Ferrocene as a scaffold for effective bifunctional amine-thiourea organocatalysts

Article abstract of DOI:10.1039/c4cy00199k

A simple and readily accessible prototype of the ferrocene-based bifunctional amine-thioureas shows high enantioselectivity in the Michael addition of acetylacetone to nitroolefins, giving the enantioselectivity of up to 96% ee. This work demonstrates tha

Full text of DOI:10.1039/c4cy00199k

Catalysis
Science &
Technology
View Article Online
View Journal | View Issue
COMMUNICATION
Ferrocene as a scaffold for effective bifunctional
amine–thiourea organocatalysts†
Cite this: Catal. Sci. Technol., 2014,
4, 1726
*
Wei Yao,‡ Ming Chen,‡ Xueying Liu, Ru Jiang, Shengyong Zhang
*
and Weiping Chen
Received 15th February 2014,
Accepted 18th March 2014
DOI: 10.1039/c4cy00199k
www.rsc.org/catalysis
A simple and readily accessible prototype of the ferrocene-based
bifunctional amine–thioureas shows high enantioselectivity in the
Michael addition of acetylacetone to nitroolefins, giving the
enantioselectivity of up to 96% ee. This work demonstrates that,
in accord with metal catalysis, ferrocene could be an excellent
scaffold for chiral organocatalysts.
and PIP11 as acyl transfer catalysts for the kinetic resolution of
racemic alcohols and amines, as well as simple chiral
ferrocene-based phosphines as nucleophilic organocatalysts
for the enantioselective boration of olefins,12 dimerizations of
ketenes,13 [3 + 2] cyclizations14 and (aza-)Morita–Baylis–Hillman
reaction.15 As a part of our continuous research on the devel-
opment of ferrocene-based chiral ligands and catalysts,16 we
are interested in exploring the potential of the ferrocene
Since the pioneering work of Jacobsen,1 Schreiner2 and
Takemoto,3 bifunctional amine–thiourea, which incorporates
both Lewis/Brønsted acid and base functionalities into a
chiral scaffold within the same molecule, has become one of
the most versatile catalysts in asymmetric organocatalysis
and has been used in very successful catalysts for numerous
enantioselective reactions.4,5 Despite their tremendous utility,
these organocatalysts are derived from a very limited range of
chiral structural scaffolds. The typical catalysts of this family
include the 1,2-diamine derivatives A (Takemoto catalyst)
(Fig. 1),3 the cinchona-alkaloid-derived catalysts such as B,
developed independently by four research groups,6 and
binaphthyl based catalysts C.7 The key to the success of these
catalysts is their ability to activate both nucleophilic and
electrophilic substrates independently and simultaneously
by the discrete functionalities, amine and thiourea, within
the same catalyst, and to control their encounter in a well-
defined chiral environment.
Ferrocene is a “privileged framework” for the construc-
tion of effective chiral ligands in metal catalysis due to its
specific and unique geometries (adequate rigidity, steric
bulkiness and planar chirality), electronic (redox) properties,
easy accessibility and derivatization, as well as stability.8
Surprisingly, ferrocene has not been exploited as a backbone of
organocatalysts9 except for the use of the planar chiral DMAP10
School of Pharmacy, Fourth Military Medical University, 169 Changle West Road,
Xi'an, 710032, PR China. E-mail: wpchen@fmmu.edu.cn
† Electronic supplementary information (ESI) available. See DOI: 10.1039/
c4cy00199k
‡ These authors contributed equally to this work.
Fig. 1 Structure of bifunctional amine–thioureas.
1726 | Catal. Sci. Technol., 2014, 4, 1726–1729
This journal is © The Royal Society of Chemistry 2014

View Article Online
Catalysis Science & Technology
Communication
moiety as a scaffold for effective organocatalysts.17 We envisioned
that, in the ferrocene-based bifunctional amine–thioureas with
a general structure D (Fig. 1), the rigid, bulky, planar and carbon-
centered chiral ferrocene moiety provided effective spatial arrange-
ment and should be an ideal scaffold for organocatalysts, allowing
the catalytic transformation with excellent enantioselectivity.
Herein, we describe the preliminary results of a simple and
readily accessible prototype of the ferrocene-based bifunctional
amine–thioureas as an organocatalyst for the enantioselective
Michael addition of acetylacetone to nitroolefins. To the best
of our knowledge, this is the first example of ferrocene-based
bifunctional amine–thioureas.
A simple prototype of the ferrocene-based bifunctional
amine–thioureas, (RC,SFc)-1 is readily synthesized from (R)-Ugi's
amine (R)-2 in three steps (Scheme 1). Thus, lithiation of (R)-2
with t-BuLi (0 °C ~ rt, 1 h) followed by reaction with
p-toluenesulfonyl azide gave the azide (RC,SFc)-3 in 82% yield.
(RC,SFc)-3 was hydrogenated in the presence of 5% Pd–C at a
H2 pressure of 1 bar to afford the diamine (RC,SFc)-4 in 93%
yield. Finally, the bifunctional amine–thiourea (RC,SFc)-1 was
obtained in 86% yield from the reaction of (RC,SFc)-4 with
3,5-bis(trifluoromethyl)phenyl isothiocyanate.
In order to demonstrate the potential of ferrocene as a
scaffold for organocatalysts, the performance of (RC,SFc)-1
was initially evaluated in the model Michael addition of
acetylacetone 6 to trans-β-nitrostyrene 5a in the presence
of 10 mol% of catalyst at room temperature, and the results
are summarized in Table 1. The choice of solvent plays a
critical role in the reaction. Reactions in chlorinated solvents
(dichloromethane, CHCl3 and 1,2-dichloroethane) afforded
the desired Michael adduct (R)-7a with moderate to good
yields (55–78%) and enantioselectivities (41–70% ee)
(Table 1, entries 1–3). More polar solvents, such as CH3CN,
dioxane and N,N-dimethylformide, decreased remarkably
the enantioselectivity (entries 4–6) while an almost racemic
Table 1 Asymmetric Michael addition of acetylacetone to trans-β-
nitrostyrene catalyzed by amine–thiourea (RC,SFc)-1a
(RC,SFc)-1
(x mol%)
Entry Solvent
Temp (°C) Yield (%)b
ee (%)c,d
1
2
3
4
5
6
7
8
CH2Cl2
CHCl3
(ClCH2)2 10
MeCN
Dioxane
DMF
MeOH
EtOH
DMSO
THF
10
10
25
25
25
25
25
25
25
25
25
25
25
25
25
25
25
40
50
0
55
65
78
69
45
80
35
40
55
65
82
75
85
78
50
90
90
65
41
40
70
12
19
13
~0
~0
~0
64
71
81
79
80
68
78
56
73
10
10
10
10
10
10
10
10
10
10
15
5
9
10
11
12
13
14
15
16
17
18e
CCl4
Toluene
o-Xylene
Toluene
Toluene
Toluene
Toluene
Toluene
10
10
10
a
Unless otherwise specified, the reactions were performed using
0.2 mmol of 5a and 0.4 mmol of 6 in 1.0 mL of solvent for 60 h.
b
c
d
Isolated yield. Determined by chiral HPLC analysis. Absolute
configuration was assigned by comparing the optical rotation values
with those reported in the literature. e Reacted for 72 h.
mixture was obtained with a protic solvent, such as MeOH
and EtOH, or the very polar solvent dimethylsulfoxide
(entries 7–9). Nonpolar solvents improved the enantio-
selectivity (entries 10–13). Like most Michael additions of
β-nitrostyrene with acetylacetone catalyzed by bifunctional
amine–thioureas,18 toluene is the best solvent in the reaction
(entry 12), possibly due to the increased hydrogen bonding
activation of β-nitrostyrene by (RC,SFc)-1 in the nonpolar
solvent. Lowering the catalyst loading to 5 mol% led to a
significant decrease in the enantioselectivity (entry 15).
Interestingly, increasing the reaction temperature from 25 °C
to 40 °C had only a marginal effect on enantioselectivity
(entry 12 vs. 16) while the selectivity decreased remarkably
when the temperature was changed from 40 °C to 50 °C
(entry 16 vs. 17). Surprisingly, lowering the reaction tempera-
ture from 25 °C to 0 °C had no beneficial effects on the
enantioselectivity (entry 18).
Following the establishment of a set of acceptable reaction
conditions: 6 (0.40 mmol, 2.0 equiv) and 5a (0.20 mmol,
1.0 equiv) in 1.0 mL of toluene with 10 mol% of (RC,SFc)-1 at
25 °C for 60 h, the substrate scope was explored. As shown in
Table 2, all the nitrostyrenes bearing either electron-donating
or electron-withdrawing substituents on the aromatic ring
gave the desired Michael adducts in good to excellent yields
and enantioselectivities. Normally, higher enantioselectivities
were obtained with the nitrostyrenes having a substituent at
Scheme 1 Synthesis of ferrocene-based bifunctional amine–thiourea
(RC,SFc)-1. Reagents and conditions: a) t-BuLi, TBME, 0 °C–rt, 1 h;
b) p-toluenesulfonyl azide, TBME, −78 °C–rt, 5 h; c) H2, 5% Pd–C,
MeOH, rt, 4 h; d) 3,5-(CF3)2C6H3NCS; CH2Cl2, rt, 4 h.
This journal is © The Royal Society of Chemistry 2014
Catal. Sci. Technol., 2014, 4, 1726–1729 | 1727

View Article Online
Communication
Catalysis Science & Technology
Table 2 Asymmetric Michael addition of acetylacetone to trans-β-
nitrostyrene catalyzed by amine–thiourea (RC,SFc)-1a
Meanwhile, the nitrostyrene is activated by the hydrogen bond-
ing interaction of the nitro group with thiourea, in which the
bottom Cp ring of ferrocene forces the phenyl ring to orient
above the upper Cp ring of ferrocene. Then, the activated
acetylacetone enolate attacks the thiourea-activated nitroolefin
from the Si-face, leading to the formation of (R)-adducts.
Entry
Ar
Product
Yield (%)b
ee (%)c,d
Conclusions
1
2
3
4
5
6
7
8
C6H5 (5a)
7a
7b
7c
7d
7e
7f
7g
7h
7i
7j
7k
7l
7m
7n
7o
7p
7q
7r
75
85
88
92
78
81
95
90
78
75
72
70
64
65
75
68
72
72
65
80
81
96
92
89
90
83
95
83
85
80
74
84
80
90
78
89
78
88
77
81
2-Cl-C6H4 (5b)
2-Br-C6H4 (5c)
2-F-C6H4 (5d)
In summary, a simple and readily accessible ferrocene-based
bifunctional amine–thiourea (RC,SFc)-1 is highly effective in the
Michael addition of acetylacetone to nitroolefins, giving the
enantioselectivity of up to 96% ee. This work demonstrates that,
in accord with metal catalysis, ferrocene could be an excellent
scaffold for chiral organocatalysts for the first time. Work is
actively under way in our lab to modify the structure of
ferrocene-based bifunctional amine–thioureas, unravel its struc-
ture–reactivity–enantioselectivity relationships, expand its appli-
cation to other valuable transformations and develop other
types of organocatalysts based on the ferrocene backbone.
2-NO2-C6H4 (5e)
2-CH3O-C6H4 (5f)
2,3-Cl2-C6H3 (5g)
2,4-Cl2-C6H3 (5h)
3-CH3O-C6H4 (5i)
3-Br-C6H4 (5j)
4-CH3O-C6H4 (5k)
4-Cl-C6H4 (5l)
4-Br-C6H4 (5m)
4-F-C6H4 (5n)
4-CF3-C6H4 (5o)
4-CH3-C6H4 (5p)
4-EtO-C6H4 (5q)
1-naphthyl (5r)
2-naphthyl (5s)
2-furyl (5t)
9
10
11
12
13
14
15
16
17
18
19
20
Acknowledgements
7s
7t
We thank the National Natural Science Foundation of China
(21272271) for financial support.
a
Reaction conditions: 6 (0.40 mmol, 2.0 equiv) and 5 (0.20 mmol,
1.0 equiv) in 1.0 mL of toluene with 10 mol% of (RC,SFc)-1 at 25 °C for
60 h. b Isolated yield. c Determined by chiral HPLC analysis. d Absolute
configuration was assigned by comparing the optical rotation values
with those reported in the literature.
Notes and references
1 M. S. Sigman and E. N. Jacobsen, J. Am. Chem. Soc., 1998,
120, 4901.
2 P. R. Schreiner and A. Wittkopp, Org. Lett., 2002, 4, 217.
3 (a) T. Okino, Y. Hoashi and Y. Takemoto, J. Am. Chem. Soc.,
2003, 125, 12672; (b) T. Okino, Y. Hoashi, T. Furukawa,
X. Xu and Y. Takemoto, J. Am. Chem. Soc., 2005, 127, 119; (c)
H. Miyabe and Y. Takemoto, Bull. Chem. Soc. Jpn., 2008, 81,
785; (d) Y. Takemoto, Chem. Pharm. Bull., 2010, 58, 593.
4 (a) For recent reviews on bifunctional amine-thiourea, see:
Z. Zhang and P. R. Schreiner, Chem. Soc. Rev., 2009, 38, 1187;
(b) S. J. Connon, Synlett, 2009, 354; (c) W.-Y. Siau and J. Wang,
Catal. Sci. Technol., 2011, 1, 1298; (d) S. Narayanaperumal,
D. G. Rivera, R. C. Silva and M. W. Paixao, ChemCatChem, 2013,
5, 2756; (e) O. V. Serdyuk, C. M. Heckel and S. B. Tsogoeva, Org.
Biomol. Chem., 2013, 11, 7051; ( f ) G. Jakab and P. R. Schreiner,
in Comprehensive Enantioselective Organocatalysis, ed. P. I. Dalko,
Wiley-VCH, Weinheim, 2013, pp. 315–342.
the 2-position of the phenyl ring (Table 1, entries 2–5),
and the 2-chloro derivative gave the highest enantioselectivity
(96% ee, entry 2). The preliminary results indicate that the
rigid, bulky, planar and carbon-centered chiral ferrocene
moiety is indeed an excellent scaffold for bifunctional
amine–thiourea, which catalyzes the Michael addition of a
variety of trans-β-nitrostyrenes with acetylacetone in high
enantioselectivity.
The absolute configuration of the Michael adducts was
assigned as (R) by comparing the optical rotation values with
those reported in the literature.18 A plausible transition state
for (RC,SFc)-1 catalyzed Michael reaction was proposed
(Fig. 2). The dimethylamino group of (RC,SFc)-1 deprotonates
an acidic proton of acetylacetone, generating the enolate.
5 (a) For some of the most recent examples of bifunctional
amine-thioureas in organocatalysis, see: M. P. Lalonde,
M. A. McGowan, N. S. Rajapaksa and E. N. Jacobsen, J. Am.
Chem. Soc., 2013, 135, 1891; (b) Y. Fukata, K. Asano and
S. Matsubara, J. Am. Chem. Soc., 2013, 135, 12160; (c) Z. Jin,
J. Xu, S. Yang, B.-A. Song and Y. R. Chi, Angew. Chem.,
Int. Ed., 2013, 52, 12354; (d) B. Qiao, Y. An, Q. Liu, W. Yang,
H. Liu, J. Shen, L. Yan and Z. Jiang, Org. Lett., 2013, 15,
2358; (e) C. B. Tripathi and S. Mukherjee, Angew. Chem.,
Int. Ed., 2013, 52, 8450; ( f ) R. L. Farmer and K. A. Scheidt,
Chem. Sci., 2013, 4, 3304; (g) R. Rohlmann, C.-G. Daniliuc
and O. G. Mancheño, Chem. Commun., 2013, 49, 11665; (h)
Fig. 2 Plausible transition-state model of Michael reaction.
1728 | Catal. Sci. Technol., 2014, 4, 1726–1729
This journal is © The Royal Society of Chemistry 2014

View Article Online
Catalysis Science & Technology
Communication
T. Tite, M. Sabbah, V. Vincent Levacher and J.-F. Brière,
Chem. Commun., 2013, 49, 11569; (i) J. Li, T. Du, G. Zhang
and Y. Peng, Chem. Commun., 2013, 49, 1330; ( j) R. Miyaji,
K. Asano and S. Matsubara, Org. Lett., 2013, 15, 3658; (k)
S. Meninno, G. Croce and A. Lattanzi, Org. Lett., 2013, 15,
3436; (l) B. Qiao, Y. An, Q. Liu, W. Yang, H. Liu, J. Shen,
L. Yan and Z. Jiang, Org. Lett., 2013, 15, 2358; (m)
S. Rajkumar, K. Shankland, J. M. Goodman and
A. J. A. Cobb, Org. Lett., 2013, 15, 1386; (n) Q. Guo and
J. C.-G. Zhao, Org. Lett., 2013, 15, 508; (o) D. Bastida, Y. Liu,
X. Tian, E. Escudero-Adán and P. Melchiorre, Org. Lett.,
2013, 15, 220; ( p) A. Quintavalla, F. Lanza, E. Montroni,
M. Lombardo and C. Trombini, J. Org. Chem., 2013, 78,
12049; (q) S. Perera, D. Sinha, N. K. Rana, V. Trieu-Do and
J. C.-G. Zhao, J. Org. Chem., 2013, 78, 10947; (r) W.-Y. Han,
J.-Q. Zhao, Z.-J. Wu, X.-M. Zhang and W.-C. Yuan, J. Org.
Chem., 2013, 78, 10541; (s) Z. Mao, A. Lin, Y. Shi, H. Mao,
W. Li, Y. Cheng and C. Zhu, J. Org. Chem., 2013, 78, 10233;
(t) B.-D. Cui, W.-Y. Han, Z.-J. Wu, X.-M. Zhang and
W.-C. Yuan, J. Org. Chem., 2013, 78, 8833; (u) A. Noole,
K. Ilmarinen, I. Järving, M. Lopp and T. Kanger, J. Org. Chem.,
2013, 78, 8117; (v) B. Zhou, Y. Yang, J. Shi, Z. Luo and Y. Li,
J. Org. Chem., 2013, 78, 2897; (w) Z.-X. Jia, Y.-C. Luo,
X. N. Cheng, P.-F. Xu and Y.-C. Gu, J. Org. Chem., 2013, 78,
6488; (x) W.-T. Meng, Y. Zheng, J. Nie, H.-Y. Xiong and
J.-A. Ma, J. Org. Chem., 2013, 78, 559; (y) Y. Gao and D.-M. Du,
Tetrahedron: Asymmetry, 2013, 24, 1312; (z) J. Guang and
J. C.-G. Zhao, Tetrahedron Lett., 2013, 54, 5703.
P. G. Cozzi, Chem. – Eur. J., 2013, 19, 7696; (b) L. A. Al-Momani
and A. Lataifeh, Inorg. Chim. Acta, 2013, 394, 176; (c) A. Patti
and S. Pedotti, Eur. J. Org. Chem., 2014, 624.
10 (a) G. C. Fu, Acc. Chem. Res., 2000, 33, 412; (b) G. C. Fu,
Acc. Chem. Res., 2004, 37, 542; (c) S. Y. Lee, J. M. Murphy,
A. Ukai and G. C. Fu, J. Am. Chem. Soc., 2012, 134, 15149,
and references therein.
11 B. Hu, M. Meng, Z. Wang, W. Du, J. S. Fossey, X. Hu and
W.-P. Deng, J. Am. Chem. Soc., 2010, 132, 17041.
12 A. Bonet, H. Gulyás and E. Fernández, Angew. Chem., Int. Ed.,
2010, 49, 5130.
13 A. A. Ibrahim, P.-H. Wei, G. D. Harzmann and N. J. Kerrigan,
J. Org. Chem., 2010, 75, 7901.
14 (a) A. Voituriez, A. Panossian, N. Fleury-Brégeot, P. Retailleau
and A. Marinetti, J. Am. Chem. Soc., 2008, 130, 14030; (b)
A. Voituriez, A. Panossian, N. Fleury-Brégeot, P. Retailleau and
A. Marinetti, Adv. Synth. Catal., 2009, 351, 1968; (c) N. Pinto,
M. Neel, A. Panossian, P. Retailleau, G. Frison, A. Voituriez
and A. Marinetti, Chem. – Eur. J., 2010, 16, 1033; (d)
A. Voituriez, N. Pinto, M. Neel, P. Retailleau and A. Marinetti,
Chem. – Eur. J., 2010, 16, 12541; (e) N. Pinto, P. Retailleau,
A. Voituriez and A. Marinetti, Chem. Commun., 2011, 47, 1015.
15 (a) S. I. Pereira, J. Adrio, A. M. S. Silva and J. C. Carretero,
J. Org. Chem., 2005, 70, 10175; (b) S. Takizawa, E. Rémond,
F. A. Arteaga, Y. Yoshida, V. Sridharan, J. Bayardon, S. Jugé
and H. Sasai, Chem. Commun., 2013, 49, 8392.
16 (a) W. Chen, W. Mbafor, S. M. Roberts and J. Whittall, J. Am.
Chem. Soc., 2006, 128, 3922; (b) W. Chen, S. M. Roberts,
J. Whittall and A. Steiner, Chem. Commun., 2006, 2916; (c)
W. Chen, P. J. McCormack, K. Mohammed, W. Mbafor,
S. M. Roberts and J. Whittall, Angew. Chem., Int. Ed., 2007, 46,
4141; (d) W. Chen, F. Spindler, B. Pugin and U. Nettekoven,
Angew. Chem., Int. Ed., 2013, 52, 8652; (e) H. Nie, G. Zhou,
Q. Wang, W. Chen and S. Zhang, Tetrahedron: Asymmetry,
2013, 24, 1567; ( f ) Q. Wang, X. Liu, X. Liu, B. Li, H. Nie,
S. Zhang and W. Chen, Chem. Commun., 2014, 50, 978.
17 Recently, we reported bifunctional ferrocene-based
squaramide-phosphine as an organocatalyst for the highly
enantioselective intramolecular Morita-Baylis-Hillman reac-
tion. X. Zhang, P. Ma, D. Zhang, Y. Lei, S. Zhang, R. Jiang
and W. Chen, Org. Biomol. Chem., 2014, 12, 2423.
6 (a) B. Vakulya, S. Varga, A. Csámpai and T. Soós, Org. Lett.,
2005, 7, 1967; (b) S. H. McCooey and S. J. Connon, Angew.
Chem., Int. Ed., 2005, 44, 6367; (c) J. Ye, D. J. Dixon and
P. S. Hynes, Chem. Commun., 2005, 4481; (d) B.-J. Li, L. Jiang,
M. Liu, Y.-C. Chen, L.-S. Ding and Y. Wu, Synlett, 2005, 603.
7 J. Wang, H. Li, W. Duan, L. Zu and W. Wang, Org. Lett.,
2005, 7, 4713.
8 (a) Selected reviews on ferrocene-based chiral ligands, see:
T. Hayashi in Ferrocenes, ed. A. Togni and T. Hayashi, Wiley-
VCH, Weinheim, 1995, pp. 105–142; (b) R. G. Arrayás,
J. Adrio and J. C. Carretero, Angew. Chem., Int. Ed., 2006, 45,
7674; (c) H.-C. Blaser, W. Chen, F. Camponovo and A. Togni, in
Ferrocenes: Ligands, Materials and Biomolecules, ed. P. Stepnicka,
Wiley-VCH, Weinheim, 2008, pp. 205–236; (d) W. Chen and
H.-U. Blaser, in Phosphorus Ligands in Asymmetric Catalysis,
ed. A. Börner, 2008, pp. 359–392; (e) Chiral Ferrocenes in Asym-
metric Catalysis, ed. L. Dai and X. Hou, Wiley-VCH, 2010.
9 (a) Very recently, ferrocenyl pyrrolidines, O-ferrocenoyl
hydroxyprolines and ferrocenyl proline amides have been
used as organocatalysts in enamine catalysis. However, in
these studies, the ferrocenyl moiety should be regarded as a
mere substituent rather than a backbone because the most
important feature of a ferrocene scaffold, planar chirality,
is not involved. D. Petruzziello, M. Stenta, A. Mazzanti and
18 (a) J. M. Andres, R. Manzano and R. Pedrosa, Chem. – Eur. J.,
2008, 14, 5116; (b) P. Gao, C. Tang, C. Wang, Y. Wu and
Z. Zhou, Eur. J. Org. Chem., 2008, 4563; (c) X. Jiang, L. Lai,
X. Liu, R. Wang, L. Wu, G. Zhang, J. Zhang, Y. Zhang, X. Jiang,
L. Lai and R. Wang, J. Org. Chem., 2009, 74, 5562; (d)
F.-Z. Peng, X.-W. Pu, Z.-H. Shao and H.-B. Zhang, Tetrahedron,
2010, 66, 3655; (e) D. P. Gavin and J. C. Stephens, ARKIVOC,
2011, 407; ( f ) X. Gao, L.-J. Huo, Y.-X. Liu, Z.-W. Ma, J.-C. Tao,
Y.-X. Liu and J.-C. Tao, Tetrahedron: Asymmetry, 2012, 23, 443;
(g) V. Bloch, M. Gandelman, G. Nisnevich and G. Suez, Eur. J.
Org. Chem., 2012, 2118.
This journal is © The Royal Society of Chemistry 2014
Catal. Sci. Technol., 2014, 4, 1726–1729 | 1729