Synthesis of novel molecular clips via click chemistry based on diethoxy carbonyl glycoluril

Article abstract of DOI:10.1055/s-0028-1087670

A novel class of molecular clips based on diethoxycarbonyl glycoluril was synthesized via click chemistry. Seven different arylacetylenes were used in this research, and all reaction products gave satisfying ¹H NMR, ¹³C NMR, HRMS, an

Full text of DOI:10.1055/s-0028-1087670

LETTER
315
Synthesis of Novel Molecular Clips via Click Chemistry Based on Diethoxy
carbonyl Glycoluril
N
ovel Molecular Clips
e
via Click C
n
hemistry g Gao, Liping Cao, Zihua Wang, Jingjing Sun, Nengfang She, Anxin Wu*
Key Laboratory of Pesticide & Chemical Biology, Ministry of Education, Central China Normal University, Wuhan 430079, P. R. of China
E-mail: chwuax@mail.ccnu.edu.cn
Received 8 September 2008
In connection with our ongoing program on the synthesis
Abstract: A novel class of molecular clips based on diethoxycarbo-
of novel molecular clips and the interesting structural fea-
tures of the 1,2,3-triazole ring, we were interested in ex-
ploring this versatile reaction on diethoxycarbonyl
nyl glycoluril was synthesized via click chemistry. Seven different
arylacetylenes were used in this research, and all reaction products
1
gave satisfying H NMR, 13C NMR, HRMS, and IR spectra. The
structure and conformation of 7a were further confirmed by single- glycoluril to synthesize novel molecular clips with 1,2,3-
crystal X-ray diffraction. A primary study on the metal-ion binding
triazole rings in flexible side arms.
ability of 7f showed that its fluorescence was obviously quenched
by Fe3+, Ag+, and Cu2+ among metal ions examined.
Table 1 Synthesis of 7a–g from Arylacetylenes
Key words: clip receptor, diethoxycarbonyl glycoluril, molecular
recognition, 1,2,3-triazole, fluorescence
Entry
1
R2C≡CH
Product
Yield (%)a
80
7a
Natural receptors play fundamental roles in molecular
recognition, catalysis, and transport processes. Inspired
by nature, artificial receptors have been developed to
mimic these functions.1 The rigid concave shape of gly-
coluril makes it a versatile building block to construct var-
ious supramolecular objects, such as molecular capsules,2
molecular clips,3 and cucurbit[n]uril family.4 Especially
glycoluril-based clips with a preorganized cleft and rigid
aromatic arms have been proved to be effective receptors
for hydroxybenzene derivatives,5 metal ions,6 and an-
ions.7 In order to expand the range of possible guest struc-
tures, we would like to introduce new binding sites and
flexible side arms in the glycoluril-based clips.1a
‘Click chemistry’ developed by Meldal8 and Sharpless9
for constructing 1,2,3-triazole compounds has found
growing applications in bioconjugation, drug discovery,
materials and supramolecular chemistry.10 Besides the ad-
vantages of its reliability, regioselectivity, and compati-
bility with reaction conditions, the unique structural
features of the 1,2,3-triazole ring–free electron pairs on
nitrogen, polarized proton, large dipole moment, and aro-
matic stability, render it active participation in hydrogen
bonding, metal–lone pair bonding, dipole–dipole and p–p
stacking interactions.10e,11 Recently, some triazole-based
receptors with interesting binding properties have been re-
ported, such as cyclodextrins,12 calix[4]arenes,13 dendrim-
ers,14 macrocycles,15 foldmers,16 and bile acids.17 These
results demonstrate that 1,2,3-triazole ring can play an im-
portant role in molecular recognition. However, to our
knowledge, the potential application of 1,2,3-triazole ring
in glycoluril-based clips has not yet been explored.
2
7b
70
N
3
4
5
7c
7d
7e
78
74
75
OMe
NH2
NH2
O
6
7
7f
82
82
7g
Fe
a Isolated yield.
The synthetic route used to obtain clips 7a–g is shown in
Scheme 1. Diethoxycarbonyl glycoluril 3, which has a
high degree of organic solubility unlike most other gly-
colurils, was synthesized according to the literature
procedure18 and used for these studies. As we previously
reported,19 compound 4 could be smoothly prepared by
treatment of 3 with ethanolamine in the presence of form-
aldehyde in MeOH. Tosylation of the hydroxyl group of 4
with TsCl afforded compound 5 in 61% yield.20 The tosy-
late 5 was reacted with sodium azide in DMF to give com-
pound 6 in 88% yield.21 Treatment of azide 6 with a
number of arylacetylenes in t-BuOH–H2O (1:1) in the
presence of CuSO4 and ascorbic acid (click reaction)9
SYNLETT 2009, No. 2, pp 0315–0319
x
x.
x
x.
2
0
0
9
Advanced online publication: 15.01.2009
DOI: 10.1055/s-0028-1087670; Art ID: W14008ST
© Georg Thieme Verlag Stuttgart · New York

316
M. Gao et al.
LETTER
O
O
HO
HO
O
O
CO2Et
CO2Et
NH
COOEt
NH
HN
EtOOC
OH
OH
(iv)
(iii)
(i)
(ii)
HN
O
2
1
3
O
O
O
OTs
(vi)
N
N
N
N
N
N
N
OH
TsO
HO
R1
R1
N
R1
N
N
N
R1
(v)
N
5
4
O
O
O
O
R2
R2
N3
N
N
N
N
N
N
N
N
N
N3
N
N
N
N
R1
N
R1
R1
R1
(vii)
N
N
N
N
R1 = COOEt
2 = Aryl
7
6
O
O
R
Scheme 1 Reagents and conditions: (i) AcOH, Br2, H2O; (ii) EtOH, HCl(g), 0 °C; (iii) PhH, H2NCONH2, TFA, reflux; (iv) NH2CH2CH2OH,
37% aq HCHO, MeOH, reflux; (v) TsCl, Et3N, CH2Cl2, r.t., 61%; (vi) NaN3, DMF, 60 °C, 88%; (vii) R2C≡CH, CuSO4, ascorbate acid, t-BuOH–
H2O, r.t., 70–82%.
gave the expected molecular clips 7a–g in 70–82% yield
(Table 1).
Taking into consideration that the nature of the substitu-
ents in arylacetylene could affect the binding properties, a
variety of arylacetylenes were used in this research. In ad-
dition to phenylacetylene, we had synthesized 4-
ethynylpyridine22 and three para-substituted phenyl-
acetylenes23 with electron-donating or -withdrawing
groups. To further enlarge the scope of the clip receptor as
a fluorescent or electrochemical sensor, we attached 1-
naphthalenyl group24 and ferrocenyl group25 to the tri-
azole units, which may allow us to monitor the binding
ability of the 1,2,3-triazole ring by fluorescence or cyclic
voltammetry (CV). All reaction products gave satisfacto-
ry 1H NMR, 13C NMR, HRMS, and IR data.26 In addition,
the structure and conformation of compound 7a were fur-
ther elucidated by single-crystal X-ray diffraction,27 as
shown in Figure 1. Compound 7a has a well-defined ge-
Figure 1 X-ray crystal structure of compound 7a; hydrogen atoms
are omitted for clarity
ometry due to the rigidity that the fused rings confer on the
in THF–MeOH (v/v, 50:1) was found to exhibit maximum
emission at 350 nm (excitation at 301 nm). Seventeen
metal ions were screened by comparing the fluorescence
intensities of the solutions before and after adding 10
equivalents metal ions as their chloride or nitrate salts: K+,
Mg2+, Ca2+, Sr2+, Ba2+, Mn2+, Fe3+, Co2+, Ni2+, Cu2+, Ag+,
Zn2+, Cd2+, Hg2+, Al3+, Sn2+, and Pb2+. The results are
shown in Figure 2.
molecule. It is built up from four fused rings, namely two
nearly planar imidazole five-membered rings and two
nonplanar triazine six-membered rings. Both six-mem-
bered rings have chair conformations. The distance be-
tween two carbonyl oxygen atoms (O1–O2) of the
glycoluril moiety is 5.299 Å. The distance between the
centers of two trizole rings is 6.388 Å, and the dihedral an-
gle between two trizole rings is 9.00°. The flexible spacer
connecting the glycoluril cavity and 1,2,3-triazole ring
can contribute to the conformational flexibility of this
host, allowing it to adopt the most favorable conformation
for guest complexation.
We found that the fluorescence of 7f (10 mM) was obvi-
ously quenched by Fe3+, Ag+, and Cu2+ ions. Comparing
these results with what we previously reported,6 we spec-
ulated that Fe3+ was bound in the cavity of the clip, the
fluorescent aromatic units of side arms probably behaved
as PET donors whereas the bound Fe3+ as electron accep-
tors, which led to a severe fluorescence quenching.29
However, complexation of Ag+ and Cu2+ ions requires the
Because fluorescent chemosensors have advantages of
high selectivity, sensitivity, and simplicity in molecular
recognition,28 we first chose 7f to screen its metal-ion
binding ability. The fluorescence spectrum of 7f (10 mM)
Synlett 2009, No. 2, 315–319 © Thieme Stuttgart · New York

LETTER
Novel Molecular Clips via Click Chemistry
317
Figure 4 Fluorescence emission spectra (excitation at 301 nm) of 7f
(1.0×10–5 M) in THF–MeOH (50:1, v/v) in the presence of AgNO3;
concentration of Ag+: 0, 1.0, 2.0, 4.0, 6.0, 8.0, 10.0, 14.0, 18.0, and
Figure 2 Fluorescence emission spectra of 7f (10 mM) in THF–
MeOH (v/v, 50:1) in the presence of 10 equiv various metal ions (ex-
citation at 301 nm)
24.0×10–5
M
coordination of the two triazole groups of 7f. This was
supported by silver(I) ion induced chemical shift of 7f
changes in the 1H NMR spectra (see Figure 3). In the pres-
ence of 5.0 equivalents of Ag+ ions, chemical shifts of
protons Ha–Hd downfield shifted by 0.02, 0.06, 0.06, and
0.07 ppm, respectively. The peak of He on the triazole
group was downfield shifted by 0.19 ppm, but the peaks
of Hf and Hg were upfield shifted by 0.31 and 0.09 ppm.
These results suggest that the two triazole groups are in-
volved in the complexation with Ag+.
In summary, we have synthesized a novel class of molec-
ular clips based on diethoxycarbonyl glycoluril via click
chemistry. The primary study on the metal ion binding
ability of 7f shows that its fluorescence was obviously
quenched by Fe3+, Ag+, and Cu2+ among seventeen metal
ions examined. Further studies on the quenching mecha-
nism and other molecular clips’ binding behavior are un-
der way in our laboratory.
Supporting Information for this article is available online at
http://www.thieme-connect.com/ejournals/toc/synlett.
Acknowledgment
This work was supported by the National Natural Science Founda-
tion of China (Nos. 20672042 and 20872042). We also thank Zhe-
jiang University for performing the structural analysis.
References and Notes
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Figure 3 (a) 1H NMR (600 MHz, CD3CN) spectra of 7f (5 mM); (b)
in the presence of 5.0 equiv of silver nitrate
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Chem. Res. 1999, 32, 995. (b) Wang, Z. G.; Zhou, B. H.;
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The fluorescence spectra of 7f (10 mM) at various concen-
trations of Ag+ ions are depicted in Figure 4; as can be
seen, no shift in the fluorescence maximum was observed.
However, the fluorescence intensities of 7f gradually de-
creased as the concentration of Ag+ increased from 10 to
240 mM. In the Job plot (see inset of Figure 4),30 a maxi-
mum fluorescence change was observed when the molar
fraction of 7f vs Ag+ was 0.5, indicative of a 1:1 complex.
Similar fluorescence titration behavior and a 1:1 binding
stoichiometry was also observed in the cases of 7f with
Cu2+ and Fe3+ ions, respectively (see supporting informa-
tion for details).
(5) Reek, J. N. H.; Priem, A. H.; Engelkamp, H.; Rowan, A. E.;
Elemans, J. A. A. W.; Nolte, R. J. M. J. Am. Chem. Soc.
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Synlett 2009, No. 2, 315–319 © Thieme Stuttgart · New York

318
M. Gao et al.
LETTER
(6) Hu, S. L.; She, N. F.; Yin, G. D.; Guo, H. Z.; Wu, A. X.;
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was isolated by column chromatography on SiO2 to get the
pure product 7.
Compound 7a: IR (KBr): 3244, 2985, 1705, 1444, 1282,
1047, 767, 697 cm–1. 1H NMR (400 MHz, CDCl3): d = 7.86
(d, J = 7.6 Hz, 4 H), 7.60 (s, 2 H), 7.45 (t, J = 7.6 Hz, 4 H),
7.35 (t, J = 7.6 Hz, 2 H), 4.82 (d, J = 13.6 Hz, 4 H), 4.36 (t,
J = 6.0 Hz, 4 H), 4.28 (q, J = 7.2 Hz, 4 H), 4.19 (d, J = 13.6
Hz, 4 H), 2.93 (t, J = 6.0 Hz, 4 H), 1.30 (t, J = 7.2 Hz, 6 H).
13C NMR (100 MHz, CDCl3): d = 164.9, 158.6, 147.3,
130.6, 128.9, 128.1, 125.6, 120.3, 76.0, 63.5, 60.0, 50.1,
47.0, 13.8. ESI-HRMS: m/z [M + Na]+ calcd for
(8) Tornoe, C. M.; Christensen, C.; Meldal, M. J. J. Org. Chem.
2002, 67, 3057.
(9) Rostovtsev, V. V.; Green, L. G.; Fokin, V. V.; Sharpless,
K. B. Angew. Chem. Int. Ed. 2002, 41, 2596.
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36, 1249. (b) Wang, Q.; Chan, T. R.; Hilgraf, R.; Fokin,
V. V.; Sharpless, K. B.; Finn, M. G. J. Am. Chem. Soc. 2003,
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Int. Ed. 2006, 45, 1435.
C34H38N12NaO6: 733.2929; found: 733.2898.
Compound 7b: IR (KBr): 3423, 3137, 2988, 1766, 1730,
1707, 1612, 1411, 1286, 1234, 1045, 907, 718 cm–1. 1H
NMR (400 MHz, CDCl3): d = 8.68 (d, J = 5.6 Hz, 4 H), 7.88
(s, 2 H), 7.77 (d, J = 5.6 Hz, 4 H), 4.81 (d, J = 13.6 Hz, 4 H),
4.45 (t, J = 5.2 Hz, 4 H), 4.31–4.22 (m, 8 H), 3.03 (t, J = 5.2
Hz, 4 H), 1.31 (t, J = 7.2 Hz, 6 H). 13C NMR (100 MHz,
CDCl3): d = 164.7, 158.5, 150.3, 144.9, 137.9, 122.0, 119.8,
75.9, 63.6, 59.9, 50.1, 47.6, 13.8. ESI-HRMS: m/z [M+Na]+
calcd for C32H36N14NaO6: 735.2834; found: 735.2814.
Compound 7c: IR (KBr): 3447, 3134, 2939, 2840, 1760,
1718, 1459, 1299, 1248, 1231, 1027, 825, 717 cm–1. 1H
NMR (400 MHz, CDCl3): d = 7.78 (d, J = 8.4 Hz, 4 H), 7.50
(s, 2 H), 6.97 (d, J = 8.4 Hz, 4 H), 4.82 (d, J = 13.2 Hz, 4 H),
4.33–4.25 (m, 8 H), 4.18 (d, J = 13.2 Hz, 4 H), 3.80 (s, 6 H),
2.88 (t, J = 6.0 Hz, 4 H), 1.30 (t, J = 7.2 Hz, 6 H). 13C NMR
(100 MHz, CDCl3): d = 164.9, 159.5, 158.5, 147.2, 126.9,
123.3, 119.5, 114.2, 76.0, 63.5, 60.0, 55.2, 50.1, 46.8, 13.8.
ESI-HRMS: m/z [M + Na]+ calcd for C36H42N12NaO8:
793.3141; found: 793.3100.
(12) Maisonneuve, S.; Fang, Q.; Xie, J. Tetrahedron 2008, 64,
8716.
(13) Chang, K. C.; Su, I. H.; Senthilvelan, A.; Chung, W.-S. Org.
Lett. 2007, 9, 3363.
(14) Ornelas, C.; Aranzaes, J. R.; Cloutet, E.; Alves, S.; Astruc,
D. Angew. Chem. Int. Ed. 2007, 46, 872.
(15) Li, Y.; Flood, A. H. Angew. Chem. Int. Ed. 2008, 47, 2649.
(16) Juwarker, H.; Lenhardt, J. M.; Pham, D. M.; Craig, S. L.
Angew. Chem. Int. Ed. 2008, 47, 3740.
(17) Pandey, P. S.; Kumar, A. Org. Lett. 2008, 10, 165.
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Fettinger, J. C.; Day, A. I.; Isaacs, L. Tetrahedron 2003, 59,
1961.
(19) Li, Y. T.; Yin, G. D.; Guo, H. Z.; Zhou, B. H.; Wu, A. X.
Synthesis 2006, 2897.
Compound 7d: IR (KBr): 3448, 3367, 2982, 2849, 1720,
1621, 1502, 1416, 1291, 1233, 1029, 906, 837, 715 cm–1. 1H
NMR (400 MHz, CDCl3): d = 7.61 (d, J = 8.0 Hz, 4 H), 7.48
(s, 2 H), 6.68 (d, J = 8.0 Hz, 4 H), 4.79 (d, J = 13.6 Hz, 4 H),
4.28–4.24 (m, 8 H), 4.12 (d, J = 13.6 Hz, 4 H), 3.90 (br s, 4
H), 2.87 (t, J = 6.0 Hz, 4 H), 1.27 (t, J = 7.2 Hz, 6 H). 13
C
(20) Compound 5: IR (KBr): 2984, 1721, 1598, 1415, 1358,
1176, 907, 817, 663, 554 cm–1. 1H NMR (400 MHz, CDCl3):
d = 7.78 (d, J = 8.4 Hz, 4 H), 7.35 (d, J = 8.4 Hz, 4 H), 4.73
(d, J = 14.4 Hz, 4 H), 4.30–4.25 (m, 8 H), 4.04 (t, J = 5.2 Hz,
4 H), 2.90 (t, J = 5.2 Hz, 4 H), 2.48 (s, 6 H), 1.30 (t, J = 7.2
Hz, 6 H). 13C NMR (150 MHz, CDCl3): d = 164.8, 158.3,
144.8, 132.5, 129.7, 127.7, 75.8, 67.2, 63.3, 59.7, 49.7, 21.4,
13.6. ESI-MS: m/z = 765.0 [M + H]+.
(21) Compound 6: IR (KBr): 2989, 2100, 1767, 1709, 1407,
1276, 899, 725 cm–1. 1H NMR (400 MHz, CDCl3): d = 4.88
(d, J = 14.0 Hz, 4 H), 4.36 (d, J = 14.0 Hz, 4 H), 4.30 (q,
J = 7.2 Hz, 4 H), 3.36 (t, J = 5.6 Hz, 4 H), 2.90 (t, J = 5.6 Hz,
6 H), 1.32 (t, J = 7.2 Hz, 6 H). 13C NMR (100 MHz, CDCl3):
d = 165.0, 158.4, 76.0, 63.4, 59.9, 50.4, 48.6, 13.8. ESI-MS:
m/z = 506.2 [M]+.
NMR (100 MHz, CDCl3): d = 164.9, 158.5, 147.6, 146.7,
126.7, 120.6, 119.1, 115.1, 76.0, 63.5, 59.9, 50.0, 46.7, 13.8.
ESI-HRMS: m/z [M + Na]+ calcd for C34H40N14NaO6:
763.3147; found: 763.3120.
Compound 7e: IR (KBr): 3362, 3150, 2984, 2844, 1726,
1678, 1617, 1414, 1235, 1030, 982, 910, 858, 776, 713 cm–1.
1H NMR (400 MHz, DMSO-d6): d = 8.50 (s, 2 H), 8.02 (s, 2
H), 7.98 (d, J = 8.0 Hz, 4 H), 7.91 (d, J = 8.0 Hz, 4 H), 7.42
(s, 2 H), 4.78 (d, J = 13.6 Hz, 4 H), 4.50 (t, J = 6.0 Hz, 4 H),
4.29–4.21 (m, 8 H), 3.01 (t, J = 6.0 Hz, 4 H), 1.21 (t, J = 7.2
Hz, 6 H). 13C NMR (100 MHz, DMSO-d6): d = 167.5, 164.9,
158.3, 145.4, 133.4, 133.3, 128.3, 124.8, 122.5, 75.8, 63.5,
59.3, 49.6, 46.9, 13.7. ESI-HRMS: m/z [M + Na]+ calcd for
C36H40N14NaO8: 819.3046; found: 819.2999.
Compound 7f: IR (KBr): 3423, 3137, 2988, 1766, 1730,
1707, 1612, 1411, 1286, 1234, 1045, 907, 718 cm–1. 1H
NMR (400 MHz, CDCl3): d = 8.43 (d, J = 8.0 Hz, 2 H), 7.82
(d, J = 8.0 Hz, 4 H), 7.70 (d, J = 8.0 Hz, 2 H), 7.55–7.47 (m,
8 H), 4.82 (d, J = 13.6 Hz, 4 H), 4.34 (t, J = 6.0 Hz, 4 H),
4.27 (q, J = 7.2 Hz, 4 H), 4.20 (d, J = 13.6 Hz, 4 H), 2.95 (t,
J = 6.0 Hz, 4 H), 1.28 (t, J = 7.2 Hz, 6 H). 13C NMR (100
MHz, CDCl3): d = 164.8, 158.5, 146.5, 133.7, 130.7, 128.8,
128.3, 127.8, 127.1, 126.7, 126.0, 125.4, 125.3, 122.9, 75.9,
63.5, 60.0, 50.2, 47.1, 13.8. ESI-HRMS: m/z [M + Na]+
calcd for C42H42N12NaO6: 833.3242; found: 833.3220.
Compound 7g: IR (KBr): 3448, 3122, 2982, 1719, 1414,
1232, 1035, 906, 821, 712, 505, 488 cm–1. 1H NMR (400
MHz, CDCl3): d = 7.47 (s, 2 H), 4.81–4.78 (m, 8 H), 4.34–
4.30 (m, 8 H), 4.27 (q, J = 7.2 Hz, 4 H), 4.20 (d, J = 13.2 Hz,
4 H), 4.10 (s, 10 H), 2.94 (t, J = 5.6 Hz, 4 H), 1.29 (t, J = 7.2
(22) Ciana, L. D.; Haim, A. J. Heterocycl. Chem. 1984, 21, 607.
(23) Takahashi, S.; Kuroyama, Y.; Sonogashira, K.; Hagihara, N.
Synthesis 1980, 627.
(24) Crisp, G. T.; Jiang, Y. L. Synth. Commun. 1998, 28, 2571.
(25) Polin, J.; Schottenberger, H. Org. Synth. 1996, 73, 262.
(26) General Procedure for the Synthesis of Compound 7
The azide 6 (0.20 mmol) and arylacetylene (0.40 mmol)
were suspended in 1:1 mixture of H2O and t-BuOH (10 mL).
Ascorbate acid (0.020 mmol, 20 mL of freshly prepared 1.0
M solution in H2O, 10 mol%) was added, followed by
copper(II) sulfate pentahydrate (0.010 mmol, 100 mL of 0.10
M solution in H2O, 5.0 mol%). The heterogeneous mixture
was stirred vigorously for 24 h under Ar protection. After
total consumption of the starting material (determination by
TLC), the solvent was removed in vacuo, and the residue
Synlett 2009, No. 2, 315–319 © Thieme Stuttgart · New York

LETTER
Novel Molecular Clips via Click Chemistry
319
Hz, 6 H). 13C NMR (100 MHz, CDCl3): d = 164.9, 158.5,
which were used in all calculations. The final wR2 (F2) was
0.1531. CCDC 699457 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.
146.4, 119.5, 76.0, 75.5, 69.6, 68.6, 66.5, 63.5, 60.0, 50.3,
47.1, 13.8. ESI-HRMS: m/z [M + Na]+ calcd for
C42H46Fe2N12NaO6: 949.2273; found:949.2215.
(27) Crystal Data for Compound 7a
C34H38N12O6, MW = 710.78, triclinic, a = 10.0153 (5),
b = 12.0495 (6), c = 16.3995 (8) Å, a = 102.658 (0)°,
b = 103.272 (0)°, g = 108.398 (0)°, V = 1734.28 (15) Å3,
T = 294 (2) K, space group Z = 4, m(MoKa) = 0.094 mm–1,
15230 reflections measured, 6724 unique (Rint = 0.0981)
(28) Callan, J. F.; de Silva, A. P.; Magri, D. C. Tetrahedron 2005,
61, 8551.
(29) Valeur, B.; Leray, I. Coord. Chem. Rev. 2000, 205, 3.
(30) Job, P. Ann. Chim. 1928, 9, 113.
Synlett 2009, No. 2, 315–319 © Thieme Stuttgart · New York

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