Zinc triflate catalysed synthesis of β-enamino ketones(esters) under solvent-free conditions

Article abstract of DOI:10.3184/174751913X13787959859506

An efficient and mild procedure is described for the synthesis of a series of β-enamino ketones(esters) from 1,3-dicarbonyl compounds and aliphatic and aromatic amines using zinc triflate as the catalyst.

Full text of DOI:10.3184/174751913X13787959859506

626 RESEARCH PAPER
OCTOBER, 626–629
JOURNAL OF CHEMICAL RESEARCH 2013
Zinc triflate catalysed synthesis of β-enamino ketones(esters) under
solvent-free conditions
Chengliang Feng, Shuguang Zhang, Jin Cai, Junqing Chen, Huayou Hu and Min Ji*
Institute of Pharmaceutical Engineering, School of Chemistry and Chemical Engineering, Southeast University, Nanjing, Jiangsu,210096,
P. R. China
An efficient and mild procedure is described for the synthesis of a series of β-enamino ketones(esters) from 1,3-dicarbonyl
compounds and aliphatic and aromatic amines using zinc triflate as the catalyst.
Keywords: zinc triflate, β-enamino ketones(esters), solvent-free conditions
Enamination of 1,3-dicarbonyl compounds (β-diketones
and β-ketoesters) to form β-enamino ketones (esters) is an
important and widely used organic reaction.1 β-Enamino
ketones and esters have served as versatile intermediates for
the synthesis of some biologically important natural products
such as alkaloids, spiropyrrolines2 and some compounds with
anti-bacterial,3 anti-convulsant,4 anti-inflammatory,5 and anti-
tumour activities.6 As a result, much effect has been devoted
to the development of effective methods for the synthesis of
β-enamino ketones and esters.
our knowledge, the synthesis of β-enamino ketones (esters)
by the enamination of 1,3-dicarbonyls using catalysis with
Zn(OTf)2 has not yet been reported. Herein, we report an easy
and mild procedure for the Zn(OTf)2-catalysed enamination of
1,3-dicarbonyls with amines at room temperature.
Results and discussion
In our initial study, the reaction of acetylacetone (β-diketone)
with aniline was chosen as a model substrate for optimisation
of the reaction conditions (Scheme 1) using the following
procedures: a mixture of acetylacetone (1.1 mmol), aniline
(1 mmol) and Zn(OTF)2 (0.5 mol%–10 mol%) was stirred
at room temperature for certain times. As shown in Table 1,
the mixture was stirred in the presence of 0.5 mol%, 1 mol%
and 2 mol% of Zn(OTf)2 and the product a was isolated in 78,
85 and 96% yield (Table 1, entries 1, 2, 3), respectively. The
reaction was then examined with 5 mol% and 10 mol% of
catalyst and the product was obtained in 94 and 96% yield,
respectively. Consequently 2 mol% of catalyst was sufficient
to obtain the best yield. The reaction conditions were used to
check the reaction of ethyl acetoacetate (β-ketoesters) with
benzylamine and aniline (Scheme 2). The reactions proceeded
well and excellent yields were obtained (Table 1, entries 6, 7).
Then a variety of 1,3-dicarbonyl compounds and aliphatic
and aromatic amines (primary and secondary amine) were
examined under our reaction conditions. The results are
summarised in Table 2, from which it can be seen that the
majority of 1,3-dicarbonyl compounds could have reacted
with primary amines (Table 2, entries 1–21) and afforded good
yields. The reaction with aliphatic amines proceeded easily in a
shorter time than those with aromatic amines. The reaction with
Synthesis of β-enamino ketones (esters); general procedure
Enamination of 1,3-dicarbonyls with amines using various of
catalystsincludingBF3·OEt2,7 Mg(ClO4)2,8 Bi(OTf )3,9 Sc(OTf )3,10
Yb(OTf)3,11 NaHSO4,12 InBr3,13 CoCl2·6H2O,14 CeCl3·7H2O,15
ZrOCl2·8H2O,16 Zn(ClO4)2·6H2O,17 Zn(OAc)2·2H2O,18 ZrCl4,19
NaAuCl4,20 SnCl42,421 I2,22 silica-supported H2SO4,23 silica-
supported SbCl3,
phosphotungstic acid,25 SO2-4ZrO2,26
copper nanoparticles,27 Ag nanoparticles,28 Cu(NO3)2 3(H2O),29
Silica-Supported LiHSO4,30 Fe(HSO4)3,31 trimethylsilyl
trifluoromethanesulfonate (TMSTf),32 montmorillonite K10,33
nano ZnO,34 VO(acac)2),35 KH2PO4 and Ag nanoparticles in
36
hollow magnetic mesoporous spheres.37
Many of the reported catalysts for this transformation have
some drawbacks such as long reaction times, high catalyst
loading, low yields, use of solvents, and deactivation of catalyst
on repeated use. Thus, there is a need to develop a new catalyst
for synthesis of β-enamino ketones and esters.
Zinc triflate [Zn(OTf)2)] is an important Lewis acid catalyst
that has been used to catalyse many organic reactions, e.g.
cascade reaction of anilines with aromatic aldehydes and
carbonyl compounds,38 Mannich-type reaction of the electron-
deficient aromatic amines with the electron-deficient aromatic
aldehydes and diethyl malonic ester39 and intermolecular
hydroamination of vinylarenes and anilines.40 To the best of
Table 1 Optimisation of reaction condition using Zn(OTf)2 catalysta
Catalyst[Zn(OTf)2]
Times
/min
Entry
Product
Yields/%b
/mol%
1
2
3
4
5
6
7
0.5
1
2
5
10
2
30
20
10
10
10
5
a
a
a
3
a
b
c
78
85
96
94
95
96
95
NH2
Zn(OTf)2
O
O
+
NH
rt. solvent-free
O
2
12
(a)
aAcetylacetone (1.1 mmol), aniline (1 mmol), ethyl acetoacetate (1.1 mmol).
Scheme 1
bIsolated yields.
NH2
O
O
Zn(OTf)2
NH2
NH
(c)
N
H
+
OEt
or
OEt
or
rt. solvent-free
O
OEt
O
(b)
Scheme 2
* Correspondent. E-mail: jimin@seu.edu.cn
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JOURNAL OF CHEMICAL RESEARCH 2013 627
Table 2 Enamination of 1,3-dicarbonyls catalysed by Zn(OTf)2
Experimental
All reagents were purchased from commercial sources and used
without further purification. Melting points were determined
on a RY-1 hot stage microscope and are uncorrected. IR spectra
Zn(OTf)2
O
O
R
+
R1 NH
R1 NH2
O
R
Solvent-free
Entry
R
R1
Times/min
Yields/%
were determined as KBr pellets on
a Thermo–Nicolet 6700
spectrophotometer. 1H NMR and 13C NMR spectra were recorded
on a Bruker Avance DPX-300 MHz/500 MHz instrument in CDCl3,
chemical shifts (δ) were given in part per million (ppm) relative to
TMS as an internal standard. The HRMS spectra were obtained on a
Thermo Finnigan spectrometer, model MAT 95XP. All reactions were
monitored by TLC on silica gel GF-254 glass plates (E. Merck) and
viewed under UV light at 254 nm.
1
2
3
4
5
6
7
8
9
10
11
12
13
14
15
16
17
18
19
20
21
22
23
CH3
Ph
Ph
Ph
Ph
10
8
12
12
5
15
12
4
2
3
2
6
2
15
3
15
20
4
6
10
5
60
60
96
92
94
94
97
94
96
90
95
94
96
97
95
94
97
92
53
96
97
95
96
OMe
OEt
OEt
OEt
CH3
OMe
OMe
OEt
OEt
OEt
CH3
CH3
CH3
OMe
OEt
Ph
Ph-
1-Np-(CH3)CH-
2-Cl-6-MeC6H4-
2-Cl-6-MeC6H4-
C6H5CH2-
4-MeC6H4-
C6H11-
Synthesis of β-enamino ketones and esters; general procedure
Zn(OTf)2 (0.05 mmol) was added into a mixture of primary amines
(1 mmol) and the 1,3-dicarbonyl compounds (1.1 mmol) in a 50 mL
round bottomed flask and it was kept stirring at room temperature.
The completion of the reaction was monitored by TLC. The product
was dissolved in ethyl acetate (50 mL) and filtered. The pure product
was obtained by directly passing through a silica gel (200–300 mesh)
column using petroleum ether/ethyl acetate and identified by IR, H
NMR, 13C NMR, MS, and HRMS.
Most of the products are known, except for entries 5, 6, 7 in Table 2.
All new compounds were characterised by IR, 1H NMR, 13C NMR and
HRMS. All known products were characterised by IR, 1H NMR and 13
NMR.
(CH3)2CH-
4-MeOC6H4-
4-MeC6H4-
4-ClC6H4-
4-MeOC6H4-
4-ClC6H4-
4-NO2C6H4-
4-MeOC6H4-
4-MeC6H4-
4-ClC6H4-
1
C
Ph
Ph
OEt
OEt
OMe
(Z)-4-(Phenylamino)pent-3-en-2-one (1): Yellow solid, m.p. 46–48 °C,
(lit.13 45–47 °C). IR (KBr, cm–1): 3443, 3348, 3234, 2988, 2916, 1632,
C6H11CH2-
Piperidine
Piperidine
1
1558, 1510, 1287, 1088. H NMR (CDCl3, 300 MHz) δ 12.42(s, 1H),
7.03–7.46(m, 5H), 5.16(s, 1H), 2.12 (s, 3H), 2.01(s, 3H). 13C NMR (CDCl3,
75 MHz) δ 19.82, 29.20, 97.68, 124.63, 124.63, 125.54, 129.08, 129.08,
138.68, 160.31, 196.07.
aromatic amines with electron-donating (p-OMe) substituents
proceeded more easily and gave better yields than those with
electron-withdrawing substituents (p-NO2). For example, the
yield of compound 15 is higher than compound 17.
However, it was noted that 1,3-dicarbonyl compounds could
not react with any secondary amines (22 and 23). For example,
the mixture of ethyl acetolacetate or methyl acetolacetate/
piperidine (22 and 23) and Zn(OTf)2 was stirred at 1 h, no
products were obtained. The reaction still did not occur on
increasing the catalyst (10, 20, 30 50 mol%) and raising the
temperature (40, 60, 80 °C).
(Z)-1-Phenyl-3-(phenylamino)but-2-en-1-one (2): Yellow solid, m.p.
108–109 °C, (lit.30 109–111 °C). IR (KBr, cm–1): 3435, 1621, 1589, 1547,
1
1325, 1267. H NMR (CDCl3, 300 MHz) δ 13.14(s, 1H), 7.21–7.96(m,
10H), 5.93(s, 1H), 2.18(s, 3H). 13C NMR (CDCl3, 75 MHz) δ 21.03, 94.71,
125.24, 125.24, 126.17, 127.53, 127.53, 128.66, 129.12, 129.12, 129.64,
129.64, 131.44, 139.02, 162.67, 189.12.
(Z)-Methyl 3-(phenylamino)but-2-enoate (3): Yellow oil;13 IR (KBr,
cm–1): 3244, 2992, 1651, 1589, 1467, 1423, 1321, 1267, 1165. H NMR
(CDCl3, 300 MHz) δ 10.37(s, 1H), 7.34(m, 2H), 7.12(m, 3H), 4.69(s, 1H),
3.68(s, 3H), 1.98(s, 3H). 13C NMR (CDCl3, 75 MHz) δ 20.20, 50.19, 85.63,
124.44, 124.44, 125.03, 128.98, 128.98, 139.31, 159.04, 170.02.
1
Additionally, we found that all 1,3-dicarbonyl compounds
including β-diketones and β-ketoesters reacted with primary
amines to give the Z-β-enaminones and Z-β-enaminoesters
with 100% stereoselectivity, respectively. The Z-form
configuration of the products was confirmed by 1HNMR
analysis and NOE experiments. The proton of the –NH-group
appearing at a lower field (δ<8.0) indicated the formation of an
intramolecular hydrogen bond, which stabilised the products.
Therefore this reaction was stereospecific.
This method was successfully applied to enamination of
1,3-dicarbonyls. In all cases, we found that amine attack took
place only at the methyl ketone carbonyl for 1,3-dicarbonyls.
The use of Zn(OTf)2 as a heterogeneous catalyst showed short
reaction times, high yields and mild reaction conditions. The
other advantage of the use of this catalyst for this reaction,
is the recyclability. Since Zn(OTf)2 is not soluble in organic
solvents, it could be separated by simple filtration, washed with
ethyl acetate and dried.
(Z)-Ethyl 3-(phenylamino)but-2-enoate (4): Yellow oil;13 IR (KBr, cm–
1): 3256, 2979, 1651, 1596, 1503, 1440, 1359, 1272, 1164. 1H NMR (CDCl3,
300 MHz) δ 10.38(s, 1H), 7.30–7.34(m, 2H), 7.07–7.17(m, 3H), 4.70(s,
1H), 4.11–4.18(m, 2H), 1.99(s, 3H), 1.28(m, 3H). 13C NMR (CDCl3,
75 MHz) δ 14.48, 20.16, 58.63, 85.94, 124.26, 124.78, 128.93, 139.23,
158.81, 170.25.
(Z)-Ethyl 3-(1-(naphthalen-1-yl)ethylamino)but-2-enoate (5): Yellow
oil. IR (KBr, cm–1):3279, 3061, 1650, 1607, 1493, 1443, 1266, 1170.
1H NMR (CDCl3, 300 MHz) δ 8.47(b, 1H), 7.02–7.71(m, 7H), 4.64(s,
1H), 4.06–4.17(m, 3H), 1.92(s, 3H), 1.48(s, 3H), 1.15(m, 3H). 13C NMR
(CDCl3, 75 MHz) δ 13.68, 21.46, 22.52, 51.23, 62.05, 82.79, 122.36,
123.69, 124.76, 124.85, 126.88, 127.13, 128.42, 134.53, 134.69, 166.46,
168.59. HRMS calcd for C18H22NO2 [M+H]+: 284.1651, found: 284.1653.
(Z)-Ethyl 3-(2-chloro-6-methylphenylamino)but-2-enoate (6): Yellow
oil. IR (KBr, cm–1): 3256, 3058, 1652, 1605, 1483, 1436, 1232, 1127. 1H
NMR (CDCl3, 500 MHz) δ 9.79(b, 1H), 6.58(m, 1H), 6.92(d, J= 7.35, 1H ),
7.27(m, 1H), 4.75(b, 1H), 4.17(m, 2H), 1.29(m, 3H), 1.65(s, 3H), 2.16(s,
3H). 13C NMR (CDCl3, 125 MHz) δ 14.45, 17.74, 18.43, 58.59, 84.86,
118.11, 123.36, 126.90, 127.92, 128.55, 141.08, 160.59, 170.45. HRMS
calcd for C13H16ClNO2 [M+Na]+: 276.0767, found: 276.0769.
In conclusion, we report an efficient catalyst for chemo-and
regioselective synthesis β-enamino ketones and esters using
primary amines and 1,3-dicarbonyl compounds. The reaction
was catalysed by Zn(OTf)2 at room temperature under solvent-
(Z)-4-(2-Chloro-6-methylphenylamino)pent-3-en-2-one (7): Yellow
oil. IR (KBr, cm–1): 3450, 3061, 2922, 1617, 1566, 1496, 1463, 1274, 1172.
1H NMR (CDCl3, 500 MHz) δ 11.97(b, 1H), 7.12–7.29(m, 3H), 5.25(s,
1H), 2.25(s, 3H), 2.11(s, 3H), 1.68(s, 3H). 13C NMR (CDCl3, 125 MHz) δ
14.76, 21.55, 28.79, 96.87, 118.43, 124.72, 126.85, 128.46, 131.68, 146.44,
1
free conditions. All the products were identified by H NMR,
13C NMR, MS, HRMS, and IR.
JCR1302023_FINAL.indd 627
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628 JOURNAL OF CHEMICAL RESEARCH 2013
160.58, 198.02. HRMS calcd for C12H14ClNONa [M+Na]+: 246.0662,
found: 246.0665.
(Z)-3-(p-Toluidino)-1-phenylbut-2-en-1-one (19) Yellow solid, m.p.
83–85 °C, (lit.31 90–92). IR (KBr, cm–1): 3446, 3047, 2912, 1993, 1899,
1
(Z)-Methyl 3-(benzylamino)but-2-enoate (8): Yellow oil, (lit.35 Yellow
oil). IR (KBr, cm–1): 3446, 3133, 1646, 1605, 1451, 1400, 1316, 1284,
1238, 1172. 1H NMR (CDCl3, 300 MHz) δ 8.94(s, 1H), 7.26–7.32(m, 2H),
7.23(m, 3H), 4.53(s, 1H), 4.42(m, 2H), 3.63(s, 3H), 1.93(s, 3H). 13C NMR
(CDCl3, 75 MHz) δ 19.43, 46.80, 50.01, 82.70, 126.51, 126.51, 127.18,
128.60, 128.60, 138.46, 161.69, 170.59.
1622, 1572, 1446, 1377. H NMR (CDCl3, 300 MHz) δ 13.03(s, 1H),
7.07–7.94(m, 9H), 5.88(s, 1H), 2.37(s, 3H), 2.12(s, 3H). 13C NMR (CDCl3,
75 MHz) δ 20.81, 21.43, 94.34, 125.31, 125.31, 127.52, 128.68, 128.68,
130.23, 131.27, 131.27, 136.17, 136.17, 140.48, 163.11, 190.02.
(Z)-3-(4-Chlorophenylamino)-1-phenylbut-2-en-1-one (20): Yellow
solid, m.p. 126–127 °C, (lit.31 126–128). IR (KBr, cm–1): 3411, 3080,
3051, 1623, 1588, 1549, 1446, 1326, 1247. 1H NMR (CDCl3, 300 MHz) δ
13.10(s, 1H), 7.10–7.95(m, 9H), 5.91(s, 1H), 2.16(s, 3H). 13C NMR (CDCl3,
75 MHz) δ 20.51, 94.69, 125.93, 125.93, 127.06, 128.43, 128.43, 129.44,
131.11, 131.11, 131.28, 137.33, 137.33, 139.90, 161.84, 189.16.
(Z)-Methyl 3-(p-toluidino)but-2-enoate (9): Yellow solid, m.p. 56–
57 °C, (lit.31. 57–58 °C). IR (KBr, cm–1): 3246, 2992, 1652, 1602, 1489,
1
1364, 1277, 1167. H NMR (CDCl3, 300 MHz) δ 10.24(s, 1H), 6.96–
7.13(m, 4H), 4.66(s, 1H), 3.68(s, 3H), 2.34(s, 3H), 1.96(s, 3H). 13C NMR
(CDCl3, 75 MHz) δ 20.09, 20.81, 50.14, 85.02, 124.69, 124.69, 129.58,
130.83, 130.83, 136.57, 159.44, 170.71.
(Z)-Ethyl 3-(benzylamino)but-2-enoate (21): Yellow oil,35 IR (KBr,
1
cm–1): 3315, 2979, 1652, 1606, 1543, 1477, 1423, 1337, 1276. H NMR
(Z)-Ethyl 3-(cyclohexylamino)but-2-enoate (10): Yellow oil,13 IR
(KBr, cm–1): 3230, 2930, 1654, 1608, 1596, 1448, 1276, 1172. 1H NMR
(300 MHz, CDCl3) δ 8.64 (s, 3H), 4.38(s, 1H), 4.10(m, 2H), 3.29–3.34(m,
1H), 1.92(s, 3H), 1.30–1.91(m, 10H), 1.24(m, 3H). 13C NMR (75 MHz,
CDCl3) δ 14.72, 19.14, 24.63, 25.41, 34.52, 51.40, 58.09, 60.38, 81.78,
160.80, 170.52.
(500 MHz, CDCl3) δ 10.29(s, 1H), 7.11(m, 2H), 6.96(m, 3H), 4.66(s, 1H),
4.15(m, 4H), 1.91(s, 3H), 1.25(m, 3H). 13C NMR (125 MHz, CDCl3) δ
14.71, 19.52, 24.13, 46.84, 58.49, 83.18, 126.78, 127.44, 128.79, 138.83,
162.81, 170.60.
We are grateful to China Postdoctoral Science Foundation (No.
2012M511645) and NSFC (No. 21202058) and the National
Basic Research of China (No. 2011CB933503) for financial
support.
(Z)-Ethyl 3-(isopropylamino)but-2-enoate (11): Yellow oil,13 IR (KBr,
1
cm–1): 3274, 2972, 2933, 1656, 1601, 1498, 1444, 1276, 1157. H NMR
(300 MHz, CDCl3) δ 8.48(s, 1H), 4.40 (s, 1H), 4.10(m, 2H), 3.62–3.73 (m,
1H), 1.92 (s, 3H), 1.21(m, 9H). 13C NMR (75 MHz, CDCl3) δ 14.49, 19.12,
24.02, 44.28, 58.01, 81.59, 160.73, 170.54.
Received 29 July 2013; accepted 6 August 2013
Paper 1302023 doi:10.3184/174751913X13787959859506
Published online: 7 October 2013
(Z)-Ethyl 3-(4-methoxyphenylamino)but-2-enoate (12): Yellow solid,
m.p. 44–46, (lit.30 45–47). IR (KBr, cm–1): 3260, 2948, 2835, 1650, 1616,
1514, 1478, 1424, 1247, 1211, 1164. 1H NMR (300 MHz, CDCl3) δ 10.16(s,
1H), 6.86–7.05(m, 4H), 4.65(s, 1H), 4.11–4.14(m, 2H), 3.80(s, 3H), 1.87(s,
3H), 1.28(m, 3H). 13C NMR (75 MHz, CDCl3) δ 14.16, 19.72, 55.01, 58.17,
84.44, 113.82, 113.82, 126.39, 131.83, 131.83, 157.14, 159.58, 170.08.
(Z)-4-(p-Toluidino)pent-3-en-2-one (13): Yellow solid, m.p. 64–65 °C,
(lit.31 66–68). IR (KBr, cm–1): 3440, 3031, 2990, 2933, 2860, 1610, 1576,
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18 R.K. Vohra, J.L. Renaud and C. Bruneau, Collect. Czech. Chem. Commun.,
2005, 70, 1943.
1
2916, 1620, 1567, 1487, 1243. H NMR (300 MHz, CDCl3) δ 12.34(s,
1H), 6.90–7.06(m, 4H), 5.15(s, 1H), 3.74(s, 3H), 2.03(s, 3H), 1.90(s, 3H).
13C NMR (75 MHz, CDCl3) δ 19.63, 29.04, 55.44, 96.85, 114.28, 114.28,
126.65, 126.65, 131.46, 157.73, 161.32, 195.76.
(Z)-Methyl 3-(4-chlorophenylamino)but-2-enoate (16): Yellow solid,
m.p. 62–63 °C, (lit.31 61–62). IR (KBr, cm–1): 3276, 2944, 1652, 1598,
1
1487, 1352, 1277, 1167. H NMR (CDCl3, 300 MHz) δ 10.34(s, 1H),
7.01–7.28(m, 4H), 4.73(s, 1H), 3.68(s, 3H), 1.99(s, 3H). 13C NMR (CDCl3,
75 MHz) δ 20.09, 50.31, 86.42, 125.53, 125.53, 129.07, 137.94, 137.94,
158.52, 162.34, 170.62.
(Z)-4-(4-Nitrophenylamino)pent-3-en-2-one (17): Yellow solid, m.p.
110–112 °C, (lit.13 111–112). IR (KBr, cm–1): 3306, 3095, 2992, 1650, 1592,
1509, 1334, 1287, 1179. 1H NMR (300 MHz, CDCl3) δ 10.92(s, 1H), 8.20(d,
J=9.0 Hz, 2H), 7.11(d, J=9.0 Hz, 2H), 4.86(s, 1H), 4.21(m, 2H), 2.21(s,
3H), 1.30(m, 3H). 13C NMR (75 MHz, CDCl3) δ 14.45, 21.26, 61.57, 84.46,
119.14, 119.14, 122.65, 122.65, 139.05, 152.25, 162.04, 167.23.
19 J. Lin and L.F. Zhang, Monatsh. Chem., 2007, 138, 77.
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(Z)-3-(4-Methoxyphenylamino)-1-phenylbut-2-en-1-one (18): Yellow
solid, m.p. 103–104 °C, (lit.31 103–105). IR (KBr, cm–1): 3051, 3010,
2949, 2830, 1623, 1607, 1587, 1508, 1330. H NMR (CDCl3, 300 MHz)
δ 12.96(s, 1H), 6.90–7.94(m, 9H), 5.90(s, 1H), 3.85(s, 3H), 2.10(s, 3H).
13C NMR (CDCl3, 75 MHz) δ 20.31, 55.54, 93.61, 126.59, 126.59, 127.14,
128.32, 128.32, 130.84, 131.49, 131.49, 140.18, 140.18, 157.90, 163.23,
188.41.
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29 G.C. Li, Monatsh. Chem., 2008, 139, 789.
1
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