Palladium-Catalyzed Stereoselective Formation of Substituted Allylic Thioethers and Sulfones

Article abstract of DOI:10.1021/acs.orglett.6b02981

A general method is reported for the stereoselective preparation of highly functionalized allylic thioethers. This protocol is based on a Pd-catalyzed thiolation of modular vinyl cyclic carbonate substrates and features high (Z)-selectivity, good yields, minimal waste, ample product scope, and operational simplicity. A one-pot strategy was used for the stereoselective formation of pharma-relevant allylic sulfones derived from their in situ prepared thioether precursors.

Full text of DOI:10.1021/acs.orglett.6b02981

Letter
pubs.acs.org/OrgLett
Palladium-Catalyzed Stereoselective Formation of Substituted Allylic
Thioethers and Sulfones
Jose Enrique Gomez,† Wusheng Guo,*,† and Arjan W. Kleij*,†,§
́
́
†Institute of Chemical Research of Catalonia (ICIQ), Barcelona Institute of Science and Technology, Av. Països Catalans 16, 43007
Tarragona, Spain
§Catalan Institute of Research and Advanced Studies (ICREA), Pg. Lluís Companys 23, 08010 Barcelona, Spain
S
* Supporting Information
ABSTRACT: A general method is reported for the stereo-
selective preparation of highly functionalized allylic thioethers.
This protocol is based on a Pd-catalyzed thiolation of modular
vinyl cyclic carbonate substrates and features high (Z)-
selectivity, good yields, minimal waste, ample product scope,
and operational simplicity. A one-pot strategy was used for the
stereoselective formation of pharma-relevant allylic sulfones
derived from their in situ prepared thioether precursors.
rganosulfur compounds such as thioethers1 and sulfones2
Scheme 1. Different Strategies for Catalytic Stereoselective
Synthesis of Allylic Thioethers and Sulfones
Oare very important building blocks in synthetic and
pharmaceutical chemistry, and their derivatives are representa-
tive in ∼20% of the best-selling US pharmaceuticals in 2012.3
As a subclass of sulfur-containing compounds, a number of
allylic thioethers have been found to be bioactive4 and
important reaction intermediates,5 and they continue to attract
interest from various synthetic communities.6,7 Effective
methodologies for the synthesis of allylic thioether/sulfones
and related compounds include allylic substitution,8 hydro-
thiolation of allenes7,9 or alkynes,10 and C−H bond
functionalization.11 Apart from these catalytic methodologies,
stoichiometric approaches have also proven to be effective in
this context.12 Despite the impressive progress noted in this
area, general catalytic methodology for the stereoselective
synthesis of highly substituted allylic thioethers (see structure
in the blue box of Scheme 1) remains underdeveloped.13
A
recent example of (Z)-selective allylic thioether and sulfone
synthesis was reported by Breit et al. using hydrothiolation of
allenes under Rh catalysis (Scheme 1, a),7 but this approach is
limited in substitution diversity around the allylic double bond.
A stereoselective construction of multifunctionalized olefin
scaffolds represents a rather challenging task in synthetic
chemistry.14
Our group recently developed a Pd-catalyzed stereoselective
methodology for the decarboxylative functionalization of vinyl
carbonates toward synthetically useful, highly substituted (Z)-
configured allylic amines and 1,4-but-2-enediols.15 Key to the
high (Z) selectivity found in these transformations was the in
situ generation of a six-membered palladacycle revealed by
DFT calculations (Scheme 1, b).15b We envisaged that, under
suitable reaction conditions, the reaction of modular vinyl
carbonates and thiol nucleophiles would give access to (Z)-
allylic thioethers through nucleophilic attack onto the in situ
formed Pd intermediate. Such a manifold would thus offer a
practical route toward the challenging stereoselective synthesis
of tri- and even tetrasubstituted allylic thioethers and sulfones
(upon oxidation) from simple and accessible precursors
(Scheme 1, b). (Allylic) sulfone scaffolds are frequently
observed in relevant pharmaceutical compounds,16 and thus,
their synthesis is of significant importance.17
We began our investigations using vinyl carbonate A (Table
1) and thiophenol as a benchmark reaction. Inspired by our
previous research,15 the combination of the White catalyst
precursor (2.0 mol %) and bidentate phosphine L1 (DPEPhos,
5.0 mol %) was first examined at rt (Table 1, entries 1−5).
Unfortunately, in different solvents no reaction was observed
(DMF, THF, MeOH, and CH3CN). To our delight, when
Pd(dba)2 in CH3CN was used, a 16% yield of allylic thioether
Received: October 3, 2016
© XXXX American Chemical Society
A
DOI: 10.1021/acs.orglett.6b02981
Org. Lett. XXXX, XXX, XXX−XXX

Organic Letters
Letter
Table 1. Screening Data for the Optimization of the
Reaction Conditions toward Allylic Thioether 1a
Table 2. Investigated Scope in Thiol Partners To Produce
Allylic Thioethers 1a−l
a
a
b
entry
R
product
yield (%)
Z/E
1
2
3
C6H5
1a
1b
1c
1d
1e
1f
90
73
93
70
76
95
83
98
93
87
98
50
94:6
92:8
4-Me-C6H4
2-MeC6H4
4-CF3C6H4
4-BrC6H4
92:8
c
b
c
4
83:17
80:20
90:10
93:7
entry
[Pd]
white
L/solvent
T (°C) yield (%)
Z/E
c
5
1
2
3
4
5
6
7
8
9
L1, none
rt
rt
0
0
6
7
3-MeC6H4
4-MeOC6H4
4-COOHC6H4
2-pyridyl
white
L1, DMF
1g
1h
1i
white
L1, THF
rt
0
c
8
91:9
white
L1, CH3CN
L1, MeOH
L1, CH3CN
L1, CH3CN
L1, CH3CN
L1, CH3CN
L1, CH3CN
L2, CH3CN
L3, CH3CN
L4, CH3CN
L5, CH3CN
L6, CH3CN
L7, CH3CN
L1, CH3CN
L1, CH3CN
rt
0
9
>99:1
91:9
white
rt
0
c
10
4-HOC6H4
adamantyl
benzyl
1j
Pd(dba)2
Pd(OAc)2
Pd(dba)2
Pd(dba)2
Pd(dba)2
Pd(dba)2
Pd(dba)2
Pd(dba)2
Pd(dba)2
Pd(dba)2
Pd(dba)2
Pd(dba)2
Pd(dba)2
rt
16
0
86:14
c
11
1k
1l
>99:1
92:8
rt
c
12
50
70
70
70
70
70
70
70
70
70
70
77
84
91
83
0
91:9
92:8
94:6
94:6
a
Reaction conditions unless otherwise stated: carbonate substrate
(0.20 mmol), thiol (1.5 equiv), CH3CN (0.20 mL), Pd(dba)2 (3.0 mol
%), L1 (5.0 mol %), 70 °C, 12 h. Isolated yield. Thiol (0.20 mmol),
d
10
b
c
11
12
13
14
15
16
carbonate substrate (0.22 mmol).
0
ortho-substitutions (Table 2, entries 2, 3, and 6) are endorsed.
Deactivated thiophenols also showed sufficient reactivity (cf.,
synthesis of 1d and 1h). Variation of the thiol partners allows
us to include synthetically useful aryl halides (1e), benzoic acid
(1h), pyridyl (1i), and phenoxy (1j) groups. Apart from aryl
thiols, alkyl thiols also proved to be feasible substrates leading
to their respective allylic thioethers in high stereoselectivty (1k
and 1l, Z/E > 92:8). In some cases, the catalytic procedure was
optimized by using a slight excess of vinyl carbonate substrate.
The (Z) configuration of the major isomer in all cases was
supported by 1H NOESY (Supporting Information, SI) and for
1a also by X-ray analysis (see the SI).
0
0
0
e
17
f
73
92
90:10
91:9
18
a
Reaction conditions unless otherwise stated: carbonate substrate
(0.20 mmol), thiophenol (1.5 equiv), solvent (0.20 mL), catalyst (2.0
b
mol %), L (5.0 mol %), open to air, 12 h. NMR yield using toluene as
c
d
an internal standard. Based on 1H NMR integration. [Pd] = 3.0 mol
e
f
%. [Pd] = 3.0 mol %, thiophenol (1.2 equiv). [Pd] = 3.0 mol %,
thiophenol (0.20 mmol) and carbonate substrate (0.22 mmol).
1a (Z/E = 86:14) was noted (Table 1, entry 6). Increasing the
temperature (Table 1, entries 8 and 9) gave a significantly
improved yield of 1a of up to 84% and higher selectivity (Z/E =
92:8).
Subsequently, the vinyl carbonate reaction partner was varied
(Table 3) to give access to a wider range of highly
functionalized allylic thioethers 2a−l in moderate to excellent
yields (up to 97%) with high stereocontrol in most cases (Z/E
ratios of at least >80:20). Both aryl and alkyl substituents in the
carbonate reagent (Table 3, “R”) were tolerated. Different
functionalities such as benzoic esters, aryl nitrile, and furyl
groups (2e,f,i) can be readily introduced to further amplify the
product diversity. Upon use of more sterically demanding vinyl
carbonates that incorporate naphthyl or cyclohexyl substituents,
the catalytic procedure was less productive (2g and 2j; 51% and
48% yield, respectively). In the case of product 2l, we observed
the formation of a branched allylic thioether which affected the
yield of the linear derivative (see the SI for details). The lower
Z/E ratios observed for 2f and 2k (Table 3) may be the result
of the oxa-palladacycle (see Scheme 1b) being in equilibrium
with a noncyclic intermediate at 70 °C. The carbonate R
substituent in the acyclic species can affect the olefin geometry
prior to attack of the thiol nucleophile through possible π−σ−π
alkene isomerization.19 A higher tendency to form such an
acyclic Pd intermediate causes loss in stereocontrol.
The catalysis was further enhanced with a higher Pd loading
(3.0 mol %; 91% yield, Z/E = 94:6; see Table 1, entry 10).
Other phosphine ligands L2−L7 were also tested but proved to
be less productive (Table 1, entries 11−16), and a lower
thiophenol amount also resulted in erosion of the yield of 1a
(1.2 equiv, Table 1, entry 17). Under the optimized conditions,
the use of an excess of carbonate gave fairly similar results
(Table 1, entry 18; 92% yield, Z/E = 91:9). Thus, the best
conditions toward the formation of allylic thioether (Z)-1a
were the use of Pd(dba)2 (3.0 mol %) and L1 (5.0 mol %) in
CH3CN at 70 °C (Table 1, entry 10). It is worth noting that no
special precautions or base additives18 were required, making
the present protocol highly attractive from a practical point of
view.
With the optimized conditions in hand, we then systemati-
cally varied the nature of both reaction partners, and first the
scope in thiols was examined (Table 2). Generally, the
decarboxylative thiolation approach proceeded with high
stereoselectivity to provide the allylic thioethers 1a−l in good
yields and Z/E ratios of typically >90:10. The presence of both
electron-withdrawing (1d,e,h,i) and -donating groups
(1b,c,f,g,j) in the aryl thiols is tolerated, and para- meta-, and
In order to further challenge the newly developed catalytic
protocol for stereoselective allylic thioether formation, the
synthesis of elusive tetrasubstituted derivatives was attempted,
and the results are listed in Table 4. To our delight, the
installation of both alkyl and aryl substituents (R2 = Ph, Me) at
B
DOI: 10.1021/acs.orglett.6b02981
Org. Lett. XXXX, XXX, XXX−XXX

Organic Letters
Letter
a
Table 3. Investigated Scope in Vinyl Carbonate Partners To
Produce Allylic Thioethers 2a−l
Table 5. One-Pot Synthesis of Highly Substituted Sulfones
a
b
entry
R1
R2
product yield (%)
Z/E
b
1
2
3
4
Ph
Ph
Ph
Ph
Ph
Ph
4a
4b
4c
4d
4e
4f
90
83
89
48
91
76
77
72
94:6
93:7
entry
R
product
yield (%)
Z/E
4-MeOC6H4
3-MeC6H4
benzyl
2-pyridyl
Ph
1
2
3
4
5
6
7
8
9
4-Me-C6H4
4-F-C6H4
4-Br-C6H4
4-Ph-C6H4
4-CO2Me-C6H4
4-CN-C6H4
2-naphthyl
3-Cl-C6H4
2-furyl
2a
2b
2c
2d
2e
2f
70
84
88
97
80
91
51
71
99
48
87
94:6
83:17
87:13
94:6
90:10
91:9
c
5
87:13
84:16
93:7
6
7
8
4-CO2MeC6H4
4-FC6H4
84:16
67:33
89:11
80:20
>99:1
84:16
70:30
80:20
Ph
4g
4h
4-MeC6H4
Ph
93:7
2g
2h
2i
a
Reaction conditions: (i) carbonate substrate (0.20 mmol), thiol (1.5
equiv), CH3CN (0.20 mL), Pd(dba)2 (3.0 mol %), L1 (5.0 mol %), 70
°C, 12 h; (ii) (NH4)6Mo7O21·4H2O, H2O2 (4 equiv, 30 wt % in H2O),
MeOH (0.20 mL), rt, 1 h. X-ray structure measured for sulfone (Z)-
4a; see the SI. Isolated yield. Thiol (0.20 mmol), carbonate substrate
(0.22 mmol).
c
10
Cy
2j
c
11
C10H21
2k
2l
b
c
d
12
Me
40
a
Reaction conditions unless otherwise stated: carbonate substrate
(0.20 mmol), thiophenol (1.5 equiv), CH3CN (0.20 mL), Pd(dba)2
b
(3.0 mol %), L1 (5.0 mol %), 70 °C, 12 h. Isolated yield.
c
Thiophenol (0.20 mmol), carbonate substrate (0.22 mmol).
Branched product (30%) also formed.
(Table 5, entries 2−8). Gratifyingly, the oxidation step did not
interfere with the presence of other (functional) groups under
these conditions including benzyl (4d), pyridyl (4e), methyl
ester (4f), halide (4g), and in all cases primary alcohol and
internal alkene groups. The same level of stereoselectivity was
found in the sulfone synthesis, and generally good to excellent
yields for 4a−h were obtained. X-ray analysis of 4a
unambiguously confirmed the (Z)-configuration of this
trisubstituted allylic sulfone (see the SI).
d
Table 4. Preparation of Elusive Tetrasubstituted Thioethers
3a−d
a
In summary, we herein report a general catalytic method for
the (Z)-selective preparation of a diverse series of highly
functionalized and substituted allylic thioethers and sulfones.
This methodology is based on a Pd-catalyzed decarboxylative
functionalization of readily available and modular vinyl cyclic
carbonates and features minimal waste release, wide scope, and
operationally simplicity. Based on our previous mechanistic
investigations,15b the (Z)-selectivity in the present catalytic
protocol is ascribed to a nucleophilic attack of the thiol reagent
onto an in situ generated (Z)-configured six-membered
palladacycle that guides the stereoselective course of the
developed process for the (Z) allylic thioethers.
b
yield
entry
R1
R2
R3
product
(%)
Z/E
1
2
3
4
p-MeC6H4
Ph
Ph
Ph
3a
3b
3c
3d
48
62
65
37
>99:1
94:6
Ph
Ph
Ph
Me
Me
Me
4-MeOC6H4
2-MeC6H4
98:2
90:10
a
Reaction conditions unless otherwise stated: carbonate substrate
(0.20 mmol), thiophenol (1.5 equiv), CH3CN (0.20 mL), Pd(dba)2
(3.0 mol %), L1 (5.0 mol %), 70 °C, 12 h. Isolated yield.
b
the β-position of the allylic scaffold is feasible while maintaining
excellent stereoselectiviy (Table 4, entries 1 and 2: 3a and 3b,
Z/E > 94:6). However, the preparation of α-functionalized
allylic thioethers, derived from internal olefin substrates, failed
under these conditions probably for steric reasons (see the SI
for details). Other combinations of R1 and R2 were then also
probed (cf. the synthesis of 3c and 3d) and gave the targeted
allylic thioethers in high to excellent stereoselectivity.
We then set out to develop a one-pot strategy toward the
stereoselective synthesis of highly substituted allylic sulfones by
combining the Pd-catalyzed allylic thioether formation and in
situ oxidation. Various conditions were tested using vinyl
carbonate A and thiophenol as model reaction (see the SI for
more details). We were pleased to find that a combination of
(NH4)6Mo7O21·4H2O and H2O2 (after initial and in situ
formation of allylic thioether 1a) gave 90% isolated yield of
sulfone product 4a (Table 5, entry 1) with excellent
stereocontrol (Z/E = 94:6).20 This one-pot strategy was then
utilized to prepare different, highly functionalized sulfones
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.6b02981.
Experimental details and copies of relevant NMR and IR
spectra for all new products (PDF)
X-ray data for compound 1a (CIF)
X-ray data for compound 4a (CIF)
AUTHOR INFORMATION
Corresponding Authors
■
*E-mail: wguo@iciq.es.
*E-mail: akleij@iciq.es.
ORCID
Arjan W. Kleij: 0000-0002-7402-4764
C
DOI: 10.1021/acs.orglett.6b02981
Org. Lett. XXXX, XXX, XXX−XXX

Organic Letters
Letter
T.; Breit, B. J. Am. Chem. Soc. 2014, 136, 16124. (c) Nurhanna Riduan,
S.; Ying, J. Y.; Zhang, Y. Org. Lett. 2012, 14, 1780. (d) Yang, Y.; Rioux,
R. M. Green Chem. 2014, 16, 3916. (e) Kondoh, A.; Takami, K.;
Yorimitsu, H.; Oshima, K. J. Org. Chem. 2005, 70, 6468. (f) Ananikov,
V. P.; Malyshev, D. A.; Beletskaya, I. P.; Aleksandrov, G. G.;
Eremenko, I. L. Adv. Synth. Catal. 2005, 347, 1993.
Notes
The authors declare no competing financial interest.
ACKNOWLEDGMENTS
■
We thank ICIQ, ICREA, and the Spanish Ministerio de
Economıa y Competitividad (MINECO) through projects
CTQ-2014-60419-R and the Severo Ochoa Excellence
(11) Rao, W.-H.; Zhan, B.-B.; Chen, K.; Ling, P.-X.; Zhang, Z.-Z.;
Shi, B.-F. Org. Lett. 2015, 17, 3552.
́
(12) (a) Reddy, L. R.; Hu, B.; Prashad, M.; Prasad, K. Angew. Chem.,
Int. Ed. 2009, 48, 172. For examples of syntheses, see: (b) Chu, X.-Q.;
Meng, H.; Xu, X.-P.; Ji, S.-J. Chem. - Eur. J. 2015, 21, 11359. (c) Ando,
K.; Wada, T.; Okumura, M.; Sumida, H. Org. Lett. 2015, 17, 6026.
(13) Only a few noncatalytic methods are known but with limited
product diversity: (a) Das, B.; Chowdhury, N.; Damodar, K.; Banerjee,
J. Chem. Pharm. Bull. 2007, 55, 1274. (b) Karnakar, K.; Ramesh, K.;
Murthy, S. N.; Nageswar, Y. V. D. Helv. Chim. Acta 2013, 96, 2276.
(c) Liu, Y.; Xu, X.; Zheng, H.; Xu, Z.; Zhang, Y. Synlett 2006, 571.
(14) Flynn, A. B.; Ogilvie, W. W. Chem. Rev. 2007, 107, 4698.
(15) (a) Guo, W.; Martínez-Rodríguez, L.; Martin, E.; Escudero-
Accreditation 2014−2018 through project SEV-2013-0319.
́
Eduardo C. Escudero-Adan and Dr. Eddy Martin (ICIQ) are
acknowledged for the X-ray analysis of compounds 1a and 4a.
J.E.G. acknowledges MINECO and ICIQ for a Severo Ochoa/
FPI predoctoral fellowship. W.G. thanks the Cellex foundation
for a postdoctoral fellowship.
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D
DOI: 10.1021/acs.orglett.6b02981
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