Synthesis of vinylsulfones via palladium-catalyzed decarboxylative coupling of cinnamic acids with aromatic sulfinic acid sodium salts

Article abstract of DOI:10.1007/s10562-014-1286-5

A highly efficient synthesis of vinylsulfones was achieved via decarboxylative cross-coupling reaction of cinnamic acids with aromatic sulfinic acid sodium salts catalyzed by Pd(OAc)₂ and dppb in the presence of Ag₂CO₃. The reaction was found to furnish various vinylsulfones in good yields instead of the expected desulfitative product stilbenes. Mechanistic investigation also suggested that the decarboxylation is likely to take place after the transmetallation step. Graphical Abstract: [Figure not available: see fulltext.]

Full text of DOI:10.1007/s10562-014-1286-5

Catal Lett
DOI 10.1007/s10562-014-1286-5
Synthesis of Vinylsulfones Via Palladium-Catalyzed
Decarboxylative Coupling of Cinnamic Acids with Aromatic
Sulfinic Acid Sodium Salts
•
•
•
Ruqing Guo Qingwen Gui Dadian Wang
Ze Tan
Received: 16 March 2014 / Accepted: 23 May 2014
Ó Springer Science+Business Media New York 2014
Abstract A highly efficient synthesis of vinylsulfones
was achieved via decarboxylative cross-coupling reaction
of cinnamic acids with aromatic sulfinic acid sodium salts
catalyzed by Pd(OAc)2 and dppb in the presence of Ag2-
CO3. The reaction was found to furnish various vinylsulf-
ones in good yields instead of the expected desulfitative
product stilbenes. Mechanistic investigation also suggested
that the decarboxylation is likely to take place after the
transmetallation step.
Myers [12–14] and others [15–44] that the field of Pd-
catalyzed decarboxylative coupling has blossomed. By
using different coupling partners, a number of different Pd-
catalyzed decarboxylative couplings have been developed
to access styrenes [12–17], biaryls [5–11, 18–32], ketones
[33–38], esters [39], nitriles [40], diarylmethanes [41, 42]
and phenanthrenes [43]. Among these couplings, those
relying on the direct arylation of arene approach [24–26,
34, 35, 38, 43, 44, 49] are particularly noteworthy since
these transformations are C–H activation based, thus
making the synthesis extremely efficient in terms of step
economy and overall yield. For example, Larrosa et al. and
Su et al. reported the intermolecular decarboxylative cou-
plings between indole or thiophene derivatives and arene
carboxylic acids [24–26]. Greaney and our group discov-
ered that benzothiazoles and oxazoles can undergo inter-
molecular decarboxylative couplings with aryl carboxylic
acids [27, 28]. Although the decarboxylative coupling of
arene carboxylic acids has been studied extensively, reports
on the decarboxylative coupling of vinyl carboxylic acids
are quite few [17]. Given the fact that a large number of
cinnamic acid derivatives are commercially available or
can be conveniently prepared by the classical Perkin
reaction from the corresponding benzaldehydes, utilization
of them in Pd-catalyzed decarboxylative couplings seems
highly desirable. Along this line, Wu in 2009 reported that
stilbenes can be efficiently constructed by the coupling of
cinnamic acids with aryl iodides and bromides in the pre-
sence of PdCl2 catalyst and using CyJohnphos as the ligand
[Eq. (1)] [17].
Keywords Pd(AcO)2 ꢀ Ag2CO3 ꢀ Cinnamic acid ꢀ
Aromatic sulfinic acid sodium salts ꢀ Decarboxylative
coupling
1 Introduction
The development of Pd-catalyzed decarboxylative cou-
plings (For some reviews on decarboxylative couplings,
see [1–3]) has attracted significant attention lately since
these couplings utilize readily available carboxylic acids
instead of the usually expensive organometallic reagents
[4–44]. Moreover, these carboxylic acids, after undergoing
decarboxylations, only produce CO2, an innocuous com-
pound, as the side-product. Though Nilsson reported the
first transition metal mediated decarboxylative biaryl cou-
pling [4], it was through the efforts of Goossen [5–11],
Electronic supplementary material The online version of this
article (doi:10.1007/s10562-014-1286-5) contains supplementary
material, which is available to authorized users.
Aryl sulfinic acid sodium salts which are relatively
stable, easily prepared from sulfonyl chlorides, are mainly
used as sulfonylation reagents for preparing organosulfonyl
compounds (Recent examples [45–48]). However, recent
studies revealed that aryl sulfinic acids and their salts could
R. Guo ꢀ Q. Gui ꢀ D. Wang ꢀ Z. Tan (&)
College of Chemistry and Chemical Engineering, Hunan
University, Changsha 410082, People’s Republic of China
e-mail: ztanze@gmail.com
123

R. Guo et al.
also serve as the arene sources via desulfitative reactions,
especially under transition metal catalysis [49–62]. Graves
was the first to report a desulfitative biaryl synthesis in 1970
using aryl sulfinic acids and their salts as aryl donors [49].
In 1992, Sato and Okoshi showed that biaryls can be syn-
thesized via Pd-catalyzed desulfitative couplings between
aryl bromides with aromatic sulfinic acids in NMP [50].
Later on Li and Duan showed that aryl triflates were also
viable substrates [61]. In addition to the reactions with aryl
halides, Pd-catalyzed desulfitative Heck type coupling with
olefins was also successfully developed by Deng to provide
various stilbenes [Eq. (2)] [62]. By using different Pd-cat-
alyst combinations, Li and Duan showed that the reaction of
olefins with aromatic sulfinic acids can be stopped after the
desulfitation-addition step and the saturated products
instead of the alkenes (Heck coupling product) were
obtained [60]. More recently this type of desulfitative
couplings was successfully extended to azoles, indoles and
other heterocycles by Wang, Deng and You [51, 52, 57].
Inspired by these results and as a continuation of our
research on transition-metal-catalyzed decarboxylative
coupling, we envision that 1,2-diaryl olefins can be syn-
thesized via Pd-catalyzed decarboxylative/desulfitative
coupling between cinnamic acids and aromatic sulfinic acid
sodium salts (Eq. 3). As a result, we systematically studied
the coupling of cinnamic acids with arenesulfinic acid
sodium salts under Pd-catalysis and we found that cinnamic
acids did undergo decarboxylation under the reaction con-
ditions. However vinyl sulfones instead of the desulfitative
product stilbenes were obtained in good to excellent yields.
(For some recent syntheses of vinylsulfones from alkenes or
alkynes and aryl sulfinates, please see [63, 64]).
2 Experimental
2.1 Instruments and Materials
All solvents and reagents were purchased from the sup-
1
pliers and used after further dried. H NMR and 13C NMR
were recorded in CDCl3 at room temperature on the
spectrometer (400 MHz 1H, 100 MHz 13C). The chemical-
1
shifts scale is based on internal TMS. Data for H NMR
and 13C NMR are reported as follows: chemical shift (d,
ppm), multiplicity, integration, and coupling constant (Hz).
All reactions were carried out under dry nitrogen
atmosphere.
2.2 General Procedure for Palladium-Catalyzed
Decarboxylative Cross-Coupling Reaction
of Cinnamic Acids with Aromatic Sulfinic Acid
Sodium Salts
To a 15-mL sealed tube were added Pd(OAc)2 (11.2 mg,
0.05 mmol), dppb (22 mg, 0.05 mmol), Ag2CO3 (264 mg,
1 mmol), cinnamic acid (0.5 mmol) and aromatic sulfinic
acid sodium salt (1.0 mmol). The tube was capped and
stirred under N2 at 75 °C for 6 h. The reaction mixture was
cooled to room temperature and diluted with EtOAc, fil-
tered through a short pad of Celite, washed with EtOAc,
and concentrated in vacuo. The resulting residue was
purified by flash column chromatography using hex-
anes:EtOAc (4:1) as the eluent. All compounds are char-
acterized by 1H NMR, 13C NMR, LRMS and their
comparison to literature values.
ð1Þ
ð2Þ
ð3Þ
123

Synthesis of Vinylsulfones
Table 1 Reaction condition optimization
Entry
PdX2 (mol %)
Additive (equiv)
Ligand
Solvent
T (oC)
Yielda
1
PdCl2
Ag2CO3
Ag2CO3
Ag2CO3
Ag2CO3
Ag2CO3
Ag2CO3
Ag2CO3
AgOAc
K2CO3
PPh3
DMF
DMF
DMF
DMF
DMF
DMF
DMF
DMF
DMF
DMF
DMF
Cl(CH2)2Cl
toluene
H2O
150
125
100
75
50
25
75
75
75
75
75
75
75
75
75
75
75
75
75
75
75
75
75
75
25 %
27 %
53 %
75 %
27 %
trace
62 %
44 %
trace
trace
76 %
10 %
7 %
2
PdCl2
PPh3
3
PdCl2
PPh3
4
PdCl2
PPh3
5
PdCl2
PPh3
6
7c
PdCl2
PPh3
PdCl2
PPh3
8
PdCl2
PPh3
9
PdCl2
PPh3
10
11
12
13
14
15
16d
17
18
19
20
21
22
23
24
PdCl2
K3PO4
PPh3
Pd(OAc)2
Pd(OAc)2
Pd(OAc)2
Pd(OAc)2
Pd(OAc)2
Pd(OAc)2
Pd(OAc)2
Pd(OAc)2
Pd(OAc)2
Pd(OAc)2
Pd(OAc)2
Pd(OAc)2
Pd(OAc)2
Ag2CO3
Ag2CO3
Ag2CO3
Ag2CO3
Ag2CO3
Ag2CO3
Ag2CO3
Ag2CO3
Ag2CO3
Ag2CO3
Ag2CO3
Ag2CO3
Ag2CO3
Ag2CO3
PPh3
PPh3
PPh3
PPh3
31 %
63 %
44 %
74 %
30 %
73 %
94 %
68 %
48 %
57 %
0 %
PPh3
DMSO
DMF
DMF
DMF
DMF
DMF
DMF
DMF
DMF
DMF
PPh3
dppf
Xantphos
Tri(o-tolyl)phosphine
dppb
dppm
dppe
dppp
dppb
Conditions: 1a (0.5 mmol), 2a (1.0 mmol), PdX2 (0.05 mmol), Additive (1.0 mmol), Ligand (0.1 mmol for monodentate ligand, 0.05 mmol for
bidentate ligand), 3 mL of solvent, at 75 °C for 6 h unless otherwise noted
Only 5 mol % Pd catalyst and 10 mol % ligand were used
The amount of DMF used was decreased to1.5 mL
a
Yields are based on 1a
3 Results and Discussion
and 0.2 equiv of PPh3 in DMF (6 mL/mmol) at 150 °C for
12 h, much to our surprise, we found that the product
isolated in 25 % yield was actually vinyl sulfone 3a instead
of the expected stilbene product 4a (Table 1, entry 1).
Clearly, the desulfination did not take place as expected. It
is important to note that the stereochemistry of the isolated
product was exclusively trans based on the large coupling
constant of the two olefinic protons. In order to improve the
reaction yield, several reaction parameters were screened
and the results were summarized in Table 1. From the
table, we can see that lowering the reaction temperature
benefited the reaction. In fact, best yield (75 %) was
reached at 75 °C while further lowering of the reaction
3.1 Optimization of Reaction Conditions for Pd-
Catalyzed Decarboxylative Coupling of Cinnamic
Acids with Aromatic Sulfinic Acid Sodium Salts
In order to study Pd-catalyzed decarboxylative-desulfita-
tive coupling between cinnamic acids and aromatic sulfinic
acid sodium salts, we decided to use the coupling of cin-
namic acid 1a and benzenesulfinic acid sodium salt 2a as
our model reaction. When cinnamic acid 1a was heated
with 2 equiv of benzenesulfinic acid sodium salt and 2
equiv. of Ag2CO3 in the presence of 10 mol % of PdCl2
123

R. Guo et al.
Table 2 Palladium-catalyzed decarboxylative cross-coupling of cinnamic acids with arenesulfinic acid sodium salts
Isolated yields based on cinnamic acid 1a
123

Synthesis of Vinylsulfones
Scheme 1 Proposed
mechanism for Palladium-
catalyzed decarboxylative
cross-coupling of cinnamic
acids with aromatic sulfinic acid
sodium salts
temperature actually was detrimental to the reaction
(Table 1, entries 2–6). When the reaction was run at room
temperature, no desired product was observed (Table 1,
entry 6). When the amount of Pd-catalyst used was reduced
to 5 mol %, the yield dropped to 62 % (Table 1, entry 7).
When the oxidant Ag2CO3 was switched to AgOAc, the
yield dropped to 44 % (Table 1, entry 8). When Ag2CO3
was substituted with either K2CO3 or K3PO4, almost no
reaction took place (Table 1, entries 9, 10). This is to be
expected because Ag2CO3 acted not only as a base but also
as an oxidant. The yield did not change much when
Pd(OAc)2 was utilized in place of PdCl2 as the catalyst
(Table 1, entry 11). Tests also showed that DMF gave the
best yield (76 %) whereas other solvents such as ClCH2-
CH2Cl, toluene, DMSO and H2O were less effective
(Table 1, entries 12–15). When the volume of DMF was
switched to 3 mL/mmol, the yield of reaction also suffered
(44 %, Table 1, entries 16). Comparable results were also
encountered when the ligand was changed to dppf, or
Tri(o-tolyl)phosphine while Xantphos gave inferior results
(Table 1, entries 17,18 and 19). However, much to our
delight, we found that the yield could be improved to 94 %
when ligand dppb was used in combination with Pd(OAc)2
while the use of dppm, dppe, dppp proved less efficient
(Table 1, entry 20–23). Control reaction also proved that
no desired reaction took place in the absence of Pd-catalyst
(Table 1, entry 24).
and electron-withdrawing groups such as methyl, chloro,
bromo, fluoro, methoxy as well as nitro group are well
tolerated on the phenyl ring of the cinnamic acids. They
can also be placed at the ortho, meta and para positions,
resulting little difference on the product yields. The results
of nitro-substituted substrates are important because the
nitro group can be easily transformed into other functional
groups. The results of chloro and bromo-substituted sub-
strates are also important because they offered convenient
handles for future transition metal catalyzed C–C and C–X
bond formation. As for the substituted aryl sulfinic acid
sodium salts, we found methyl, tert-butyl, fluoro and
bromo-substituted phenyl sulfinic acid sodium salts worked
satisfactorily to afford the desired vinyl sulfones in
50–88 % yield (Table 2, 3am–3bc). 2-Naphthyl sulfinic
acid sodium salt worked well too (Table 2, 3bd). It is
worthwhile to point out that 2-methyl cinnamic acid can
also participate in the cross coupling with phenyl sulfinic
acid sodium salt successfully, furnishing the desired tri-
substituted vinyl sulfone 3az in 45 % yield. Though the
yield is rather low, it should be noted that only one ste-
reoisomer was obtained and analysis of the product indi-
cated that it was the
E
product, showing the
stereochemistry of the starting cinnamic acid was retained
during the coupling. However, when we tried to use alkyl
sulfinic acid sodium salt such as sodium methanesulfinate
as the coupling partner, the reaction became very messy
(Table 2, 3be) (Scheme 1)
3.2 General Procedure for Palladium-Catalyzed
Decarboxylative Cross-Coupling Reaction
of Cinnamic Acid with Aromatic Sulfinic Acid
Sodium Salt
3.3 Investigation into the Reaction Mechanism
Based on previous reports [5–11, 51–62], we have con-
sidered two possible reaction mechanisms for this cou-
pling. As expected, the cinnamic acid first underwent
anion exchange with Pd(OAc)2 to generate palladium
cinnamate I. In catalytic cycle A, this intermediate next
reacted with arene sulfinic acid sodium salt to form
intermediate II. After decarboxylation, intermediate II
turned into intermediate III which after reductive elimi-
nation should furnish the desired vinyl sulfone product.
The Pd(0) is oxidized by the silver salt back to Pd(II), thus
With the optimized protocol in hand, we next set out to
explore the scope and the limitation of the reaction
(Table 2). We found that the reaction worked very well for
a wide variety of substituted cinnamic acids, affording the
desired vinyl sulfones in yields ranging from 45 to 88 %. It
is important to note that, in all the reactions, only one
stereoisomer was obtained and the stereochemistry of the
isolated vinylsulfones were all E. Both electron-donating
123

R. Guo et al.
(a)
(b)
(e)
(f)
(c)
(g)
(d)
Scheme 2 1H NMR (400 MHz) study of the reaction between
cinnamic acid and aromatic sulfinic acid sodium salt catalyzed by
the combination of Pd(OAc)2 and PPh3 in the presence of Ag2CO3.
spectrum after the addition of Pd(OAc)2, PPh3 and Ag2CO3 at 25 °C.
d–e 1H NMR spectra after 5 min at 50 °C and 1 h at 75 °C. f 1H
1
NMR spectrum after the addition of PhSO2Na at 75 °C. g H NMR
a
1H NMR spectrum of cinnamic acid, mesitylene and DMF in
spectrum after 1 h at 75 °C
DMSO-d6 at 25 °C. b reaction mixture 1 h at 75 °C. c 1H NMR
completing the catalytic cycle. However, another possible
pathway is catalytic cycle B. In cycle B, palladium cin-
namate I will decarboxylate first before reacting with
arene sulfinic acid sodium to form intermediate IV. This
intermediate will react with the arene sulfinic acid sodium
to form intermediate III which is also formed in cycle
A. In order to differentiate these two pathways, several
mechanistic experiments were performed. We tried to
monitor the coupling reaction in situ using NMR spec-
troscopy using mesitylene as the internal standard
(Scheme 2). For solubility reasons, we opted to add some
DMSO-d6 to the reaction mixture as the NMR solvent.
From the spectra, we can see that even at 75 °C with Pd
catalyst, PPh3 and Ag2CO3, the signal at 6.61 ppm which
we assigned to be the proton of the 2-position of cinnamic
acid did not decrease. This showed that decarboxylation
did not take place at the reaction temperature. However,
after we added arene sulfinic acid sodium to the NMR
tube, the intensity of the peak decreased drastically in just
1 h at 75 °C (Scheme 2, g), showing decarboxylation took
place in the presence of arene sulfinic acid sodium. Based
on the above results, we concluded that the catalytic cycle
A is more likely though other mechanisms may operate
here as well.
123

Synthesis of Vinylsulfones
26. Hu P, Zhang M, Jie X, Su W (2012) Angew Chem Int Ed 51:227
27. Xie K, Yang Z, Zhou X, Li X, Wang S, Tan Z, An X, Guo CC
(2010) Org Lett 12:1564
28. Zhang F, Greaney MF (2010) Angew Chem Int Ed 49:2768
29. Zhao H, Wei Y, Xu J, Kan J, Su W, Hong M (2011) J Org Chem
76:882
30. Cornella J, Lahlali H, Larrosa I (2010) Chem Commun 46:8276
31. Xie K, Wang S, Yang Z, Liu J, Wang A, Li X, Tan Z, Guo CC,
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4 Conclusions
In conclusion, we have developed a highly effective
method of synthesizing vinyl sulfones via Pd-catalyzed
decarboxylative cross-coupling reaction of cinnamic acid
with arene sulfinic acid sodium salts. The best result was
obtained by using catalyst combination of Pd(OAc)2 and
dppb in DMF using Ag2CO3 as the base and oxidant. The
reaction was found to tolerate both electron withdrawing
and electron donating groups, affording various vinyl
sulfones in 45–94 % yield. Currently efforts are underway
to expand the reaction scope and the results will be
reported in due course.
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Acknowledgments We thank the financial support of National
Natural Science Foundation of China (Grants J1210040, J1103312).
40. Shang R, Sheng D, Chu L, Fu Y, Liu L (2011) Angew Chem Int
Ed 123:4562
41. Shang R, Yang Z, Wang Y, Zhang S, Liu L (2010) J Am Chem
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