Sustainable H2O/ethyl lactate system for ligand-free Suzuki-Miyaura reaction

Article abstract of DOI:10.1039/c2ra21632a

A sustainable catalyst system consisted of H₂O/ethyl lactate (EL), Pd(OAc)₂ and K₂CO₃ has been developed as a generally applicable protocol for Suzuki-Miyaura reactions using various aryl bromides and iodides to incorporate arylboronic acids under ligand-free conditions. This protocol is advantageous owing to the employment of water and non-toxic, biomass available ethyl lactate as green media. In addition, as a co-solvent with water, ethyl lactate displayed evident superiority over conventional polar organic solvents such as DMF, DMSO and dioxane etc., which implied the additional potential function of EL as a ligand in these reactions.

Full text of DOI:10.1039/c2ra21632a

View Article Online / Journal Homepage / Table of Contents for this issue
Dynamic Article Links
RSC Advances
Cite this: RSC Advances, 2012, 2, 8789–8792
www.rsc.org/advances
PAPER
Sustainable H2O/ethyl lactate system for ligand-free Suzuki–Miyaura
reaction{
Jie-Ping Wan,ab Chunping Wang,a Rihui Zhoua and Yunyun Liu*ab
Received 1st August 2012, Accepted 1st August 2012
DOI: 10.1039/c2ra21632a
A sustainable catalyst system consisted of H2O/ethyl lactate (EL), Pd(OAc)2 and K2CO3 has been
developed as a generally applicable protocol for Suzuki–Miyaura reactions using various aryl
bromides and iodides to incorporate arylboronic acids under ligand-free conditions. This protocol is
advantageous owing to the employment of water and non-toxic, biomass available ethyl lactate as
green media. In addition, as a co-solvent with water, ethyl lactate displayed evident superiority over
conventional polar organic solvents such as DMF, DMSO and dioxane etc., which implied the
additional potential function of EL as a ligand in these reactions.
water–acetone,7 water–ionic liquid,8 water–MeCN,9 water–
DMF,3d water–alcohol (EtOH, i-PrOH, BuOH) etc.,10 to name
Introduction
The discovery of the Suzuki–Miyaura reaction has brought
but a few. On the other hand, using only water as solvent for the
revolutionary progress to synthetic organic chemistry by provid-
Suzuki–Miyaura reactions had also been achieved with elegant
ing a universally applicable method of constructing sp2 C–C
results, the limit for these reactions was the need for prior
bonds via the coupling of aryl/vinyl halides and boranes.1 The past
decorated palladium catalysts with functional organic auxili-
several decades have witnessed spectacular advances on the
in most cases.11 For those reactions performed under conven-
research of the Suzuki–Miyaura reaction, and the application of
aries, harsh heating, special ligands or other external assistance
this classical transformation has nowadays permeated to the
tional heating with simple palladium catalysts such as PdCl2,
synthesis of natural products, agrochemicals, clinical medicines,
Pd(OAc)2 etc., it seems that the presence of organic solvent is still
currently inevitable to guarantee satisfactory results.12 In this
intermediates of the chemical industry as well as functionalized
materials.2 Among the current extensive research interests into the
regard, the design of a catalyst system employing authentically
field of the Suzuki–Miyaura reaction, developing green and
nontoxic and biodegradable organic medium is a promising
sustainable catalyst protocols represents one of the most widely
direction for establishing greener Suzuki–Miyaura reactions.
concerned directions. In order to attain ideal results with eco-
In recent years, nontoxic biomass available solvents have been
friendly processes, numerous efforts have been made to establish-
regarded as highly potential sustainable media for organic
synthesis.13 Solvents of this type such as glycerol,14 glycerol
ing green methodologies for the Suzuki–Miyaura reaction.
ethers15 etc. have already shown broad applicability as media in
Currently, routes of designing eco-friendly protocols for the
Suzuki–Miyaura reaction include alternating traditional phos-
organic synthesis. Ethyl lactate is another biomass-derivable
phine ligands with environmentally benign ligands or ligand-free
solvent that possesses unique advantages of low production cost,
catalysis,3 reactions under mild conditions,4 reactions at low
nontoxicity (used as additive in food industry), low environmental
catalyst loading, transition metal-free conditions or employing
impact (completely biodegradable), good compatibility with water,
reusable supported catalysts.5 However, most efforts on green
high boiling point (154 uC) and excellent solubility to organic
compounds.16 Therefore, ethyl lactate is theoretically an ideal
Suzuki–Miyaura methodology have focused on the search of
green and sustainable solvent mediated catalyst systems. A lot of
solvent as medium for sustainable organic synthesis. However, the
different green media have been employed for developing the
Suzuki–Miyaura reaction in eco-friendly conditions. Typical
examples in this area include water-poly(ethylene glycol) (PEG),6
research on EL mediated organic synthesis is presently in its
infancy,17 to our best knowledge, the Suzuki–Miyaura reaction
using EL or similar biomass derived solvents is not yet available.
Herein, we report the first water–EL mixed green solvent mediated
Suzuki–Miyaura reaction using a simple commercially available
palladium catalyst under ligand free conditions.
aKey Laboratory of Functional Small Organic Molecule, Ministry of
Education, Jiangxi Normal University, Nanchang, 330022, P. R. China
bCollege of Chemistry and Chemical Engineering, Jiangxi Normal
University, Nanchang, 330022, P. R. China.
E-mail: chemliuyunyun@gmail.com; Fax: +86 791 88120380;
Tel: +86 791 88120380
Results and discussion
{ Electronic Supplementary Information (ESI) available: General
experimental information, characterization data as well as copies of 1H
and 13C NMR spectra of all products. See DOI: 10.1039/c2ra21632a
We started to explore the method on the model reaction of
iodobenzene 1a (0.5 mol) and phenylboronic acid 2a (0.75 mol)
This journal is ß The Royal Society of Chemistry 2012
RSC Adv., 2012, 2, 8789–8792 | 8789

View Article Online
with palladium catalysis. In the presence of K2CO3, 1 mol%
Pd(OAc), reactions performed at 60 uC in 3 mL of EL solvent,
pure water and EL : water (2 : 1) and (1 : 1) mixture suggested
that 1 : 1 mixed EL and water was better than other media
systems (entries 1–4, Table 1), further modification of the solvent
proved that 2 mL of 1 : 1 mixed EL : water gave even better
result (entry 5, Table 1). With this established mixed solvent
system, extensive investigation was conducted to evaluate the
effect of temperature (entry 6, Table 1), base species (entries 7–9,
Table 1) and catalyst loading (entries 10–11, Table 1). Through
comparing the results from these experiments, optimal condi-
tions turned out to be 1 mol% Pd(OAc)2, 2 eq of K2CO3 in 2 mL
mixed EL : water (1 : 1) and 60 uC (entry 5, Table 1). Further
experiments performed with PdCl2 as catalyst or with DMF–
water, DMSO–water, ethyl acetate–water, dioxane–water as
mixed media all gave inferior yield of the product 3a (entries 12–
16, Table 1).
Table 2 Suzuki–Miyaura cross coupling reactions of aryl iodides with
boronic acids in ethyl lactate : H2O systema
Entry
R1
Ar
Product
Yield (%)b
1
2
3
4
5
6
7
8
H
Ph
Ph
Ph
Ph
3a
3b
3c
3d
3e
3f
3g
3b
3h
3i
93
99
91
81
80
98
97
82
70
97
60
92
4-MeO
3-NO2
2-MeO
2-OH
H
H
H
H
H
Ph
4-FC6H4
4-ClC6H4
4-MeOC6H4
4-MeC6H4
2-MeC6H4
Thiophene-2-yl
Naphth-1-yl
9
10
11
12
a
H
H
3j
3k
Based on the optimized results, we first examined the
applicable scope by using different iodobenzenes and arylboro-
nic acid derivatives. The results from this section were shown in
Table 2. It was found that most aryl iodides reacted with
phenylboronic acids to give corresponding biaryls with excellent
yields since aryl iodides are known as active electrophiles in
various cross coupling reactions. The heteroaryl substrate
thiophen-2-boronic acid provided relatively lower yield of the
corresponding product when reacted with 1a (entry 11, Table 2).
Based on these typical examples, it was undoubted that the
palladium-catalyzed green protocol consisted of EL–water was
well suited for the Suzuki–Miyaura cross coupling of aryl iodide
substrates with boronic acids. With these acquired results, we
believed that this method should be also applicable for the
reactions of aryl bromides. Therefore, we turned to examine the
Suzuki–Miyaura reactions using aryl bromides as these reactions
represented more practical application of a catalytic system for
the Suzuki–Miyaura reaction. The reaction results acquired from
General reaction conditions: 1 (0.5 mmol), 2 (0.75 mmol) mixed in
1 : 1 EL : H2O, stirred at 60 uC for 2 h in the presence of 0.005 mmol
Pd(OAc)2 and 1 mmol K2CO3. Isolated yield based on 1.
b
a broad array of aryl/heteroaryl bromides and aryl boronic acids
were outlined in Table 3. As expected, the reactions of aryl
bromides have also shown excellent tolerance to this protocol
albeit the reaction temperature was enhanced because of lower
reactivity of these bromide substrates. Generally, aryl bromides
with electron withdrawing as well donating groups in the ortho-,
meta- and para-positions, and heteroaryl bromides were able to
smoothly incorporate a class of different aryl boronic acids to
give corresponding biaryls 3 in moderate to excellent yields.
Based on the results from these present entries, the properties of
the functional groups in both reactants didn’t show any regular
effects on the reaction efficiency. On the other hand, an attempt
Table 3 Suzuki–Miyaura cross coupling reactions of aryl bromides with
boronic acids in ethyl lactate : H2O systema
Table 1 Optimization of reaction conditionsa
Entry Catalyst (mol %) Base
Solvent (mL)
Yield (%)b
Entry
Ar1
Ar2
Product
Yield (%)b
1
2
3
4
Pd(OAc)2 (1)
Pd(OAc)2 (1)
Pd(OAc)2 (1)
Pd(OAc)2 (1)
Pd(OAc)2 (1)
Pd(OAc)2 (1)
Pd(OAc)2 (1)
Pd(OAc)2 (1)
Pd(OAc)2 (1)
Pd(OAc)2 (0.5)
Pd(OAc)2 (0.25) K2CO3
PdCl2 (1)
Pd(OAc)2 (1)
Pd(OAc)2 (1)
Pd(OAc)2 (1)
Pd(OAc)2 (1)
K2CO3
K2CO3
K2CO3
K2CO3
K2CO3
K2CO3
Na2CO3 EL : H2O (1 : 1)
Et3N EL : H2O (1 : 1)
NaHCO3 EL : H2O (1 : 1)
EL : H2O (2 : 1)
EL (3)
H2O (3)
EL : H2O (1.5 : 1.5)
EL : H2O (1 : 1)
EL : H2O (1 : 1)
26
62
60
56
93
47
62
53
44
52
44
23
70
1
2
3
4
5
6
7
8
Ph
Ph
Ph
Ph
Ph
3a
3l
81
93
76
65
60
43
56
62
81
88
96
62
43
50
48
45
4-OHC6H4
3-MeOC6H4
4-AcC6H4
2-OHCC6H4
Ph
3m
3n
3o
3b
3f
3k
3p
3q
3r
3s
3t
3u
3v
3w
5
Ph
6c
7
4-MeOC6H4
4-FC6H4
Naphtha-1-yl
4-MeC6H4
4-FC6H4
4-ClC6H4
4-FC6H4
4-ClC6H4
Napth-1-yl
Ph
Ph
Ph
8
9
9
4-AcC6H4
4-AcC6H4
2-OHCC6H4
2-OHCC6H4
4-MeC6H4
3-MeOC6H4
Naphtha-2-yl
Pyridine-2-yl
10
11
12
13
14
15
16
a
K2CO3
EL : H2O (1 : 1)
EL : H2O (1 : 1)
EL : H2O (1 : 1)
DMF : H2O (1 : 1)
DMSO : H2O (1 : 1) 57
Dioxane : H2O (1 : 1) 58
10
11
12
13
14
15
16
a
K2CO3
K2CO3
K2CO3
K2CO3
K2CO3
EA : H2O (1 : 1)
15
Ph
Unless specified, the general conditions: 1a (0.5 mmol), 2a (0.75 mmol)
and base (2 eq) at 60 uC for 2 h. EL = ethyl lactate, EA = ethyl acetate.
General reaction conditions: 4 (0.5 mmol), 2 (0.75 mmol) mixed in
1 : 1 EL : H2O, stirred at 80 uC for 2 h in the presence of 0.005 mmol
Pd(OAc)2 and 1 mmol K2CO3. Isolated yield based on 4.
b
b
Isolated yield based on 1a. c This reaction was performed at 45 uC.
8790 | RSC Adv., 2012, 2, 8789–8792
This journal is ß The Royal Society of Chemistry 2012

View Article Online
General procedure for Suzuki–Miyaura reactions using aryl
on employing functional aryl chlorides such as p-tolyl chloride
as a reaction partner showed only trace transformation under
present conditions.
bromides and bromophenyl iodides.
To a 25 mL round bottom flask was charged aryl bromide/
bromophenyl iodide (0.5 mmol), aryl boronic acid (0.75 mmol),
Pd(OAc)2 (0.005 mmol) and K2CO3 (1 mmol), EL (1 mL) and
water (1 mL) were added and the mixture was heated at 80 uC for
2 h (TLC). After cooling down to room temperature, the mixture
was extracted with ethyl acetate (3 6 10 mL). The combined
organic phases were dried over anhydrous Na2SO4. The solvent
was then removed under reduced pressure and the residue was
subjected to silica gel chromatography to give pure product.
The practical reaction results of aryl boronic acids to both aryl
iodides and bromides encouraged us to further explore the
reactivity of different bromo- and iodoaryl dihalides to
investigate the chemoselectivity of these bihaloaryls in the
Suzuki–Miyaura coupling. Interestingly, when subjected to
80 uC heating in the presence of 2 equiv of phenylboronic acid,
the mono-coupled biaryl products 3x–3z were obtained as single
or major products and the doubly coupled triaryl product was
not observed or isolated as a minor product (Scheme 1). The
chemoselectivity was different from most previously reported
catalyst protocols wherein triaryls were usually provided as the
main or sole products.5k,7,18 The excellent chemoselectivity of
our approach on the production of bromo-substituted biaryls
was advantageous for the synthesis of unsymmetrical triaryls via
stepwise cross coupling using different aryl boronic acids.
Acknowledgements
Financial support from NSFC of China (No. 21102059), NSF of
Jiangxi Province (No. 20114BAB213005), a Sponsored Program
for Cultivating Youths of Outstanding Ability in Jiangxi Normal
University and a grant from the Open Project Program of Key
Laboratory of Functional Small Organic Molecule, Ministry of
Education, Jiangxi Normal University (No. KLFS-KF-201220)
are gratefully acknowledged.
References
1 (a) N. Miyaura and A. Suzuki, J. Chem. Soc., Chem. Commun., 1979,
866–867; (b) N. Miyaura, K. Yamada and A. Suzuki, Tetrahedron
Lett., 1979, 20, 3437–3440.
Scheme 1 Chemoselective Suzuki–Miyaura reactions of bromoaryl
2 (a) N. Miyaura and A. Suzuki, Chem. Rev., 1995, 95, 2457–2483; (b)
A. Suzuki, Angew. Chem., Int. Ed., 2011, 50, 6722–6737; (c) J.
Dupont, C. S. Consorti and J. Spencer, Chem. Rev., 2005, 105,
2527–2572; (d) L. Yin and J. Liebscher, Chem. Rev., 2007, 107,
133–173; (e) J. D. Sellars and P. G. Steel, Chem. Soc. Rev., 2011, 40,
5170–5180; (f) A. Fihri, M. Bouhrara, B. Nekoueishahraki, J.-M.
Basset and V. Polshettiwar, Chem. Soc. Rev., 2011, 40, 5181–5203; (g)
F. A. Littke and G. C. Fu, Angew. Chem., Int. Ed., 2002, 41,
4176–4211; (h) S. Kotha and K. Mandal, Chem.–Asian J., 2009, 4,
354–362.
3 (a) C. Zhou, J. Wang, L. Li, R. Wang and M. Hong, Green Chem.,
2011, 13, 2100–2106; (b) S.-S. Zhang, Z.-Q. Wang, M.-H. Xu and
G.-Q. Lin, Org. Lett., 2010, 12, 5540–5549; (c) L.-W. Xu, X.-H.
Chen, H. Shen, Y. Deng, J.-X. Jiang, K.-Z. Jiang, G.-Q. Lai and
C.-Q. Shen, Eur. J. Org. Chem., 2012, 290–297; (d) C. Liu, Q. Ni, F.
Bao and J. Qiu, Green Chem., 2011, 13, 1260–1266; (e) C. Liu, Q. Ni,
P. Hu and J. Qiu, Org. Biomol. Chem., 2011, 9, 1054–1060; (f) Y.
Kitamura, S. Sako, A. Tsutsui, Y. Monguchi, T. Maegawa, Y.
Kitade and H. Sajiki, Adv. Synth. Catal., 2010, 351, 718–730; (g) B.
Basu, K. Biswas, S. Kundu and S. Ghosh, Green Chem., 2010, 12,
1734–1738.
4 (a) L. Wu, E. Drinkel, F. Gaggia, S. Capolicchio, A. Linden, L.
Falivene, L. Cavallo and R. Dorta, Chem.–Eur. J., 2011, 17,
12886–12890; (b) B. H. Lipshutz, T. B. Petersen and A. R. Abela,
Org. Lett., 2008, 7, 1333–1336; (c) B. Karimi and P. F. Akhavan,
Chem. Commun., 2011, 47, 4686–4688; (d) P. Das, C. Sarmah, A.
Tairai and U. Bora, Appl. Organomet. Chem., 2011, 25, 283–288; (e)
Y.-Y. Peng, J. Liu, X. Lei and Z. Yin, Green Chem., 2010, 12,
1072–1075.
5 (a) A. Fihri, D. Luart, C. Len, A. Solhy, C. Chevrin and V.
Polshettiwar, Dalton Trans., 2011, 40, 3116–3121; (b) M. Cui, J. Li,
A. Yu, J. Zhang and Y. Wu, J. Mol. Catal. A: Chem., 2008, 290,
67–71; (c) O. Navarro, N. Marion, J. Mei and S. P. Nolan, Chem.–
Eur. J., 2006, 19, 5142–5148; (d) N. E. Leadbeater and M. Marco,
Angew. Chem., Int. Ed., 2003, 12, 1407–1409; (e) R. K. Arvela, N. E.
Leadbeater and M. J. Collins, Tetrahedron, 2005, 61, 9349–9355; (f)
K. Inamoto, C. Hasegawa, K. Hiroya, Y. Kondo, T. Osako, Y.
Uozumi and T. Doi, Chem. Commun., 2012, 48, 2912–2914; (g) R. K.
Arvela, N. E. Leadbeater, M. S. Sangi, V. A. Williams, P. Granados
and R. D. Singer, J. Org. Chem., 2005, 70, 161–168; (h) B. Karimi, D.
Elhamifar, J. H. Clark and J. Hunt, Chem.–Eur. J., 2010, 16,
8047–8053; (i) L. Wu, B.-L. Li, Y.-Y. Huang, H.-F. Zhou, Y.-M. He
iodides.
Conclusions
In conclusion, we developed a sustainable system consisting of
EL–water media in the presence of 1 mol% Pd(OAc)2. This
protocol tolerated the reactions of a broad array of aryl halides
and aryl boronic acids under mild reaction conditions. Unique
advantages of this methodology were the nontoxicity, no
environmental impact, low cost and easy availability of the
reaction media as well as ligand-free conditions. More impor-
tantly, since the biomass available EL was rarely employed in
organic synthesis as an eco-friendly solvent, successful employ-
ment of EL as solvent for Suzuki–Miyaura reactions marks a
notable extension on biomass organic solvent-based green
synthesis. The present methodology also provides a useful
complement to known catalyst systems for Suzuki–Miyaura
reactions.
Experimental
General procedure for Suzuki–Miyaura reactions using aryl iodides
To a 25 mL round bottom flask was charged aryl iodide (0.5 mmol),
aryl boronic acid (0.75 mmol), Pd(OAc)2 (0.005 mmol) and K2CO3
(1 mmol). EL (1 mL) and water (1 mL) were added and the mixture
was heated at 60 uC for 2 h (TLC). After cooling down to room
temperature, the mixture was extracted with ethyl acetate (3 6
10 mL). The combined organic phases were dried over anhydrous
Na2SO4. The solvent was then removed under reduced pressure and
the residue was subjected to silica gel chromatography to give pure
product.
This journal is ß The Royal Society of Chemistry 2012
RSC Adv., 2012, 2, 8789–8792 | 8791

View Article Online
and Q.-H. Fan, Org. Lett., 2006, 8, 3605–3608; (j) D.-H. Lee, M.
Choi, B.-W. Yu, R. Ryoo, A. Taher, S. Hossain and M. J. Jin, Adv.
Synth. Catal., 2009, 351, 2912–2920; (k) J.-F. Wei, J. Jiao, J.-J. Feng,
J. Lv, X.-R. Zhang, X.-Y. Shi and Z.-G. Chen, J. Org. Chem., 2009,
74, 6283–6286.
6 (a) L. Liu, Y. Zhang and Y. Wang, J. Org. Chem., 2005, 70,
6122–6125; (b) S. Shi and Y. Zhang, Green Chem., 2008, 10, 868–872;
(c) A. L. F. de Souza, A. D. Silva and O. A. C. Autunes, Appl.
Organomet. Chem., 2009, 23, 5–8.
7 L. Liu, Y. Zhang and B. Xin, J. Org. Chem., 2006, 71, 3994–3997.
8 (a) B. Xin, Y. Zhang, L. Liu and Y. Wang, Synlett, 2005, 3083–3086;
(b) N. Liu, C. Liu and Z. Jin, Green Chem., 2012, 14, 592–597.
9 I. Cobo, M. I. Matheu, S. Castillo´n, O. Boutureira and B. G. Davis,
Org. Lett., 2012, 14, 1728–1731.
10 (a) M. Gruttadauria, L. F. Liotta, A. M. P. Salvo, F. Giacalone,
V. L. Parola, C. Aprile and R. Noto, Adv. Synth. Catal., 2011, 253,
2119–2130; (b) T. Maegawa, Y. Kitamura, S. Sako, T. Udzu, A.
Sakurai, A. Tanaka, Y. Kobayashi, K. Endo, U. Bora, T. Kurita, A.
Kozaki, Y. Monguchi and H. Sajiki, Chem.–Eur. J., 2007, 13,
5937–5943; (c) C. Liu and W. Yang, Chem. Commun., 2009,
6267–6269; (d) S. Li, Y. Lin, J. Cao and S. Zhang, J. Org. Chem.,
2007, 72, 4067–4072; (e) C. A. Fleckenstein and H. Plenio, J. Org.
Chem., 2008, 73, 3236–3244.
Chem. Soc., 2009, 131, 16346–16347; (c) B. Yuan, Y. Pan, Y. Li, B. Yin
and H. Jiang, Angew. Chem., Int. Ed., 2010, 49, 4054–4058; (d) J. Zhi,
D. Song, Z. Li, X. Lei and A. Hu, Chem. Commun., 2011, 47,
10707–10709; (e) A. N. Marziale, D. Jantke, S. H. Faul, T. Reiner, E.
Herdtweck and J. Eppinger, Green Chem., 2011, 13, 169–177; (f) N. E.
Leadbeater, Chem. Commun., 2005, 2881–2092; (g) N. E. Leadbeater
and M. Marco, J. Org. Chem., 2003, 68, 888–892; (h) C. Baleiza˜o, A.
Corma, H. Garc´ıa and A. Leyva, Chem. Commun., 2003, 606–607.
12 For a review on aqueous media-based Suzuki–Miyaura reaction,
see: V. Polshettiwar, A. Decottignies, C. Len and A. Fihri,
ChemSusChem, 2010, 3, 502–522.
13 P. G. Jessop, Green Chem., 2011, 13, 1391–1398.
14 Y. Gu and F. Je´roˆme, Green Chem., 2010, 12, 1127–1138.
15 J. I. Garc´ıa, H. Garc´ıa-Mar´ın, J. A. Mayoral and P. Pe´rez, Green
Chem., 2010, 12, 426–434.
16 (a) S. Aparicio and R. Alcalde, Green Chem., 2009, 11, 65–78; (b)
C. S. M. Pereira, V. M. T. M. Silva and R. E. Rodrigues, Green
Chem., 2011, 13, 2658–2671.
17 J. S. Bennett, K. L. Charles, M. R. Miner, C. F. Heuberger, E. J.
Spina, M. F. Bartels and T. Foreman, Green Chem., 2009, 11,
166–168.
18 (a) S. R. Borhade and S. B. Waghmode, Beilstein J. Org. Chem.,
2011, 7, 310–319; (b) N. Mej´ıas, R. Pleixats, A. Shafir, M. Medio-
Simo´n and G. Asensio, Eur. J. Org. Chem., 2010, 5090–5099; (c)
X.-H. Fan and L.-M. Yang, Eur. J. Org. Chem., 2011, 1467–1471.
11 (a) L. Botella and C. Na´jera, Angew. Chem., Int. Ed., 2002, 41,
179–182; (b) J. M. Chalker, C. S. C. Wood and B. G. Davis, J. Am.
8792 | RSC Adv., 2012, 2, 8789–8792
This journal is ß The Royal Society of Chemistry 2012