Synthesis and characterization of mesoporous zirconium phosphonates: A novel supported cinchona alkaloid catalysts in asymmetric catalysis

Article abstract of DOI:10.1016/j.catcom.2009.10.031

In this paper, cinchonidine (CD) with the different arm lengths (n = 2-6) was covalently immobolized onto the backbone of zirconium phosphonate to afford a series of mesoporous zirconium phosphonates (4a-e) for the first time. It was found that zirconium phosphonates (4a-e) were conglomerated with the globular aggregates with the diameters of about 100-150 nm and possessed the surface areas of 20.8-36.2 m² g^-1, pore volumes of 0.219-0.498 cc g^-1 and average pore sizes of 11.7-41.5 ?. The TEM images of 4a-e in organic solvent showed that these materials were easily swollen into the filiform structure with the length about several micrometers and thickness about 30-50 nm. In the field of the asymmetric catalysis, the preliminarily enantioselective addition of diethylzinc to various aldehydes catalyzed by 4a-e (35-62%e.e. in the 68-92% yield) was carried out and can be reused ten times without loss in the yield and enantiomeric excess.

Full text of DOI:10.1016/j.catcom.2009.10.031

Catalysis Communications 11 (2010) 401–407
Contents lists available at ScienceDirect
Catalysis Communications
journal homepage: www.elsevier.com/locate/catcom
Synthesis and characterization of mesoporous zirconium phosphonates:
A novel supported cinchona alkaloid catalysts in asymmetric catalysis
*
Xuebing Ma , Yonghua Wang, Wei Wang, Jun Cao
College of Chemistry and Chemical Engineering, Southwest University, Chongqing 400715, PR China
a r t i c l e i n f o
a b s t r a c t
Article history:
In this paper, cinchonidine (CD) with the different arm lengths (n = 2–6) was covalently immobolized
onto the backbone of zirconium phosphonate to afford a series of mesoporous zirconium phosphonates
Received 30 September 2009
Accepted 28 October 2009
Available online 16 November 2009
(4a–e) for the first time. It was found that zirconium phosphonates (4a–e) were conglomerated with the
globular aggregates with the diameters of about 100–150 nm and possessed the surface areas of 20.8–
2
ꢀ1
ꢀ1
3
6.2 m g , pore volumes of 0.219–0.498 cc g and average pore sizes of 11.7–41.5 Å. The TEM images
Keywords:
Zirconium phosphonate
Cinchonidine
Immobolized
Enantioselective addition
Diethylzinc
of 4a–e in organic solvent showed that these materials were easily swollen into the filiform structure
with the length about several micrometers and thickness about 30–50 nm. In the field of the asymmetric
catalysis, the preliminarily enantioselective addition of diethylzinc to various aldehydes catalyzed by 4a–
e (35–62%e.e. in the 68–92% yield) was carried out and can be reused ten times without loss in the yield
and enantiomeric excess.
Ó 2009 Elsevier B.V. All rights reserved.
1
. Introduction
Only within the past thirty years cinchona alkaloids emerged as
catalysis of zirconium phosphonates 4a–e was preliminarily car-
ried out. The experimental results indicated that zirconium phos-
phonates (4a–e) possessed the mesoporous and filiform structure
with length and thickness up to several micrometers and 30–
50 nm respectively, which was beneficial for the reactant species
to be accessible to the catalytic sites of cinchonidine on the surface
or in the internal pores.
versatile chiral basic catalysts, ligands, chromatographic selectors
and NMR discriminating agents. Quite obviously, they conformed
to generally accepted requirements: they are commercially avail-
able and relatively inexpensive, stable, and their structure can be
readily modified for diverse catalytic applications. Especially, the
tremendous interest in supported cinchona alkaloids is as a result
of rapid development in two main areas: chiral stationary phases
in chromatographic separation techniques and chiral polymer-sup-
ported catalysts [1–4]. In the field of asymmetric catalysis, Cin-
chona alkaloids were generally immobilized onto silica gel [5–9]
and organic polymer [10–16].
Zirconium phosphate (ZrP) and its derivatives with the layered
and porous structure are the important class of compounds in the
field of catalysis [17–21], adsorption, ion exchange and/or func-
tional materials [22–26]. Due to the high structural flexibility of
these compounds, it is relatively easy to prepare modified inor-
ganic–organic functional derivatives by means of covalently bond-
ing, intercalation and ion-exchange under mild conditions [27–29].
In this paper, cinchonidine (CD) was covalently anchored onto the
layered and porous backbone of zirconium phosphate through the
quinuclidine olefinic side chain to obtain the functional com-
pounds 4a–e for the first time (Scheme 1). Furthermore, the enan-
tioselective addition of diethylzinc to various aldehydes under the
2
. Experimental
2.1. Materials
Diethyl
2 n 2
x-bromoalkylphosphonates [Br(CH ) P(O)(OEt) ] (n =
2
–6) were synthesized according to the known procedures and
1
13
well confirmed by H, C NMR and FT-IR [30]. All the other mate-
rials used are of analytical grade obtained from commercial
sources without further purification.
2.2. Preparation of the phosphonates 2a–e
In the dried three-necked flask (250 mL) was charged anhy-
drous K
at room temperature, added dithioglycol (10.90 g, 116 mmol) and
00 mL of DMF by syringe and cooled to 0–5 °C. 25 mL of DMF
solution containing -bromoalkylphosphonates 1a–e (58 mmol)
2 3
CO (16.0 g, 116 mmol), flushed three times with nitrogen
1
x
was added dropwise over 1 h and stirred for 30 min. Then the
ice-bath was removed and stirred at room temperature for another
4
8 h with the tracking of TLC. The reaction mixture was added dis-
*
Corresponding author. Tel.: +86 23 68252360; fax: +86 23 68254000.
E-mail address: zcj123@swu.edu.cn (X. Ma).
tilled water till the solid K CO was dissolved and then separated.
2
3
1
566-7367/$ - see front matter Ó 2009 Elsevier B.V. All rights reserved.
doi:10.1016/j.catcom.2009.10.031

4
02
X. Ma et al. / Catalysis Communications 11 (2010) 401–407
O
O
HSCH CH SH
CD, AIBN
2
2
(
C H O) P
Br
(C H O) P
S
2
5
2
2
5
2
(
)
(
)
SH
n
K CO , DMF, 0-20 C
n
2
3
CHCl , 48 h, 84 C
3
1
a-e (n=2-6)
2
a-e
S
(
)
3
n P(O)(OC H )
S
S
2
5 2
( )
]
[
S
n
PO Zr(OH)
4-2x
2
4
x
3
H 7
5
H
H
N
H
HO
8
1
N
6
HCl/AcOH ZrOCl2 8H O
.
9
HO
2
5'
4'
3'
6'
84 C, 24 h
.
7'
1'
n H O
2
2
'
N
a-e
8'
N
3
4
a-e (n=2-6)
Scheme 1. The synthetic route to zirconium phosphonates 4a–e.
The water phase was extracted by mixed ethyl acetate/benzene
30 mL ꢁ 3, v/v = 3/1). Then the organic phases were combined,
washed with distilled water till DMF was removed completely,
dried over anhydrous Na SO and concentrated under reduced
pressure to give the pale yellow liquid. The crude products were
purified by silica gel column chromatography using petroleum
ether (60–90 °C)/ethyl acetate (v/v = 12/1 ? 10/1 ? 6/1) and con-
centrated under reduced pressure to afford 2a–e in 60–70% yields.
was added dropwise at 0 °C under nitrogen atmosphere. After stir-
ring for 1 h, benzaldehyde (50 uL, 0.5 mmol) was added dropwise,
stirred for 72 h at room temperature, quenched by a saturated
(
2
4
aqueous NH
(10 mL ꢁ 3). The combined organic layer was washed with brine
SO and evaporated under re-
4
Cl solution, filtered and extracted with ethyl acetate
(5 mL ꢁ 2), dried over anhydrous Na
2
4
duced pressure to give an oily residue. The residue was purified by
silica gel column using ethyl acetate/petroleum ether (60–90 °C)
(
v/v = 5:1) to give 1-phenyl-1-propanol.
2.3. Preparation of the phosphonates 3a–e
3
. Results and discussion
In the dried flask (100 mL) was charged cinchonidine (3.3 g,
1
1.1 mmol), 2a–e (44.5 mmol) and AIBN (0.46 g, 2.8 mmol) consec-
utively, flushed three times with nitrogen at room temperature,
added 100 mL of CHCl by syringe and then refluxed for 48 h with
the tracking of TLC. The reaction mixture was washed with water
SO
3
4
.1. Characterization of the supported cinchona alkaloid catalysts
a–e
3
3.1.1. IR spectra
(
15 mL ꢁ 3), the organic phase was dried over anhydrous Na
2
4
The infrared spectra of 4a–e were recorded in the range 4000–
and concentrated under reduced pressure to give the pale yellow
liquid. The crude product was purified by silica gel column chro-
ꢀ1
4
00 cm in order to ascertain whether the attachment of cincho-
nidine molecule onto the backbone of zirconium phosphate hap-
pened. From the IR spectra shown in Fig. 1, it was found that the
strong and wide absorption band extending from 3700 to
matography using CHCl
to remove the side products with poor polarity and then CHCl
methanol (v/v = 80/1 ? 60/1 ? 40/1) to afford 3a–e in 79–98%
yields.
3
/petroleum ether (60–90 °C) (v/v = 40/1)
3
/
ꢀ1
ꢀ1
2
500 cm and centered at 3409 cm was assigned to the AOAH
stretching vibration of hydroxyl group in organic moieties and
crystal water in the internal or external surface. Due to the increase
2.4. Preparation of zirconium phosphonates 4a–e
in the arm chain length of A(CH
2
)
n
- organic groups (n from 2 to 6),
A mixture of 3a (1.0 g, 2 mmol), 45 mL of acetic acid and 15 mL
of hydrochloric acid (36%) was stirred at 84 °C for 24 h and concen-
trated under reduced pressure to 10 mL volume. Zirconium oxy-
chloride (1.7 g, 6 mmol) in 15 mL of deionized water was added
dropwise and stirred at room temperature for another 16 h. The
white solid was filtered, dispersed in 50 mL of water, adjusted to
pH = 6–7 by using sodium carbonate (0.1 mol/L) and filtered again.
The mud cake was washed with deionized water till chloride ion
was not detected by using ion chromatography, and then dried at
6
0 °C for 24 h under reduced pressure to afford zirconium phos-
phonate 4a in 58% yield. The other zirconium phosphonates 4b–e
were prepared according to the same procedure as 4a in 50–60%
yields.
2
2.5. Enantioselective addition of Et Zn to benzaldehyde catalyzed by
4
a–e
In the dried flask (10 mL) was charged zirconium phosphonate
a (31.8 mg, 0.075 mmol), flushed three times with N atmosphere
4
2
and sealed, added anhydrous toluene (5 mL) by syringe and stirred
for 12 h at room temperature. Then Et Zn (1 mL, 1 mol/L in hexane)
Fig. 1. The infrared spectra of 4a–e in the 4000–400 cm 1 range.
ꢀ
2

X. Ma et al. / Catalysis Communications 11 (2010) 401–407
403
Fig. 2. Thermal gravimetric curves of zirconium phosphonates 4b–e.
ꢀ
1
the stronger CAH stretching vibration at 2854 cm was observed.
desorption of the surface-bound or intercalated water in the pores
with weight losses of 6.8, 5.3, 4.8, 3.7 and 3.5 wt.% respectively, fol-
lowed by the sharp second curve with a single DTG peak owing to
the volatilization of appended organic fragments between 250 °C
and 400 °C, while the third stage appeared from 400 °C to
ꢀ1
ꢀ1
The absorption bands at 1636–1462 cm and 803, 763 cm were
attributed to the characteristic absorption and flexural vibration of
aryl group in appended organic moieties respectively. Further-
more, the strong and wide absorption bands at about 1031 cm
for all the samples 4a–e indicated that the reaction between the
phosphonates 3a–e and ZrOCl were performed to form the
ꢀ1
2
ZrAOAP bonds. Therefore, from FT-IR spectra it was concluded that
cinchonidine was successfully anchored onto the framework of zir-
conium phosphonate by covalently bonding through the different
Table 2
The BET surface areas, pore volumes and average pore sizes of 4a–e.
a
Samples
n = 2
n = 3
n = 4
n = 5
n = 6
arm lengths of A(CH
2
)
n
–organic chains (n = 2–6).
BET (m2
ꢀ1)b
S
g
20.8
0.219
41.5
31.4
0.423
31.2
34.1
0.498
28.8
35.9
0.280
25.3
36.2
0.322
11.7
Pore volume (cc g )c
ꢀ
1
0
Average pore size ( AÅ )d
3.1.2. TG analysis
a
b
c
The samples were degassed at 100 °C for 5 h.
Surface based on multipoint BET method.
Pore volume based on the desorption data of BJH method.
Pore size based on the desorption data using BJH method.
On heating the samples 4a–e, the similar thermolysis curves
over a broad temperature range of 40–1200 °C with a three-step
weight loss behavior were observed, which were shown in Fig. 2.
The first slope curve of 4a–e below 200 °C was attributed to the
d
Table 1
The formulae of 4a–e and the data of elemental analysis and TG analysis.
Samples
x
n
Elemental analysis calcd [%] (found [%])
The weight lossa calcd [%] (found [%])
4
4
4
4
4
a
b
c
d
e
0.49
0.52
0.59
0.78
0.82
2.08
1.62
1.47
1.22
1.15
C, 32.05 (31.92); H, 4.34 (4.43), N, 3.26 (3.12)
C, 33.11 (32.98); H, 4.47 (4.56), N, 3.32 (3.16)
C, 37.38 (37.61); H, 4.98 (5.13), N, 3.49 (3.32)
C, 42.49 (42.34); H, 5.50 (5.63), N, 3.80 (3.78)
C, 44.06 (44.0); H, 5.74 (5.90), N, 3.80 (3.69)
48.1 (47.0)
48.6 (48.0)
54.8 (54.0)
62.4 (61.3)
64.1 (61.3)
a
The weight loss of organic moieties monitored by TG analysis.

4
04
X. Ma et al. / Catalysis Communications 11 (2010) 401–407
1
200 °C corresponding to the oxidation of the cokes generated dur-
losses of appended organic moieties in 4a–e were 47.0, 48.0, 54.0,
61.3 and 61.3 wt.% respectively and increased with n from 2 to 6.
ing the second stage. From Fig. 2, it was found that the total weight
Fig. 3. The nitrogen adsorption–desorption isotherms and pore distribution of 4a–e.
Fig. 4. SEM images of zirconium phosphonates 4b–e.

X. Ma et al. / Catalysis Communications 11 (2010) 401–407
405
The pronounced inclination of the less surface-bound or interca-
lated water from n = 2 to 6 below 200 °C was ascribed to the lower
hydrophilicity in the interlayer region owing to the more appended
organic moieties (the increasing x values from 0.49 to 0.82). In gen-
eral, the structures of zirconium phosphonates 4a–e were similar
was confirmed by ion chromatography. In order to detect the zirco-
nium (IV) amount, a mixture of 2 mL of deionized water and 50 mg
of 4a–e in a sealed plastic container was added 4 mL of hydroflu-
oric acid (30%) and stirred for 6 h at room temperature. Due to
2
ꢀ
the formation of the water-soluble complex ZrF , 4a–e were
6
to that of the parent compound
a
-ZrP, and the organic groups point
decomposed, and the contents of zirconium (IV) and phosphonic
acid with cinchonidine in the resulting solution were determined
by ICP-AES and P NMR respectively. Taking into account the mo-
towards the interlayer region. The free area associated to each or-
2
31
ganic group is 24 Å and filled by these organic groups [27]. In the
confined areas, due to the increasing arm chain lengths n from 2 to
lar ratios (x) of the appended organic moieties to zirconium (IV)
3
1
6
, the interlayer spacings were expanded and the bulky cinchoni-
monitored by P NMR and ICP-AES and the number (n) of crystal
water detected by TG curve below 200 °C, the formulae of 4a–e
were given in Table 1, which were in the good agreement with
the data by using elemental analysis and TG analysis.
dine was more remote to zirconium layer. Therefore, there were
the more larger free spaces for zirconium phosphonates with long-
er arm chain lengths n from 2 to 6 to accommodate the more
cinchonidines.
3.1.4. Nitrogen adsorption–desorption isotherms
3.1.3. The chemical composition of 4a–e
2
The textural parameters of 4a–e calculated by N adsorption
Because it was difficult for 4a–e to work up single crystals to
and desorption isotherms at 77 K were presented in Table 2. As ex-
elucidate the architectural structure, only the chemical composi-
tion was determined by using elemental analysis, TG analysis
and chemical analysis. According to the reported literature, Cl an-
ion could be topotactically replaced by the inorganic anions such as
HO and Br [31]. In the preparation process of 4a–e sodium car-
bonate (0.1 mol/L) and deionized water were used to dealt with
the samples enoughly to completely remove the Cl anions, which
pected, it was shown that 4a–e (n = 2–6) were porous solid mate-
2
ꢀ1
rials with the surface areas of 20.8–36.2 m g , pore volumes of
ꢀ
1
0.219–0.498 cc g and average pore sizes of 11.7–41.5 Å. With
the increase in arm chain lengths n from 2 to 6, it was observed
that the surface areas of 4a–e increased and average pore sizes de-
creased respectively, which was related to the bulky size and ap-
pended amount of the organic moieties containing cinchonidine.
ꢀ
ꢀ
Fig. 5. TEM images of zirconium phosphonates 4b–e.

4
06
X. Ma et al. / Catalysis Communications 11 (2010) 401–407
A reasonable and plausible explanation of this change in surface
area and average pore size was that the longer arm chain lengths
n from 2 to 6 made the inorganic zirconium layers more expanded
and amorphous, and then the more surface areas were obtained. In
the other hand, the organic moieties with the arm chain lengths n
from 2 to 6 between the neighbour zirconium layers tended to be
more close packing in the interlayer region ascribled to the in-
crease in x values (x = 0.49–0.82) for 4a–e (Table 1), which caused
the less free mesopores and the decrease in the average pore sizes
from 41.5 to 11.7 Å in solid state.
the conglomerated particles 4a–e in the smaller size than the par-
ent materials were observed. Interestingly, 4a–e in the organic sol-
vents were easily swollen, which could be verified by the TEM
images. After the samples 4a–e were dispersed in cyclohexane
and stirred for 2 h, their morphologies of the filiform structure
with thickness about 30–50 nm and length about several microm-
eters was observed (Fig. 5).
3.2. The enantioselective addition of diethylzinc to benzaldehyde
catalyzed by 4a–e
The nitrogen adsorption–desorption isotherms of all these com-
pounds 4a–e presented type IV isotherms according to the IUPAC
classification with a sharp increase in N adsorption at higher P/
2
Enantioselective addition of diethylzinc to various aldehydes is
one of the most studied catalytically asymmetric reactions, which
was usually used to evaluate the catalytic activities and enantiose-
lectivities of the chiral ligands. In the homogeneous CD-induced
enantioselective addition of diethylzinc to benzaldehyde, 58%e.e.
of (R)-1-phenylpropan-1-ol was achieved. Under the catalysis of
zirconium phosphonates 4a–e, the enantioselective addition of
diethylzinc to benzaldehyde was preliminarily carried out. From
Table 3, it was found that the increase of the isolated yields from
P
0
value (ꢂ0.95) and a distinct hysteresis loop (type H1) possessing
an almost parallel adsorption and desorption (Fig. 3). Based on the
desorption isotherms of 4a–e, BJH analysis gave several mesopores
in different pore sizes with the non-uniform distribution in the
range 20–80 Å. From Fig. 3, it was found that there were three
characteristic cavities centered at about 25–28, 30–45 and 55–
7
0 Å. Especially, it was noted that 4e with the highest x value
(
0.82), which implied the organic moieties in the interlayer region
6
4% to 80% and enantiomeric excesses from 19% to 35%e.e. with
arm chain lengths (n) from 2 to 6 for 4a–e was observed (Table
, entries 2–6). In the toluene/hexane mixed solvents (v/v = 1:1)
at 20 °C for 84 h, the enantiomeric excess of (R)-1-phenylpropan-
-ol with 49%e.e. in 88% yield for 4e was obtained (Table 3, entry
were crowded, possessed the weakened nitrogen desorption (Dv)
in the mesopores, and the micropores below 20 Å strongly
appeared.
3
1
3
.1.5. Surface morphology
Generally, SEM micrographs of metal (IV) phosphonates are
16), which was 9%e.e. lower than the homogeneous CD-induced
enantiomeric excess 58%e.e. in 89% yield (Table 3, entry 1).
The ligand 4e was further used as the catalyst for the addition of
diethylzinc to the other aromatic aldehydes (Table 3, entries 18–
23). Under the optimized conditions at 20 °C in toluene/hexane
(v/v = 1:1) for 84 h, the reactions catalyzed by 10 mol% of catalyst
4e were performed. From the catalytic experimental results, it
seemed that the electronic effects of the aromatic ring substituents
bearing a strong electron-donating group such as methoxyl or elec-
spheroid or featureless and submicron in size with a layered crys-
talline structure in solid state [27–29]. From the SEM images
shown in Fig. 4, 4a–e were conglomerated with the globular aggre-
gates with the diameters of about 100–150 nm. It was highlighted
that the interaction between two neighbour zirconium layers was
greatly weakened owing to the functionalization by the bulky
cinchonidine with the different arm chain lengths (n = 2–6), and
Table 3
a
Enantioselective addition of diethylzinc to various aldehydes catalyzed by 4a–e under different solvents .
H
OH
1
5mol% 4a-e
ArCHO + ZnEt2
Ar
Entry
Catalyst
Ar
Temperature (°C)
Solvent
Time (h)
Yield (%)b
e.e. (%)c
1
2
3
4
5
6
7
8
9
CD
4a
4b
4c
4d
4e
4e
4e
4e
4e
4e
4e
4e
4e
4e
4e
4e (10th)
4e
4e
4e
Ph
Ph
Ph
Ph
Ph
Ph
Ph
Ph
Ph
Ph
Ph
Ph
Ph
Ph
Ph
Ph
20
20
20
20
20
20
20
20
20
0
ꢀ10
ꢀ20
20
20
20
20
20
20
20
20
20
20
20
Toluene
Toluene
Toluene
Toluene
Toluene
Toluene
THF
72
72
72
72
72
72
72
72
72
72
72
72
12
24
48
84
72
84
84
84
84
84
84
89
64
68
70
74
80
50
66
85
70
62
48
30
42
70
88
84
90
86
78
92
74
68
58(R)
19(R)
20(R)
23(R)
31(R)
35(R)
15(R)
26(R)
42(R)
32(R)
29(R)
25(R)
25(R)
28(R)
32(R)
49(R)
35(R)
56(R)
48(R)
37(R)
62(R)
35(R)
43(R)
Hexane
Toluene/Hexaned
d
10
11
12
13
14
15
16
17
18
19
20
21
22
23
Toluene/Hexane
Toluene/Hexaned
Toluene/Hexaned
d
Toluene/Hexane
Toluene/Hexaned
d
Toluene/Hexane
Toluene/Hexaned
Toluene/Hexaned
Ph
p-MeO-C
d
6
H
4
Toluene/Hexane
p-Br-C
o-Br-C
p-Cl-C
p-O N-C
p-Me N-C
6
H
4
Toluene/Hexaned
Toluene/Hexaned
Toluene/Hexaned
Toluene/Hexaned
Toluene/Hexaned
6
H
4
4e
4e
4e
6
H
4
2
6 4
H
2
6
H
4
a
b
c
Reaction was carried out with a molar ratio Et
Isolated yield.
Determined by GC (30 m, chiral cyclodex-B column, agilent) for Ar = Ph and HPLC for the other substrates on chiralcel OD, OJ-H and AD-H with hexane-2-propanol as eluant.
v/v = 1:1.
2
Zn/aldehyde/chiral ligand of catalyst = 2/1/0.15.
d

X. Ma et al. / Catalysis Communications 11 (2010) 401–407
[7] Y.M. Song, J.S. Choi, J.W. Yang, H. Han, Tetrahedron Lett. 45 (2004) 3301.
407
tron-withdrawing groups such as bromo and chloro had a little
[
8] B.M. Choudary, N.S. Chowdari, K. Jyothi, S. Madhi, M.L. Kantam, Adv. Synth.
Catal. 344 (2002) 503.
influence on the yield (86–92%) and great effect on the enantiose-
lectivities varied from 48% to 62%e.e. (Table 3, entries 18, 19, 21).
[
9] I. Motorina, C.M. Crudden, Org. Lett. 3 (2001) 2325.
However, the strong electron-donating group p-Me
tron-withdrawing group p-NO decreased the enantioselectivities
43% and 35%e.e. respectively) and yields of the products (68%
and 74% respectively) (Table 3, entries 22 and 23). Fortunately,
there was no significant loss in the yield and enantiomeric excess
after the catalyst 4e was reused ten times (Table 3, entry 17).
2
N and elec-
[10] P. Besenius, P.A.G. Cormack, J. Liu, S. Otto, J.K.M. Sanders, D.C. Sherrington,
Chem. Eur. J. 14 (2008) 9006.
2
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