Chiral octahedral complexes of CoIII as a family of asymmetric catalysts operating under phase transfer conditions

Article abstract of DOI:10.1021/cs400409d

Stereochemically inert and positively charged chiral complexes of Co ^III prepared from Schiff bases derived from chiral diamines and salicylaldehydes were shown to be efficient catalysts of the asymmetric phase transfer benchmark reaction of alkylation of O'Donnell's substrate with alkyl halides. The enantiomeric purities of the reaction products were up to 92%.

Full text of DOI:10.1021/cs400409d

Research Article
pubs.acs.org/acscatalysis
Chiral Octahedral Complexes of CoIII As a Family of Asymmetric
Catalysts Operating under Phase Transfer Conditions
Yuri N. Belokon,*,† Victor I. Maleev,† Michael North,‡ Vladimir A. Larionov,† Tat’yana F. Savel’yeva,†
Aike Nijland,§ and Yuliya V. Nelyubina†
†Nesmeyanov Institute of Organoelement Compounds, 119991, Moscow, Vavilov 28, Russia
‡School of Chemistry, Newcastle University, Bedson Building, Newcastle upon Tyne, NE1 7RU, United Kingdom
§Stratingh Institute for Chemistry, University of Groningen, Nijenborgh 4, 9747AG, Groningen, The Netherlands
S
* Supporting Information
ABSTRACT: Stereochemically inert and positively charged
chiral complexes of CoIII prepared from Schiff bases derived
from chiral diamines and salicylaldehydes were shown to be
efficient catalysts of the asymmetric phase transfer benchmark
reaction of alkylation of O’Donnell’s substrate with alkyl
halides. The enantiomeric purities of the reaction products
were up to 92%.
KEYWORDS: cobalt Schiff base, phase transfer asymmetric catalysis
complexes of CoIII as catalysts of asymmetric enolate alkylation
under PT conditions.
INTRODUCTION
■
The field of asymmetric phase transfer (APT) catalytic enolate
alkylation,1−7 as a part of a larger field of ion-pairing
catalysis,8−10 has been intensively studied in recent years. The
reasons for this are that this type of catalysis is intrinsically
“green,” inexpensive, and versatile. APT catalysis is already
being applied to industrial production.11,12 Denmark et al.
recently summarized the existing mechanistic understandings
and catalyst types present in APT catalyzed enolate
alkylation.13,14 The area is still dominated by positively charged,
chiral, quaternary ammonium salts derived from cinchona
alkaloids.1−7 In recent years, purely synthetic chiral ammonium
catalysts were also developed by Maruoka, Lygo, Denmark, and
other groups.13−18 Unfortunately, the cinchona skeleton is
difficult to modify, and the preparation of purely synthetic
Maruoka type catalysts is synthetically challenging. Addition-
ally, the catalysts themselves are expensive. Thus, there is a
need to develop novel, simple, inexpensive, and versatile APT
catalysts, capable of easy modification to fine-tune them to
particular reactions. In addition, it would be highly desirable to
introduce to the catalysts some features not present in the
classic ammonium type agents. An emerging class of novel
bifunctional chiral quaternary ammonium salt catalysts with
additional hydroxyl groups as hydrogen bond donors is an
example of recent modifications introduced to the family of the
catalysts.19
RESULTS AND DISCUSSION
■
Complexes 1−4 were prepared from Schiff bases derived from
chiral diamines and salicylaldehydes by reaction with the
corresponding CoIII salts, as illustrated in Scheme 1 for a
cyclohexenediamine example. Complex 5, analogous to 1d but
with a CrIII central metal ion instead of CoIII, was prepared
from the same Schiff base, starting with CrCl3 as described in
the Experimental Section, Supporting Information.
The structures of all the positively charged complexes (1−5)
prepared in this study are depicted in Figure 1. The structure of
anionic complex CoIII, 6, is also presented in Figure 1.
Compounds 1d and 3 were analyzed by X-ray crystallography.
Their X-ray structures are presented in Figure 2 and indicate
Λ(R,R)-1d and Δ(S,S)-3 configurations with Λ or Δ
configurations attributed according to left or right helical
mutual orientations of their tridentate ligands relative to the C2
symmetry axis.2 The absolute configurations of 1a,b,c,e, 2, 4,
and 5 were assigned by the comparison of their CD spectra
with those of Λ(R,R)-1d and Λ(S,S)-3 (see the Supporting
Information). The formation of all the positively charged
complexes is highly enantioselective, with complexes of only
the Λ configuration obtained from Schiff bases of (R)-
cyclohexenediamine and complexes of Δ configuration
obtained from (S)-cyclohexenediamine. The Schiff bases
derived from (S)-aminomethylpyrrolidine were obtained
exclusively with the Λ configuration.
In previous work, we successfully elaborated chiral negatively
charged stereochemically inert complexes of CoIII as ACDC
type9 catalysts for asymmetric synthesis.20−22 Recently,
positively charged CoIII complexes were developed by some
of us as catalysts for the cyanosilylation of aldehydes with high
catalytic efficiency, but low enantioselectivity.23 In this
manuscript, we report the use of novel chiral positively charged
Received: May 30, 2013
Revised: July 9, 2013
© XXXX American Chemical Society
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Research Article
Scheme 1. The Synthetic Protocol Used for Catalyst Preparation
Figure 1. Structures of catalysts 1−6. (a) R = R′ = R″ = H. (b) R = R″ = H, R′ = allyl. (c) R = R″ = H, R′ = t-Bu. (d) R = H, R′ = R″ = tBu. (e) R =
R′ = H, R″ = Ph. (f) R = Ph, R′ = R″ = H.
NH···Cl− equal to 2.448 Å in the case of 1d and 3.168 Å in the
case of 3.
Complex 1a was tested in its capacity to exchange chloride
anions with aqueous solutions of KF, KBr, and KI (see Scheme
2). As can be seen from Table 1, all the anions are easily
Scheme 2. Ion Exchange of Chlorine Counteranions of 1a
with Those of Potassium Salts in a System Water/CH2Cl2
Figure 2. X-ray structures of Λ(R,R)-1d and Δ(S,S)-3 with chlorine
counteranions.
A salient feature of the stereochemically inert complexes is a
set of hydrogen bonds connecting the complex cation with the
halide counteranion. Only two NH bonds were engaged in this
interaction in the case of 1d. The other two NH bonds
remained unattached and oriented toward π planes of the
aromatic moieties of the second tridentate ligand. In the case of
3, both present NH groups were engaged in the hydrogen
bonds with the chlorine anion as indicated by the distances of
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Table 1. Ion Exchange of [1a]Cl with KX in Water/CH2Cl2
Mixture
Table 2. Alkylation of O’Donnell’s Substrate by Benzyl
Bromide at Ambient Temperature, Promoted by Δ(S,S)-1a
a
a
b
b
run
KX
[1a]+Cl−, %
[1a]+X−, %
time
b
c
run base (MOH)
solvent
(h)
yield (%)
e.r.
1
2
3
KF
KBr
KI
45
13
10
55
87
90
1
2
3
4
5
6
7
8
50% aq KOH CH2Cl2
1.5
1.5
1.5
1.5
22
4
67
69
66
73
32
n.d.
75
76.5(R):23.5(S)
79.5(R):20.5(S)
80.5(S):19.5(R)
80(R):20(S)
72.5(R):27.5(S)
n.d.
KOH
CH2Cl2
CH2Cl2
CH2Cl2
CH2Cl2
CH2Cl2
toluene
d
KOH
a
Reaction conditions: 10 mg of [1a]+Cl− (0,0189 mmol), 2 mL of
CsOH.H2O
NaOH
LiOH
CH2Cl2, 10 equiv of 50% aq. KX (0.189 mmol) was stirred at ambient
b
temperature for 2 h. The ratio of [1a]+Cl− and [1a]+X− in CH2Cl2
was assessed by employing X-ray fluoresence analysis determining the
ratio of Co/Cl in the organic layer and additionally with 19F NMR in
the case of X = F−.
KOH
1
69(R):31(S)
84.5(R):15.5(S)
e
KOH
1,4-
dioxane
1
62 (80−
90)
9
KOH
THF
Et2O
1.5
1.5
60
54
82.5(R):17.5(S)
65.5(R):34.5(S)
transferred by the positively charged complex in CH2Cl2
solution. Notably, this also included the fluorine ion. The
structure of 1a was not changed, as shown by 1H NMR and CD
spectra of the series of [1a]+X−. Most likely, it was the set of
the hydrogen bonds provided by 1a (Figure 2) that stabilized
the anions in CH2Cl2. The phenomena of anion coordination
by multiple hydrogen bonds of synthetic receptors is well-
known and reviewed.24,25 It underlies the leveling effect of the
complex on the equilibrium of Scheme 2 making the extraction
coefficient of Br and I very similar and fluorine anion exchange
with Cl anion and extraction possible.
10 KOH
a
Reaction conditions: 0.101 mmol (30 mg) O’Donnell’s substrate, 10
mol % catalyst, 1 mL of solvent, 3 equiv solid metal hydroxide, and
1.25 equiv BnBr unless indicated otherwise. Yield of pure alkylated
Schiff base after purification by silica gel chromatography. Enantio-
meric purity was established by chiral HPLC analysis. Promoted by
Λ(R,R)-1a. The figures in brackets are the chemical yields assessed by
NMR, using an internal standard.
b
c
d
e
Other catalysts 1−6 were tested in the reaction under the
conditions of Table 2, run 8, and the data are summarized in
Table 3. Evidently, the introduction of large alkyl groups in
The asymmetric phase transfer alkylation of the O’Donnell
substrate by alkyl halides was chosen as a model reaction to test
the catalytic activity of complexes 1−6 (Scheme 3). The
Table 3. Alkylation of O’Donnell’s Substrate by Benzyl
Bromide at Ambient Temperature, Promoted by Catalysts
1−6
a
Scheme 3. Alkylation of O’Donnell’s Substrate Catalyzed by
CoIII Complexes 1−6
b
c
run
catalyst
yield (%)
54
55
e.r.
1
Λ(R,R)-1b
Λ(R,R)-1c
Λ(R,R)-1d
Λ(R,R)-1d
Λ(R,R)-1e
Λ(R,R)-1f
Λ(R,R)-2
Λ(S,S)-3
Λ(R,R)-4
Λ(R,R)-1d
Λ(R,R)-5
Λ(R,R)-6
83(S):17(R)
86(S):14(R)
88.5(S):11.5(R)
85(S):15(R)
89(S):11(R)
racemate
2
3
4
5
6
7
8
9
65 (80−90)
d
80
65
57
64
34
55
79
65
20
positively charged complex was expected to form an ion pair
with the carbanion to be transferred into the organic phase
where the chiral ion pair would undergo the alkylation reaction.
An NH−O hydrogen bond and electrostatic plus lipophilic
interactions could lead to the prevalence of one enantiogenic
form of the enolate in the ion pair.
The catalytic activities of the complexes were initially
investigated using benzyl bromide as the alkylating agent.
First, different bases were tested in the reaction promoted by
complex 1a (Table 2, runs 1−6). The best base was found to be
either solid or aqueous potassium hydroxide (runs 1−3).
Sodium hydroxide was much less efficient, and lithium
hydroxide gave no product (runs 5, 6). Another good base
was CsOH × H2O (run 4). Under the reaction conditions, the
enantiomeric purity of the resulting phenylalanine was 53−61%
with Δ(S,S)-1a furnishing (R)-amino acid (runs 1, 2, 4) and
Λ(R,R)-1a giving the (S)-enantiomer (run 3). There was no
influence of the nature of the base cation on the enantiomeric
purity of the product. Different solvents were also tested, and
the best enantioselectivity was obtained using 1,4-dioxane (run
8, compare with runs 2, 3, 7, 9, 10).
86.5(S):13.5(R)
53.5(S):46.5(R)
57(S):43(R)
83(S):17(R)
85(S):15(R)
racemate
de
,
10
11
12
a
Reaction conditions: 0.101 mmol (30 mg) O’Donnell’s substrate, 10
mol % catalyst, 1 mL 1,4-dioxane, 3 equiv 50% aq. KOH and 1.25
equiv BnBr for 1 h at ambient temperature unless indicated otherwise;
see footnotes to Table 1. No dependence of the enantioselectivity on
b
the reaction volume was observed. Yield of pure alkylated Schiff base
c
after purification by silica gel chromatography. Enantiomeric purity
d
e
was established by chiral HPLC analysis. Solid KOH was used. The
catalyst was recovered from the previous experiment and reused.
positions 3 and 5 of the salicylaldehyde moiety increased the
enantioselectivity of the alkylation (compare Table 2, run 8 and
Table 3, runs 1−5). In the case of complex 1d, the asymmetric
induction reached 78%. Almost the same effect was brought
about by the enlargement of the aromatic Schiff base to a
naphthol derivative 2 (Table 3, run 7). The introduction of a
phenyl group at the imine carbon atom (1f) resulted in a
complete loss of any stereoselectivity during the catalysis
(Table 3, run 6). The aminomethylpyrrolidine derived complex
Δ(S,S)-3 was a sluggish catalyst, and there was almost no
asymmetric induction in the reaction (Table 3, run 8).
A control experiment proved that a significant loss of product
occurred during chromatographic separation of the product on
silica. An NMR experiment of the reaction mixture with a
standard added proved that the real yield of the alkylation
product was in the range of 80−90% (Table 2, run 8, figures in
brackets).
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Surprisingly, diphenylethylenediamine derivative 4, although
catalytically active, furnished almost no asymmetric induction in
the reaction (Table 3, run 9). Thus, complex 1d was the most
active APT catalyst under the experimental conditions.
Remarkably, aqueous KOH was a superior base to the solid
version (Table 2, run 3 and 4). The complexes could be
recovered from the reaction mixture and reused without a
significant loss of catalytic activity or enantioselectivity (Table
3, run 10). The CrIII derived catalyst was almost as active as
analogous 1d, furnishing the same chemical yield of the product
and enentioselectivity (Table 3, run 11). Under the same
conditions, catalyst 6 was sluggish, and the product was racemic
(Table 3, runs 12). The conversion is most likely a result of a
blank reaction, which was shown to take place under the
experimental conditions with the same rate.
Table 4. Alkylation of O’Donnell’s Substrate with Different
Alkyl Halides at Varying Temperatures Promoted by
Λ(R,R)-1d in 3:1 Toluene/Dioxane in the Presence of aq.
a
KOH
b
c
run
alkyl halide
BnBr
T (°C) T (h) yield (%)
e.r.
d
1
65
25
1.5
1.5
7
65
65
66
61
81(S):19(R)
89(S):11(R)
89(S):11(R)
92.5(S):7.5(R)
95(S):5(R)
d
2
BnBr
e
3
BnBr
25
4
BnBr
−15
−40
−78
−40
−40
−40
−40
−40
3
f
5
BnBr
12
62 (88 )
6
BnBr
6
12
12
12
12
12
10
61
30
21
58
97(S):3(R)
7
4-FC6H4CH2Br
MeI
96(S):4(R)
8
93.5(S):6.5(R)
96(S):4(R)
9
EtI
10
11
Allyl Br
Propargyl Br
93.5(S):6.5(R)
93(S):7(R)
Figure 3 illustrates the dependence of the enantioselectivity
and the chemical yield of the alkylation catalyzed by Δ(S,S)-1d
41
a
b
Reaction conditions, see footnote to Table 3. Yield of pure alkylated
Schiff base after purification by silica gel chromatography. Enantio-
meric purity was established by chiral HPLC analysis. Conducted in
pure dioxane. Conducted in a 1:1 mixture of dioxane and toluene.
c
d
e
f
Determined by NMR analysis of the reaction mixture before
chromatography.
lectivity of the alkylation was almost independent of the alkyl
halide structure and gave amino acid derivatives with 85−94%
enantiomeric excess (Table 4, runs 5, 7−11). As expected,
aliphatic alkyl iodides were slow to react compared to activated
halides but still gave reasonable chemical yields (Table 4, runs
8,9).
Another closely related PT reaction catalyzed by the novel
catalyst family was Michael addition of methyl acrylate to
O’Donnel’s substrate (Scheme 4). The condensation was
catalyzed by Λ(R,R)-1d in CH2Cl2 at ambient temperature to
give (S)-glutamic acid with e.r. 87.5(S):12.5(R) in a chemical
yield of 89%.
Figure 3. Variation of the enantiomeric purity and the chemical yield
of the alkylation product with reaction time carried out using a solid
KOH/dioxane system promoted by complex 1d.
The mechanism of the catalytic activity of the positively
charged complexes is, at present, a subject of debate. Although
the direct participation of the central metal ion in the catalytic
stage of the C−C bond forming reactions could not be entirely
excluded at this experimental stage, it looked more likely that
the mechanism of the reaction was that of a regular phase
transfer conversion. The following arguments support this
notion: The complexes are capable of promoting ion exchange
similarly to traditional ammonium salts and other anion
receptors (see Table 1).
on the reaction time. Evidently, there was no racemization of
the product during the alkylation, but a sizable decrease in the
chemical yield of the reaction was observed at longer reaction
times.
There was no dependence of the enantiomeric excess of the
product on the 1d catalyst loading in the range of 3−10 mol %
in dioxane. However, when using just 1 mol % of the catalyst,
the enantiomeric excess of the product dropped to just 34%.
The competitive uncatalyzed alkylation reaction was shown to
be responsible for the adverse effect of low catalyst loading on
the enantioselectivity. A further increase of the catalyst loading
to 15 mol % led to a decrease in the enantioselectivity, probably
due to catalyst self-association, as was observed earlier in the
case of anionic CoIII complexes.20
The complexes of CoIII (d6) are stereochemically inert and
coordinatively saturated. Any direct involvement of the metal
ion in the transition state of the alkylation would demand a
significant energy expenditure of preliminary ligand dissociation
and structure reorganization.26 Still, the complex catalyst can be
easily recovered from the reaction mixture without any loss of
its catalytic activity (Table 3, run 10) and with unchanged
chemical structure and configuration.
Further improvement in the enantioselectivity of the reaction
came from varying the temperature of the experiments, as
summarized in Table 4.
The experiments at low temperatures were conducted in a
3:1 mixture of toluene and dioxane to avoid solvent
crystallization. There was a clear tendency for the enantiose-
lectivity of the alkylation to increase as the reaction temperature
was reduced (Table 4, runs 1−6). Although the maximum
enantioselectivity was observed at −78 °C (Table 4, run 6), the
reaction was too slow at this temperature, and −40 °C was
chosen as the optimal temperature to conduct further
alkylations with other alkyl halides. Notably, the enantiose-
Complexes of Co and Cr, 1d and 5, have almost the same
catalytic activity and asymmetric inducing ability (Table 3, runs
3 and 11). Had the central metal ion directly participated in the
transition state of the reaction, such dependence would have
been highly unusual. On the other hand, if the coordinated
ligand had the main role to play in the phase transfer, it would
be a highly expected event.
Negatively charged Co complex 6 was a very poor catalyst of
the alkylation reactions (Table 3, runs 12). Evidently, had the
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Scheme 4. Michael Addition of O’Donnell Substrate to Methyl Acrylate, Catalyzed by Λ(R,R)-1d
(9) Mahlau, M.; List, B. Angew. Chem., Int. Ed. 2013, 52, 518−533.
(10) Lacour, L.; Moraleda, D. Chem. Commun. 2009, 7073−7089.
(11) Dolling, U.-H.; Davies, P.; Grabowski, E. J. J. Am. Chem. Soc.
1984, 106, 446−447.
central metal ion a decisive influence on the catalytic activity,
the catalyst would be expected to have much higher activity.
Michael addition was also catalyzed by 1d with the same
sense of chirality of the final product as in the case of alkyl
halide alkylation. Such behavior is expected for classical APTC
reactions.
(12) Howell, G. P. Org. Process Res. Dev. 2012, 16, 1258−1272.
(13) Denmark, S. E.; Gould, N. D.; Wolf, L. M. J. Org. Chem. 2011,
76, 4260−4336.
(14) Denmark, S. E.; Gould, N. D.; Wolf, L. M. J. Org. Chem. 2011,
76, 4337−4357.
CONCLUSIONS
■
(15) Ooi, T.; Kameda, M.; Maruoka, K. J. Am. Chem. Soc. 1999, 121,
6519−6500.
The experiments demonstrated that the strategy of using chiral,
stereochemically inert, positively charged complexes of CoIII as
a catalyst system in the benchmark reaction of O’Donnell
substrate alkylation under APTC conditions was successful.
The complexes are easy to prepare and modify. Another feature
of the catalysts not present in chiral ammonium cations was
their hydrogen bond donating properties and potential redox
properties. Work is ongoing to further modify the complexes
and explore their applications in other areas of organic catalysis
and anion coordination applications.
(16) Lygo, B.; Allbutt, B.; James, R. S. Tetrahedron Lett. 2003, 44,
5629−5632.
(17) Shibuguchi, T.; Fukuta, Y.; Akachi, Y.; Sekine, A.; Ohshima, T.;
Shibasaki, M. Tetrahedron Lett. 2002, 43, 9539−9543.
(18) Arai, S.; Truji, R.; Nishida, A. Tetrahedron Lett. 2002, 43, 9535−
9537.
(19) Novacek, J.; Waser, M. Eur. J. Org. Chem. 2013, 637−648.
(20) Belokon, Y. N.; Maleev, V. I.; Kataev, D. A.; Mal’fanov, I. L.;
Bulychev, A. G.; Moskalenko, M. A.; Saveleva, T. F.; Skrupskaya, T. V.;
Lyssenko, K. A.; Godovikov, I. A.; North, M. Tetrahedron: Asymmetry
2008, 19, 822−831.
(21) Belokon, Y. N.; Maleev, V. I.; Kataev, D. A.; Saveleva, T. F.;
Skrupskaya, T. V.; Nelyubina, Y. V.; North, M. Tetrahedron: Asymmetry
2009, 20, 1746−1752.
(22) Maleev, V. I.; Skrupskaya, T. V.; Yashkina, L. Y.; Mkrtchyan, A.
F.; Saghyan, A. S.; Il’in, M. M.; Chusov, D. A. Tetrahedron: Asymmetry
2013, 24, 178−183.
(23) Belokon, Y. N.; Larionov, V. A.; Mkrtchan, A. F.; Khrustalev, V.
N.; Nijland, A.; Saghiyan, A. S.; Godovikov, I. A.; Peregudov, A. S.;
Babievsky, K. K.; Ikonnikov, N. S.; Maleev, V. I. Izv. Akad. Nauk, Ser.
Khim. 2012, 12, 2231−2239 (in Russian). Russ. Chem. Bull. 2012, in
press (in press).
ASSOCIATED CONTENT
* Supporting Information
■
S
All procedures for the preparation of novel complexes 1−5 and
their characterization; general procedures alkylation and
Michael addition of glycine tert-butyl ester benzophenone
Schiff base; proton and carbon NMR as well as IR and CD
spectra of novel reported compounds 1−5; X-ray diffraction
data for complexes 1d and 3; and CIF files of single crystals 1d
and 3. This material is available free of charge via the Internet at
http://pubs.acs.org.
(24) Gale, P. Acc. Chem. Res. 2011, 44, 216−226.
AUTHOR INFORMATION
Corresponding Author
*Fax: (+7) 499-135-5085. E-mail: yubel@ineos.ac.ru.
Notes
■
(25) Gale, P. Chem. Commun. 2011, 47, 82−86.
(26) Kemmitt, R. D. W.; Russell, D. R. In Comprehensive
Organometallic Chemistry. The Synthesis, Reactions and Structures of
Organometallic Compounds; Abel, E. W., Wilkinson, G., Stone, F. G. A.,
Eds.; Pergamon Press Lt: New York, 1982; Vol. 5, pp 2−264.
The authors declare no competing financial interest.
ACKNOWLEDGMENTS
■
The authors gratefully acknowledge financial support from
RFBR, research project no. 13-03-92601-KO-a. The authors are
also grateful to Dr. Michail M. Il’yin for HPLC analysis, Dr.
Kirill K. Babievsky for CD measurments, and Dr. Olga L.
Lependina for X-ray fluoresence analysis.
REFERENCES
■
(1) Donnell, M. J. Aldrichimica Acta 2001, 34, 3−15.
(2) Lygo, B.; Andrews, B. I. Acc. Chem. Res. 2004, 37, 518−526.
(3) Ooi, T.; Maruoka, K. Acc. Chem. Res. 2004, 37, 526−533.
(4) Hashimoto, T.; Maruoka, K. Chem. Rev. 2007, 107, 5656−5682.
(5) Maruoka, K.; Ooi, T.; Kano, T. Chem. Commun. 2007, 1487−
1495.
(6) Maruoka, K. Chem. Rec. 2010, 10, 254−259.
(7) Shirakawa, S.; Maruoka, K. Angew. Chem., Int. Ed. 2013, 52,
4312−4348.
(8) Brak, K.; Jacobsen, E. N. Angew. Chem., Int. Ed. 2013, 52, 534−
561.
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