Air-stable zirconocene bis(perfluorobutanesulfonate) as a highly efficient catalyst for synthesis of α-aminophosphonates via Kabachnik-Fields reaction under solvent-free condition

Article abstract of DOI:10.1016/j.catcom.2013.10.013

Zirconocene bis(perfluorobutanesulfonate) [Cp₂Zr(OSO ₂C₄F₉)₂·2H₂O] was successfully synthesized by treatment of Cp₂ZrCl₂ with C₄F₉SO₃Ag, and was found to have the nature of air-stability, water tolerance, high thermal stability and strong Lewis acidity. This complex showed high catalytic efficiency for the synthesis of α-aminophosphonates via Kabachnik-Fields reaction of aldehydes/ketones, amines and diethyl phosphite under mild and solvent-free conditions. Furthermore, it can be reused without loss of activity in a test of five cycles. Compared with our previously reported complex of Cp₂Zr(OSO ₂C₈F₁₇)₂·3H ₂O·THF, this complex showed better catalytic activity.

Full text of DOI:10.1016/j.catcom.2013.10.013

Catalysis Communications 43 (2014) 184–187
Contents lists available at ScienceDirect
Catalysis Communications
journal homepage: www.elsevier.com/locate/catcom
Short Communication
Air-stable zirconocene bis(perfluorobutanesulfonate) as a highly
efficient catalyst for synthesis of α-aminophosphonates
via Kabachnik–Fields reaction under solvent-free condition
⁎
⁎
Ningbo Li, Xie Wang, Renhua Qiu, Xinhua Xu , Jinyang Chen, Xiaohong Zhang, Sihai Chen, Shuangfeng Yin
College of Chemistry and Chemical Engineering, Hunan University, Changsha 410082, PR China
a r t i c l e i n f o
a b s t r a c t
Article history:
Received 17 August 2013
Received in revised form 10 October 2013
Accepted 10 October 2013
Available online 17 October 2013
Zirconocene bis(perfluorobutanesulfonate) [Cp2Zr(OSO2C4F9)2·2H2O] was successfully synthesized by treatment
of Cp2ZrCl2 with C4F9SO3Ag, and was found to have the nature of air-stability, water tolerance, high thermal
stability and strong Lewis acidity. This complex showed high catalytic efficiency for the synthesis of α-
aminophosphonates via Kabachnik–Fields reaction of aldehydes/ketones, amines and diethyl phosphite under
mild and solvent-free conditions. Furthermore, it can be reused without loss of activity in a test of five cycles.
Compared with our previously reported complex of Cp2Zr(OSO2C8F17)2·3H2O·THF, this complex showed better
catalytic activity.
Keywords:
Zirconocene
Perfluorobutanesulfonate
Lewis acid
© 2013 Elsevier B.V. All rights reserved.
Kabachnik–Fields reaction
α-Aminophosphonates
1. Introduction
bis(triflate) complexes of titanium and zirconium Cp2M(OTf)2
(Cp = C5H5; M = Ti, Zr) as catalysts to accomplish carbon–carbon
As a kind of natural amino acid analogs, α-aminophosphonates
constitute an important class of compounds owing to their biological
and physical properties as well as their utility as synthetic intermediates
[1–3]. The synthesis of α-aminophosphonates has received increasing
attentions over the last 20 years. The Kabachnik–Fields reaction
represents one of the most direct and efficient methods for the synthesis
of α-aminophosphonates. The process involves a three-component
reaction of an aldehyde, an amine, and a dialkyl or trialkyl phosphite
catalyzed by Lewis acid catalysts such as ZnCl2 [4], SnCl4 [5], FeCl3 [6],
TaCl5–SiO2 [7], Mg(ClO4)2 [8], Cd(ClO4)2·xH2O [9], Yb(OTf)3 [10],
Cu(OTf)2 [11], Sc(OTf)3 [12] and Sn(OTf)2 [13]. Unfortunately, these
catalysts have one or more disadvantages such as air or moisture
sensitivity (metal chlorides), potential explosivity (perchloride salts)
and the use of stoichiometric and/or toxic (heavy metal pollution),
relatively expensive metals (Sc, Yb), and most of these catalysts are
not recyclable. Furthermore, the organic solvents are needed in most
cases, which can't fulfill the requirement of the green chemistry. Thus,
developing an air-stable, highly efficient and recyclable catalyst for the
synthesis of α-aminophosphonates via solvent-free Kabachnik–Fields
reaction is still highly desirable.
bond-forming reactions [16,17]. The anhydrous bis(triflate) complexes
are hydroscopic and readily form air-stable, aquo complexes [18],
analogous to those reported here, and which are also active as Brønsted
and/or Lewis acid catalysts [16]; Brønsted acidity is disadvantageous
for applications involving asymmetric catalysis [17c]. Later, Otera and
co-worker found that the longer perfluorooctanesulfonate groups
could be used as effective counter anions to provide air-stable
and water-tolerant organometallic species in sharp contrast to the
corresponding hygroscopic organometallic triflates [19,20]. With this
in mind, we have synthetized a series of air-stable metallocene
bis(perfluorooctanesulfonate) complexes Cp2M(OPf)2 (M = Ti, Zr;
Pf = OSO2C8F17), which showed moderate to high catalytic efficiency in
C\C and C\O bond formation reactions [21–23]. However, their
relatively low solubility in organic solvents owing to the strongly
lipophobic nature of C8F17 group [24] may decline the catalytic efficiency.
Besides, perfluorooctanesulfonate (PFOS) compounds have been
found to be potentially toxic to animals and human beings [25]
and will result in environmental pollution [26]. Hence, we envisioned
that perfluorobutyl groups, which are less lipophilic, may increase
the solubility as well as the catalytic efficiency. In addition, perfluoro-
butanesulfonate (PFBS) compounds also exhibited lower toxicity to
humans and animals than PFOS [27]. Based on this idea and as part
of our ongoing efforts devoted to metallocene complexes, we pre-
pared the complex of zirconocene bis(perfluorobutanesulfonate)
[Cp2Zr(OSO2C4F9)2·2H2O], studied its physiochemical properties
(e.g. acidity, thermal stability), and examined its catalytic efficiency
In recent years, cationic group metallocene compounds as Lewis acid
catalysts have attracted much attention [14,15]. For example, the
groups of Bosnich and Collins have adopted anhydrous metallocene
⁎
Corresponding authors. Fax: +86 731 88821546.
E-mail addresses: xhx1581@hnu.edu.cn (X. Xu), sf_yin@hnu.edu.cn (S. Yin).
1566-7367/$ – see front matter © 2013 Elsevier B.V. All rights reserved.
http://dx.doi.org/10.1016/j.catcom.2013.10.013

N. Li et al. / Catalysis Communications 43 (2014) 184–187
185
for the synthesis of α-aminophosphonates via Kabachnik–Fields
reaction of aldehydes/ketones, amines, and diethyl phosphite
(Scheme 1).
adopted it as a catalyst for the Kabachnik–Fields reaction of
aldehydes/ketones, amines and diethyl phosphite.
3.2. Screening optimal conditions
2. Experimental
To determine the best experimental conditions, the reaction of
benzaldehyde, aniline, and diethyl phosphite (DEP) was considered
as the model reaction (Table 1). The reaction was tested in various
organic solvents in the presence of Cp2Zr(OSO2C4F9)2·2H2O. It can
be seen that the reaction can occur effectively in MeOH, CH3CN,
THF and Et2O, but slowly in CH2Cl2, n-hexane and toluene (Table 1,
entries 1–4). Obviously, longer reaction time was required and low
yield was obtained when organic solvents were used as reaction
media in some cases. While under solvent-free conditions, only
2.5 h is required to get the 96% yield of desired product (Table 1,
entry 9). Also, we investigated the loading of catalyst and found
that the optimal dosage of Cp2Zr(OSO2C4F9)2·2H2O was 5.0 mol%
(Table 1, entries 9–11). Thus, the best results were obtained in
the presence of Cp2Zr(OSO2C4F9)2·2H2O (5.0 mol%) affording the
desired α-aminophosphonate after 2.5 h at room temperature
under solvent-free condition.
Preparation of zirconocene bis(perfluorobutanesulfonate)
Cp2Zr(OSO2C4F9)2·2H2O [23]:
A solution of Cp2ZrCl2 (292 mg,
1 mmol) in Et2O (20 mL) was added with a solution of AgOSO2C4F9
(854 mg, 2.1 mmol) in Et2O (10 mL), and the resultant mixture was
stirred in the dark at room temperature for 1 h. Then the reaction
mixture was filtered and evaporated, and the residue was diluted
with Et2O (4 mL) and toluene (10 mL). The solution was kept standing
in a refrigerator for 24 h, and the white crystals were precipitated
(642mg, 75%).
Typical procedure for synthesis of α-aminophosphonates (using
benzaldehyde, aniline and diethyl phosphate as an example): The
complex of Cp2Zr(OSO2C4F9)2·2H2O (41.1 mg, 0.05 mmol), PhCHO
(106 mg, 1.0 mmol), PhNH2 (93 mg, 1.0 mmol), and diethyl phosphite
(166 mg, 1.2 mmol) were mixed in a 50 mL round-bottom flask. Then
the mixture was stirred at room temperature for 2.5 h under the
TLC analysis until the PhCHO and PhNH2 as well as the intermediate
N-benzylideneaniline obtained from PhCHO and PhNH2 were consumed
completely. Then the solvents of the resulted mixture were removed by
evaporation in a vacuum, followed by adding the petroleum ether
(3 × 10 mL), and the recovery catalyst was precipitated and filtered for
the next cycle of reaction. The combined organic layer was evaporated
to a yellowish solid, then it was subjected to silica gel column
chromatography (petroleum ether/ethyl acetate = 5:1), and the pale
yellow solid of 4a was obtained (306 mg, isolated yield = 96%).
3.3. The scope of Kabachnik–Fields reaction catalyzed
by Cp2Zr(OSO2C4F9)2·2H2O
With the optimal condition in hand, various aldehydes and amines
as reactants (Table 2) were screened with Cp2Zr(OSO2C4F9)2·2H2O in
direct Kabachnik–Fields reaction, and good-to-excellent yields
were obtained. Both of the electron-donating groups and electron-
withdrawing groups attached in para-position of phenyl plane in
aromatic aldehydes were employed and showed high reactivity
(Table 2, entries 2–16). The aldehydes with electron-withdrawing
groups (e.g., Cl, Br, CF3 and NO2) exhibited higher reactivity in
Kabachnik–Fields reaction than those with electron-donating
groups in the para-position of the phenyl plane (e.g., methyl,
methoxyl and hydroxyl) (Table 2, entries 2–16). Tereph-thaldehyde also
showed high reactive activity (Table 2, entry 17). The cinnamaldehyde
with double bond was tolerant in current catalytic system (Table 2,
entry 18). The amines with electron-donating groups were beneficial
for catalytic reaction comparing with the amines bearing electron-
withdrawing groups (Table 2, entries 19–24). α-Naphthylamine,
benzidine and 8-aminoquinoline showed high reactive activity in
this catalytic system (Table 2, entry 25–26). Acetophenone and
cyclohexanone can be also used in this reaction with satisfactory
yields (Table 2, entries 25–26). We also investigated n-butylamine,
cyclohexylamine, piperidine, 2-aminopyridine and 4-aminopyridine
(Table 2, entries 31–34). However, for aliphatic amines, piperidine
and 2-aminopyridine, the corresponding α-aminophosphonates were
3. Results and discussion
3.1. Characterization of Cp2Zr(OSO2C4F9)2·2H2O
In this study, we used NMR, TG–DSC, fluorescence spectra and
Hammett indicator methods to investigate the physiochemical properties
of Cp2Zr(OSO2C4F9)2·2H2O (see Supplementary material). The complex
remained as a crystal or powder and suffered no color change after
being kept in open air for one year. The thermal behavior of the complex
of Cp2Zr(OSO2C4F9)2·2H2O was investigated by TG–DSC in O2
atmosphere (Figure S1, see Supplementary material). The curves
showed that they were thermally stable up to about 250 °C. We
estimated the Lewis acidity of Cp2Zr(OSO2C4F9)2 by the red shift
(λem) of Lewis acid metal ions (Zr2+) with 10-methylacridone on
the basis of fluorescence spectra [28], which showed that the
fluorescence maximum (λmax) of complex was 475 nm (Figure S2,
see Supplementary material). The acidity of the complex was also
determined by Hammett indicator method. It has relatively strong
acidity with acid strength of 0.8 b Ho ≤ 3.3 (Ho being the Hammett
acidity function). In addition, the complex was highly soluble in
methanol and in common polar organic solvents (Table S1, see
Supplementary material). In view of the water tolerance, high
thermal stability and strong acidity of Cp2Zr(OSO2C4F9)2·2H2O, we
Table 1
Cp2Zr(OSO2C4F9)2·2H2O catalyzed synthesis of α-aminophosphonates under various
conditionsa.
Entry
Solvent
Catalyst (mol%)
Time (h)
Yield (%)b
1
2
3
4
5
6
7
8
MeOH
CH3CN
THF
5
5
5
5
5
5
5
5
5
3
7
6
7
6
82
79
84
73
68
58
48
64
96
81
96
Et2O
8
CH2Cl2
n-Hexane
Toluene
H2O
Neat
Neat
10
10
10
10
2.5
5
9
10
11
Neat
2.5
a
PhCHO: 1.0 mmol; PhNH2: 1.0 mmol; diethyl phosphite: 1.2 mmol.
Isolated yield.
b
Scheme 1. Kabachnik–Fields reaction using Cp2Zr(OSO2C4F9)2·2H2O as catalyst.

186
N. Li et al. / Catalysis Communications 43 (2014) 184–187
Table 2
Product yield for reaction of aldehydes/ketones, amines and diethyl phosphite catalyzed by Cp2Zr(OSO2C4F9)2·2H2Oa.
Entry
R1
R2
R3
Product
Time (h)
Yield (%)b
1
2
3
4
5
6
7
8
Ph
H
H
H
H
H
H
H
H
H
H
H
H
H
H
H
H
Ph
Ph
Ph
Ph
Ph
Ph
Ph
Ph
Ph
Ph
Ph
Ph
Ph
Ph
Ph
Ph
4a
4b
4c
4d
4e
4f
4g
4h
4i
4j
4k
4l
4m
4n
4o
4p
4q
4r
4s
4t
4u
4v
4w
4x
4y
4z
4aa
4ab
4ac
4ad
4ae/5a
4af/5a
4ag/5a
4ah/5a
5a
2.5
3
3
96
90
91
89
97
96
98
96
92
93
93
91
89
90
91
p-CH3OC6H4
p-CH3C6H4
p-(CH3)2NC6H4
m-NO2C6H4
o-NO2C6H4
p-CF3C6H4
o-CF3C6H4
o-FC6H4
p-ClC6H4
p-BrC6H4
o-BrC6H4
o-HOC6H4
m-HOC6H4
p-HOC6H4
3-MeO-4-HO-C6H3
Tereph-thaldehyde
C6H5CH_CH
Ph
p-HOC6H4
Ph
Ph
Ph
Ph
Ph
Ph
Ph
3
2.5
2.5
2.5
2.5
3
2.5
2.5
3
3
3
3
3
9
10
11
12
13
14
15
16
17
18
19
20
21
22
23
24
25
26
27
28
29
30
31
32
33
34
35
90
Ph
Ph
PhCH2
PhCH2
p-CH3C6H4
p-ClC6H4
p-NO2C6H4
p-CF3OC6H4
α-Naphthylamine
Benzidine
Ph
3.5
3.5
4
90c
87
H
H
H
H
H
H
H
H
H
Me
86
84
95
91
89
87
4
2.5
2.5
3
3
3
3.5
4.5
4
8
5
90
89d
85
Cyclohexanone
n-C3H7
Ph
Ph
Ph
Ph
Ph
Ph
Ph
84
78
81
H
H
H
H
H
H
H
p-CH3C6H4
8-Aminoquinoline
n-C4H9
Cyclohexyl
Piperidine
2-Aminopyridine
4-Aminopyridine
8
73/19e
68/24e
55/35e
51/38e
49
10
10
10
12
a
PhCHO: 1.0 mmol; PhNH2: 1.0 mmol; diethyl phosphite: 1.2 mmol; rt.; neat; Cp2Zr(OSO2C4F9)2·2H2O: 0.05 mmol.
Isolated yield.
Two equivalents of aniline were used.
Two equivalents of benzaldehyde were used.
The products are α-aminophosphonate (4ae–4ah)/α-hydroxyphosphonate (5a).
b
c
d
e
obtained, and another solid byproduct was obtained in a very
using the traditional metal chlorides as catalysts owing to possible
hydrolysis (Table 3, entries 1–2). With the catalysts InBr3 and
Mg(ClO4)2, the yields were 76% and 58%, respectively (Table 3,
entries 3–4). We also investigated metal triflates as catalysts, such
as Sc(OTf)3 and Bi(OTf)3, and the yields were about 80% (Table 3,
entries 5–6). These catalysts are hydrolytic in air, and Godeau and co-
workers proposed that the presence of water molecules in the vicinity
of the bismuth cation could generate a hybrid Lewis/Brønsted acid
(LBA) as active catalytic species [29]. When the precursor Cp2ZrCl2 was
used as catalyst, the yield was only 19% (Table 3, entry 7). It is interesting
to note that the complex of Cp2Zr(OSO2C4F9)2·2H2O showed high
catalytic efficiency and the yield was 96%, which was higher than those
of Cp2Zr(OSO2CF3)2·3H2O·THF and Cp2Zr(OSO2C8F17)2·3H2O·THF as
catalysts (Table 3, entries 8–10), though it should be mentioned that
these two complexes feature lower Lewis and Brønsted acidity at the
same 5.0 mol% loadings. Furthermore, after being stored in air for one
year, the complex of Cp2Zr(OSO2C4F9)2·2H2O also exhibited catalytic
efficiency comparable to the freshly prepared sample (Table 3, entry 11).
short time, which was characterized as the corresponding α-
hydroxyphosphonate 5a. While, for 4-aminopyridine, the only
byproduct α-hydroxyphosphonate was afforded (Table 2, entry 35).
3.4. Catalyst comparison investigation
As shown in Table 3, several catalysts were evaluated by the current
reaction. It can be seen that the isolated yields were lowered to 30%
Table 3
Catalyst comparison in the Kabachnik–Fields reactiona.
Entry
Catalyst (mol%)
Yield (%)b
1
2
3
4
5
6
7
8
AlCl3
ZrCl4
InBr3
Mg(ClO4)2
Sc(OTf)3
Bi(OTf)3
Cp2ZrCl2
Cp2Zr(OSO2CF3)2·3H2O·THF
Cp2Zr(OSO2C8F17)2·3H2O·THF
Cp2Zr(OSO2C4F9)2·2H2O
Cp2Zr(OSO2C4F9)2·2H2O
25
32
76
58
82
72
19
84
88
96
93c
3.5. Catalyst recovery investigation
9
10
11
To test the reusability of the catalyst and reproducibility of catalytic
performance, Cp2Zr(OSO2C4F9)2·2H2O was subject to recycling experi-
ments of the Kabachnik–Fields reaction of benzaldehyde, aniline and
diethyl phosphite (see Supplementary material). The change in product
yield was negligible in a trial of five recycling experiments, demon-
strating that the catalyst was stable and suitable for reuse.
a
PhCHO: 1.0 mmol; PhNH2: 1.0 mmol; diethyl phosphite: 1.2 mmol; rt.; neat;
cat.: 0.05 mmol; time: 2.5 h.
b
Isolated yield.
Catalyst kept in open air for one year.
c

N. Li et al. / Catalysis Communications 43 (2014) 184–187
187
Scheme 2. Proposed mechanism on the Kabachnik–Fields reaction.
3.6. A plausible mechanism
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Appendix A. Supplementary data
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