Asymmetric Michael addition of boronic acids to a γ-hydroxy-α, β-unsaturated aldehyde catalyzed by resin-supported peptide

Article abstract of DOI:10.1039/c2ob25431j

A resin-supported peptide catalyst effective for the asymmetric Michael addition of boronic acids to (E)-4-hydroxybut-2-enal was developed. From a spectral study, it was revealed that the optimum peptide consisted of both a β-turn and helix. Such a combination of secondary structures was essential for achieving a high catalytic ability.

Full text of DOI:10.1039/c2ob25431j

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COMMUNICATION
Asymmetric Michael addition of boronic acids to a
γ-hydroxy- α,β-unsaturated aldehyde catalyzed by resin-supported peptide†‡
Kengo Akagawa, Masahide Sugiyama and Kazuaki Kudo*
Received 28th February 2012, Accepted 21st May 2012
DOI: 10.1039/c2ob25431j
A resin-supported peptide catalyst effective for the asym-
metric Michael addition of boronic acids to (E)-4-hydroxy-
but-2-enal was developed. From a spectral study, it was
revealed that the optimum peptide consisted of both a β-turn
and helix. Such a combination of secondary structures was
essential for achieving a high catalytic ability.
Fig. 1 Resin-supported peptide catalyst.
Recently, peptides have been paid attention to in the field of
organocatalysis.1–3 Peptide catalysts can be considered as sim-
plified enzymes. Enzymes have complicated three-dimensional
frameworks consisting of secondary structures such as an
α-helix, a β-sheet, and a turn moiety. By combining such struc-
tural units, catalytically active reaction sites are constructed,
which makes enzymatic reactions highly efficient and stereo-
selective. Most of the peptide catalysts reported so far are rela-
tively short, thus, it is suspected that the potential of peptide
catalysis has not been fully demonstrated. A peptide with mul-
tiple secondary structural motifs is expected to have a high cata-
lytic performance.4
Our group has developed peptide catalyst 1 for organocatalytic
reactions (Fig. 1).5 Peptide catalyst 1 possesses an N-terminal
five residues including a turn motif, D-Pro-Aib (Aib: 2-amino-
isobutyric acid),6 and a helical part. The whole peptide is sup-
ported by an amphiphilic resin,7 which enables efficient synthesis
and recovery of the catalyst and facile handling of the peptide
with low solubility. In the peptide sequence, both the turn motif
and the helical unit are necessary for high enantioselectivity.
Especially, it is intriguing that the presence of the helical part
significantly improves the stereochemical outcome of the reac-
tions, though this moiety is somewhat distant from the terminal
prolyl group which works as the catalytic center. In the organo-
catalytic reactions performed with peptide catalyst 1, aromatic
aldehydes have been mainly employed, and use of aliphatic
aldehydes was limited to those without functional groups.
Because the structures and the substituents of peptide chains can
Scheme 1 Preliminary result with peptide 1.
be easily modified simply by changing amino acid monomers, it
is expected that the substrate scope for peptide-catalyzed reac-
tions can be extended by exploring peptide sequences.
The Michael addition to an α,β-unsaturated aldehyde having a
hydroxyl group at the γ-position is a versatile reaction to syn-
thesize precursors of β-substituted γ-lactones. Kim8 reported the
Michael addition to (E)-4-hydroxybut-2-enal with boronic acids
catalyzed by a diarylprolinol silyl ether.9–11 However, in some
cases this reaction suffers low chemical yield and/or quite low
enantioselectivity. When peptide catalyst 1 was employed for
this type of reaction, only moderate enantioselectivity was
attained (Scheme 1). Therefore, we set out an examination to
develop a new peptide sequence effective for this reaction and to
investigate the structure–activity relationship of the peptide
catalysts.
Initially, resin-supported proline was used to test the catalytic
ability of the simple prolyl group. The Michael addition of the
alkenyl group on the boron atom and the subsequent cyclization
proceeded smoothly. However, the corresponding lactone
formed by oxidation of the lactol was nearly racemic (Table 1,
entry 1).12 The reaction with the dipeptide D-Pro-Aib, which is
known as a turn motif, also gave low enantioselectivity (entry 2).
The introduction of another prolyl residue to the N-terminus of
this moiety somewhat improved the ee value (entry 3). The same
tendency was observed for longer peptides (compare entries
4 and 17). Next, whilst preserving the Pro-D-Pro-Aib sequence,
Institute of Industrial Science, University of Tokyo, 4-6-1 Komaba,
Meguro-ku, Tokyo 153-8505, Japan. E-mail: kkudo@iis.u-tokyo.ac.jp;
Fax: +81 3 5452 6357; Tel: +81 3 5452 6359
†This article is part of the joint ChemComm–Organic & Biomolecular
Chemistry ‘Organocatalysis’ web themed issue.
‡Electronic supplementary information (ESI) available: Experimental
details, preparation of peptides and ROESY spectra. See DOI:
10.1039/c2ob25431j
This journal is © The Royal Society of Chemistry 2012
Org. Biomol. Chem., 2012, 10, 4839–4843 | 4839

Table 1 Screening of peptide sequence
Table 2 Asymmetric Michael addition of boronic acids to (E)-4-
hydroxybut-2-enal
Time
(h)
Isolated yielda
(%)
eeb
(%)
Entry
Peptide
Conversiona (%)
eeb (%)
Entry
1
R
1
2
3
4
5
6
7
8
Pro
D-Pro-Aib
Pro-D-Pro-Aib
72
72
75
73
78
93
99
99
99
99
66
88
99
99
99
99
85
72
46
60
28
99
99
79
72
63
99
99
99
99
7
24
24
36
24
24
24
92
90
81
64
67
71
90
83
92
92
96
90
−18
37
−22
55
81
82
71
70
74
72
64
82
83
54
64
64
81
46
76
58
80
81
52
50
78
82
77
91
90
D-Pro-Aib-Trp-Trp
Pro-D-Pro-Aib-Gly
Pro-D-Pro-Aib-Ala
Pro-D-Pro-Aib-Leu
Pro-D-Pro-Aib-Val
Pro-D-Pro-Aib-Ile
Pro-D-Pro-Aib-Trp
Pro-D-Pro-Aib-Ser
Pro-D-Pro-Aib-(Gly)2
Pro-D-Pro-Aib-(Ala)2
Pro-D-Pro-Aib-(Leu)2
Pro-D-Pro-Aib-(Val)2
Pro-D-Pro-Aib-(Ile)2
Pro-D-Pro-Aib-(Trp)2
Pro-D-Pro-Aib-(Ser)2
Pro-D-Pro-Aib-(Thr)2
Pro-D-Pro-Aib-(Tyr)2
Pro-D-Pro-Aib-(Cys)2
Pro-D-Pro-Aib-(Ala)3
Pro-D-Pro-Aib-(Leu)3
Pro-D-Pro-Aib-(Val)3
Pro-D-Pro-Aib-(Ile)3
Pro-D-Pro-Aib-(Ser)3
Pro-D-Pro-Aib-(Ala)4
Pro-D-Pro-Aib-(Leu)4
Pro-D-Pro-Aib-(Ala)5 (2)
2
2
3
4
5
6
9
10
11
12
13
14
15
16
17
18
19
20
21
22
23
24
25
26
27
28
29
30c
7
24
48
87
84
37
71
8c
9
36
94
39
a Based on the amount of boronic acid. b Determined by HPLC analysis
after oxidation to the corresponding lactone. In entries 1 to 6,
hydrogenation of the olefin part was conducted before the measurement.
c The amount of aldehyde was 4 molar equiv.
a Determined by 1H NMR analysis of the crude mixture based on
consumed boronic acid. b Determined by HPLC analysis after oxidation
to the corresponding lactone and hydrogenation of the olefin part.
c Catalyst loading was 10 mol%.
similar to those with the (Ala)2 and (Leu)2 residues, it was indi-
cated that the hydroxy groups of serines did not play any positive
role for controlling the enantioselectivity. The insertion of
trimers and tetramers of the amino acid residues between the
N-terminal tripeptide and PEG linker showed a similar tendency
(entries 22 to 28); the peptides with alanyl and leucinyl groups
maintained good enantioselectivity. Interestingly, when penta-
alanyl residues were introduced, the highest enantioselectivity
was observed (entry 29).13 With this peptide, the catalyst loading
could be reduced from 20 mol% to 10 mol% to afford the
product with full conversion and high selectivity (entry 30).
Using the optimum catalyst 2, substrate scope was examined
(Table 2). Styryl-type boronic acids with various substituents on
benzene rings gave the products with good to moderate yield in
a highly enantioselective manner (entries 1 to 6). A control reac-
tion using 2-(4-chlorophenyl)vinyl boronic acid (cf. entry 3)
catalyzed by (S)-(−)-α,α-diphenyl-2-pyrrolidinemethanol trimethyl-
silyl ether (3) was sluggish under the same reaction conditions
(38% yield and 69% ee after 36 h). This result demonstrates the
usefulness of the present peptide catalyst. According to the
literature, use of aryl boronic acids as Michael donors affords the
products with quite low enantioselectivity.8a For example, the ee
value of the reaction with 2-furylboronic acid catalyzed by 3 was
various amino acid residues were introduced at the C-terminus.
While the introduction of even glycine improved enantioselectiv-
ity (entry 5), other amino acids with diverse sizes and structures
of the side chains further enhanced the selectivity (entries 6 to
10). Takemoto et al. reported that a thiourea catalyst with a
hydroxyl group effectively activates an alkenyl boronic acid for
the asymmetric Michael addition to a γ-hydroxy-α,β-unsaturated
ketone.9d In the case of the peptide catalyst with a serinyl group,
selectivity was the same level as those with other amino acids
(entry 11). To investigate the effect of elongation of the peptide
chain, two amino acid residues were introduced next to Aib
(entries 12 to 21). While the peptide with (Ala)2 or (Leu)2
sequences showed an enantioselectivity comparable to that with
mono Ala or Leu residue (entries 13 and 14), the insertion of
(Val)2, (Ile)2, and (Trp)2 moieties somewhat decreased the selec-
tivity (entries 15 to 17). It is interesting to note that Ala and Leu
groups have the α-helical propensity, while Val, Ile, and Trp do
not. Further, amino acids with hydroxyl groups or a thiol group
were also tested (entries 18 to 21). Among them, the peptide
with the (Ser)2 moiety showed the best enantioselectivity (entry
18). Because the level of enantioinduction with this peptide was
4840 | Org. Biomol. Chem., 2012, 10, 4839–4843
This journal is © The Royal Society of Chemistry 2012

Fig. 3 Schematic illustration of terminal structure.
Fig. 2 CD spectra of TFA·Pro-D-Pro-Aib-(Ala)n-NH2 (n = 0 to 5) in
2,2,2-trifluoroethanol.
reported to be only 4%. With this substrate, the peptide-catalyzed
reaction gave the product in a more enantioselective way
(entry 7). The reactions with other heteroarylboronic acids
also proceeded with higher enantioselectivity than the previous
results, though the selectivity left room for improvement
(entries 8 and 9).
To obtain information about the structure of peptide catalyst 2,
the spectroscopic analyses of 2-related peptides were conducted.
The circular dichroism (CD) spectra of the peptides having a
–CONH2 group at the C-terminus were measured for the
sequences of Pro-D-Pro-Aib-(Ala)n (n = 0 to 5) (Fig. 2).14 While
the spectrum of the peptide with no alanine indicated a lack of
any specific secondary structures, the formation of a turn struc-
ture was suggested for the one with a single alanyl residue.15
Incorporation of two or more alanines brought about a further
change in the spectra; a negative band at around 200 nm and a
shoulder at around 220 nm were observed, implying the for-
mation of a helical structure. Next, two-dimensional NMR
spectra were measured for the same set of peptides. The NMR
measurements were conducted in CDCl3 for the Pro-D-Pro-Aib-
(Ala)n (n = 0 to 2) sequences. Because of the limited solubility
of the peptides with more alanyl residues toward halogenated
solvents, the spectra were measured in CD3CN for the peptides
having three and four alanines, and in DMF-d7 for the one
having a pentaalanyl moiety. The ROESY spectra of the peptides
with (Ala)n (n = 1 to 5) residues showed the presence of the
NOEs between CαH of Pro and CδH protons of D-Pro, and also
between CαH of D-Pro and NH of Aib (Fig. 3), while TFA·Pro-
D-Pro-Aib-NH2 did not show these NOE signals (Fig. S1 to S6
in ESI‡). This indicates that the N-termini of the alanine-contain-
ing peptides adopt a common secondary structure, possibly a
β-turn. It has been reported that the change in the chemical shift
of an amide proton upon treatment with polar solvents can be
used as an indicator for the formation of a β-turn structure by
intramolecular hydrogen bonding.6a,16 For the peptide TFA·Pro-
D-Pro-Aib-(Ala)2-NH2 in CDCl3, the NH of one alanyl group
shifted in a small range (Δδ < 0.2 ppm), whereas the ones of Aib
Fig. 4 Chemical shift changes for amide protons of TFA·Pro-D-Pro-
Aib-(Ala)2-NH2 in CDCl3 with DMSO-d6.
and another Ala shifted significantly (Δδ = 1.2 and 0.5 ppm,
respectively) to a lower magnetic field upon addition of DMSO-
d6 (Fig. 4). The small shift of the alanyl amide proton was simi-
larly observed when the peptide with one alanyl group, TFA·Pro-
D-Pro-Aib-Ala-NH2, was employed for the same experiment.
This indicates that the amide proton of the alanyl residue is
involved in intramolecular hydrogen bonding. From these data
together with the CD and ROESY spectra, the peptides with
alanyl residues are considered to form a β-turn structure at the
N-terminus. The IR spectra of the resin-supported peptides
swollen with CH2Cl2 afforded information on the structures of
them used in the catalytic reaction. The N–H stretching region of
the IR spectra for resin-supported Pro-D-Pro-Aib-(Ala)n (n = 0 to
5) is shown in Fig. 5. For the peptide Pro-D-Pro-Aib, the absorp-
tions with peak tops at around 3350 cm−1 and 3420 cm−1 were
observed. The former peak derives from a hydrogen bonded
N–H group and the latter is due to a free N–H group. When one
alanyl group was added at the C-terminus of this peptide, the
peak top of the absorbance around 3350 cm−1 shifted to a lower-
energy region and the peak at around 3420 cm−1 disappeared.
This means that the intramolecular hydrogen bonding is strength-
ened by the introduction of alanine,17 which is consistent with
the CD and NMR studies in the solution state, and strongly indi-
cates the formation of the β-turn structure. The peptides with
more than two alanines at the C-terminus of the Pro-D-Pro-Aib
part gave the spectra covering the region around 3300 cm−1
.
This journal is © The Royal Society of Chemistry 2012
Org. Biomol. Chem., 2012, 10, 4839–4843 | 4841

2 For recent selected examples of organocatalytic reactions with peptides,
see: (a) V. D’Elia, H. Zwicknagl and O. Reiser, J. Org. Chem., 2008, 73,
3262; (b) M. Wiesner, J. D. Revell, S. Tonazzi and H. Wennemers, J. Am.
Chem. Soc., 2008, 130, 5610; (c) C. A. Lewis, J. L. Gustafson, A. Chiu,
J. Balsells, D. Pollard, J. Murry, R. A. Reamer, K. B. Hansen and
S. J. Miller, J. Am. Chem. Soc., 2008, 130, 16358; (d) C. E. Müller,
L. Wanka, K. Jewell and P. R. Schreiner, Angew. Chem., Int. Ed., 2008,
47, 6180; (e) K. W. Fiori, A. L. A. Puchlopek and S. J. Miller, Nat.
Chem., 2009, 1, 630; (f) F.-C. Wu, C.-S. Da, Z.-X. Du, Q.-P. Guo,
W.-P. Li, L. Yi, Y.-N. Jia and X. Ma, J. Org. Chem., 2009, 74, 4812;
(g) B. J. Cowen, L. B. Saunders and S. J. Miller, J. Am. Chem. Soc.,
2009, 131, 6105; (h) J. L. Gustafson, D. Lim and S. J. Miller, Science,
2010, 328, 1251; (i) M. Wiesner, G. Upert, G. Angelici and
H. Wennemers, J. Am. Chem. Soc., 2010, 132, 6; ( j) B. S. Fowler,
P. J. Mikochik and S. J. Miller, J. Am. Chem. Soc., 2010, 132, 2870;
(k) P. Chen and J. Qu, J. Org. Chem., 2011, 76, 2994; (l) F. Kolundzic,
M. N. Noshi, M. Tjandra, M. Movassaghi and S. J. Miller, J. Am. Chem.
Soc., 2011, 133, 9104; (m) C. E. Müller, R. Hrdina, R. C. Wende and
P. R. Schreiner, Chem.–Eur. J., 2011, 17, 6309; (n) G. Rulli, N. Duangdee,
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H. Schedel, J. Am. Chem. Soc., 2007, 129, 12890.
3 For selected reviews of organocatalytic reactions, see: (a) A. Erkkilä,
I. Majander and P. M. Pihko, Chem. Rev., 2007, 107, 5416;
(b) S. Mukherjee, J. W. Yang, S. Hoffmann and B. List, Chem. Rev.,
2007, 107, 5471; (c) H. Pellissier, Tetrahedron, 2007, 63, 9267;
(d) D. Almaşi, D. A. Alonso and C. Nájera, Tetrahedron: Asymmetry,
2007, 18, 299; (e) A. Dondoni and A. Massi, Angew. Chem., Int. Ed.,
2008, 47, 4638; (f) P. Melchiorre, M. Marigo, A. Carlone and G. Bartoli,
Angew. Chem., Int. Ed., 2008, 47, 6138.
4 For selected examples of relatively long peptide catalysts, see:
(a) G. T. Copeland and S. J. Miller, J. Am. Chem. Soc., 2001, 123, 6496;
(b) M. M. Vasbinder, J. E. Imbriglio and S. J. Miller, Tetrahedron, 2006,
62, 11450; (c) M. Sánchez-Roselló, A. L. A. Puchlopek, A. J. Morgan
and S. J. Miller, J. Org. Chem., 2008, 73, 1774; (d) D. Coquière, J. Bos,
J. Beld and G. Roelfes, Angew. Chem., Int. Ed., 2009, 48, 5159;
(e) R. Sambasivan and Z. T. Ball, J. Am. Chem. Soc., 2010, 132, 9289.
5 (a) K. Akagawa, H. Akabane, S. Sakamoto and K. Kudo, Org. Lett.,
2008, 10, 2035; (b) K. Akagawa, H. Akabane, S. Sakamoto and
K. Kudo, Tetrahedron: Asymmetry, 2009, 20, 461; (c) K. Akagawa,
T. Yamashita, S. Sakamoto and K. Kudo, Tetrahedron Lett., 2009, 50,
5602; (d) K. Akagawa, T. Fujiwara, S. Sakamoto and K. Kudo, Org.
Lett., 2010, 12, 1804; (e) K. Akagawa, T. Fujiwara, S. Sakamoto and
K. Kudo, Chem. Commun., 2010, 46, 8040; (f) K. Akagawa and
K. Kudo, Org. Lett., 2011, 13, 3498; (g) K. Akagawa and K. Kudo, Adv.
Synth. Catal., 2011, 353, 843.
6 (a) G. T. Copeland, E. R. Jarvo and S. J. Miller, J. Org. Chem., 1998, 63,
6784; (b) S. Aravinda, V. V. Harini, N. Shamala, C. Das and P. Balaram,
Biochemistry, 2004, 43, 1832; (c) S. J. Miller, Acc. Chem. Res., 2004, 37,
601; (d) J. T. Blank and S. J. Miller, Pept Sci., 2006, 84, 38.
7 Uozumi et al. have demonstrated that the PEG-PS-resin-support is effec-
tive for transition-metal-catalyzed reactions in water. For examples, see:
(a) Y. Uozumi, Bull. Chem. Soc. Jpn., 2008, 81, 1183; (b) Y. Uozumi,
Y. Matsuura, T. Arakawa and Y. M. A. Yamada, Angew. Chem., Int. Ed.,
2009, 48, 2708; (c) T. Suzuka, Y. Okada, K. Ooshiro and Y. Uozumi,
Tetrahedron, 2010, 66, 1064; (d) Y. Hirai and Y. Uozumi, Chem.–Asian
J., 2010, 5, 1788.
Fig. 5 IR spectra of Pro-D-Pro-Aib-(Ala)n-(PEG-PS resin) swollen
with CH2Cl2.
Such a change might reflect the formation of a helical structure
along with the N-terminal turn structure. The peptide with the
pentaalanyl group showed a peak at an even smaller wavenum-
ber. This implies the formation of strong intramolecular hydro-
gen bonds derived from a robust helical structure. From the
observed enantioselectivity with the various peptide catalysts
shown in Table 1, it might be safely concluded that not only the
presence of the turn structure, but also the helical structure are
important for high enantioselectivity. The finding achieved in
this paper totally coincides with our recent study on a peptide-
catalyzed asymmetric Friedel–Crafts-type reaction in the point
that the efficient peptide catalyst should have both an N-terminal
β-turn structure and a subsequent helical part.18 Such a coopera-
tivity of the structural motifs seems interesting and might be
applicable to the design of other peptide catalysts, although the
interaction between these motifs is waiting to be clarified.
In conclusion, a new resin-supported peptide catalyst effective
for the Michael addition to
a γ-hydroxy-α,β-unsaturated
aldehyde with boronic acids was developed. From spectroscopic
analyses, the optimum peptide was indicated to possess both a
turn structure and a rigid helical part. The present study demon-
strates an example for the creation of an efficient peptide catalyst
by combining secondary structural units. This approach might
contribute to the advancement in the field of peptide-based
organocatalysts.
8 (a) S.-G. Kim, Tetrahedron Lett., 2008, 49, 6148; (b) S.-G. Kim, Bull.
Korean Chem. Soc., 2009, 30, 2519.
9 For examples of asymmetric Michael additions with organoboron com-
pounds by organocatalysts, see: (a) T. R. Wu and J. M. Chong, J. Am.
Chem. Soc., 2005, 127, 3244; (b) T. R. Wu, J. M. Chong and
J. M. Chong, J. Am. Chem. Soc., 2007, 129, 4908; (c) S. Lee and
D. W. C. MacMillan, J. Am. Chem. Soc., 2007, 129, 15438;
(d) T. Inokuma, K. Takasu, T. Sakaeda and Y. Takemoto, Org. Lett.,
2009, 11, 2425; (e) M. Sugiura, M. Tokudomi and M. Nakajima, Chem.
Commun., 2010, 46, 7799; (f) K.-S. Choi and S.-G. Kim, Tetrahedron
Lett., 2010, 51, 5203; (g) K.-S. Choi and S.-G. Kim, Synthesis, 2010,
3999; (h) B. J. Lundy, S. Jansone-Popova and J. A. May, Org. Lett.,
2011, 13, 4958.
Acknowledgements
This work was partially supported by a Grant-in-Aid for
Scientific Research (C) (23550116 for K.K.) from Japan Society
for the Promotion of Science.
10 For other examples of organocatalyzed reactions with organoboronic
acids, see: (a) S. Lou, P. N. Moquist and S. E. Schaus, J. Am. Chem.
Soc., 2006, 128, 12660; (b) S. Lou, P. N. Moquist and S. E. Schaus,
J. Am. Chem. Soc., 2007, 129, 15398; (c) S. Lou and S. E. Schaus,
J. Am. Chem. Soc., 2008, 130, 6922; (d) J. A. Bishop, S. Lou and
Notes and references
1 For reviews of enantioselective peptide catalysts, see: (a) E. A. C. Davie,
S. M. Mennen, Y. Xu and S. J. Miller, Chem. Rev., 2007, 107, 5759;
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4842 | Org. Biomol. Chem., 2012, 10, 4839–4843
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14 The concentration of the peptides was 0.5 mM. In the case of the
sequence of Pro-D-Pro-Aib-(Ala)5, the solubility of the peptide was not
enough. This might be the cause of somewhat smaller molar ellipticity
than those of the peptides with (Ala)n (n = 2 to 4).
15 (a) S. R. Raghothama, S. K. Awasthi and P. Balaram, J. Chem. Soc.,
Perkin Trans. 2, 1998, 137; (b) C. Das, G. A. Naganagowda, I. L. Karle
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16 R. Rai, S. Raghothama, R. Sridharan and P. Balaram, Pept. Sci., 2007,
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11 For selected reviews of metal-catalyzed Michael reactions with organo-
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103, 2829; (b) N. Miyaura, Bull. Chem. Soc. Jpn., 2008, 81, 1535;
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1745; (e) C. S. Marques and A. J. Burke, ChemCatChem, 2011, 3, 635.
12 The diastereomeric ratio of the lactol product was about 6 : 4. In all cases
in Table 1, diastereoselectivity was almost the same.
13 Córdova et al. reported that a dialanyl peptide effectively catalyzes the
asymmetric Michael addition of nitroolefins to ketones. Y. Xu, W. Zou,
H. Sundén, I. Ibrahem and A. Córdova, Adv. Synth. Catal., 2006, 348,
418.
17 T. S. Haque, J. C. Little and S. H. Gellman, J. Am. Chem. Soc., 1996,
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18 K. Akagawa, R. Suzuki and K. Kudo, Adv. Synth. Catal., 2012, 354,
1280.
This journal is © The Royal Society of Chemistry 2012
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