources to activate the other phenols
which avoid preparation steps; 3) phenols generated after
aryl triflates after Tf transfer or other additional added
alcohols can be employed as nucleophiles to synthesize the
corresponding carboxylic acid esters.
a)
Conventional alkoxycarbonylation of phenols to carboxylic acid esters using
Tf2O as activating reagent
C-OTf bond activation
O
OH
OTf
Tf2O
Base
CO
R'OH
R
R
OR'
R
b)
This work
gement
exchan
bond
O-Tf
O
O
ArOH
Tf
OAr
OR'
R
R
O
OTf
Ar
OH
We started our studies by using phenyl triflate (1a) and
4-(methylthio)phenol (2a) as the substrates, in the
presence of commercially available Pd(OAc)2 as the catalyst
and PtBu3·HBF4 as the ligand, the alkoxycarbonylation
reaction was carried out under 5 bar of CO using 2.0 equiv
of K2CO3 as the base in 1.5 mL of MeCN at 100°C for 24 h. To
CO
R
R
''Tf transfer''
C-OTf bond activation
R'OH
Scheme 1. Methods for alkoxycarbonylation of phenols.
our delight, the desired product
phenyl 4-
Phenols are widely present in numerous compounds.
Additionally, as an important industrial commodity,
phenols are produced on the scale of millions of tons per
year from fuel and biomass.[5] Concerning the synthetic
applications, one of the most popular pathway is reacting
(methylthio)benzoate 3a was formed in 24% yield (Table 1,
entry1). Meanwhile, byproducts Bp1, Bp2 and Bp3 were
produced as well and 20% yield of byproduct 4-
(methylthio)phenyl 4-(methylthio)benzoate Bp3 was
detected. As we expected, no desired product was

determined in the absence of palladium catalyst and
phosphine ligand (Table 1, entry 2). To further improve the
yield of the desired product 3a, a series of optimizations
were performed, and the results are summarized in Table 1.
Several bases (Na2CO3, NaHCO3, Cs2CO3, DBU and CsF) and
ligands (PPh3 and PCy3) were screened, no better yields
were obtained (Table 1, entries 3-6). A series of solvents
were also screened subsequently, no desired product was
detected when toluene, THF, 1,4-dioxane, DMSO or NMP
was used as the solvent (Table 1, entries 7-10). When
toluene or THT was employed as the solvent, phenyl triflate
1a is easier to be directly carbonylated with 4-
(methylthio)phenol 2a without undergoing Tf transfer
process and produced a large amount of byproduct 1 4-
(methylthio)phenyl benzoate (Table 1, entries 8 and 9). By
employing DMAc as the solvent, the yield of 3a was
improved slightly and no or traces of the other byproducts
were detected (Table 1, entry 11). The effects of the reaction
temperature and CO gas pressure were also tested (Table 1,
entries 12-15). By using DMAc as the solvent, 53% yield of
the target product was obtained when the reaction was
performed under 3 bar of CO at 90 °C (Table 1, entry 15). By
extending the reaction time, the yield of 3a can be further
improved. The reaction can give the best yield (68%) of 3a
and a 60% isolated yield when the reaction was carried out
for 48 hours (Table 1, entry 16). Furthermore, the loading
of palladium catalyst and ligand were studied as well, no
better yields were obtained. (Table 1, entries 17 and 18).
After systematic studies, the optimal conditions were
established as Pd(OAc)2 (10 mol %), PtBu3·HBF4 (20 mol
%), K2CO3 (2.0 equiv), CO (3 bar) in DMAc (1.5 mL) at 90 °C
for 48 h (Table 1, entry 16).
11
12
DMAc instead of MeCN
3a: 29%
DMAc instead of MeCN, 3 bar CO
3a: 40%;
Bp3: 12%
13
14
15
16
DMAc instead of MeCN, 1 bar CO
3a: 12%
3a: 35%
3a: 53%
DMAc instead of MeCN, 3 bar CO, 80 oC
DMAc instead of MeCN, 3 bar CO, 90 oC
DMAc instead of MeCN, 3 bar CO, 90 oC, 48 h
3a: 68%
(60%)
17
18
5 mol% of Pd(OAc)2, 10 mol% of PtBu3·HBF4,
DMAc instead of MeCN
3a: trace
3a: 40%
15 mol% of Pd(OAc)2, 30 mol% of PtBu3·HBF4,
DMAc instead of MeCN
[a] Reaction conditions: phenyl triflate 1a (0.3 mmol), 4-
(methylthio)phenol 2a (0.2 mmol), Pd(OAc)2 (10 mol %),
PtBu3·HBF4 (20 mol %), K2CO3 (0.4 mmol), CO (5 bar), MeCN
(1.5 mL), 100 °C, 24 h. Abbreviations used in this table: n.d. =
not detected, Byproduct = Bp, DMAc = N,N-Dimethylacetamide.
[b] As determined by gas chromatography (GC), using
hexadecane as the internal standard.
With the optimized reaction conditions in hand, the scope
of various aryl triflates 1 with different phenols 2 were
examined (Scheme 2). The para-position of aryl triflates
with electron-rich groups such as Me, Et, tBu, Ph can deliver
the corresponding products (3b-3e) in moderate to good
yields (53-83%). Aryl triflates containing electron-deficient
groups such as F, Cl at the para position can also afford
corresponding products (3f, 3g) in 47 and 65% yields,
respectively. The para-position of aryl triflates with CHO,
Ac, COOMe were also tested, but unfortunately no desired
product was detected. The possible reason may be that the
strong electron-deficient phenols generated from aryl
triflates after Tf transfer have low nucleophilic activity,
which is not conducive for the carbonylation process. Aryl
triflates with ortho/meta-substitution can be successfully
applied in this reaction as well and gave the corresponding
products in 58-65% yields (3h-3k). We also tested
disubstituted aryl triflates as the substrates and resulting
55-85% yields of the corresponding esters (3l-3o). Notably,
benzo[d][1,3]dioxol-5-yl trifluoromethanesulfonate was
also successfully transformed into the product 3p in 72 %
yield. Subsequently, the scope of phenols was considered, p-
methoxyphenol and naphthol can participate in the desired
alkoxycarbonylation reaction and 45%-75% yields of the
corresponding esters (3q-3w) were obtained. Some other
phenols such as phenol, p-cresol, p-chlorophenol were
tested as well, but unfortunately low yields of the desired
products were detected, and a large amount of byproducts
were produced. Hence, in this system, the Tf transfer step is
the most important step and driven by relatively basic
phenol derivative.
Table 1. Optimization of reaction conditions.[a]
O
OH
OTf
Pd(OAc)2 (10 mol%), PtBu3HBF4 (20 mol%)
+
O
K2CO3 (2.0 equiv), MeCN (1.5 mL)
CO (5 bar), 100 oC, 24 h
S
S
1a
2a
3a
S
S
O
O
S
O
O
O
O
Byproduct Bp1
Byproduct Bp2
Byproduct Bp3
Entry
1
Variations from the standard conditions
----
Yield[b] (%)
3a: 24%;
Bp3: 20%
2
3
4
5
6
7
8
without catalyst/ligand
3a: n.d
Na2CO3, NaHCO3 or Cs2CO3 instead of K2CO3
DBU instead of K2CO3
3a: trace
3a: 12%
3a: 22%
3a: trace
3a: 19%
CsF instead of K2CO3
PPh3 or PCy3 instead of PtBu3·HBF4
DMF instead of MeCN
Toluene instead of MeCN
3a: n.d;
Bp1: 36%
9
THF instead of MeCN
3a: n.d;
Bp1: 48%
10
1,4-Dioxane, DMSO or NMP instead of MeCN
3a: n.d

O
but unfortunately no desired product was detected besides
byproduct Bp5. It is worth mentioning that it is quite a
challenge to purify the desired product due to the similar
polarity of the desired product and byproduct Bp4 when
aryl triflates with R-substituted (R = Me, Et, tBu, Ph, OMe, Cl,
COOMe) were employed as the Tf source.
OH
OTf
Pd(OAc)2 (10 mol%), PtBu3HBF4 (20 mol%)
O
O
K2CO3 (2.0 equiv.), DMAc (1.5 mL)
CO (3 bar), 90 oC, 48 h
R
R'
R
R'
1
2
3
Et
F
O
O
O
O
O
S
S
S
S
3a
3d
3g
3b
3c
, 60%
,53%
, 61%
O
O
tBu
Ph
OH
OTf
Pd(OAc)2 (10 mol%), PtBu3HBF4 (20 mol%)
O
O
MeOH
4a
O
R
K2CO3 (2.0 equiv), DMAc (1.5 mL)
CO (3 bar), 110 oC, 48 h
O
O
O
S
S
1a
2a
5a
S
S
3e
3f
, 80%
O
, 83%
O
, 47%
O
O
Cl
O
R
O
O
O
O
S
S
iPr
S
S
S
Byproduct Bp4
Byproduct Bp5
3h
3i
3l
, 65%
O
, 58%
, 63%
O
O
OTf
OTf
OTf
OTf
OTf
OTf
O
O
Ph
O
Ph
Ph
Et
tBu
Ph
S
S
S
S
79 %
80 %
81%
OTf
73%
OTf
82%
3j
3k
, 62%
, 65%
Ph
, 64%
OTf
OTf
O
O
O
O
O
MeO
Br
Cl
O
45%
n.d
76%
78%
50%
70%
O
O
S
OTf
OTf
OTf
OTf
O
O
S
3m
3n
3q
3o
, 64%
, 55%
, 59%
TfO
O2N
NC
MeOOC
O
O
O
O
O
O
79%
82%
n.d
50 %
O
O
O
O
S
Scheme 3. Substrate scope of aryl triflate. Reaction
conditions: all reactions were carried out with 0.2 mmol of
phenyl triflates 1 (2.0 equiv), phenols 2 (1.0 equiv), MeOH
(2.0 equiv), Pd(OAc)2 (10 mol %), PtBu3·HBF4 (20 mol %),
K2CO3 (2.0 equiv), CO (3 bar), DMAc (1.5 mL), 110 °C, 48 h.
The yield of 5a was determined by GC using hexadecane as
an internal standard.
3p
3r
, 72%
O
, 45%
, 70%
O
S
O
O
O
O
O
O
O
3s
3v
3t
3u
, 68%
, 72%
, 64%
O
O
O
O
O
O
tBu
3w
, 75%
, 55%
O
OTf
OH
Pd(OAc)2 (10 mol%), PtBu3HBF4 (20 mol%)
ROH
OR
Scheme 2. Substrate scope of aryl triflates and phenols.
Reaction conditions: all reactions were carried out with 0.2
mmol of phenyl triflates 1 (1.5 equiv), phenols 2 (1.0 equiv),
Pd(OAc)2 (10 mol %), PtBu3·HBF4 (20 mol %), K2CO3 (2.0
equiv), CO (3 bar), DMAc (1.5 mL), 90 °C, 48 h.
K2CO3 (2.0 equiv), DMAc (1.5 mL)
CO (3 bar), 110 oC, 48 h
NC
R1
R1
2
4
5
O
O
O
OMe
OEt
O
CF3
S
S
S
S
5a
5d
5b
5e
5c
, 70%
, 62%
O
, 64%
O
Delightfully, we find this method was also applicable to
the synthesis of carboxylic acid esters when other
additional aliphatic alcohol was added as a new nucleophile.
Under the condition of Pd(OAc)2 (10 mol %), PtBu3·HBF4
(20 mol %), K2CO3 (2.0 equiv), CO (3 bar), DMAc (1.5 mL),
110 °C, 48 h, the alkoxycarbonylation reaction of 4-
(methylthio)phenol 1a with methanol (MeOH) using phenyl
triflate as Tf source proceeded well and the corresponding
carboxylic acid ester methyl 4-(methylthio)benzoate 5a
was detected in 79 % GC yield; meanwhile, byproduct Bp4
and Bp5 were detected as well. Under these reaction
conditions, the testing of various aryl triflates as the triflate
exchanging reagents were examined subsequently (Scheme
3). It shows that both electron-rich (Me, Et, tBu, Ph, OMe)
and electron-deficient (Cl, CN, COOMe) substituted aryl
triflates can be applied successfully and gave the desired
product 5a in moderate to good yields (45-82%). It is worth
mentioning that we also tested 4-bromophenyl
O
OnPr
OnBu
OnHex
S
S
5f
, 62%
O
, 56%
, 58%
O
O
OnOct
OBn
O
S
S
S
5g
5h
5i
, 60%
, 55%
O
, 55%
S
O OMe
O
O
OMe
S
5j
5k
5l
, 75%
, 56%
, 65%
Scheme 4. Substrate scope of phenols and aliphatic
alcohols. Reaction conditions: all reactions were carried
out
with
0.2
mmol
of
4-cyanophenyl
trifluoromethanesulfonate (2.0 equiv), phenols 2 (1.0
equiv), alcohols 4 (2.0 equiv), Pd(OAc)2 (10 mol %),
PtBu3·HBF4 (20 mol %), K2CO3 (0.4 mmol), CO (3 bar), DMAc
(1.5 mL), 110 °C, 48 h.
trifluoromethanesulfonate
and
4-nitrophenyl
trifluoromethanesulfonate as triflate exchanging reagents,

various aryl triflates and alcohols were tested and the
corresponding esters were isolated in moderate to good
yields. Notably, not only phenols generated from aryl
triflates after Tf transfer, but also other additional aliphatic
alcohols could be employed as nucleophiles to synthesize
the corresponding carboxylic acid esters under these
reaction conditions.
In order to overcome the challenge mentioned, 4-
cyanophenyl trifluoromethanesulfonate was selected as the
Tf source to active phenols. The alkoxycarbonylation
reaction of 4-(methylthio)phenol 1a or naphthol with
different aliphatic alcohols were performed (Scheme 4).
The tested alcohols, including ethanol, trifluoroethanol,
propanol, butanol, hexanol, octanol, benzyl alcohol and 2-
phenylethanol were all able to give the corresponding
esters (5b-5i) in moderate yields (55-64%). Moreover,
when 3-thiopheneethanol was used as a nucleophilic
reagent, the reaction gave the corresponding product 5j in
56% yield. Using 1-naphthol or 2-naphthol as the substrate
instead of 4-(methylthio)phenol were also successfully
employed in this alkoxycarbonylation reaction, and the
corresponding products (5k, 5l) were obtained in 75% and
55% yield, respectively. It is worth to mention that no
desired product could be detected when isopropanol or
tert-butanol was tested as the substrate.
Notes
The authors declare no competing financial interest.
ACKNOWLEDGMENT
We thank the analytical department of Leibniz-Institute for
Catalysis at the University of Rostock for their excellent
analytical service.
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OH
OTf
OTf
OH
Base
+
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Base•HOTf
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Scheme 5. Proposed reaction mechanism.
On the basis of our experimental results and literatures,[4]
plausible reaction pathway is proposed for this
a
alkoxycarbonylation of phenols (Scheme 5). Firstly, in the
presence of K2CO3, phenyl triflate reacted with 4-
(methylthio)phenol to produce 4-(methylthio)phenyl
triflate and phenol via Tf transfer. Then, the newly formed
4-(methylthio)phenyl triflate reacts with LnPd0 species A to
generate the oxidative addition complex MeSPhLPdIIOTf B.
Subsequently, coordination and insertion of CO with
intermediate B forms the key intermediate acyl-palladium
complex C. Nucleophilic attack of phenol or methanol on
intermediate C will provide the target carboxylic acid ester
3a or 5a, together with the simultaneous formation of
intermediate D. Under the assistance of base, intermediate
D undergoes reductive elimination to regenerate the active
Pd0Ln species A for the next catalytic cycle.
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In conclusion, a novel methodology for the palladium-
catalyzed alkoxycarbonylation of phenols with aryl triflates
as triflyl source has been developed. By using commercially
available Pd(OAc)2 and PtBu3·HBF4 as the catalyst system,
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O
OH
Pd(OAc)2, PtBu3HBF4
OTf
O
CO, K2CO3, DMAc, 90 o
C
R
R'
R
R'
Graphic abstract:
5

Highlights
1. A novel methodology for the palladium-catalyzed alkoxycarbonylation of phenols with aryl triflates as
triflate source has been developed.
2. Commercially available Pd(OAc)2 and PtBu3·HBF4 has been used as the catalyst system.
3. Aryl triflates were explored as Tf transfer reagents and applied in carbonylation.
6