Palladium-Catalyzed Decarboxylative Carbonylative Transformation of Benzyl Aryl Carbonates: Direct Synthesis of Aryl 2-Arylacetates

Article abstract of DOI:10.1021/acs.orglett.8b02631

A procedure on palladium-catalyzed decarboxylative alkoxycarbonylation of carbonates for the synthesis of aryl 2-arylacetates has been developed. A broad range of aryl 2-arylacetates were obtained in good yields under mild conditions under a carbon monoxide atmosphere. Interestingly, other alcohols can be added as nucleophiles as well, and the corresponding esters were also obtained in good yields.

Full text of DOI:10.1021/acs.orglett.8b02631

Letter
pubs.acs.org/OrgLett
Cite This: Org. Lett. XXXX, XXX, XXX−XXX
Palladium-Catalyzed Decarboxylative Carbonylative Transformation
of Benzyl Aryl Carbonates: Direct Synthesis of Aryl 2‑Arylacetates
Jian-Xing Xu and Xiao-Feng Wu*
Leibniz-Institut fur Katalyse an der Universitat Rostock e. V., Albert-Einstein-Straße 29a, 18059 Rostock, Germany
̈
̈
S
* Supporting Information
ABSTRACT: A procedure on palladium-catalyzed decarboxylative
alkoxycarbonylation of carbonates for the synthesis of aryl 2-
arylacetates has been developed. A broad range of aryl 2-arylacetates
were obtained in good yields under mild conditions under a carbon
monoxide atmosphere. Interestingly, other alcohols can be added as
nucleophiles as well, and the corresponding esters were also obtained in good yields.
sters are widely present in fine chemicals, agrochemicals,
and natural products.1 Among the different types of esters,
oxidant. Moreover, concerning the nucleophiles or ester
E
products, aliphatic alcohols are more commonly studied to
give alkyl benzoates, while phenol, as the reaction partner, is
much less studied.10
arylacetates are common skeletons found in pharmaceuticals.
In the past decades, numerous synthetic approaches have been
developed for their synthesis.1 Among them, transition-metal-
catalyzed carbonylation, which uses CO as an inexpensive and
readily available C1 building block, has emerged as one of the
most attractive and powerful strategies for the efficient
construction of various esters, in terms of atom and step
economy.2 In the developed carbonylative methodologies, the
usage of benzylic halides as the substrates is one of the choices.
However, the formation of a stoichiometric amount of halogen
wastes is unavoidable.3 Oxidative carbonylation of benzylic
C(sp3)−H bond is an ideal option, but the reaction efficiency
still must be improved.4 Alternatively, palladium-catalyzed
alkoxycarbonylation of benzyl amines or benzyl alcohols also
are possible. However, the reaction should be conducted in
dimethyl carbonate (DMC), which is also acting as the
activator.5
On the other hand, transition-metal-catalyzed decarboxyla-
tive couplings have recently emerged as a powerful strategy for
C−C and C−heteroatom bond formations.6 Compared with
traditional cross-coupling reactions; decarboxylative coupling
reactions are milder and more environmentally friendly.
Carboxylic acids or carbonatese, as the applied substrates,
are nontoxic, stable, and readily available.7,8 As expected,
palladium-catalyzed decarboxylative carbonylations have also
been reported.9 In 2011, Lee and co-workers reported a
palladium-catalyzed decarboxylative carbonylation of aryl
alkynyl carboxylic acids and aryl iodides for synthesis of α,β-
alkynyl ketone under CO pressure (10 bar).9a Shortly
afterward, they were able to use the same method to synthesize
various alkynyl amides.9c Our group also developed a
palladium-catalyzed decarboxylative carbonylation synthesis
of α,β-alkynyl ketone, employing formic acid as the CO
source.9d Herein, we developed the first decarboxylative
carbonylation procedure for the synthesis of aryl 2-arylacetates
from benzyl aryl carbonates under mild conditions. Compared
with the reported alkoxycarbonylation procedures, this process
avoided the use of halides or stoichiometric amounts of
To begin this study, we chose benzyl phenyl carbonate (1a)
as the model substrate to establish the reaction conditions.
After extensive experimentation, we found that the decarbox-
ylative carbonylation synthesis of phenyl-2-phenylacetate (3a)
can be achieved in 85% yield with [(cinnamyl)PdCl]2 (2.5
mol %)/dppp (5.5 mol %) as the catalytic system in o-xylene
under 1 bar of CO gas without any additive (Table 1, entry 1).
As we expected, no desired product was determined in the
absence of CO (Table 1, entry 2). Both palladium salt and
phosphine are required for the reaction to proceed (Table 1,
entry 3). Alternative palladium precursors, including PdCl2,
Pd(OAc)2, and Pd2(dba)3 were tested as well, and only
Pd2(dba)3 can deliver 3a in 71% yield (Table 1, entry 6). We
also investigated different ligands, and dppp was found to be
superior to other ligands (Table 1, entries 7 and 8). In
addition, the results show that the bite angle of the ligands has
a great influence on the reaction outcome. Other solvents, such
as CH3CN, toluene, or 1,4-dioxane, was less effective than o-
xylene (Table 1, entry 11). Moreover, increasing or decreasing
the amount of catalyst led to a reduced yield of 3a (Table 1,
entries 10 and 11). Here, it is also important to mention that
the addition of nitrogen is done to avoid the overflow of
reagents from the reaction vial under 120 °C and maintain the
reproducibility of our procedure.
With the optimized reaction conditions in hand, we next
investigated the scope of this decarboxylative carbonylation
procedure (Table 2). A variety of benzyl aryl carbonates
containing electron-donating or electron-withdrawing groups
are compatible for this transformation. Benzyl group bearing
substituents with fluoride-, trifluoromethyl-, and methoxy-
groups at the ortho- or para-position all give high yields of the
corresponding esters (see Table 2, entries 2−5). Benzo[d]-
Received: August 17, 2018
© XXXX American Chemical Society
A
DOI: 10.1021/acs.orglett.8b02631
Org. Lett. XXXX, XXX, XXX−XXX

Organic Letters
Letter
a
Table 1. Optimization of the Reaction Conditions
Table 2. Decarboxylative Carbonylation of Different
Carbonates
a
variation from the standard
b
entry
conditions
yield (%)
1
2
3
4
85
n.r.
n.r.
without CO
without catalyst/ligand
PdCl2 instead of [(cinnamyl) n.r.
PdCl]2
5
6
7
Pd(OAc)2 instead of
[(cinnamyl)PdCl]2
Pd2(dba)3 instead of
[(cinnamyl)PdCl]2
PPh3, or dppm instead of
dppp
9
71
n.r.
8
9
dppe, or dppb instead of dppp 18 (using dppe), 77 (using dppb)
CH3CN, toluene, or 1,4-
62 (using CH3CN), 77 (using
toluene), 75 (using 1,4-dioxane)
dioxane instead of o-xylene
10
11
1 mol % [(cinnamyl)PdCl]2,
2.2 mol % dppp
5 mol % [(cinnamyl)PdCl]2,
11 mol % dppp
68
64
a
Reaction conditions: benzyl phenyl carbonate (0.2 mmol),
[(cinnamyl)PdCl]2 (2.5 mol %), dppp (5.5 mol %), CO (1 bar), N2
(9 bar), o-xylene (2 mL), 120 °C, 12 h. Abbreviations used in this
table: n.r. = no reaction, [(cinnamyl)PdCl]2 = dichlorobis[(1,2,3-)-1-
phenyl-2-propenyl]dipalladium(II), dppp
= 1,3-bis-
(diphenylphosphino)propane, dppm = bis(diphenylphosphino)-
methane, dppe = 1,2-bis(diphenylphosphino)ethane, and dppb =
b
1,4-bis(diphenylphosphino)butane. As determined by gas chroma-
tography (GC), using hexadecane as the internal standard. .
[1,3]dioxol-5-ylmethyl phenyl carbonate and naphthalen-1-
ylmethyl phenyl carbonate, including a sterically hindered
group, can also give the desired esters in excellent yields (see
Table 2, entries 7 and 8). Similar to benzyl group, a phenyl
group with electron-withdrawing groups (F, Cl, CN, and
COCH3), or electron-donating groups all generated the
corresponding esters in good to excellent yields (Table 2,
entries 9−13). In addition, heteroaromatic carbonates such as
furan-2-ylmethyl phenyl carbonate and benzylpyridin-3-yl
carbonate, can be applied as well and gave high yields of the
desired products (Table 2, entries 6 and 16). Moreover,
multifluoro-substituted benzyl (perfluorophenyl) carbonate,
and benzyl naphthalen-2-yl carbonate can all be transformed to
the desired esters in good yields (Table 2, entries 15 and 17).
However, the reaction failed with substrates substituted with
bromide (for a list of failed examples, see Table S1 in the
Supporting Information (SI)). This phenomenon is mainly
due to the low activating energy of the C−Br bond, compared
with the carbonate, and palladium catalyst reacted faster with
bromide and then decomposed. In addition, under the
optimized conditions, cinnamyl phenyl carbonate can be
transformed as well; however, a mixture of two products was
obtained (see Scheme 1).
a
Reaction conditions: benzyl phenyl carbonate (0.2 mmol),
[(cinnamyl)PdCl]2 (2.5 mol %), dppp (5.5 mol %), CO (1 bar), N2
(9 bar), o-xylene (2 mL), 120 °C, 12 h, isolated yield.
Since the importance of alkyl arylacetates such as
methylphenidate and Plavix is well-known,11 we investigated
the reaction with aliphatic alcohols as well (see Table 3). As we
expected, when the decarboxylative carbonylation was
performed in methanol, the benzyl phenyl carbonate could
be completely converted to the desired methyl 2-phenylacetate
in good yield (see Table 3, entry 1). Inspired by this result, we
also tested other alcohols, such as ethanol, 1-pentanol,
cyclohexanol, and benzyl alcohol, and the desired products
were all generated in moderate to good yields (see Table 3,
entries 2−5).
B
DOI: 10.1021/acs.orglett.8b02631
Org. Lett. XXXX, XXX, XXX−XXX

Organic Letters
Letter
terminal product 3a and Pd0 were obtained by reductive
elimination of acyl palladium species IM4 or IM5.
Scheme 1. Decarboxylative Carbonylation of Cinnamyl
Phenyl Carbonate
In summary, we have successfully developed a new
palladium-catalyzed decarboxylative carbonylation of benzyl
aryl carbonates for the synthesis of aryl 2-arylacetates. Without
any additive or external phenol addition, the desired aryl
arylacetic acid esters were obtained in good to excellent yields.
In addition, alkyl arylacetic acid esters can also be prepared by
adding the corresponding alcohols.
Table 3. Decarboxylative Carbonylation of Carbonates with
other Alcohols
a
ASSOCIATED CONTENT
* Supporting Information
■
S
The Supporting Information is available free of charge on the
ACS Publications website at DOI: 10.1021/acs.or-
glett.8b02631.
Experimental details, additional reaction data, and NMR
spectra of products. General comments, procedure for
substrates and products preparation, analytic data for
substrates and products, and NMR spectra for substrates
and products (PDF)
AUTHOR INFORMATION
■
Corresponding Author
*E-mail: xiao-feng.wu@catalysis.de.
ORCID
a
Reaction conditions: benzyl phenyl carbonate (0.3 mmol), R3OH (2
Xiao-Feng Wu: 0000-0001-6622-3328
Notes
mL), [(cinnamyl)PdCl]2 (2.5 mol %), dppp (5.5 mol %), CO (1 bar),
N2 (14 bar), 120 °C, 15 h, isolated yield (NMR yield given in
parentheses, using dibromomethane as an internal standard). b1-
The authors declare no competing financial interest.
c
pentanol (5 equiv), o-xylene (2 mL). R3OH (1 mL).
ACKNOWLEDGMENTS
■
The authors thank the Chinese Scholarship Council for
financial support. We also thank the analytical department of
Leibniz-Institute for Catalysis (LIKAT) at the University of
Rostock for their excellent analytical service. We appreciate the
Based on the experimental data and precedents regarding
relevant palladium-catalyzed carbonylation reactions,2,8
a
possible reaction pathway is proposed and shown in Scheme
2. Initially, the reactive Pd0 species was generated by the ligand
exchange, followed by the oxidative addition with 1a to form
IM1. After the elimination of one molecule of CO2, IM2 was
generated. Both the IM4 and IM5 then are possible after the
coordination and insertion of CO with IM2. Finally, the
̈
general support from Professor Armin Borner and Professor
Matthias Beller in LIKAT.
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D
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