2-pyridyl sulfoxide: A versatile and removable directing group for the PdII-catalyzed direct C-H olefination of arenes

Article abstract of DOI:10.1002/chem.201003633

Removable and versatile: The 2-pyridylsulfinyl group has proved to be an efficient directing group in the Pd^II-catalyzed aryl ortho C-H olefination. This catalyst system enables the sequential double olefination to give asymmetrically di-ortho-functionalized arenes. The sulfinyl directing group can be easily cleaved, providing access to 1,3-disubstituted arenes, or transformed into a thiol group.

Full text of DOI:10.1002/chem.201003633

COMMUNICATION
DOI: 10.1002/chem.201003633
2-Pyridyl Sulfoxide: A Versatile and Removable Directing Group for the
II
ꢀ
Pd -Catalyzed Direct C H Olefination of Arenes
Alfonso Garcꢀa-Rubia, M. ꢁngeles Fernꢂndez-IbꢂÇez, Ramꢃn Gꢃmez Arrayꢂs,* and
Juan Carlos Carretero*[a]
Dedicated to the memory of Professor Rafael Suau
The Mirozoki–Heck reaction, which couples aryl halides
(or triflates) with olefins, ranks among the most powerful
oxides remain, to the best of our knowledge, undocumented.
II
ꢀ
Herein, we describe a practical Pd -catalyzed ortho C H al-
and reliable C C bond-forming reactions.[1] As a very attrac-
kenylation assisted by a 2-pyridyl sulfoxide group, a group
that can be readily removed or transformed into other func-
tionalities such as a thiol group.
ꢀ
tive alternative, from chemical efficiency and environmental
II
ꢀ
impact aspects, the Pd -catalyzed oxidative direct C H alke-
nylation (Fujiwara–Moritani reaction), in which the aryl
halide is replaced by an unactivated arene, is receiving in-
We have previously reported the direct PdII-catalyzed
ꢀ
C2 H alkenylation and oxidative homocoupling of indoles
creasing attention.[2–8] Very efficient procedures for the PdII-
using a N-(2-pyridyl)sulfonyl group that functions as both
[2]
catalyzed aryl C H alkenylation of aniline derivatives, pyr-
directing and protecting group.[7f,h] In the pursuit of extend-
ꢀ
idine N-oxides,[3] aromatic carboxylic acids,[4] alcohols,[5] and
amines,[6] as well as heteroaromatic compounds,[7] have been
recently reported.[9] Most of these protocols require a direct-
ing this C H alkenylation strategy to non heteroaromatic
ꢀ
compounds, our initial studies focused on phenyl 2-pyridyl
sulfone (1) as substrate. However, the PdACTHNURTGNENG(U OAc)2-catalyzed
ꢀ
ing group that aids in the carbometallation of a proximal C
(10 mol%) ortho olefination of 1 with butyl acrylate in the
presence of K2S2O8 (2 equiv) as oxidant failed to provide
the desired alkenylation product 7 in useful conversions
(Table 1, entry 1). This low reactivity could be ascribed to
the high electron-deficient character of the substrate 1 com-
pared to N-sulfonyl indoles.
[10]
ꢀ
H bond, a common strategy in C H functionalization.
However, the majority of such directing groups are not
easily removable from the resulting products, thus compro-
mising the generality of the reaction.
From a synthetic practicality viewpoint, directing groups
that are not only easily attachable and removable but also
chemically versatile are highly valuable, since they provide
an additional handle to introduce diversity and complexity
in the final product.[11] In spite of this progress, critical chal-
lenges still remain in this reaction, such as achieving high se-
lectivity in the monoalkenylation product without affecting
Table 1. Screening of a sulfur-based directing group for PdII-catalyzed ar-
ꢀ
omatic ortho C H alkenylation.
ꢀ
the second ortho C H bond, thus enabling a sequential
double olefination, and expanding the scope to include
other types of substrates. In this regard, organosulfur com-
pounds play an important role in biological systems and
they serve as versatile reagents, ligands, and catalysts for or-
ganic synthesis. However, auxiliary directing groups for cata-
Entry
R (imine)
Substrate
Product
Yield [%][a]
1
2
3
4
5
6
2-pyridyl
2-pyridyl
2-pyridyl
Ph
Me
4-pyridyl
1 (n=2)
2 (n=0)
3 (n=1)
4 (n=1)
5 (n=1)
6 (n=1)
7 (n=2)
8 (n=1)
8 (n=1)
<15
55[b]
100 (79)
[c]
–
–
–
ꢀ
lytic C H carbometalation/cross coupling reactions leading
to aromatic sulfur derivatives such as thiols, sulfides, or sulf-
[c]
[c]
[a] Conversion yields (determined by 1H NMR spectroscopy from the
crude reaction mixture). In parenthesis, isolated yield after chromatogra-
phy. [b] Phenyl 2-pyridyl sulfoxide (45%) was also detected in the mix-
ture. [c] Oxidation to sulfone was only detected. DCE=1,2-dicloro-
ethane.
[a] A. Garcꢀa-Rubia, Dr. M. ꢁ. Fernꢂndez-IbꢂÇez,
Dr. R. Gꢃmez Arrayꢂs, Prof. Dr. J. C. Carretero
Departamento de Quꢀmica Orgꢂnica
Universidad Autꢃnoma de Madrid (UAM)
Facultad de Ciencias, Cantoblanco
Cantoblanco, 28049 Madrid (Spain)
Fax : (+34)91-497-3966
Therefore, we turned our attention to less electron-with-
drawing sulfur-directing groups. Thus, both the phenyl 2-pyr-
idyl sulfide (2) and the phenyl 2-pyridyl sulfoxide (3) were
examined in the model reaction with butyl acrylate using
E-mail: ramon.gomez@uam.es
juancarlos.carretero@uam.es
Supporting information for this article is available on the WWW
under http://dx.doi.org/10.1002/chem.201003633.
Chem. Eur. J. 2011, 17, 3567 – 3570
ꢄ 2011 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
3567

K2S2O8 (2.0 equiv) as oxidant. The reaction of sulfide 2
(Table 1, entry 2) afforded a 45:55 mixture of the sulfoxide 3
and its ortho-alkenylation product (8). Pleasingly, the sulfox-
ide 3 served as a very effective directing group, leading to
complete ortho alkenylation without detecting oxidation to
either K2S2O8 or PhIACTHGUNTERN(NUG OAc)2 as oxidant] turned out to be ap-
plicable to a wide range of substituted aryl 2-pyridyl sulfox-
ides[13] (Scheme 1). Both electron-rich substrates bearing Me
sulfone. Under the optimized conditions [PdACTHNUGTRENUNG(OAc)2 (10
mol%), K2S2O8 (2.0 equiv), DCE, 1108C], the olefination
product 8 was obtained with complete ortho-selectivity and
very high mono-selectivity[12] in 79% isolated yield.
We next investigated whether the 2-pyridyl or the sulfinyl
moiety was responsible for the directing ability of the 2-pyri-
dylsulfinyl group. As shown in Table 1, neither the diphenyl
sulfoxide (4, entry 4) nor the methyl phenyl sulfoxide (5,
entry 5) led to even traces of the corresponding alkenylation
products under the optimized conditions; only oxidation of
the starting material to sulfone being observed after extend-
ed reaction times. Furthermore, the absence of the alkenyla-
tion reaction observed in the case of the phenyl 4-pyridyl
sulfoxide (6, entry 6), an isomer of 3, highlights the key role
of the 2-pyridylsulfinyl group likely through stabilization of
the cyclopalladated intermediate.
ꢀ
With a regioselective C H alkenylation protocol in hand,
the scope with regard to the alkene component was exam-
ined (Table 2). Monosubstituted electrophilic alkenes (2–
Table 2. PdII-catalyzed selective monoolefination of phenyl sulfoxide 3.
Scheme 1. PdII-catalyzed mono-olefination of sulfoxide 3. Method A:
alkene (2.0 equiv), K2S2O8 (2.0 equiv). Method B: [a] alkene (3.0 equiv),
Entry
R (x)
Oxidant (y)
Product
Yield [%][a]
PhI
alkene (3.0 equiv), PhI
(OAc)2 (3.0 equiv).
G
ACHTUNGRTEN(NUGN OAc)2 (2.0 equiv); [c]
AHCTUNGTRENNUNG
AHCTUNGTRENNUNG
1
2
3
4
5
6
7
8
CO2Bu (2.0)
CO2Me (2.0)
SO2Ph (3.0)
P(O)ACHTUNGTRENNUNG(OMe)2 (3.0)
Ph (3.0)
4-OAcC6H4 (3.0)
4-OMeC6H4 (3.0)
4-BrC6H4 (3.0)
K2S2O8 (2.0)
K2S2O8 (2.0)
K2S2O8 (2.0)
K2S2O8 (2.0)
8
9
79
72
65
70
63
49
62
57
10
11
12
13
14
15
PhI
PhI
PhI
PhI
ACHTUNGTRENNUNG
or OMe substituents and electron-deficient substrates con-
taining Cl, Br, or CO2Me groups were efficiently converted
into the corresponding products with complete mono-selec-
tivity in most cases. In general, butyl acrylate showed higher
efficiency (typically 60–80% yield) than styrene (typically
40–60% yield). Substrates with meta substituents underwent
alkenylation to form a single detectable regioisomeric prod-
ACHTUNGTRENNUNG
AHCTUNGTRENNUNG
AHCTUNGTRENNUNG
[a] Yield of isolated product after chromatography.
3 equiv) cross coupled efficiently with substrate 3, affording
the corresponding monoalkenylated products in good yields
(65–79%, Table 2, entries 1–4). The more challenging sty-
rene derivatives showed, however, very low reactivity under
these reaction conditions (less than 10% conversion). Grati-
fyingly, this low reactivity problem could be solved by using
ꢀ
uct (with the new C C bond installed at the less hindered
ortho position), regardless of the electronic nature of the
substituent (products 23–26). The only exception to this
trend was found with the 2-naphthyl 2-pyridyl sulfoxide,
which provided a substantial amount of C1 alkenylation
(C3/C1=2.7:1, products 27).
PhI
ACHTUNGTRENNUNG
A
a
Even substrates that contained an ortho substituent to the
sulfinyl directing group proved to be suitable for this reac-
tion (products 28–33), although in some cases the reaction
required 2–3 equivalents of the oxidant PhIACTHNURGTNEUNG(OAc)2 for ach-
ieving good conversions (products 28, 29, 32, and 33).
(3.0 equiv) were found to be competent coupling partners
regardless of the electronic nature of the substituents on the
phenyl ring, although the yields were slightly lower (49–
63%, Table 2, entries 5–8).
These optimized conditions for selective mono-olefination
using butyl acrylate and styrene as model olefins [with
Interestingly, increasing the amount of alkene to five
equivalents and the amount of PhIACTHNUTRGNENUG(OAc)2 to three equiva-
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Chem. Eur. J. 2011, 17, 3567 – 3570

ꢀ
Direct C H Olefination of Arenes
COMMUNICATION
lents resulted in a clean and high-yielding di-ortho-olefina-
tion with both Michael acceptor olefins and styrene-type
substrates (products 34–37, 70–89% yield, Scheme 2).
Scheme 2. PdII-catalyzed di-ortho-olefination of sulfoxide 3.
Scheme 4. Transformations and removal of the directing group.
MCPBA=m-chloroperbenzoic acid.
Furthermore, the installation of two different alkenes at
the two ortho positions of the arene by sequential double
ꢀ
C H alkenylation reactions is also feasible, as demonstrated
in Scheme 3. The second ortho-olefination reaction was per-
cleanly attained with the ortho-styryl-substituted products
12 and 33, affording the corresponding phenethyl-substitut-
ed products 46 and 47 in good yields.
On the other hand, efficient cleavage of the 2-pyridylsul-
finyl group was achieved by sulfoxide/lithium exchange[14]
with nBuLi (2.0 equiv) in THF at ꢀ988C (products 48 and
49).[15] Globally, considering installation of the sulfoxide–ole-
fination-removal of the SOPy, compound 49 results from a
three-step formal meta alkenylation of anisole. The practical
utility of this sequence is illustrated in the formal synthesis
of resveratrol,[16,17] a naturally occurring product of the phy-
toalexin family with important biological activities including
anticancer properties (Scheme 5).[18]
Scheme 3. Sequential dialkenylation process.
formed by using the more reactive catalyst system Pd-
ACHTUNGTRENNUNG(OAc)2/PhIACHTUNGTRENNUNG(OAc)2 under the same reaction conditions.
Products asymmetrically substituted with two orthogonally
protected acrylates (e.g., product 38), two different types of
Michael acceptor olefins (vinyl sulfone/acrylate, products 39
and 40), as well as two electronically dissimilar olefins (acry-
late/styrene, products 41 and 42) were obtained generally in
acceptable yields.
Scheme 5. Application to the synthesis of resveratrol.
Finally, to fully demonstrate the synthetic potential of this
methodology, we briefly explored both the removal of the
directing group and its chemical versatility (Scheme 4).
First, exploiting the ability of the sulfur atom to pass fairly
readily from one oxidation state to another, the alkenylated
sulfoxide 8 was converted into the corresponding sulfide
(43) and sulfone (44). In addition, the 2-pyridyl sulfoxide 8
was found to undergo smooth conversion into the corre-
sponding C2-alkylated thiophenol 45 by reduction with Mg
(75% yield). In this reaction, the reduction of the carbon–
carbon double bond of the acrylate moiety was also ob-
served, along with transesterification to methyl ester. The
same sulfoxide!thiol/double bond reduction sequence was
In conclusion, we have demonstrated the directing ability
of the 2-pyridyl sulfinyl group in promoting the PdII-cata-
ꢀ
lyzed ortho C H olefination of arenes. Electron-deficient al-
kenes and styrene-type olefins serve as efficient coupling
partners, providing access to either mono-alkenylated or
asymmetrically di-ortho-alkenylated products (through a se-
ꢀ
quential double C H alkenylation) in good yields and high
selectivity. The 2-pyridylsulfinyl group can be readily re-
moved from the products or transformed into a thiol group.
Chem. Eur. J. 2011, 17, 3567 – 3570
ꢄ 2011 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
www.chemeurj.org
3569

J. C. Carretero et al.
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Experimental Section
Representative procedure for the ortho-alkenylation of 2-pyridyl sulfox-
ides: Reaction of phenyl 2-pyridyl sulfoxide (3) with methyl acrylate to
afford methyl 2-[2-(2-pyridyl)phenyl]acrylate (9): A glass tube with a
septum was charged with the sulfoxide 3 (51 mg, 0.25 mmol), PdACTHNUTRGNEUNG(OAc)2
(5.6 mg, 0.025 mmol), and K2S2O8 (135 mg, 0.5 mmol). The mixture was
degassed with two vacuum-nitrogen cycles, before DCE (2.5 mL) and
methyl acrylate (45 mL, 0.5 mmol) were added at room temperature. The
septum was replaced by a teflon-lined screw cap and the tube was stirred
at 1108C for 20 h. The reaction mixture was then allowed to reach room
temperature and it was treated with H2O (2 mL). The aqueous phase was
separated and extracted with CH2Cl2 (2ꢅ5 mL), and the combined or-
ganic phase was dried (Na2SO4) and concentrated under reduced pres-
sure. The residue was purified by flash chromatography (n-hexane-
EtOAc 7:3) to afford 9 as a white solid; yield: 52.1 mg (72%); m.p. 86–
888C. See Supporting Information for spectroscopic data.
[8] For meta-selective direct olefination of electron-deficient arenes,
see: Y.-H. Zhang, B.-F. Shi, J.-Q. Yu, J. Am. Chem. Soc. 2009, 131,
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Acknowledgements
ꢀ
We thank the Ministerio de Ciencia e Innovaciꢃn (MICINN, CTQ2009–
07791) and the Consejerꢀa de Educaciꢃn de la Comunidad de Madrid
(programme AVANCAT, S2009/PPQ-1634) for financial support. A.G.R.
and M.A.F.I thank the MICINN for a predoctoral fellowship and a Juan
de la Cierva contract, respectively. We also thank Johnson Matthey PLC
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ꢀ
Keywords: C H activation
pyridine · sulfoxide
· olefination · palladium ·
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reaction mixture (ꢁ5%). Using PhI
ACHTUNGRTEN(NUNG OAc)2 (2 equiv) as oxidant in-
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ucts was obtained.
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Received: December 16, 2010
Published online: February 24, 2011
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Chem. Eur. J. 2011, 17, 3567 – 3570