Highly selective activation of vinyl C-S bonds over aryl C-S bonds in the Pd-catalyzed coupling of (E)-(β-trifluoromethyl)vinyldiphenylsulfonium salts: Preparation of trifluoromethylated alkenes and dienes

Article abstract of DOI:10.1002/ejoc.201200758

We describe the Suzuki coupling reaction of (E)-(β-trifluoromethyl) vinyldiphenylsulfonium salts with arylboronic acid. The highly efficient and selective reaction provides a useful and mild method for the synthesis of trifluoromethylated alkenes and dienes. Subsequent DFT studies showed that the oxidative addition transition state of the vinyl C-S bond is much more favorable (11.7 kcal mol^-1) than that of the aryl C-S bond. Copyright

Full text of DOI:10.1002/ejoc.201200758

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SHORT COMMUNICATION
DOI: 10.1002/ejoc.201200758
Highly Selective Activation of Vinyl C–S Bonds Over Aryl C–S Bonds in the
Pd-Catalyzed Coupling of (E)-(β-Trifluoromethyl)vinyldiphenylsulfonium Salts:
Preparation of Trifluoromethylated Alkenes and Dienes
Hao Lin,[a] Xicheng Dong,[a] Yuxue Li,*[b] Qilong Shen,*[a] and Long Lu*[a]
Keywords: Sulfur / Fluorine / Cross-coupling / Palladium / Alkenes
We describe the Suzuki coupling reaction of (E)-(β-trifluoro-
methyl)vinyldiphenylsulfonium salts with arylboronic acid.
The highly efficient and selective reaction provides a useful
and mild method for the synthesis of trifluoromethylated alk-
enes and dienes. Subsequent DFT studies showed that the
oxidative addition transition state of the vinyl C–S bond is
much more favorable (11.7 kcalmol–1) than that of the aryl
C–S bond.
propanes and heterocycles[7] have never been considered as
cationic cross-coupling reactants. Likely, the activation en-
ergy of a vinylic C–S bond and an aryl C–S bond is too
close to be differentiated. As a result, selective formation of
new vinylic carbon–carbon bonds from vinyl diphenylsulf-
onium salts will be a challenging target [Equation (1)].
Introduction
Since its discovery in the late 1960s, the palladium-cata-
lyzed Suzuki–Miyaura reaction to form new carbon–carbon
bonds between electrophiles and organometallic reagents
has become an essential tool for the preparation of agro-
chemically and pharmaceutically active ingredients and
other fine chemicals.[1] The most commonly used electro-
philic reagents in these reactions involve aryl or vinyl hal-
ides (such as I, Br, Cl)[2] and other pseudohalides (such as
OTf, OTs, OMs, SMe, SO2Cl et al.).[3] Surprisingly, sulf-
onium salts,[4] which are usually crystalline solids that can
be easily prepared from thiols or thioethers, have been ra-
rely employed as the electrophilic coupling partners.[5] To
the best of our knowledge, there are only a few reports de-
tailing the Suzuki coupling of sulfonium salts. In 1997,
Liebeskind and co-worker documented the first palladium-
catalyzed Suzuki–Miyaura coupling of sulfonium salts.[6] In
this context, a variety of tetramethylenesulfonium salts were
coupled with aryl boronic acids in good to excellent yields.
Meanwhile, vinyldiphenylsulfonium salts that are typically
used as good Michael acceptors for the synthesis of cyclo-
In this communication, we report the first Suzuki cross-
coupling of (E)-(β-trifluoromethyl)vinyldiphenylsulfonium
salts that are chemoselectively activated at the vinyl C–S
bond over the aryl C–S bond to give (E)-trifluoromethyl-
ated aryl alkenes in high yields. DFT calculations show that
the oxidative addition of the vinyldiphenylsulfonium salt to
the Pd0 species is different from those of carbon–halogen
bonds. The oxidative addition of the vinyldiphenylsulfon-
ium salt proceeds through a path involving (a) coordination
of the double bond to Pd0 to form a π complex, (b) filling
of the empty π* orbital (the antibonding π orbital) of the
double bond by a pair of 4d electrons of the Pd intermedi-
ate to form new a Pd–C bond, and (c) shifting of the two
electrons of the S+–C bond to S+ to kick off the SR2 leaving
group. The trifluoromethyl group is very important for the
selectivity of the oxidative addition, as it lowers the energy
of the π* bond orbital of the vinyl group, which increases
its oxidation ability.
[a] Key Laboratory of Organofluorine Chemistry, Shanghai
Institute of Organic Chemistry, Chinese Academy of Sciences,
345 Lingling Road, Shanghai 200032
Fax: +86-21-64166128
+86-21-61453998
E-mail: shenql@mail.sioc.ac.cn
lulong@mail.sioc.ac.cn
Homepage:
http://sourcedb.sioc.cas.cn/cn/expert/yxyjhx/
201005/t20100521_2852566.html
http://sourcedb.sioc.cas.cn/cn/expert/yxyjhx/200906/t20090621_
1750793.html
[b] Key Laboratory of Organometallic Chemistry, Shanghai
Institute of Organic Chemistry, Chinese Academy of Sciences,
345 Lingling Road, Shanghai 200032
Results and Discussion
E-mail: liyuxue@mail.sioc.ac.cn
Homepage: http://sourcedb.sioc.cas.cn/cn/expert/jsyjhx/200906/
t20090621_1750753.html
Supporting information for this article is available on the
WWW under http://dx.doi.org/10.1002/ejoc.201200758.
Our investigation of the coupling of sulfonium salts be-
gan with the reaction of the (E)-(β-trifluoromethyl)vinyl-
tetramethylenesulfonium salt[7g,7h] with p-diphenylboronic
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H. Lin, X. Dong, Y. Li, Q. Shen, L. Lu
SHORT COMMUNICATION
acid and a series of palladium catalysts. Surprisingly, no wide range of arylboronic acids were readily converted into
trace amount of the desired coupled product was observed. the corresponding trans-aryl-3,3,3-trifluoropropenes in
In contrast, when the (E)-(β-trifluoromethyl)vinyldiphenyl- moderate to good yields with excellent selectivity. Reactions
sulfonium salt was used as the electrophile, in the presence of both arylboronic acids with weak electron-donating
of 2.5 mol-% Pd(OAc)2 and 5 mol-% P(oTol)3, the reaction groups (Table 2, entries 2–4) and electron-withdrawing
occurred to full conversion to give the coupled product in groups (Table 2, entries 6–9) gave the corresponding prod-
18% yield (Table 1, entry 1). Encouraged by this result, we ucts in good to excellent yields. Interestingly, reactions of
further studied the effect of different palladium precursors arylboronic acids with a strong electron-donating methoxy
and ligands on the yield of the reaction. Switching the pal- group or an electron-rich 2-benzofuran boronic acid did not
ladium precursor to Pd2(dba)3 led to a higher 41% yield, occur to full conversion even with prolonged reaction times.
whereas Pd(PPh3)2Cl2 and Pd(CH3CN)2Cl2 gave no trace Increasing the reaction temperature to 70 °C led to lower
amount of the desired product (Table 1, entries 2–4). Biden- yields and the formation of some unidentified side products.
tate ligands such as BINAP, DPPP, DPPB, and DPPF were Reactions of these substrates occurred with excellent yields
ineffective in the current reaction, possibly due to the slow if Pd(PPh3)4 was used as the catalyst. In some cases
oxidative addition of the biarylphosphanes (Table 1, en- (Table 2, entries 7–9), protodeboronation byproducts were
tries 5–8). To accelerate the oxidative addition of the cata- formed in 10–20% yield. This problem could be easily
lytic cycle, we studied sterically bulky alkylphosphanes such solved by switching the palladium precursor to Pd(OAc)2.
as PCy3 and PtBu3.[8] To our delight, reaction in the pres- It is also worth noting that the reaction conditions were
ence of Pd2(dba)3 and PtBu3 occurred to 55% conversion compatible with various functional groups. Reactions of ar-
and gave the desired coupled adduct in 47% yield after 4 h
at 50 °C. The yield increased to 73% yield if the reaction
time was prolonged to 29 h (Table 1, entries 10 & 11).
Table 2. The Suzuki reaction of sulfonium salt 1 with arylboronic
acid.
Moreover, using K3PO4 as the base further improved the
yield to 89% when dioxane was used as the solvent (Table 1,
entries 13–15). The loading of the catalyst could be further
decreased to 1.0 mol-% without any deleterious effect on
the yield of the reaction (Table 1, entry 15).
Table 1. Optimization of the Suzuki–Miyaura reaction of (E)-(β-
trifluoromethyl)vinyldiphenylsulfonium salt (1).[a]
[a] Reaction conditions: 4-biphenylboronic acid (0.24 mmol), sulf-
onium salt 1 (0.2 mmol), Pd0 or PdII (2.0 mmol-%), ligand
(4.0 mmol-%), base (1.2 equiv.), solvent (1 mL). [b] Yield was deter-
mined by 19F NMR spectroscopy of the crude reaction mixture
with trifluoromethylbenzene as an internal standard. [c] 1.0 mol-%
Pd2(dba)3 and 2.0 mol-%PtBu3·HBF4 were used.
[a] Reaction conditions: arylboronic acid (0.48 mmol), sulfonium
salt
(2.0 mmol-%), base (1.2 equiv.), solvent (2 mL). [b] Isolated yield.
us to further explore the scope of the reaction (Table 2). A [c] 1 mol-% Pd(PPh3)4 was used. [d] 2.5 mol-% Pd(OAc)2 was used.
1
(0.4 mmol), Pd2(dba)3 (1.0 mmol-%), PtBu3·HBF4
The higher efficiency of the palladium catalyst prompted
2
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Preparation of Trifluoromethylated Alkenes and Dienes
ylboronic acids with functional groups such as enolizable Table 3. The Suzuki reaction of sulfonium salts 2 with arylboronic
acids.[a]
ketones, aldehyde, ester, amide, alcohol, and chloride oc-
curred in good to excellent yields (Table 2, entries 6–11, 15).
It is interesting to note that alkenylboronic acids were also
converted into the corresponding carbon–carbon bond-
forming product in good yield (Table 2, entry 18). Tri-
mfluoroethylated alkenes obtained from this reaction repre-
sent a family of important biologically active compounds
that are capable of replacing a peptide bond in a peptidomi-
metic to improve biological activity, due to their similar ste-
ric and electronic properties.[9] Classic methods for the
preparation of trifluoromethylated alkenes rely on Wittig
olefination of trifluoromethylated aldehydes and ketones, or
direct trifluoromethylation of β-halostyrenes using various
trifluoromethylating reagents. However, these methods suf-
fer from limited substrate scopes, difficulty in controlling
in E/Z isomers, or require harsh conditions.[10] The current
approach is very easy to perform, efficient, and simple, and
thus,thisprocessallowsthefacileandselectivesynthesisoftri-
fluoromethylated alkenes with a wide array of functional
groups.[11]
Encouraged by the efficiency of the palladium-catalyzed
Suzuki coupling of 1, we synthesized E-(β-trifluoromethyl-
β-vinyl)vinyldiphenylsulfonium salts 2 and studied their Su-
zuki coupling reactions (Table 3), because efficient and
highly selective syntheses of trisubstituted alkenes are still
major challenges for synthetic chemists. As we expected, the
[a] Reaction conditions: aryl boronic acid (0.48 mmol), sulfonium
reaction underwent stereospecific cross-coupling to form
salt
2
(0.4 mmol), Pd2(dba)3 (1.0 mmol-%), PtBu3·HBF4
the trifluoromethylated dienes in excellent yields. A variety
of boronic acids with electron-donating groups and elec-
tron-withdrawing groups are readily tolerated. Again, alk-
enylboranes also could be cross-coupled under the standard
(2.0 mmol-%), base (1.2 equiv.), solvent (2 mL). [b] Isolated yield.
[c] 2.5 mol-% Pd(OAc)2 was used. [d] 1 mol-% Pd(PPh3)4 and
1.2 equiv. of K2CO3 was used.
reaction conditions to give the trifluoromethylated triene in tion (3)]. In addition, side product 7 from activation of the
excellent yield. phenylic C–S bond was observed by GC–MS. The ratio of
To probe the effect of the trifluoromethyl group on the 6/7 was 15:1 as determined by GC–MS. These experiments
reaction, we compared the reactivity of (E)-(β-trifluoro- suggest that the trifluoromethyl groups are important not
methyl)vinyldiphenylsulfonium salt 1 with those of (E)-(β- only for the reactivity of the sulfonium salt but also for the
phenyl)vinyldiphenylsulfonium salts 5 under the same con- selective activation of the vinyl C–S bond and the phenylic
ditions. It was found that reaction of 1 with p-tolylboronic C–S bond.
acid (3b) occurred to full conversion to give the desired
To rationalize the chemoselectivity of the vinyl C–S bond
product (E)-1-methyl-4-(3,3,3-trifluoroprop-1-enyl)benzene and the aryl C–S bond in sulfonium salt 1, DFT[12] studies
in 61% isolated yield [Equation (2)]. No product derived were performed[13] using the M06[14] method. As shown in
from activation of the phenylic C–S bond was observed by Figure 1, the oxidative addition transition state of the vinyl
GC–MS. In contrast, under the same conditions, reaction C–S bond is much more favorable (11.7 kcal/mol) than that
of (E)-(β-phenyl)vinyldiphenylsulfonium salt 5 with 3b oc- of the aryl C–S bond, which is consistent with the experi-
curred to form coupled product 6 in 24% yield [Equa- mental results. The proposed mechanism and possible or-
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H. Lin, X. Dong, Y. Li, Q. Shen, L. Lu
SHORT COMMUNICATION
bital interactions are shown in Scheme 1. Firstly, the Pd active one. Obviously, it is the electron-withdrawing CF3
catalyst and the double bond form a π complex.[15] Then, a group that lowers the energy of the π* bond orbital of the
pair of 4d electrons fill in the empty π* orbital (the anti- vinyl group, which increases its oxidation ability. Thus, the
bonding π orbital) of the double bond to form a new Pd– vinyl group is eager to accept electrons, significantly affect-
C bond. Meanwhile, the two electrons of the S+–C bond ing the selectivity.
return back to S+, kicking off the SR2 leaving group. The
SR2 leaving group is neutral, and therefore, the oxidative
Conclusions
addition mechanism is very different from that of carbon–
halide bonds. The 4d electrons of Pd have the tendency to
fill in the empty π* orbital with lowest energy, that is, the
C–S bond attached to the double bond that has the lowest
empty π* orbital should be the most active one in sulfo-
nium salt 1.
In summary, selective activation of the vinyl C–S bond
over the aryl C–S bond has been achieved in the palladium-
catalyzed coupling of trifluoromethylated vinyldiphenyl-
sulfonium salts. DFT calculations showed that the oxidative
addition of the vinyldiphenylsulfonium salt proceeds
through a path involving (a) coordination of double bond
to Pd0 to form a π complex, (b) filling of the empty π*
orbital (the antibonding π orbital) of the double bond by a
pair of 4d electrons of the Pd intermediate to form a new
Pd–C bond, and (c) shifting of the two electrons of the S+–
C bond to S+ to kick off the leaving group SR2. The Suzuki
reaction of E-(β-trifluoromethyl)vinyldiphenylsulfonium
salt 1 and E-(β-trifluoromethyl-β-vinyl)vinyldiphenylsulf-
onium salt 2 provided a useful and mild method for the
synthesis of trifluoromethylated alkenes and dienes. Full de-
tails of this new sulfonium salt Suzuki reaction will be
forthcoming.
Figure 1. The optimized oxidative addition transition states of the
vinyl C–S bond (TS-OA-vinyl) and the aryl C–S bond (TS-OA-
aryl). The selected bond lengths are in angstroms, and the relative
free energies in solvent ΔGsol are in kcalmol–1. Calculated at the
M06/6-311+G**/Lanl2DZ/SMD level.
Experimental Section
General Procedure: To a Schlenk tube was added Pd2(dba)3
(0.004 mmol), P(tBu)3·HBF4 (0.008 mmol), K3PO4 (0.48 mmol),
and dioxane (2.0 mL) at room temperature under an argon atmo-
sphere. The mixture was stirred for 5 min. Arylboronic acid
(0.48 mmol) and sulfonium salt 1 or 2 (0.4 mmol) were added. The
mixture was stirred to 50 °C for 16 h. The solvent was then re-
moved under reduced pressure. The residue was purified by flash
column chromatography to give the corresponding trifluoromethyl-
ated alkene as a pale yellow liquid.
Supporting Information (see footnote on the first page of this arti-
1
cle): Experimental and computational details and copies of the H
NMR and 13C NMR spectra.
Scheme 1. Proposed mechanism and description of possible orbital
interactions.
Acknowledgments
According to the NBO analysis (Figure 2), the energy of
the π* bond orbital of the vinyl group is –0.50 eV, which is
0.27 eV lower than that of the antibonding π orbital on the
phenyl ring. Therefore, the vinyl C–S bond is the most
The authors gratefully acknowledge financial support from the
National Basic Research Program of China (2010CB126103), Spe-
cial Fund for Agro-scientific Research in the Public Interest
(201103007), National Key Technologies R&D Program
(2011BAE06B05), Shanghai Scientific Research Program
(10XD1405200), National Natural Science Foundation of China
(21032006), and National Natural Science Foundation of China
(21172245/B020304/21172248).
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4
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Received: June 6, 2012
Published Online: ᭿
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Job/Unit: O20758
/KAP1
Date: 24-07-12 11:06:52
Pages: 6
H. Lin, X. Dong, Y. Li, Q. Shen, L. Lu
SHORT COMMUNICATION
Cross-Coupling
H. Lin, X. Dong, Y. Li,* Q. Shen,*
L. Lu* ............................................... 1–6
Highly Selective Activation of Vinyl C–S
Bonds Over Aryl C–S Bonds in the Pd-Cat-
alyzed Coupling of (E)-(β-Trifluoromethyl)-
vinyldiphenylsulfonium Salts: Preparation
of Trifluoromethylated Alkenes and Dienes
The highly selective activation of vinyl C–
S bonds over aryl C–S bonds in the pal-
ladium-catalyzed coupling of trifluoro-
methylated vinyldiphenylsulfonium salts is
described. DFT calculations showed that
the oxidative addition transition state of
the vinyl C–S bond is much more favorable
(11.7 kcalmol–1) than that of the aryl C–S
bond.
Keywords: Sulfur / Fluorine / Cross-cou-
pling / Palladium / Alkenes
6
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