Synthesis and structures of air-stable binuclear hafnocene perfluorobutanesulfonate and perfluorobenzenesulfonate and their catalytic application in C-C bond-forming reactions

Article abstract of DOI:10.1002/adsc.201300439

The two air-stable m2-hydroxy-bridged binuclear hafnocene perfluorobutanesulfonate and perfluorobenzenesulfonate complexes were successfully synthesized. The high catalytic activity and recyclability of these complexes were exemplified for various carbon-carbon bond formation reactions. Compared with our previously reported hafnocene perfluorooctanesulfonate, these complexes show stronger Lewis acidity and better catalytic activity, and should find broad applications in organic synthesis.

Full text of DOI:10.1002/adsc.201300439

UPDATES
DOI: 10.1002/adsc.201300439
Synthesis and Structures of Air-Stable Binuclear Hafnocene
Perfluorobutanesulfonate and Perfluorobenzenesulfonate and
ꢀ
their Catalytic Application in C C Bond-Forming Reactions
Ningbo Li,a Xiaohong Zhang,a Xinhua Xu,a,b,* Yun Chen,a Renhua Qiu,a,*
Jinyang Chen,a Xie Wang,a and Shuang-Feng Yina,*
a
State Key Laboratory of Chemo/Biosensing and Chemometrics, College of Chemistry and Chemical Engineering, Hunan
University,
Fax : (+86)-731-8882-1546; e-mail: xhx1581@hnu.edu.cn or renhuaqiu@hnu.edu.cn or sf_yin@hnu.edu.cn
State Key Laboratory of Elemento-Organic Chemistry, Nankai, University, Tianjing 300071, Peopleꢀs Republic of China
b
Received: May 20, 2013; Revised: June 13, 2013; Published online: August 13, 2013
Supporting information for this article is available on the WWW under http://dx.doi.org/10.1002/adsc.201300439.
Abstract: The two air-stable m2-hydroxy-bridged bi-
nuclear hafnocene perfluorobutanesulfonate and
perfluorobenzenesulfonate complexes were success-
fully synthesized. The high catalytic activity and re-
cyclability of these complexes were exemplified for
various carbon-carbon bond formation reactions.
Compared with our previously reported hafnocene
perfluorooctanesulfonate, these complexes show
stronger Lewis acidity and better catalytic activity,
and should find broad applications in organic syn-
thesis.
perfluorooctanesulfonate
[SO3C8F17]4·4H2O·2THF (2·4H2O·2THF) Lewis acids
catalysts, which showed moderate to high catalytic ef-
(m2-OH)2]
[{CpHfACHTNGUTREN(NUG OH2)3}2ACHTGNUTRENNUNG
AHCTUNGTRENNUNG
[5]
ꢀ
ꢀ
ficiency in C C and C O bond formation reactions.
However, it should be noted that their stabilities and
Lewis acidity/catalytic activities are not only related
to the special binuclear cations but also to the per-
ꢀ
fluoro anions, because the strong C F bonds in the
perfluoro anions can enhance the robustness and anti-
oxidative role as well as the relative lipophobic abili-
ty.[6] Furthermore, the longer fluorocarbon chain will
cause lower catalytic activity which, owing to the fact
that the perfluoro anions well wrap the cationic metal
center, make it hard for the lipophilic substrates to
approach the center metal atom for activation. Be-
sides, due to the strongly lipophobic property of the
perfluorooctyl group, the complex usually exhibited
low solubility in common organic solvents, such as
hafnocene complex (2·4H2O·2THF),[7] which may be
ꢀ
Keywords: C C bond formation; hafnocenes; ho-
mogeneous catalysis; Lewis acids; perfluorobutane-
ACHTUNGTRENNUNG(benzene)sulfonates
Organometallic Lewis acid catalysis has attracted another reason for the decline of catalytic efficiency.
much attention in organic synthesis, especially the bi- In addition, the features of the perfluorooctanesulfo-
metallic Lewis acid catalysis, which is commonly ob- nate (PFOS) result in severe environmental pollution
served in metalloenzyme activities.[1] Recently, various and potential toxicity to animals and human beings.[8]
scientific activities have been devoted to construct bi-
According to the literature,[9] the toxicities of
(benzene)sulfonates were lower than
metallic Lewis acid catalysis systems to mimic such perfluorobutaneACHTGNUTRENNUNG
actions in the organic synthesis.[2] For example, we that of PFOS, and their lipophobicities were weaker
have prepared the binuclear organobismuth com- than that of the perfluorooctyl group.[7] Herein, by
plexes bridged by sulfur atoms, and found that they combining the binuclear hafnocene cation with the
showed higher cooperative catalytic effect in the perfluorobutaneACTHNUTRGNEUG(N benzene)sulfonates anions, we suc-
chemical fixation of CO2,[3] illustrating that by fixation cessfully synthesized the hafnocene perfluorobutane-
of two metal centers in a suitable framework, the bi- sulfonate and perfluorobenzenesulfonate complexes,
nuclear organometallic complexes show high catalytic and assessed their catalytic activities in various
efficiency compared with their monomer analogues.[4] carbon-carbon bond-forming reactions, such as the
Besides, we have synthesized anther two air-stable Mukaiyama–aldol reaction, Mukaiyama–Michael ad-
and water-tolerant binuclear zirconocene per- dition reaction, Michael addition reaction and Man-
A
(m2-OH)2] nich reaction.
[{CpZrACHTNGUTREN(NUG OH2)3}2ACHTGNUTRENNUNG
ACHTUNGTRENNUNG
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Synthesis and Structures of Air-Stable Binuclear Hafnocene Perfluorobutanesulfonate
Scheme 1. Synthesis of 3·4H2O·2THF and 4·6H2O.
The
[{CpHf
(3·4H2O·2THF)
[{CpHf(OH2)3}2A
(m2-OH)2]
hafnocene
(OH2)3}2A
(m2-OH)2]
and
perfluorobutanesulfonate
ACHTUNGNERT[NGNU OSO2C4F9]4·4H2O·2THF
The cationic structures of these complexes in the
solid state were confirmed by X-ray analysis. An
G
E
E
perfluorobenzenesulfonate ORTEP representation of 3·4H2O·2THF and 4·6H2O
E
N
[OSO2C6F5]4·6H2O and selected bonds and angles are shown in Figure 1
(4·6H2O) complexes were synthesized by treatment and Figure 2. Accoding to the X-ray structure, it can
of Cp2HfCl2 with silver perfluorobutanesulfonate and be concluded that the hafnium atoms have distorted
perfluorobenzenesulfonate (AgX, for 3, X= octahedral coordination geometry with the Cp group
OSO2C4F9; for 4, X=OSO2C6F5) (2 equiv.) in THF or being Ttrans to OH consistent with [CpHf2+
ACHTUNGTRENNUNG
CH3CN (Scheme 1), respectively.
G
Aꢀ
1H NMR and elemental analyses show that the The C4F9SO3 /C6F5SO3 ions, the dissociated H2O
freshly prepared samples after recrystallization con- molecules, and solvating ligand THF are packed
tain four H2O molecules and two THF molecules for around the complex cation in such a way that their
3, and six H2O molecules for 4. We found that oxygen atoms point towards the H2O ligands. The
3·4H2O·2THF and 4·6H2O remained as dry crystals C4F9/C6F5 side chains of the anions, on the other
or powder and suffered no color change in a test hand, are clustered together to form the hydrophobic
period of one year in the open air. Therefore, the two domains.
complexes can be considered as air-stable complexes
The thermal behavior of complexes 3·4H2O·2THF
in the ambient environment and have great advantag- and 4·6H2O was investigated by TG-DSC in an O2 at-
es over hafnocene triflates from the operational point mosphere (Figure S1 in the Supporting Information).
of view.[10]
The curves showed that they were thermally stable up
ꢀ
Figure 1. ORTEP view of crystal structure of 3·4H2O·2THF. Selected bond lengths (ꢂ) and angles (deg): Hf1 O1, 2.091 (4);
Hf1 O1A, 2.156 (4); Hf1 O2, 2.177(4); Hf1 O3, 2.187(4); Hf1 O4, 2.116(4); Hf1 C1, 2.501(6); Hf1 C2, 2.482(6); Hf1 C3,
ꢀ
ꢀ
ꢀ
ꢀ
ꢀ
ꢀ
ꢀ
ꢀ
ꢀ
ꢀ
ꢀ
ꢀ
ꢀ
ꢀ
2.482(6); Hf1 C4, 2.513(6); Hf1 C5, 2.521(5); Hf1 Hf1A, 3.4979(4); Hf1 O1 Hf1A, 110.88(16); O1 Hf1 O1A, 69.12(16);
the two Cp ring planes are parallel.
Adv. Synth. Catal. 2013, 355, 2430 – 2440
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Ningbo Li et al.
UPDATES
ꢀ
ꢀ
Figure 2. ORTEP view of crystal structure of 4·6H2O. Selected bond lengths (ꢂ) and angles (deg): Hf1 O1, 2.082 (2); Hf1
O1A, 2.138 (2); Hf1 O2, 2.146(2); Hf1 O3, 2.171(2); Hf1 O4, 2.160(2); Hf1 C1, 2.510(3); Hf1 C2, 2.485(3); Hf1 C3,
ꢀ
ꢀ
ꢀ
ꢀ
ꢀ
ꢀ
ꢀ
ꢀ
ꢀ
ꢀ
ꢀ
ꢀ
ꢀ
2.481(3); Hf1 C4, 2.504(3); Hf1 C5, 2.523(3); Hf1 Hf1A, 3.4972(2); Hf1 O1 Hf1A, 111.05(9); O1 Hf1 O1A, 68.95(8);
the two Cp ring planes are parallel.
to about 3008C, illustrating the antioxidative role of carbon bond-forming reactions, such as the Mukaiya-
the perfluoro anions. The large molar conductivity ma–aldol reaction, Mukaiyama–Michael addition re-
value (L=398 mS·cmꢀ1 molꢀ1; 295 mS cmꢀ1 molꢀ1, action, Michael addition reaction and Mannich reac-
Table S1 in the Supporting Information) were consis- tion.
tent with the complete ionization of 3·4H2O·2THF
The Mukaiyama–aldol reaction mediated by
and 4·6H2O into a 1:4 electrolyte.[11] In addition, a Lewis acid is one of the most convenient processes
3·4H2O·2THF and 4·6H2O are highly soluble in for the construction of carbon-carbon bonds in organ-
methanol and in common polar organic solvents in ic synthesis. This route provides a rapid access to syn-
sharp contrast to 2·4H2O·2THF (Table S2 in the Sup- thesize b-hydroxy carbonyl compounds, and has in-
porting Information).
duced many synthetic efforts due to these motifs
A significant red shift was observed in UV-Vis often being seen in natural products and bioactive
spectra due to the strong complex formation between molecules.[15] Several efficient Lewis acid catalysts
10-methylacridone and 3·4H2O·2THF, illustrating based on titanium, zirconium, copper and Brønsted
a large Lewis acidity of 3·4H2O·2THF (Figure S2 in acids have been reported. But in most cases, tempera-
the Supporting Information).[12] We also estimated the tures of ꢀ208C to ꢀ808C and strictly anhydrous con-
Lewis acidity of complexes 2–4 by the red shift (lem) ditions are required.[16] We hence assessed the catalyt-
of Lewis acid metal ions (Hf2+) with 10-methylacri- ic efficiency with 3·4H2O·2THF in the Mukaiyama–
done on the basis of fluorescence spectra.[13] The fluo- aldol reaction, and a high catalytic activity was ob-
rescence maxima (lmax) of complexes 2–4 are 464 nm, served (Table 1).
472 nm and 470 nm, respectively (Figure S3 in the
As shown in Table 1, the complex 3 showed high
Supporting Information). Obviously, the Lewis acidity catalytic efficency in the Mukaiyama–aldol reaction,
of 2·4H2O·2THF is lower than those of good-to-excellent yields were obtained (Table 1, 7a–
3·4H2O·2THF and 4·6H2O. Besides, they have a rela- 7r, 81–96%). Compared with our reported catalyst
tively strong acidity with acid strength of 0.8<Hoꢁ 2·4H2O·2THF, the yield was apparently improved but
3.3 (Ho being the Hammett acidity function),[14] the reaction time was relatively reduced, and such
larger than that of 2·4H2O·2THF (3.3<Hoꢁ4.8) de- special features were observed in cases of aromatic al-
temined by the Hammett indicator method.[5] These dehydes with different electron-donating and elec-
characteristic features encouraged us to evaluate their tron-withdrawing groups (Table 1, entries 2–13). For
performance as Lewis acid catalysts for carbon- example, as shown in Table 1, entry 5, the yield of 7e
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Synthesis and Structures of Air-Stable Binuclear Hafnocene Perfluorobutanesulfonate
Table 1. Product yields for reaction of aldehydes and enol silyl ethers catalyzed by 3·4H2O·2THF.[a]
Entry
R1
R2
R3
Product
Yield [%][b]
1
2
3
4
5
6
7
8
Ph (5a)
CH3 (6a)
CH3 (6a)
CH3 (6a)
CH3 (6a)
H (6b)
H (6b)
H (6b)
H (6b)
H (6b)
H (6b)
H (6b)
H (6b)
H (6b)
CH3 (6a)
CH3 (6a)
CH3 (6a)
H (6b)
OMe (6a)
OMe (6a)
OMe (6a)
OMe (6a)
Ph (6b)
Ph (6b)
Ph (6b)
Ph (6b)
Ph (6b)
Ph (6b)
Ph (6b)
Ph (6b)
Ph (6b)
OMe (6a)
OMe (6a)
OMe (6a)
Ph (6b)
7a
7b
7c
7d
7e
7f
7g
7h
7i
7j
7k
7l
7m
7n
7o
7p
7q
7r
95
92
96
94
93
90
89
92
93
91
95
94
94
92
93
90
87
81
p-CH3C6H4 (5b)
p-CF3C6H4 (5g)
p-ClC6H4 (5f)
Ph (5a)
p-CH3C6H4 (5b)
p-CH3OC6H4 (5c)
o-FC6H4 (5d)
p-ClC6H4 (5e)
p-BrC6H4 (5f)
p-CF3C6H4 (5g)
o-CF3C6H4 (5h)
o-NO2C6H4 (5i)
PhCH=CH (5j)
m-ClC6H4CH=CH (5k)
o-CH3CH=CH(5l)
PhCH=CH (5j)
CH3CH2CH2 (5m)
9
10
11
12
13
14
15
16
17
18
H (6b)
Ph (6b)
[a]
3·4H2O·2THF: 0.05 mmol; 5: 1.0 mmol; 6: 1.2 mmol; solvent: Et2O; 08C to room temperature.
Isolated yield.
[b]
was up to 93% at 5 h with 3·4H2O·2THF, while that 24 h, and the desired Michael adducts were obtained
with 2·4H2O·2THF with only 78% at 24 h.[5b] And the in high yields (Table 2, 9a–9e, 78–88%). Also the silyl
aldehydes with electron-withdrawing groups (such as enol ethers (6a–6c) with different substituents were
chloride and trifluoromethyl) in the phenyl plane ex- found to react with methyl vinyl ketone in good to ex-
hibited higher reaction activity than the aldehydes cellent yields with 3·4H2O·2THF as catalyst (Table 2,
with electron-donating groups (for example, methyl entries 1–3, 9a–9c, 83–88%). The reaction of trimeth-
and methoxy) (Table 1, entries 2–13). The parent cin- yl(1-phenyl-vinyloxy)silane also showed good reactivi-
namaldehyde and substituted cinnamaldehydes with ty with cyclopentenone and cyclohexenone giving
a double bond were also tolertaed in this catalytic yields of 80% and 78%, respectively (9d, 9e). It is in-
system, and successfully reacted with different silyl teresting to note that the reaction of the enol silyl
enol ethers to form 1,2-addition adducts in high yields ethers with a,b-unsaturated ketones (Table 2, en-
(Table 1, entries 14–17). The alkyl aldehyde also tries 3–5) is a 1,4-addition reaction, while the reaction
showed good reactivity with 81% yield with of the enol silyl ethers with a,b-unsaturated aldehydes
3·4H2O·2THF, while the yield of 7r with catalyst of is a 1,2-addition reaction (Table 1, entries 14–17). To
2·4H2O·2THF under the same conditions was only the best of our knowledge, such a difference may be
46%, illustrating a remarkable improvement of the owing to the steric hindrance and electron effect be-
catalytic efficiency.
tween the aldehydes and ketones.[19]
The Mukaiyama–Michael addition is a powerful
Later we moved to the Lewis acids-catalyzed Mi-
tool for the preparation of synthetically useful 1,5-di- chael addition reaction of indole derivatives with a,b-
carbonyl compounds, which are important building unsaturated carbonyl compounds, which is another
blocks for the synthesis of various biologically active important carbon-carbon bond-forming reaction and
molecules.[17] Presently, several catalysts are known has attracted much attention in organic synthesis in
for the direct Michael reaction, but for the Mukaiya- recent years.[20] Most of the recently developed cata-
ma–Michael reaction of silyl enol ethers with enones, lytic systems are sensitive to air or moisture.[21] The
only few catalysts have been reported.[18] Therefore, highly efficient system of silica-Sc-IL/
ACTHNUGTRNEU[GN DBIm]SbF6 de-
the development of an efficient catalytic system for veloped by Kobayashi also required ionic liquids as
the Mukaiyama–Michael reaction still remains a chal- co-catalyst.[22] We hence applied the catalysts
lenge. We applied the complex 3·4H2O·2THF in the 3·4H2O·2THF and 4·6H2O in the Michael addition
reaction of enol silyl ethers with enones in Et2O for reaction of indole derivatives with enones in CH3CN
Adv. Synth. Catal. 2013, 355, 2430 – 2440
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UPDATES
Table 2. Product yields for reaction of enol silyl ethers with enones catalyzed by 3·4H2O·2THF.[a]
Entry
1
Enol silyl ethers
Enones
Product
Yield [%][b]
88
(6a)
(6c)
(8a)
(8a)
9a
2
9b
86
3
4
(6b)
(6b)
(8a)
(8b)
9c
83
80
9d
5
(6b)
(8c)
9e
78
[a]
3·4H2O·2THF: 0.05 mmol; 6: 1.2 mmol; 8: 1.0 mmol; solvent: Et2O, 0 8C to room temperature.
Isolated yield.
[b]
for 3 h and obtained the desired Michael adducts in alkenes as Michael acceptors in asymmetric catalytic
high yields (Table 3).
reactions were reported.[24] The Friedel–Crafts alkyla-
As shown in Table 3, the terminal but-3-en-2-one tion reaction of indole derivatives with trans-b-nitro-
can be transformed to the corresponding 3-substituted styrenes has been reported.[24d] However, Zn
(OTf)2 as
AHCTUNGTRENNUNG
indole with a monoindole group and the yield was up a catalyst is hygroscopic, and catalytic efficiency is
to 96% (11a). The internal a,b-unsaturated ketones low. Thus, we assessed 3·4H2O·2THF and 4·6H2O as
non-3-en-2-one and 4-phenylbut-3-en-2-one can be a catalyst for this reaction. In the presence of
also successfully reacted with indole derivatives to 5.0 mol% of 3·4H2O·2THF or 4·6H2O, the Michael
form the target products in yields of 94% and 85%, addition reaction of indole derivatives with trans-b-ni-
respectively (11b, 11e). And we found that the large trostyrene occurred at room temperature in CH3CN
steric effect of the phenyl group did not have an for 2.5–3 h, and good-to-excellent yields were ob-
effect on the catalytic activity. For example, the yields tained (Table 4).
with the a,b-unsaturated cyclo ketones such as cyclo-
As shown in Table 4, the complex 3 showed high
pent-2-enone and cyclohex-2-enone were up to 90% catalytic efficiency in the Michael addition reaction of
and 88%, respectively (11c, 11d). 5-Bromo-1H-indole indole derivatives with trans-b-nitrostyrene, and the
and 2-methyl-1H-indole showed high reactivity in this yield was up to 94%. The indole derivatives contain-
catalytic system with cyclopent-2-enone, the yields ing electron-withdrawing groups on the phenyl group
were 87% and 90%, respectively (11f, 11g). 2-Methyl- (F, Cl, Br) showed slightly lower yields of 84%, 86%
1H-indole and 1-methylindole also exhibited high re- and 85%, respectively (13b–13d). When an electron-
activity with non-3-en-2-one and the yields were up to donating group was introduced into the indole ring
91% and 92%, respectively (11h, 11i). In addition, (2-Me, 5-Me, 7-Me), the yield became relatively high
4·6H2O also showed a relatively high catalytic effi- from 91% to 94% (13e–13g). 1-Methylindole also had
ciency towards the Michael addition reaction, giving high reactivity in the current catalytic system and the
yields of 76–85%.
yield was up to 89% (13h). And the complex 4 was
Nitroalkenes, similar to a,b-unsaturated carbonyl also found to be an efficient catalyst in this Michael
compounds, are very attractive Michael acceptors and addition with the yields of 76–86%.
have been widely used in organic synthesis.[23] Recent-
1-Amidoalkyl-2-naphthol derivatives are of impor-
ly, many examples regarding the application of nitro- tance as they can be easily converted to biologically
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Synthesis and Structures of Air-Stable Binuclear Hafnocene Perfluorobutanesulfonate
Table 3. Product yields for reaction of indole derivatives with enones catalyzed by 3·4H2O·2THF[a] and 4·6H2O.[a]
Entry
Indoles (10)
Enones (8)
Product (11)
Yield [%][b]
3·4H2O·2THF
4·6H2O
1
2
3
G
(8a)
(8d)
(8b)
11a
11b
11c
96
94
90
85
82
81
4
5
G
(8c)
(8e)
11d
11e
88
85
78
76
6
7
8
G
(8b)
(8b)
(8d)
(8d)
11f
11g
11h
11i
87
90
91
92
80
81
82
83
9
[a]
3·4H2O·2THF/4·6H2O: 0.05 mmol; 10: 1.0 mmol; 8: 1.0 mmol; solvent:CH3CN; room temperature.
Isolated yield.
[b]
active compounds.[25] A one-pot multi-component
As shown in Table 5, the reaction proceeded well
Mannich reaction of b-naphthol with aromatic alde- for a variety of aromatic aldehydes (84–98%). The ar-
hydes and amide derivatives has been used as a practi- omatic aldehydes with electron-withdrawing groups
cal synthetic route toward 1-amidoalkyl-2-naphthols. (e.g., Cl, Br, and NO2) exhibited higher reactivity in
Several Lewis and Brønsted acids have been applied this Mannich reaction than those with electron-donat-
to catalyze this reaction, such as p-toluenesulfonic ing groups in the para-position of the phenyl plane
acid, iodine, HClO4/SiO2, 4-(1-imidazolium)butanesul- (e.g., methyl and methoxy) (Table 5, entries 2–6).
fonate.[26] However, these methods have disadvan- Benzamide and urea also showed high reactivity in
tage(s) such as low yield, long reaction time and disa- this catalytic system with benzaldehyde and b-naph-
greement with green chemistry protocols. We hence thol and the yields were 89% and 84%, respectively
addressed these problems with 4·6H2O as a catalyst (16g, 16h).
(2.0 mol%) in a one-pot Mannich reaction in reflux-
To show the advantage of 3·4H2O·2THF and
ing EtOH for 1–1.5 h, and good-to-excellent yields 4·6H2O, we compared the catalytic activities of
were obtained (Table 5).
3·4H2O·2THF and 4·6H2O with those of Cp2HfCl2,
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Ningbo Li et al.
UPDATES
Table 4. Product yields for reaction of indole derivatives with trans-b-Nitrostyrene catalyzed by 3·4H2O·2THF[a] and
4·6H2O.[a]
Entry
G
R
Time [h]
Product (13)
Yield [%][b]
3·4H2O·2THF
4·6H2O
1
2
3
4
5
6
7
8
H (10a)
H
H
H
H
H
H
H
CH3
3
3
3
3
2.5
2.5
2.5
2.5
13a
13b
13c
13d
13e
13f
13g
13h
94
84
86
85
91
94
92
89
83
77
79
76
82
85
86
84
5-F (10e)
5-Cl (10f)
5-Br (10b)
2-Me (10c)
5-Me (10g)
7-Me (10h)
H (10d)
[a]
3·4H2O·2THF/4·6H2O: 0.05 mmol; 10: 1.0 mmol; 12: 1.0 mmol; solvent: CH3CN; room temperature.
Isolated yield.
[b]
Table 5. Product yields for reaction of b-naphthol with aromatic aldehydes and amide derivatives catalyzed by 4·6H2O.[a]
Entry
R1
R2
Time [h]
Product
Yield [%][b]
1
2
3
4
5
6
7
8
Ph (5a)
CH3 (14a)
CH3 (14a)
CH3 (14a)
CH3 (14a)
CH3 (14a)
CH3 (14a)
Ph (14b)
1.0
1.0
1.0
1.0
1.0
1.0
1.5
1.5
16a
16b
16c
16d
16e
16f
16g
16h
97
90
92
97
96
98
89
84
p-CH3C6H4 (5b)
p-CH3OC6H4 (5c)
p-ClC6H4 (5e)
p-BrC6H4 (5f)
m-NO2C6H4 (5n)
Ph (5a)
Ph (5a)
NH2 (14c)
[a]
4·6H2O: 0.02 mmol; 5: 1.0 mmol; 14: 1.2 mmol; 15: 1.0 mmol; solvent: EtOH; 808C.
Isolated yield
[b]
Cp2HfACHTUNGTRENNUNG(OSO2CF3)2 (17) and 2·4H2O·2THF. As shown five recycling experiments, demonstrating that the cat-
in Table 6, high yields were constantly gained over alyst is stable and suitable for reuse (Table 7).
complex 3·4H2O·2THF or 4·6H2O as catalysts, while
In conclusion, we have synthesized and character-
the other catalysts showed much lower yields, plausi- ized two air-stable m2-hydroxy-bridged binuclear haf-
bly due to their lower Lewis acidity or moisture-sensi- nocene Lewis acid complexes with perfluorobutane-
tive features (Table 6).
sulfonate and perfluorobenzenesulfonate as counter
To test the reusability of the catalyst and the repro- anions with strong Lewis acidity. These complexes
ducibility of its catalytic performance, 3·4H2O·2THF can be used as a general catalyst in Lewis acid-cata-
was subject to recycling experiments of above reac- lyzed carbon-carbon bond-forming reactions. Due to
tions [Eq. (1): 5a+6a!7a; Eq. (2): 6a+8a!9a; Eq. their highly catalytic efficiency, stability, storability,
(3): 10a+8a!12a; Eq. (4): 5a+14a+15a!16a]. The low toxicity and reusability, they should find board
change in product yield was negligible in a trial of application in organic synthesis.
2436
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Adv. Synth. Catal. 2013, 355, 2430 – 2440

Synthesis and Structures of Air-Stable Binuclear Hafnocene Perfluorobutanesulfonate
Table 6. Catalyst comparison in carbon-carbon bond-forming reactions.
Entry
Catalyst
Yield [%][a]
Eq. (1)
Eq. (2)
Eq. (3)
Eq. (4)
1
2
3
4
5
Cp2HfCl2
8
5
15
71
96
85
46
18
79
98
91
59
2·4H2O·2THF
3·4H2O·2THF
4·6H2O
67
95
86
40
59
88
80
39
Cp2HfACHTNUTRGEN(UGN OSO2CF3)2(17)
[a]
Same conditions as shown in the previous Tables 1, 2, 3, and 5.
Table 7. Yields of the Mukaiyama aldol reaction [Eq. (1)]; Mukaiyama–Michael addition reaction [Eq. (2)]; Michael addition
reaction [Eq. (3)] and Mannich reaction [Eq. (4)] catalyzed by recovered catalyst.[a]
Cycle
Yield [%][b]
Cat [%][c]
Yield [%][b]
Cat [%][c]
Yield [%][b]
Cat [%][c]
Yield [%][b]
Cat [%][c]
Eq. (1)
Eq. (2)
Eq. (3)
Eq. (4)
1
2
3
4
5
95
94
93
95
94
97
95
98
96
97
88
87
86
87
85
91
89
90
91
90
96
95
96
92
95
97
98
99
95
96
98
96
97
98
97
99
98
98
99
98
[a]
Same conditions as shown in the previous Tables 1, 2, 3 and 5 and the equivalents are expanded to ten times.
Isolated yield of desired product.
Isolated yield of recovered catalyst.
[b]
[c]
was measured on a REX conductivity meter DDS-307. IR
spectra were recorded on a Nicolet 5700 FTR spectropho-
tometer (Thermo Electron Corporation). TG-DSC analysis
was performed on an HCT-1 (Henven, Beijing, China) in-
strument. X-ray single crystal diffraction analysis was per-
formed with SMART-APEX and RASA-7A by Shanghai
Institute Organic Chemistry, China Academy of Science.
UV/Vis (Shimadzu UV-1601) and fluorescence spectroscopy
(Hitachi F-4600) was measured in the State Key Laboratory
of Chemo/Biosensing and Chemometrics, College of
Chemistry and Chemical Engineering, Hunan University
(China). The acidity was measured by the Hammett indica-
tor method as described previously.[5] Acid strength was ex-
Experimental Section
General
All chemicals were purchased from Aldrich. Co. Ltd and
used as received unless otherwise indicated. The preparation
of catalyst was carried out under a nitrogen atmosphere
with freshly distilled solvents unless otherwise noted. THF
and hexane were distilled from sodium/benzophenone. Ace-
tonitrile was distilled from CaH2. The NMR spectra were re-
corded at 258C on an Inova-400M (USA) calibrated with
tetramethylsilane (TMS) as an internal reference. Elemental
analyses were performed with a Vario EL III. Conductivity
Adv. Synth. Catal. 2013, 355, 2430 – 2440
ꢁ 2013 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
asc.wiley-vch.de
2437

Ningbo Li et al.
UPDATES
pressed in terms of Hammett acidity function (Ho) as scaled
by pKa value of the indicators.
tained free of charge from The Cambridge Crystallographic
Data Centre via www.ccdc.cam.ac.uk/data_request/cif.
Preparation of 3·4H2O·2THF
Typical Procedure for the Mukaiyama–Aldol
Reaction of Aldehydes with Enol Silyl Ethers
Catalyzed by 3·4H2O·2THF
To a solution of Cp2HfCl2 (0.379 g, 1.0 mmol) in 20 mL THF
was added a solution of AgOSO2C4F9 (0.814 g, 2.0 mmol) in
10 mL THF. After the mixture had been stirred at 258C for
2 h in the absence of light, it was filtered. The filtrate was
placed in a small jar and then put into a larger jar to which
was added 40 mL dry hexane and then the larger jar was
obdurated. After being kept in the refrigerator for 24 h, col-
orless crystals were obtained; yield: 837 mg (82%). Recrys-
tallization of this complex in THF/hexane produced good
Complex 3·4H2O·2THF (0.05 mmol), and enol silyl ether
(6) (1.2 mmol) were added to a solution of aldehyde (5)
(1.0 mmol) in diethyl ether (3.0 mL) at 08C. Then the tem-
perature was raised to room temperature slowly. After the
mixture had been stirred at room temperature for 5 h and
monitored by TLC, it was subject to evaporation under
vacuum at room temperature, the residue was dissolved in
n-hexane (10 mLꢃ3) and the catalyst was collected by
means of filtration for the next cycle of reaction. To the
combined hexane solution, MeOH and HCl (aq.) were
added and the mixture was stirred for 15 min. NaHCO3
(aq.) was added for neutralization. The mixture was subject
to evaporation, and the solids thus obtained were dissolved
in AcOEt and water. After extraction with AcOEt (three
times), the organic layer was washed with NaCl (aq.) and
dried over MgSO4. After evaporation, the residue was sub-
ject to silica gel column chromatography (petroleum ether :
ethyl acetate=8:1), colorless crystals (7a) were obtained;
isolated yield: 95%. Aldehydes and enol silyl ethers are
commercially available.
1
crystals suitable for X-ray analysis. H NMR (400 MHz, ace-
tone-d6, 258C, TMS) d=1.77 to 1.80 (m, 2H, THF), 3.61 to
3.64 (m, 2H, THF), 4.86 (s, nH, H2O), 6.59 (s, 2H, Cp), 6.62
(s, 2H, Cp), 6.81 (s, 6H, Cp); 19F NMR (376 MHz, acetone-
d6, 258C): d=ꢀ76.21 to ꢀ76.26 (m, 3F, CF3), ꢀ109.73 to
ꢀ109.78 (m, 2F, CF2), ꢀ116.52 to ꢀ116.57 (m, 2F, CF2),
ꢀ121.10 to ꢀ121.18 (m, 2F, CF2); IR
ACHTUNGTREN(NUNG KBr): n=3430, 2965,
2925, 2359, 1670, 1515, 1430, 1260, 1117, 1023, 925, 828, 753,
666, 569 cmꢀ1
;
elemental analysis calculated (%) for
C34H48F36Hf2O26S4: C 20.00, H 2.37; found: C 20.01; H 2.35.
Crystal data for 3·4H2O·2THF: C17 H24 F18 Hf O13S2;
Mr=1020.97, triclinic, space group P-1, a=10.4026(9) ꢂ,
b=10.8999(9) ꢂ, c=15.5637(13) ꢂ; V=1656.3(2) ꢂ3; T=
173(2) K; Z=2; reflections collected/unique, 12721/7145,
R
int =0.0189, final R indices [I>2s(I)] R1 =0.0379, wR2 =
Typical Procedure for the Mukaiyama–Michael
0.1041; indices (all data), R1 =0.0410, wR2 =0.1073.
R
GOF=1.089; CCDC 916236 contains the supplementary Addition Reaction Catalyzed by 3·4H2O·2THF
crystallographic data for this paper. These data can be ob-
tained free of charge from The Cambridge Crystallographic
Data Centre via www.ccdc.cam.ac.uk/data_request/cif.
Complex 3·4H2O·2THF (0.05 mmol), and enol silyl ether
(6) (1.2 mmol) were added to a solution of enone (8)
(1.0 mmol) in diethyl ether (3.0 mL) at 08C. Then the tem-
perature was raised to room temperature slowly. After the
mixture had been stirred at room temperature for 24 h and
Preparation of 4·6H2O
monitored by TLC, it was subject to evaporation under
To a solution of Cp2HfCl2 (0.379 g, 1.0 mmol) in 20 mL
vacuum at room temperature. The residue was dissolved in
CH3CN was added a solution of AgOSO2C6F5 (0.772 g,
n-hexane (10 mLꢃ3) and the catalyst was collected by
2.0 mmol) in 10 mL CH3CN. After the mixture had been
means of filtration for the next cycle of reaction. To the
stirred in the absence of light at room temperature for 2 h,
combined hexane solution, MeOH and HCl (aq.) were
it was filtered and evaporated under vacuum and the result-
added and the mixture was stirred for 15 min. NaHCO3
ing residue was diluted with 10 mL THF followed by five
(aq.) was added for neutralization. The mixture was subject
drops of dry hexane and then was stood in the refrigerator
to evaporation, and the solids thus obtained were dissolved
for 24 h; colorless crystals were obtained; yield: 673 mg
in AcOEt and water. After extraction with AcOEt (three
(78%). Recrystallization of this complex in THF/hexane
1
times), the organic layer was washed with NaCl (aq.) and
dried over MgSO4. After evaporation, the residue was sub-
ject to silica gel column chromatography (petroleum ether:-
ethyl acetate=5:1), a colorless oil (9a) was obtained; isolat-
ed yield: 88%. Enol silyl ethers and enones are commercial-
ly available.
produced good crystals suitable for X-ray analysis. H NMR
(400 MHz, acetone-d6, 258C, TMS): d=4.32 (s, nH, H2O),
6.54 (s, 1H, Cp), 6.59 (s, 2H, Cp), 6.73 (s, 6H, Cp), 6.89(s,
1H, Cp); 19F NMR (376 MHz, acetone-d6, 258C): d=
ꢀ139.03 (s, 2F, C6F5), ꢀ155.30 (s, 1F, C6F5), ꢀ163.98 (s, 2F,
C6F5); IRACHTUNGTRENNUNG(KBr): n=3121, 2358, 1658, 1533, 1501, 1313, 1177,
1115, 1042, 979, 833, 634, 488 cmꢀ1; elemental analysis calcu-
lated (%) for C34H36F20O26S4Hf2 (as dodecahydrate): C
23.66, H 2.10; found: C 23.60, H 2.08.
General Procedure for the Michael Addition
Reaction of Indoles with Enones or Nitroalkenes by
3·4H2O·2THF/4·6H2O
Crystal data for 4·6H2O: C34H36F20Hf2O26S4; Mr=1725.85,
orthorhombic, space group Pbca, a=15.2866(8) ꢂ, b=
12.0159(7) ꢂ, c=28.7517(15) ꢂ; V=5281.2(5) ꢂ3; T=
296(2) K; Z=4; reflections collected/unique, 21924/4894,
A mixture of enone (1.0 mmol) or nitroalkene (1.0 mmol),
indole (1.0 mmol) and 3·4H2O·2THF/4·6H2O (0.05 mol) in
acetonitrile (3 mL) was stirred at room temperature for 3 h
and monitored by TLC. Then the solvents of the resulting
mixture were removed by evaporation under vacuum,
CH2Cl2 (10 mL) was added to the reaction mixture and the
R
int =0.0192, final R indices [I>2s(I)] R1 =0.0246, wR2 =
0.0567; indices (all data), R1 =0.0258, wR2 =0.0572.
R
GOF=1.092; CCDC 916237 contains the supplementary
crystallographic data for this paper. These data can be ob-
2438
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ꢁ 2013 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
Adv. Synth. Catal. 2013, 355, 2430 – 2440

Synthesis and Structures of Air-Stable Binuclear Hafnocene Perfluorobutanesulfonate
catalyst was filtered for the next cycle of reaction. From the
filtrate, after evaporation of the solvent, a pinkish solid mix-
ture was obtained. The residue was purified by short column
chromatography eluted with ethyl acetate/petroleum ether
(petroleum ether:ethyl acetate=5:1) and a colorless oil
(11a) was obtained; isolated yield: 96%. Indoles, enones
and nitroalkene are commercially available.
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Typical Procedure for the Mannich Reaction
Catalyzed by 4·6H2O
A well-ground mixture of b-naphthol (1.0 mmol), aldehyde
(1.0 mmol), amide derivative (1.2 mmol) and 4·6H2O
(0.02 mmol) was heated to reflux in ethanol until the reac-
tion was completed as indicated by TLC. After that, the
mixture was subject to silica gel column chromatography;
the product (16a) was obtained; isolated yield: 97%. After
the completion of column chromatography, the upper part
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solvent (THF) for the next cycle of reaction. Aldehydes,
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Acknowledgements
This work was supported by the NSFC (21172061, 21273068,
21273067, 21003040), the NSF of Hunan Province (10JJ1003,
11JJ5009), and the Fundamental Research Funds for the Cen-
tral Universities. All authors thank Profs. J. Otera and A.
Orita of Okayama University of Science for helpful discus-
sion. R. Qiu thanks the Prof. N. Kambe and Dr. T. Iwsakai
of Osaka University for helpful discussion and JSPS for a fel-
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