Ruthenium-catalyzed ortho -alkenylation of aromatic ketones with alkenes by C-H bond activation

Article abstract of DOI:10.1021/ol202580e

A ruthenium-catalyzed chelation-assisted C-H bond activation of aromatic ketones and the reaction with olefins to provide Heck-type products in good to excellent yields with a high regio- and stereoselective manner is described.

Full text of DOI:10.1021/ol202580e

ORGANIC
LETTERS
2011
Vol. 13, No. 23
6144–6147
Ruthenium-Catalyzed Ortho-Alkenylation
of Aromatic Ketones with Alkenes by CꢀH
Bond Activation
Kishor Padala and Masilamani Jeganmohan*
Department of Chemistry, Indian Institute of Science Education and Research,
Pune 411021, India
mjeganmohan@iiserpune.ac.in
Received August 19, 2011
ABSTRACT
A ruthenium-catalyzed chelation-assisted CꢀH bond activation of aromatic ketones and the reaction with olefins to provide Heck-type products in
good to excellent yields with a high regio- and stereoselective manner is described.
Chelation-assisted alkenylation at the ortho position of
the aromatic ring with alkenes through a metal-catalyzed
CꢀH bond activation reaction is a highly efficient and
beneficial method in organic synthesis.1,2 In 1968,
Fujiwara’s group reported the first example of alkenylation
of electron-rich aromatics with alkenes catalyzed by palla-
dium complexes.3a,b After that, several research groups
have devoted substantial effort in the area of alkenylation
of electron-rich aromatics and heteroaromatics with al-
kenes.1,3 In 1979, Diamond and his co-workers revealed a
palladium-catalyzed chelation-assisted alkenylation at the
ortho position ofaromatic amineswithalkenes.4 Carboxyl-
directed lactonization of sp3 and sp2 CꢀH bonds catalyzed
by platinum and palladium complexes has been observed
by Sen, Miura, and others previously.5 Meanwhile, Miura
et al. demonstrated the phenol-directed alkenylation of sp2
CꢀH bonds in the presence of a palladium complex.5b,c Yu
and co-workers disclosed the palladium-catalyzed COOH,
OH and carbonyl assisted CꢀH activation/CꢀC coupling
reactions of the substituted arenes and alkanes.6 van
Leeuwen and others studied alkenylation at the ortho posi-
tion of the acetanilide CꢀH bond in the presence of palla-
dium complexes.7 Recently, enormous effort has been ex-
pended, by the groups of Miura, Satoh, Fagnou, Glorius,
and others, to the development of a rhodium-catalyzed
chelation-assisted oxidative alkenylation at the ortho posi-
tion of the aromatic CꢀH bonds.8 For the chelation-assisted
(1) Reviews for oxidative alkenylation: (a) Ritleng, V.; Sirlin, C.;
Pfeffer, M. Chem. Rev. 2002, 102, 1731. (b) Beccalli, E. M.; Broggini, G.;
Martinelli, M.; Sottocornola, S. Chem. Rev. 2007, 107, 5318. (c) Bras,
J. L.; Muzart, J. Chem. Rev. 2011, 111, 1170. (d) Yeung, C. S.; Dong,
V. M. Chem. Rev. 2011, 111, 1215.
(2) Selected recent reviews for CꢀH bond activation reactions: (a)
Alberico, D.; Scott, M. E.; Lautens, M. Chem. Rev. 2007, 107, 174. (b)
Herrerias, C. I.; Yao, X.; Li, Z.; Li, C.-J. Chem. Rev. 2007, 107, 2546. (c)
Park, Y. J.; Park, J.-W.; Jun, C.-H. Acc. Chem. Res. 2008, 41, 222. (d)
Daugulis, O.; Do, H.-Q.; Shabashov, D. Acc. Chem. Res. 2009, 42, 1074.
(e) Willis, M. C. Chem. Rev. 2010, 110, 725. (f) Daugulis, O. Top. Curr.
Chem. 2010, 292, 57. (g) Ackermann, L.; Vicente, R.; Kapdi, A. R.
Angew. Chem., Int. Ed. 2009, 48, 9792. (h) Chen, X.; Engle, K. M.; Wang,
D.-H.; Yu, J.-Q. Angew. Chem., Int. Ed. 2009, 48, 5094. (i) Colby, D. A.;
Bergman, R. G.; Ellman, J. A. Chem. Rev. 2010, 110, 624. (j) Lyons,
T. W.; Sanford, M. S. Chem. Rev. 2010, 110, 1147. (k) McMurray, L.;
Hara, F. O.; Gaunt, M. J. Chem. Soc. Rev. 2011, 40, 1885. (l)
Ackermann, L. Chem. Rev. 2011, 111, 1351. (m) Liu, C.; Zhang, H.;
Shi, W.; Lei, A. Chem. Rev. 2011, 111, 1780.
(3) (a) Fujiwara, Y.; Moritani, I.; Matsuda, M.; Teranishi, S. Tetra-
hedron Lett. 1968, 3863. (b) Fujiwara, Y.; Moritani, I.; Danno, S.;
Asano, R.; Teranishi, S. J. Am. Chem. Soc. 1969, 91, 7166. (c) Shue,
R. S. Chem. Commun. 1971, 1510. (d) Aouf, C.; Thiery, E.; Bras, J. L.;
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1c. (e) Cho, S. H.; Kim, J. Y.; Kwak, J.; Chang, S. Chem. Soc. Rev. 2011,
40, 5068.
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1979, 101, 490. (b) Cai, G.; Fu, Y.; Li, Y.; Wan, X.; Shi, Z. J. Am. Chem.
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(5) (a) Kao, L.-C.; Sen, A. J. Chem. Soc., Chem. Commun. 1991, 1242.
(b) Miura, M.; Tsuda, T.; Satoh, T.; Nomura, M. Chem. Lett. 1997,
1103. (c) Miura, M.; Tsuda, T.; Satoh, T.; Pivsa-Art, S.; Nomura, M.
J. Org. Chem. 1998, 63, 5211. (d) Dangel, B. D.; Johnson, J. A.; Sames,
D. J. Am. Chem. Soc. 2001, 123, 8149. (e) Lee, J. M.; Chang, S.
Tetrahedron Lett. 2006, 47, 1375. (f) Aoki, S.; Amamoto, C.; Oyamada,
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Tunge, J. A. J. Org. Chem. 2005, 70, 2881.
r
10.1021/ol202580e
Published on Web 10/31/2011
2011 American Chemical Society

Heck-type alkenylation reactions, only palladium and
rhodium complexes have been widely used until now. On
the other hand, the use of a less-expensive ruthenium com-
plex as a catalyst has not been explored in detail for the
oxidative alkenylation at the ortho position of the aromatic
CꢀH bond.9 While the CꢀH bond activation reaction in
the presence of strong coordinating groups is well docu-
mented and facile, activation in the presence of weak-
coordinating groups such as aldehydes, esters, and ketones
is still a challenging task. In 1993, Murai’s group reported
one of the first papers in the ruthenium-catalyzed chelation-
assisted CꢀH bond functionalization of aromatic
ketones with olefins.10a,b However, in the reaction only
alkylated products were observed. Later, several other re-
search groups also reported an addition reaction of aro-
matic ketones with alkenes by using the different ruthenium
complexes.10cꢀi In all these reactions only alkylated pro-
ducts were observed. Very recently, Ackermann’s group has
reported a ruthenium-catalyzed CꢀH bond activation reac-
tion in which aromatic acids undergo oxidative cyclization
with alkenes to give the cyclic phthalides.11 Herein, we wish
to report a ruthenium-catalyzed reaction of aryl ketones
with olefins, giving ortho-alkenylated aryl ketones through a
CꢀH bond activation reaction in a highly regio- and
stereoselective fashion.12,13
Scheme 1
The reaction of 4-bromoacetophenone (1a) with n-butyl
acrylate (2a) in the presence of [{RuCl2(p-cymene)}2]
(2 mol %), AgSbF6 (10 mol %), and Cu(OAc)2 H2O
3
(25 mol %) in 1,2-dichloroethane (DCE) at 110 °C for
12h provided a Heck-type product 3ain88% isolatedyield
with excellent E-stereoselectivity (Scheme 1). It is impor-
tant to note that, in most of the rhodium-catalyzed CꢀH
activation reactions, a stoichiometric amount of oxidant
has been used.8 However, only 25 mol % of oxidant is used
in the present reaction. It is also whorthwhile to mention
that only the catalytic amount of oxidants (Ag salts or Cu
salts or air) were used in the palladium-catalyzed alkenyla-
tion reactions.1d Control experiments revealed that no 3a
was obtained in the absence of a ruthenium catalyst, silver
salt, or copper salt (for the detailed optimization studies,
see Supporting Information (SI)).
(6) (a) Wasa, M.; Engle, K. M.; Yu, J.-Q. J. Am. Chem. Soc. 2010,
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131, 9886. (c) Lu, Y.; Wang, D.-H.; Engle, K.-M.; Yu, J.-Q. J. Am.
Chem. Soc. 2010, 132, 5916. (d) Shi, B.-F.; Zhang, Y.-H.; Lam, J. K.;
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Under the optimized reaction conditions, several substi-
tuted acetophenones 1bꢀm reacted efficiently with n-butyl
acrylate (2a) to give the corresponding alkene derivatives
3bꢀm in good to excellent yields with complete E-stereo-
selectivity (Table 1). Thus, acetophenone (1b), 4-fluoroace-
tophenone (1c), and 4-iodoacetophenone (1d) gave the Heck-
type products 3bꢀ3d in 86%, 89%, and 83% yields, respec-
tively (entries 1ꢀ3). It is interesting to note that the catalytic
reaction is compatible with very sensitive halogen groups
such as I and Br (Table 1, entry 3 and Scheme 1). Similarly,
4-methylacetophenone (1e) and 4-methoxyacetophenone
(7) (a) Boele, M. D. K.; van Strijdonck, G. P. F.; de Vries, A. H. M.;
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(9) The cost of 1 g of [{RuCl2(p-cymene)}2] is $36 from Alfa-Asear,
whereas 1 g of [{Cp*RhCl2}2] is $702.
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Kakiuchi, F.; Sekine, S.; Tanaka, Y.; Kamatani, A.; Sonoda, M.;
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Org. Lett., Vol. 13, No. 23, 2011
6145

and bis alkenylated products 3j and 3j0 in 41% and 49%
yields, respectively (eq 1).
Table 1. Results of the Reaction of Aromatic Ketones 1 with n-
Butyl Acrylate (2a)a
Next, we studied the regioselectivity of the unsymmetrical
acetophenones 1kꢀm in the reaction (entries 9ꢀ11). When
3,4-dimethoxyacetophenone (1k) was treated with n-butyl
acrylate (2a), a single regioisomeric product, 3k, was
observed in 72% yield (entry 9). Clearly, it indicates that
formation of 3k is a sterically controlled process. In
contrast, 3,4-(methylenedioxy)acetophenone (1l) provided
a single product 3l in 75% yield, in which the alkene had
been added at the more sterically hindered site (entry 10).
Table 2. Results of the Reaction of 4-Bromoacetophenone (1a),
Acetophenone (1b), or 4-Methylacetophenone (1e) with Sub-
stituted Alkenes 2bꢀfa
a All reactions were carried out using aromatic ketones 1 (1.0 mmol),
n-butyl acrylate (2a) (1.5 mmol), [{RuCl2(p-cymene)}2] (2 mol %),
AgSbF6 (10 mol %), and Cu(OAc)2 H2O (25 mol %) in DCE
3
(1,2-dichloroethane) (3.0 mL) at 110 °C for 12 h. b Isolated yields.
(1f) afforded the corresponding alkene derivatives 3e and 3f
in 79% and 77% yields, respectively (entries 4 and 5).
Surprisingly, 4-methylester acetophenone 1g reacted with
2a to give the alkenylation product 3g at the next carbon to
the acetyl group in 85% yield (entry 6). The regiochemistry
of compound 3g was established by NOESY experiments
(see SI). It is well-known that the ester group serves as an
excellent directing group for the ruthenium-catalyzed
CꢀH bond activation reactions.14 The present result
shows that the acetyl group chelates with Ru better
than the ester group. The catalytic reaction was also
tested with the effect of changing the methyl group in
acetophenone to other substituents. Thus, propiophe-
none (1h) and isobutyrophenone (1i) efficiently re-
acted with 2a to afford 3h and 3i in 85% and 79%
yields, respectively (entries 7 and 8). However, bulkier
benzophenone (1j) provided a mixture of monoalkenylated
a All reactions were carried out using aromatic ketones 1a, 1b, or 1e
(1.0 mmol), alkenes 2 (2bꢀe (1.5 mmol), 2f and 2g (2.0 mmol)),
[{RuCl2(p-cymene)}2] (2 mol %), AgSbF6 (10 mol %), and Cu(OAc)2 .
H2O (25 mol %) in DCE (1,2-dichloroethane) (3.0 mL) at 110 °C for
12 h. b Isolated yields. c The reaction was carried out in tert-butanol
(3.0 mL) instead of DCE.
On the other hand, 2-napthophenone (1m) afforded 3m,
with the alkene substituent at C3 carbon, exclusively in
77% yield (entry 11). The present methodology can be
further extended to the heteroaromatic ketone 1n. Under
the optimized reaction conditions, the reaction of chro-
mone (1n) with 2a afforded 3n in 88% yields (entry 12). In
the reaction, an alkene moiety was incorporated in the C-5
(14) Sonoda, M.; Kakiuchi, F.; Kamatani, A.; Chatani, N.; Murai, S.
Chem. Lett. 1996, 109.
6146
Org. Lett., Vol. 13, No. 23, 2011

carbon of the chromone. The regiochemistry of compound
3n was established by NOESY experiments (see SI).
The present CꢀH bond functionalization reaction was
successfully extended to various alkenes (Table 2). Methyl
acrylate (2b), ethyl acrylate (2c), tert-butyl acrylate (2d),
and cyclohexyl acrylate (2e) efficiently reacted with 1a or 1e
under the optimized reaction conditions to give the corre-
sponding Heck-type products 3oꢀr in 86%, 89%, 75%,
and 83% yields, respectively (Table 2, entries 1ꢀ4).
The catalytic reaction was also tested with substituted
styrenes. Initially, the reaction of styrene (2f) with acet-
ophenone (1b) was tested with 1,2-dichloroethane under
the optimized reaction conditions. However, in the reac-
tion, the corresponding alkenylated product was not ob-
served. The same reaction was tested with other solvents
such as THF and tert-butanol. Interestingly, tert-butanol
worked nicely and gave the expected Heck-type product 3s
in 55% yield (entry 5). But, acetophenone (1b) reacted with
4-bromostyrene (2g) under similar reaction conditions to
give bis alkenylated product 3t in 59% yield (entry 6). In a
similar fashion, 4-bromoacetophenone (1a) also reacted
efficiently with styrene (2f) to afford bis alkenylated pro-
duct 3u in 62% yield (entry 7). In these reactions, no
monoalkenylated product was observed.
the keto oxygen of 1 to the ruthenium cationic species
followed by ortho metalation gives a five-membered me-
tallacycle 4.2l Coordinative insertion of alkene 2 into the
RuꢀC bond of metallacycle 4 provides an intermediate 5.
β-Hydride elimination from intermediate 5 in the presence
of Cu(OAc)2 H2O gives the final product 3 and regener-
3
ates the active ruthenium species for the next catalytic
cycle. The exact role of the copper source was not clear in the
reaction. But, we propose that Cu(OAc)2 H2O provides
3
the OAcꢀ source to the active ruthenium species in order to
accelerate the ortho-metalation.
It is interesting to compare the mechanistic differences
between the present reactions with the previously reported
Murai’s type alkylation reaction.10 The alkylation reaction
proceeds via oxidative addition of an ortho CꢀH bond of
acetophenone to the Ru(0) center giving the ruthe-
niumꢀhydride [(Ar)Ru(II)(H)] species. Coordinative in-
sertion of an alkene into the RuꢀH species followed by
reductive elimination provides the final alkylated product,
whereas, in the present reaction, CꢀH bond cleavage takes
place via a concerted metalation/deprotonation mechan-
ism. Later, coordinative insertion of the olefin into the
RuꢀC species 4 followed by β-hydride elimination gives
the alkenylated product.
Inconclusion, we havedevelopeda ruthenium-catalyzed
chelation-assisted CꢀH bond functionalization of anortho
CꢀH bond of aromatic ketones with alkenes to afford
substituted alkene derivatives in good to excellent yields.
The catalytic reaction is highly regio- and stereoselective. It
is interesting to note that acetophenones reacted with
alkenes in the presence of RuH2(CO)(PPh3)3 to give
Michael-type alkylated products, whereas the same reac-
tion provides Heck-type alkenylated products in the pre-
sence of a catalytic amount of [{RuCl2(p-cymene)}2],
Scheme 2
AgSbF6, and Cu(OAc)2 H2O. Further extension of the
3
CꢀH bond activation of other chelating group substituted
aromatics and functionalization with other π-components
and a detailed mechanistic investigation are in progress.
Acknowledgment. We thank the Indian Institute of
Science Education and Research, Pune, India for the sup-
port of this research.
On the basis of known metal-catalyzed directing group
assisted CꢀH bond activation reactions,1ꢀ8 a possible
reaction mechanism is proposed to account for the present
catalytic reaction (Scheme 2). The removal of a chloride
ligand by a Agþ salt from the [{RuCl2(p-cymene)}2] com-
plex likely initiates the catalytic reaction. Coordination of
Supporting Information Available. General experimen-
tal procedure and characterization details. This material
is available free of charge via the Internet at http://pubs.
acs.org.
Org. Lett., Vol. 13, No. 23, 2011
6147