Asymmetric inverse-electron-demand hetero-Diels-Alder reaction of six-membered cyclic ketones: An enamine/metal lewis acid bifunctional approach

Article abstract of DOI:10.1002/anie.201100160

On demand: The first example of the title reaction involving cyclic ketones and β,γ-unsaturated α-ketoesters has been achieved using the novel enamine/metal Lewis acid bifunctional catalysis (see scheme; Tf=trifluoromethanesulfonyl). Enones with both elec

Full text of DOI:10.1002/anie.201100160

Communications
DOI: 10.1002/anie.201100160
Asymmetric Catalysis
Asymmetric Inverse-Electron-Demand Hetero-Diels–Alder Reaction
of Six-membered Cyclic Ketones: An Enamine/Metal Lewis Acid
Bifunctional Approach**
Zhenghu Xu, Lu Liu, Kraig Wheeler, and Hong Wang*
The combination of organocatalysis with metal catalysis has
emerged as a potentially powerful tool in organic synthesis.[1]
This new concept aims to achieve organic transformations
that cannot be accessed by organocatalysis or metal catalysis
alone. In our effort to combine organo-enamine catalysis with
metal Lewis acid catalysis, we have developed a new class of
bifunctional enamine/metal Lewis acid catalysts.[2] These
bifunctional catalysts displayed unusually high activity and
high stereoselectivity in asymmetric direct aldol reactions.
The challenge in the development of Lewis acid/Lewis base
catalytic systems lies in the acid-base quenching reaction that
leads to catalyst inactivation.[3] A common and elegant
approach to solving this problem is the use of a soft acid
along with a hard base, or vice versa. Based on this approach,
organo-enamine catalysis has been successfully combined
with CuI,[1c,d] AgI,[1e] Pd0,[1f–h] and AuI.[1i,j] We use a different
strategy to solve the acid-base problem. This new strategy
complements the mixed soft/hard approach. In our system,
the Lewis base (primary or secondary amine) is tethered to a
chelating ligand, which serves as a “trap” for the incoming
metal. In this way, the base and the metal Lewis acid are
brought into close proximity in one molecule without
interacting with each other (Figure 1). The bifunctional
enamine/metal Lewis acid catalysts have two unique advan-
tages. First, a large number of metals can be introduced. The
Lewis acidity can be easily tuned by simply using a different
metal, thereby offering great flexibility to this system. For
example, stronger Lewis acids, such as LaIII, can be used to
activate the enamine acceptor more strongly. Second, the
bifunctional catalysts can potentially convert an intermolec-
ular reaction into a much more efficient intramolecular
reaction. In addition, the intramolecular bifunctional nature
of the catalysts would also enhance the stereoselectivity of the
reaction. With these catalysts, we intend to develop new
carbon–carbon or carbon–heteroatom bond-forming reac-
tions involving difficult organic transformations. Herein, we
report the first example of a highly chemo- and enantiose-
lective inverse-electron-demand hetero-Diels–Alder (HDA)
reaction of cyclic ketones with b,g-unsaturated-a-ketoesters
catalyzed by primary-amine-based enamine/metal Lewis acid
bifunctional catalysts.
Asymmetric inverse-electron-demand hetero-Diels–
Alder (IED/HDA) reactions of electron-rich alkenes with
an electron-deficient a,b-unsaturated ketone offers a valuable
synthetic entry into dihydropyran derivatives, which are
chemically and biologically of significant importance, allow-
ing the construction of up to three stereogenic centers in one
operation.[4] In most of the inverse-electron-demand HDA
reactions, enol ethers derived from aldehydes act as the
electron-rich alkenes (dienophiles). Very recently, enamine-
based organocatalytic asymmetric inverse-electron-demand
HDA reactions, in which an in situ formed enamine from a
chiral pyrolidine and an aldehyde serves as the dienophile,
have been made possible.[5] Ketones are much less reactive
compared to aldehydes because of electronic and steric
reasons. Asymmetric HDA reactions of ketones, in particular
cyclic ketones, have remained a long-standing challenge.[4,6]
We are interested in developing a catalytic asymmetric
enamine-based IED/HDA reaction of simple ketones with
enones, as it would greatly generalize this method, and open it
up to much wider exploitation.
Figure 1. Illustration of primary amine/metal Lewis acid bifunctional
catalysts.
To achieve this we believe that the activation of enones
should extend beyond hydrogen-bond methods,[5,7,8] for
example, by using a strong metal Lewis acid.[4,9] In contrast,
the formation of a less congested enamine intermediate using
a primary amine catalyst may also contribute to or facilitate
this transformation. The primary-amine-based enamine/metal
Lewis acid bifunctional catalysts developed in our labora-
tory[2] appear to be ideal candidates to tackle this difficult
problem. We envision that the primary amine/metal Lewis
acid bifunctional catalyst would engage enone 3 and the cyclic
ketone 2 intramolecularly (Scheme 1). The primary amine
would form an enamine in situ with the ketone (A) and the
[*] Dr. Z. Xu, Dr. L. Liu, Prof. Dr. H. Wang
Department of Chemistry and Biochemistry, Miami University
Oxford, OH (USA)
Fax: (+1)513-529-5715
E-mail: wangh3@muohio.edu
Prof. Dr. K. Wheeler
Department of Chemistry, Eastern Illinois University
Charleston, IL (USA)
[**] K.W. acknowledges an NSF grant (CHE-0722547) and Miami
University for financial support.
Supporting information for this article is available on the WWW
under http://dx.doi.org/10.1002/anie.201100160.
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IED/HDA reaction is the stability of the HDA product 4a.[5,6]
It was not clear to us if 4a was stable enough to be isolated, or
if further stabilization of 4a, for example, through the
protection of the hydroxy group, was needed. To investigate
the possibility of a HDA reaction, we first pursued screening
of the metal using ligand 1a (Table 1). The catalysts were
prepared by stirring the ligand and the metal salt in solvent
for 1–4 hours before addition of the substrates. The reaction
of cyclohexanone 2a and enone 3a was carried out in THF in
the presence of 1a and a metal salt at room temperature.
Whereas Cu(SbF6)2, Sc(OTf)3 and Eu(fod)3 essentially
yielded the aldol product 5a (entries 1, 4, and 5), we were
intrigued to discover that La(OTf)3, Yb(OTf)3, and Y(OTf)3
produced the desired HDA product 4a (entries 2, 3, and 6).
To our surprise, the HDA product 4a displayed remarkable
stability. Y(OTf)3 was selected as the metal for solvent
screening because of the higher chemo-, diastereo-, and
enantioselectivity of the resulted HDA reaction. A range of
solvents were examined (entries 6–11). Toluene gave the
highest chemoselectivity but moderate enantioselectivity of
4a (entry 8), THF and CH3CN resulted in good chemo-
selectivity and very good enantioselectivity (entries 6 and 9),
and when MeOH was used, only the aldol product 5a was
isolated (entry 11).
Scheme 1. The proposed catalytic cycle of the HDA reaction catalyzed
by primary amine/metal Lewis acid bifunctional catalyst. L=ligand.
metal could strongly activate enone 3 through chelation (B).
The activated enone would react with the enamine, thus
leading to the formation of cyclic aminal C; hydrolysis of C
will generate the HDA product hexahydrochromene 4, and
release the catalyst to complete the catalytic cycle.
As indicated in Table 1, the aldol reaction was the major
competing reaction with the HDA reaction.[7] The formal
[3+3] annulation product[8] was not observed. The hydrolysis
of the aminal C (Scheme 1) appeared to be a key step to
Initially, the primary amine/metal Lewis acid bifunctional
catalysts were applied to the reaction of cyclic ketones with
enones. The IED/HDA reaction of cyclic ketones with enones
will lead to the formation of a novel class of fused
dihydropyran derivatives (hexahydrochromenes), creating
three stereogenic centers including a quaternary carbon
center (Scheme 2, bottom). However, it has been reported
that cyclohexanone reacted with enone 3a in the presence of a
proline-derived catalyst, thereby leading to a highly enantio-
selective aldol reaction (Scheme 2, top);[7] in contrast, a novel
asymmetric formal [3+3] annulation reaction occurred
between the same reactants using a different proline deriv-
ative as the catalyst, forming a bicyclo [3,3,1] skeleton
(Scheme 2, middle).[8] Another uncertainty involved in the
Table 1: Optimization of HDA reaction.[a]
Entry Metal
Solvent t [h] Yield [%][b] d.r.[c] 4a/5a[c] ee [%][d]
1
2
3
4
5
6
7
8
Cu(SbF6)2 THF
72 72
16 95
36 61
60 60
60 37
16 61
–
0:100
95:5
70:30 70
0:100
0:100
92:8
96:4
>99:1
92:8
58:42 82
<1:99
91:9
–
49
La(OTf)3 THF
Yb(OTf)3 THF
Sc(OTf)3 THF
Eu(fod)3 THF
4:1
3:1
–
–
–
83
75
67
85
–
Y(OTf)3
Y(OTf)3
Y(OTf)3
Y(OTf)3
Y(OTf)3
Y(OTf)3
THF
7:1
4:1
4:1
4:1
4:1
–
5:1
–
–
CH2Cl2 36 75
Toluene 36 74
CH3CN 36 70
9
10
11
Neat
36 76
MeOH 72 31
CH3CN 36 50
CH3CN 72 20
CH3CN 72 20
CH3CN 72 38
–
82
–
12[e] Y(OTf)3
13[f] Y(OTf)3
14[g] Y(OTf)3
15[h] Y(OTf)3
8:92
4:96
83:17 80
–
4:1
[a] Reaction conditions: enone
3 (0.2 mmol) and cyclohexanone
(0.5 mL), room temperature in 1 mL of solvent. [b] Yield of the isolated
HDA products. [c] Determined by 1H NMR spectroscopy.[10] [d] Deter-
mined by HPLC analysis using a chiral stationary phase. [e] 50 mg of
silica gel was added. [f] 5 equivalents of H2O was added. [g] 10 equiv-
alents of H2O was added. [h] 100 mg of 4 ꢀ M.S. was added. fod=
6,6,7,7.8,8,8-heptafluoro-2,2-dimethyl-3,5-octadiene, M.S.=molecular
sieves, THF=tetrahydrofuran.
Scheme 2. Comparison of organocatalysts with the bifunctional cata-
lyst in the reaction of cyclohexanone with enone 3a. TBDPS=tert-
butyldiphenylsilyl, Tf=trifluoromethanesulfonyl.
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complete the HDA catalytic cycle, and we found that the
ketoesters with both electron-donating and electron-with-
water generated in situ in this reaction played a critical role in
the hydrolysis. When molecular sieves were included in the
reaction (Table 1, entry 15), the HDA reaction was noticeably
slower and the yield decreased significantly, suggesting that
water was essential for the HDA reaction; however, when
extra water was added to the reaction, the reaction path was
switched to favor the aldol reaction (entries 13 and 14). In
contrast to the aminals formed from secondary amines, the
hydrolysis of aminal C, which was generated from a primary
amine, did not require the presence of silica;[5] inclusion of
silica did not facilitate the reaction (entry 12).
drawing aromatic substituents at the g position reacted
Having established the possibility of an inverse-electron-
demand HDA reaction of cyclohexanone 2a and enone 3a,
we screened a series of primary-amine-based ligands (1, 6, and
7; Table 2) to enhance the activity and stereoselectivity.
Ligand 1, 6, and 7 were prepared through the coupling
reaction of the corresponding amine and N-protected l-
amino acids with subsequent deprotection (see the Support-
ing Information for experimental details). While all ligands
showed activity toward the HDA reaction, 1 f displayed the
best chemo- and enantioselectivity; however, the diastereo-
selectivity was a little low (entry 8). Lowering the temper-
ature to 48C lead to excellent chemo- and enantioselectivity,
but also with a slight decrease in diastereoselectivity (entry 9)
When the solvent was changed from CH3CN to THF at 48C,
the diastereoselectivity significantly increased, whilst excel-
lent chemo- and enantioselectivity were maintained
(entry 10).
The substrate scope of this reaction was explored by the
reaction of four six-membered cyclic ketones with various
substituted enones (Scheme 3). The b,g-unsaturated-a-
Table 2: Ligand screening.[a]
Entry Ligand Solvent t [h] Yield [%][b] d.r.[c] 4a/5a[c] ee [%][d]
1
2
3
4
5
6
7
8
9
10
1a
1b
1c
1d
1e
6
CH3CN 36
CH3CN 36
CH3CN 36
CH3CN 48
CH3CN 36
CH3CN 72
CH3CN 36
CH3CN 36
CH3CN 72
70
21
72
48
64
51
54
91
86
81
4:1
13:1
4:1
4:1
6:1
–
92:8
50:50 66
95:5 80
75:25 48
88:12 71
85
10:90
–
7
6:1
3:1
85:15 67
1 f
1 f[e]
1 f[e]
98:2
92
2.5:1 >99:1
9:1 >99:1
>99
>99
Scheme 3. Substrate scope. The reaction was performed with
0.2 mmol of enone and 0.5 mL of cyclic ketone at 48C in 1 mL of THF
unless otherwise mentioned. [a] 5.0 equivalents of cyclic ketone was
used at 48C. [b] 5.0 equivalents of cyclic ketone was used at room
temperature. [c] Only two diastereomers were detected by 1H NMR
analysis of the crude reaction mixture, similar to the reactions of
cyclohexanone.
THF
84
[a] Reaction conditions: enone 3a (0.2 mmol) and cyclohexanone
(0.5 mL) at room temperature in 1 mL of solvent. [b] Yield of the
isoalted HDA products. [c] Determined by 1H NMR spectroscopy.[10]
[d] Determined by HPLC analysis using a chiral stationary phase.
[e] Run at 48C. Bn=benzyl.
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smoothly with cyclohexanone to give the HDA product 4 in
very good to excellent enantioselectivity (4a–4m, 85–
99% ee), and good to very good diastereoselectivity (d.r. 3:1
to 9:1). No aldol product was detected in either of the
reactions except the reaction of the g-3-nitrophenyl-substi-
tuted enone (4m/5m = 12:1). The reactions of 4-methylcy-
clohexanone with enones were also highly chemo- and
stereoselective (4r–4t, no aldol product, d.r. 5:1 to 6:1, 94–
97% ee), noting four stereogenic centers were created in
these reactions. Both dihydropyran-4-one and dihydrothio-
pyran-4-one underwent HDA reaction smoothly with enones,
generating the fused dipyran in good stereoselectivity (4n–
4q, d.r. 3:1 to 8:1, 92–95% ee); however, the aldol products
were also isolated as the minor product in 10–30% yields.
Only two endo diastereomers were detected for the HDA
reaction of cyclic ketones. The IED/HDA reactions were
highly chemo- and enantioselective. It is worth noting that a
stereochemically well-controlled quaternary carbon center
was formed in this reaction. We believe that the strong
activation of the activated enone 3 through chelation to the
metal and the intramolecular nature of the bifunctional
catalyst contribute to the high activity and stereoselectivity of
the HDA reactions. In addition, a new approach has been
introduced to the emerging field involving combination of
organocatalysis and metal catalysis. In depth investigation
and detailed mechanistic study of this reaction is underway.
The application of these catalysts in other important organic
reactions will be reported in due course.
products as determined from the 1H NMR spectra of the Experimental Section
General procedure: A mixture of Y(OTf)3 (16 mg, 0.03 mmol, 15
crude reaction mixtures, even though three to four stereo-
genic centers were generated in these inverse-electron-
demand HDA reactions. The HDA reactions were highly p-
facial diastereoselective, and the final diastereomers were
produced from the hydrolysis in the last step (see
Scheme 1).[10] The structure and relative stereochemistry of
the major diastereomer was determined by 1H and 13C NMR
spectroscopy, including DEPT, COSY, NOESY, HSQC, and
NOE methods (see the Supporting Information). The abso-
lute configuration of 4g was established as 4R, 4aS, 8aR by X-
ray crystallography (Figure 2).[11] All the rest of the com-
pounds were assumed to have similar configurations as 4g.
mol%), ligand 1 f ( 12.4 mg, 0.06 mmol, 30 mol%), cyclohexanone
(0.5 mL), and THF (1.0 mL) was stirred at room temperature for 4 h.
The reaction mixture was then cooled to 48C. Enone 3 (0.2 mmol) was
added. The resulting mixture was stirred at 48C until the reaction was
complete (monitored by TLC). The reaction mixture was filtered
through a short silica gel plug to remove the catalyst. After removal of
the solvent, 1H NMR analysis of the mixture was recorded to
determine the chemoselectivity and the diastereoselectivity. The
mixture was purified through column chromatography on silica gel
(eluent: n-hexane/ethyl acetate 6:1) to give the pure products.
The ee values were determined by HPLC analysis using a chiral
stationary phase. Racemic HDA products were prepared under the
same conditions using racemic ligand 1a, which was obtained from
racemic valine.
Received: January 8, 2011
Published online: March 10, 2011
Keywords: bifunctional catalysis · cyclic ketones ·
.
cycloadditions · Lewis acids · organocatalysis
Figure 2. X-ray crystal structure of 4g.
[1] For reviews, see a) Z. H. Shao, H. B. Zhang, Chem. Soc. Rev.
2009, 38, 2745; b) C. Zhong, X. D. Shi, Eur. J. Org. Chem. 2010,
2999; for examples, see c) A. E. Allen, D. W. C. MacMillan, J.
Am. Chem. Soc. 2010, 132, 4986; d) T. Yang, A. Ferrali, L.
Campbell, D. J. Dixon, Chem. Commun. 2008, 2923; e) Q. P.
Ding, J. Wu, Org. Lett. 2007, 9, 4959; f) I. Ibrahem, A. Cordova,
Angew. Chem. 2006, 118, 1986; Angew. Chem. Int. Ed. 2006, 45,
1952; g) F. Bihelovic, R. Matovic, B. Vulovic, R. N. Saicic, Org.
Lett. 2007, 9, 5063; h) I. Usui, S. Schmidt, B. Breit, Org. Lett.
2009, 11, 1453; i) A. S. K. Hashmi, C. Hubbert, Angew. Chem.
2010, 122, 1026; Angew. Chem. Int. Ed. 2010, 49, 1010; j) J. T.
Binder, B. Crone, T. T. Haug, H. Menz, S. F. Kirsch, Org. Lett.
2008, 10, 1025; k) M. Ikeda, Y. Miyake, Y. Nishibayashi, Angew.
Chem. 2010, 122, 7447; Angew. Chem. Int. Ed. 2010, 49, 7289.
[2] a) Z. H. Xu, P. Daka, H. Wang, Chem. Commun. 2009, 6825;
b) Z. H. Xu, P. Daka, I. Budik, H. Wang, F. Q. Bai, H. X. Zhang,
Eur. J. Org. Chem. 2009, 4581; c) P. Daka, Z. H. Xu, A. Alexa, H.
Wang, Chem. Commun. 2011, 47, 224.
The bifunctional nature of the catalysts was revealed by
the fact that neither the ligand 1a nor the metal (Y(OTf)3)
alone could catalyze the reaction. The proposed transition-
state B (Figure 3), featuring an endo-selective mode of
reaction, matches well with the relative stereochemistry at
C4 and C4a. The big R (tert-butyl)
group shields the Re face of the
enamine, thus the activated enone
attacks from the Si face, accounting
for the absolute configuration of the
final products.
In conclusion, the novel concept of
enamine/metal Lewis acid bifunc-
tional catalysis was introduced for
the first time to the HDA reactions,
offering
approach to meeting the long-stand-
ing challenge of asymmetric HDA
Figure 3. Proposed
transition state of the
IED/HDA reaction cat-
alyzed by enamine/
metal Lewis acid
[3] a) D. H. Paull, C. J. Abraham, M. T. Scerba, E. Alden-Danforth,
T. Lectka, Acc. Chem. Res. 2008, 41, 655; b) H. Grꢀger, Chem.
Eur. J. 2001, 7, 5246; c) L. Y. Wu, G. Bencivenni, M. Mancinelli,
A. Mazzanti, G. Bartoli, P. Melchiorre, Angew. Chem. 2009, 121,
7332; Angew. Chem. Int. Ed. 2009, 48, 7196; d) M. Kanai, N.
Kato, E. Ichikawa, M. Shibasaki, Synlett 2005, 1491.
a convenient and viable
bifunctional catalysts.
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[4] a) H. Pellissier, Tetrahedron 2009, 65, 2839; b) K. A. Jørgensen,
3059; d) D. A. Evans, J. S. Johnson, E. J. Olhava, J. Am. Chem.
Soc. 2000, 122, 1635; e) H. Audrain, J. Thorhauge, R. G. Hazell,
K. A. Jorgensen, J. Org. Chem. 2000, 65, 4487; f) W. Zhuang, J.
Thorhauge, K. A. Jorgensen, Chem. Commun. 2000, 459; g) G.
Desimoni, G. Faita, M. Toscanini, M. Boiocchi, Chem. Eur. J.
2007, 13, 9478; h) G. Desimoni, G. Faita, M. Mella, F. Piccinini,
M. Toscanini, Eur. J. Org. Chem. 2007, 1529; i) D. H. Paull, E.
Alden-Danforth, J. Wolfer, C. Dogo-Isonagie, C. J. Abraham, T.
Lectka, J. Org. Chem. 2007, 72, 5380; j) F. Gohier, K. Bouhad-
jera, D. Faye, C. Gaulon, V. Maisonneuve, G. Dujardin, R. Dhal,
Org. Lett. 2007, 9, 211.
Eur. J. Org. Chem. 2004, 2093; c) K. A. Jørgensen, M. Johann-
sen, S. Yao, H. Audrain, J. Thorhauge, Acc. Chem. Res. 1999, 32,
605; d) R. Alfini, M. Cecchi, D. Giomi, Molecules 2010, 15, 1722.
[5] a) K. Juhl, K. A. Jørgensen, Angew. Chem. 2003, 115, 1536;
Angew. Chem. Int. Ed. 2003, 42, 1498; b) Y. Zhao, X. J. Wang,
J. T. Liu, Synlett 2008, 1017; c) S. Samanta, J. Krause, T. Mandal,
C. G. Zhao, Org. Lett. 2007, 9, 2745; d) H. X. Xie, L. S. Zu, H. R.
Oueis, H. L. J. Wang, W. Wang, Org. Lett. 2008, 10, 1923; e) B.
Han, Z. Q. He, J. L. Li, R. Li, K. Jiang, T. Y. Liu, Y. C. Chen,
Angew. Chem. 2009, 121, 5582; Angew. Chem. Int. Ed. 2009, 48,
5474; f) B. Han, J. L. Li, C. Ma, S. J. Zhang, Y. C. Chen, Angew.
Chem. 2008, 120, 10119; Angew. Chem. Int. Ed. 2008, 47, 9971.
[6] HDA reaction of enamide derived from cyclic ketones: F.
Gallier, H. Hussain, A. Martel, A. Kirschning, G. Dujardin, Org.
Lett. 2009, 11, 3060.
[10] The minor diastereomer of the HDA products was identified as
an epimer (at C8a) of the endo product that resulted from the
hydrolysis in the last step (refer to Scheme 1). See the Support-
ing Information for relevant experiments and NMR spectrosco-
py.
[7] Organocatalytic reaction of cyclic ketone with enone leading to
aldol product: C. Zheng, Y. Wu, X. Wang, G. Zhao, Adv. Synth.
Catal. 2008, 350, 2690.
[8] Organocatalytic reaction of cyclic ketone with enone leading to
formal [3+3] annulation product: C. L. Cao, X. L. Sun, Y. B.
Kang, Y. Tang, Org. Lett. 2007, 9, 4151.
[9] For reviews, see ref. [4] and a) G. Desimoni, G. Faita, K. A.
Jorgensen, Chem. Rev. 2006, 106, 3561; b) J. S. Johnson, D. A.
Evans, Acc. Chem. Res. 2000, 33, 325; For representative
examples, see: c) K. Gademann, D. E. Chavez, E. N. Jacobsen,
Angew. Chem. 2002, 114, 3185; Angew. Chem. Int. Ed. 2002, 41,
[11] The crystal was obtained from the major enantiomer of 4g (>
99% ee after recrystallization). Crystal data for 4g at 100 K:
orthorhombic, space group P212121; a = 7.5414(2), b = 9.8212(2),
c = 20.9811(4) ꢁ; a = 90.00, b = 90.00, g = 90.008; V=
1553.98(6) ꢁ3, Z = 4, m(CuKa) = 3.747 mmÀ1, 10525 reflections
(2709 unique) collected with 4.21 < q < 67.588, Rint = 0.0314;
R1 = 0.0339, wR2 = 0.0867 refined on F2. CCDC 806477 (4g)
contains the supplementary crystallographic data for this paper.
These data can be obtained free of charge from The Cambridge
Crystallographic Data Centre via www.ccdc.cam.ac.uk/data_
request/cif.
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