Synthesis and structure of an extremely air-stable binuclear hafnocene perfluorooctanesulfonate complex and its use in lewis acid-catalyzed reactions

Article abstract of DOI:10.1002/chem.200802727

An extremely air-stable μ²-hydroxy-bridged binuclear hafonocene perfluorooctanesulfoante complex was successfully synthesized. This complex showed high catalytic efficiency in the esterification of alcohols, phenol, thiol, and amines, in the Friedel-Crafts acylation of alylaryl ethers, in the Mukaiyama aldol reaction, and in the allylation of aldehydes and could be reused.

Full text of DOI:10.1002/chem.200802727

DOI: 10.1002/chem.200802727
Synthesis and Structure of an Extremely Air-Stable Binuclear Hafnocene
Perfluorooctanesulfonate Complex and Its Use in Lewis Acid-Catalyzed
Reactions
Renhua Qiu, Guoping Zhang, Yuyang Zhu, Xinhua Xu,* Lingling Shao, Yinhui Li,
Delie An,* and Shuangfeng Yin*[a]
Abstract: An extremely air-stable m2-hydroxy-bridged binuclear hafonocene per-
fluorooctanesulfoante complex was successfully synthesized. This complex showed
high catalytic efficiency in the esterification of alcohols, phenol, thiol, and amines,
in the Friedel–Crafts acylation of alylaryl ethers, in the Mukaiyama aldol reaction,
and in the allylation of aldehydes and could be reused.
Keywords: hafnocenes · homogene-
ous catalysis · Lewis acids · per-
fluorooctanesulfonate
Introduction
zation of THF.[6] Unfortunately, the compound was extreme-
ly unstable in air due to facile hydrolysis. From the stand-
point of the practical utilization of cationic hafnocene deriv-
atives as catalysts, it is highly desirable to lower the hygro-
scopic character while improving the Lewis acidity of the
substance.
Much attention has been paid to cationic hafnocene (Hf)
compounds in recent years[1] due to their application in
areas such as polymerization,[2] N2 activation,[3] glycosyla-
tion,[4] and metal complexation.[5] The most interesting of
these compounds is hafnocene dichloride ([Cp2HfCl2]; Cp=
cyclopentadiene; 2) and its derivatives because they are re-
garded as organometallic compounds that are somewhat
stable and catalytically highly reactive. However, [Cp2HfCl2]
has seldom been used as a Lewis acid in organic synthesis
on account of its weak acidity. To overcome this shortcom-
ing, AgClO4, a potentially explosive material, is added for
glycosylation[4d] and a cocatalyst is needed for polymeriza-
tion.[2a,b] In general, to enhance the activity, an organometal-
lic Lewis acid should be as strongly acidic as possible. The
dilemma is that with an increase in acidity, the compound
becomes more susceptible to hydrolysis. For example,
Angus-Dunne reported the incorporation of a triflate ion as
the counteranion to [Cp2HfCl2] for the generation of hafno-
Recently, Otera and co-workers postulated that the incor-
poration of long-chain perfluoroalkylsulfonate and perfluor-
oarylsulfonate groups to organometallic (i.e., Sn, Zr, and Ti)
cations could result in an enhancement of acidity and stabili-
ty.[7] These findings led us to envision that perfluorooctane-
sulfonate groups can help to overcome the hydrolytic insta-
bility of other cationic organometallic species. Herein, we
report the synthesis and characterization of the air-stable
cationic m2-hydroxy-bridged binuclear complex [{CpHf-
(m2-OH)2][OSO2C8F17]4·xH2O·yTHF
ACHUTNGREN(NUG OH2)3}2ACHTUNGTRENNGUN
(1·xH2O·yTHF) and an assessment of its Lewis acidity and
catalytic activity.[8]
cene bis
G
Results and Discussion
The synthesis of binuclear hafnocene perfluorooctanesulfo-
nates (1·xH2O·yTHF) is shown in Scheme 1. The treatment
[a] R. Qiu, G. Zhang, Y. Zhu, X. Xu, L. Shao, Y. Li, D. An, S. Yin
College of Chemistry and Chemical Engineering
Hunan University, Changsha, 410082 (China)
Fax : (+86)731-511-8161
E-mail: xhx1581@yahoo.com.cn
deliean@sina.com
sf_yin@hnu.cn
Supporting information for this article is available on the WWW
under http://dx.doi.org/10.1002/chem.200802727.
Scheme 1. Synthesis of 1·4H2O·2THF.
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FULL PAPER
of [Cp2HfCl2] with silver perfluorooctanesulfonate (AgO-
SO2C8F17; 2 equiv) in THF afforded the corresponding com-
plexes as hydrates. The water molecules in the complex ori-
ginated from air or the solvent, and the hydrate numbers x
and solvating ligands THF y varied according to the reaction
conditions. The results of 1H NMR spectroscopic (in dry
[D6]acetone) and elemental analyses proved that in the
freshly prepared sample after recrystallization x was approx-
imately equal to four and y equal to two. After exposure to
air for two days, the sample showed y=0 with the solvating
ligand THF replaced by water molecules and the water con-
tent increased to x=6. When 1·4H2O·2THF was kept for
one week at room temperature under vacuum (created
through pumping), there was partial dehydration of the
sample (x=0) and removal of the solvating ligand THF (y=
0). We found that 1·4H2O·2THF remained as dry crystals or
powder and suffered no color change over one year in air.
Therefore, this complex can be considered to be extremely
air stable at ambient environment and has a great advantage
over hafnocene triflates from an operational point of view.
The cationic structure of 1·4H2O·2THF in the solid state
was confirmed by X-ray diffraction analysis. Single crystals
of 1·4H2O·2THF could be obtained by diffusion of hexane
into a saturated solution of the complex in THF. An
ORTEP plot of the structure of the cation, a ball-and-stick
view of the crystal cell, and selected bonds and angles are
shown in Figure 1. The data shows that the hafnium atoms
have a distorted octahedral coordination with the Cp group
trans to the OH unit, such as in the case of [{CpHfCAHTUNGTRENUNG
[9]
AHCTUNGTERNNUNG .
(m2-OH)2]4+
ꢀ
2.086(4), 2.122(5), 2.151(4), 2.177(5), and 2.179(5) ꢁ, respec-
ꢀ
tively; the Hf C(Cp) lengths range from 2.477(7) to
2.512(7) ꢁ and the average length is 2.493 ꢁ. The two Cp
rings are parallel, the Hf-O1-HfA angle is 111.13(17)8, and
the Hf···HfA distance is 3.4952 ꢁ, thus suggesting that the
ꢀ
Hf···HfA interaction is very weak. The C8F17SO3 ions, the
dissociated H2O molecules, and solvating ligand THF are
packed around the complex cation in such a way that the
oxygen atoms of these species point towards the H2O li-
gands. The C8F17 side chains of the anions, on the other
hand, are clustered together to form the hydrophobic do-
mains.
The thermal behavior of complex 1 was investigated by
thermogravimetric–differential scanning calorimetry (TG–
DSC) in argon and oxygen (Figure 2). The TG–DSC curves
show three stages of weight loss. The endothermic step
below 1008C can be assigned to the removal of water mole-
cules. The material is stable up to about 3008C, after which
two overlapping weight losses of an exothermic nature
Figure 1. a) OTREP view of the novel cationic structure of [{CpHf-
A
2ACHTUNGTRENNUNG
(m2-OH)2]4+; b) and a ball-and-stick view of the crystal structure
ACHTUNGTRENNUNG(OH2)3}2ACHTUNGTRENNUNG
(m2-OH)2][OSO2C8F17]4·4H2O·2THF (1·4H2O·2THF).
The displacement of ellipsoids is drawn at the 30% probability level and
H atoms are shown as small spheres of arbitrary radii. Selected bonds
ꢀ
ꢀ
ꢀ
[ꢁ] and angles [8]: Hf O1 2.086(4), Hf O2 2.122(5), Hf O1A 2.151(4),
ꢀ
ꢀ
ꢀ
ꢀ
ꢀ
Hf O4 2.177(5), Hf O3 2.179(5), Hf C4 2.477(7), Hf C3 2.481(8), Hf
ꢀ
ꢀ
ꢀ
C2 2.496(8), Hf C5 2.498(7), Hf C1 2.512(7), Hf HfA 3.4952(5); O1-Hf-
O2 95.17(18), O1-Hf-O1A 68.87(17), O2-Hf-O1A 79.19(18), O1-Hf-O4
90.40(18), O2-Hf-O4 152.21(19), O1A-Hf-O4 77.51(17), O1-Hf-O3
149.61(18), O2-Hf-O3 82.3(2), O1A-Hf-O3 80.96(17), O4-Hf-O3 79.28
(19). The two Cp ring planes are parallel.
Figure 2. TG–DSC curves of complex 1.
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6489

S. Yin, X. Xu, D. An et al.
appear, plausibly due to the oxidation of organic entities. At
4008C, we observed the removal of pentafluorooctanesulfur-
yl ligands, thus leaving hafnium fluoride compounds. The re-
sults of the TG–DSC analysis are in agreement with those
ganic synthesis and extensively investigated by organic
chemists for nearly 100 years.[12] This reaction provides an
efficient and inexpensive means for protecting hydroxy and
amino groups in a multistep synthetic process. A number of
1
obtained from the H and 19F NMR spectroscopic investiga-
Lewis acids, such as Sc
ACHTUNGRTEN(NUNG OTf)3, trimethylsulfonate triflate
tions (see the Supporting Information). The empirical for-
mula of this complex can be written as 1·6H2O.
Based on the results from conductivity measurements
(Table 1), 1·4H2O·2THF can be partially dissociated into
ionic species in an anhydrous solution of CH3CN and com-
(TMSOTf), and Bi(OTf)3, are known to catalyze the acety-
ACHTUNGTRENNUNG
lation of alcohols with acetic anhydride.[13] Unfortunately,
most of the methods have one or more disadvantages, in-
cluding low yield, poor chemoselectivity, functional intoler-
ance, being environmental unfriendly, the need for organic
solvents as reaction media, and the need for strictly anhy-
drous reaction conditions. Most of the catalysts are unrecov-
erable, air or moisture sensitive, and too low or too large in
Lewis acidity.
Table 1. Conductivity
of
hafnocene
perfluorooctanesulfonate
(1·4H2O·2THF).[a]
Entry VCH CN/VH O Conductivity
Entry VCH CN/VH O Conductivity
3
2
3
2
[b]
[b]
[mScmꢀ1
ACHTUNGTERNNUNG ]
The esterification reaction was investigated with structur-
ally diverse alcohols, phenols, thiols, and amines with
1.2 equivalents of acetic anhydride in the presence of
1.0 mol% of 1·4H2O·2THF at room temperature under sol-
vent-free conditions (Table 3). We observed that fast and ef-
G
]
1
2
3
5:0
4:1
3:2
55.6 (55.6)[c]
192.6 (192.6)[d]
221 (221)[d]
4
5
6
2:3
1:4
0:5
327 (327)[d]
186.0 (186.0)[c]
13.78 (13.78)[e]
[a] The complex was freshly prepared and recrystallized from a mixture
of water and CH3CN (0.5 mmolLꢀ1) at 158C, and then left under vacuum
at room temperature for 2 h. [b] The value given in the parentheses is the
molar conductivity L [mScmꢀ1 molꢀ1]. [c] The sample was not dissolved
completely. [d] The sample was dissolved completely. [e] The sample was
not dissolved.
Table 3. Yields from the nucleophilic substitution reactions of acetic an-
hydride with alcohols catalyzed by 1·4H2O·2THF.[a]
pletely dissociated into ionic species in an aqueous solution
of CH3CN. The large molar conductivity value (L=
327 mScmꢀ1 molꢀ1; entry 4, Table 1) is consistent with the
complete ionization of 1·4H2O·2THF into a 1:4 electro-
lyte.[10] Notably, the complex has relatively strong acidity
with an acid strength of 3.3<Ho ꢁ4.8 (Ho =Hammett acidity
function).[11] In addition, this complex is highly soluble in
methanol (400 gLꢀ1; entry 1, Table 2) and in aqueous solu-
Entry
ROH
Time
AHCTUNGTERG[NNUN min]
Product
Yield [%]
1
2
3
4
PhCH2OH
PhCH2CH2CH2OH
PhCH(OH)CH2CH3
C8H17OH
PhOH
furanmethanol
geraniol
Ph3COH
PhSH
PhNH2
Ph2NH
G
5
30
30
5
120
5
5a
5b
5c
5d
5e
5 f
5g
5h
5i
99
99
97
99
94
98
99
<1
96
99
97
ACHTUNGTRENNUNG
ACHTUNGTRENNUNG
ACHTUNGTRENNUNG
5[b]
6
T
ACHTUNGTRENNUNG
7
E
5
8[b]
9[b]
10
11[b]
N
720
300
5
ACHTUNGTRENNUNG
T
5j
5k
Table 2. Solubility of 1·4H2O·2THF in organic solvents at 258C [gLꢀ1].[a]
A
60
Entry Solvent Pure[b] Mixture[c] Entry Solvent Pure[b] Mixture[c]
[a] Substrates: 4, 1.0 mmol; Ac2O, 1.2 mmol; catalyst: 0.01 mmol; room
temperature; yields are given for the isolated products. [b] Catalyst:
0.05 mmol.
1
2
3
4
5
MeOH 400
EtOAc 68
acetone 33
1783
169
6
7
8
9
CH3CN 11
972
0
0
CH2Cl2
0
0
0
1082
483
toluene[f]
hexane[f]
THF
Et2O
20
23
0
118[e]
10
fective acetylation occurred not only for alcohols (94–99%
yield; entries 1–4, Table 3) but also for amines (97–99%
yield; entries 10 and 11, Table 3). Phenol (94% yield;
entry 5, Table 3) and thiol (96% yield; entry 9, Table 3)
were also acetylated efficiently. Furthermore, in the cases of
furan, methanol, and geraniol with a carbon–carbon double
bond, 1·4H2O·2THF was found to be functionally tolerated
(98–99% yield; entries 6 and 7, Table 3). High chemoselec-
tivities were achieved, and tertiary alcohols, such as triphen-
yl methanol, were almost unaffected; only trace amounts of
the desired ester were detected.
The Friedel–Crafts acylation of aromatic compounds is
one of the most important reactions in organic chemistry.[14]
Traditionally, AlCl3 is used as a Lewis acid promoter, and a
stoichiometric amount of AlCl3 is needed due to the forma-
tion of stable adduct compounds that result from the inter-
[a] Complex 1·4H2O·2THF was freshly prepared, recrystallized, and left
under vacuum at room temperature for 2 h. [b] Pure organic solvent.
[c] Vorganic solvent/Vwater =4:1. [d] Vorganic solvent/Vwater =95:5. [e] Vorganic solvent
water =97:3. [f] The organic solvent and water did not mix with each
other.
/
V
tions of common polar organic solvents (118–1793 gLꢀ1; en-
tries 1–6, Table 2). The characterization of 1·4H2O·2THF
stimulated us to evaluate its performance as a Lewis acid
catalyst for reactions such as the esterification of alcohols,
phenols, thiols, and amines; Friedel–Crafts acylation of alky-
laryl ethers; the Mukaiyama aldol reaction; and the allyla-
tion of aldehydes.
The acetylation of alcohols, phenols, thiols, and amines
has been one of the most frequently used strategies in or-
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Chem. Eur. J. 2009, 15, 6488 – 6494

A Binuclear Hafnocene Perfluorooctanesulfonate Complex as a Lewis Acid
FULL PAPER
action between the catalyst and the carbonyl oxygen atom
of the ketone product. The compound Sc(OTf)3 was applied
in this reaction and a good result was obtained;[14b] however,
the high price of Sc(OTf)3 limited its utilization. Combina-
tions of Hf(OTf)4/LiClO4 and NbCl5/AgClO4 were also re-
rivatives,[17] because new stereogenic centers and new
carbon–carbon bonds can be formed in a single operation.
Several efficient Lewis acid catalysts based on boron, titani-
um, zirconium, copper, and so forth have been repor-
ted.[17b,18] In most cases, temperatures of ꢀ20 to ꢀ788C and
strictly anhydrous conditions are required. In the present in-
vestigation, we address these problems by preparing the ex-
tremely air-stable hafnocene perfluorooctanesulfonate cata-
lyst 1·4H2O·2THF and report the highly efficient catalytic
activity of the catalyst in Mukaiyama aldol reactions and the
allylation of aldehydes.
The reactions of aldehydes 8a–h with nucleophiles, such
as tetraallyltin (9), ketene silyl acetals 10, and enol silyl
ethers 11 were examined over 1·4H2O·2THF (Table 5). As
expected, the reactions resulted in good yields of homoallyl
alcohols 12a–h, b-hydroxy esters 13a–h, and b-hydroxy ke-
tones 14a–h (89–95, 75–94, and 65–90%, respectively) in
methanol or diethyl ether. A similar tendency in the catalyt-
ic activity of complex 1 was observed when aliphatic alde-
hydes and aromatic aldehydes with electron-donating or
-withdrawing groups were investigated.
Because of the high tolerance of 1·4H2O·2THF towards
hydrolysis, the solvents adopted in these reactions were used
as received and not subjected to any kind of drying proce-
dure. Usually a highly reactive reagent would cause poor
chemoselectivity. For example, Grignard, lithium, and titani-
um reagents often fail to discriminate between aldehydes
and ketones. Over 1·4H2O·2THF, however, we observed
good chemoselectivity. Ketones were less reactive: the addi-
tion reaction of acetophenone (8i) with nucleophiles 9–11
did not occur at room temperature, but the corresponding
homoallyl alcohol 12i was obtained in 85% yield at 658C
over 24 h in the presence of 1·4H2O·2THF.
AHCTUNGTRENNUNG
AHCTUNGTRENNUNG
ACHTUNGTRENNUNG
ported as Friedel–Crafts acylation catalysts.[14c,d] However,
larger equivalents of potentially explosive perchloride salts
have to be used as cocatalysts. Furthermore, most of these
Lewis acids are air and/or moisture sensitive.
We assessed 1·4H2O·2THF (5.0 mol%) as a catalyst for
the Friedel–Crafts acylation of structurally diverse alkyl aryl
ethers with 2.0 equivalents of acetic anhydride at room tem-
perature in CH3CN and good-to-excellent yields were ob-
tained (81–87%; entries 1–5, Table 4). The electron-donat-
Table 4. Yield from the Friedel–Crafts acylation of alkyl aryl ethers cata-
lyzed by 1·4H2O·2THF.[a]
Entry
R1
H
R2
Time [h]
Product
Yield [%]
1
2
3
4
5
CH3
U
8
12
24
6
7a
7b
7c
7d
7e
81
83
84
85
87
n-C4H9
benzyl
CH3
ACHTUNGTRENNUNG
AHCTUNGTRENNUNG
CH3
ACHTUNGTRENNUNG
n-C4H9
E
10
[a] Substrates: 6, 1.0 mmol; Ac2O, 2.0 mmol; catalyst: 0.05 mmol; solvent:
CH3CN, 2.0 mL; room temperature; yields are given for the isolated
para-substituted product.
ing groups attached to the aromatic ring enhanced the reac-
tion activity (85–87%; entries 4 and 5, Table 4). As expect-
ed, this reaction showed high
regioselectivity (para-regioiso-
mer: >99%). In addition, the
activity of AlCl3 was markedly
low under the present mild re-
action conditions. No product
was obtained in the absence of
the catalyst. Thus, complex 1
can be considered to be an ex-
cellent catalyst in terms of yield
and para-regioselectivity in the
Friedel–Crafts acylation.
The allylation of aldehydes
and the Mukaiyama aldol reac-
tion mediated by a Lewis acid
are one of the most convenient
processes for the formation of
carbon–carbon bonds in organic
synthesis.[15] The addition reac-
tions are intrinsically efficient
in the production of useful
building blocks, such as homo-
allylic alcohols[16] and b-hydroxy
Table 5. Product yield for the reactions of aldehydes 8a–h with nucleophiles 9–11 catalyzed by 1·4H2O·2THF.
Entry
RCHO
Yield [%]
12[a]
13[b]
14[c]
1
2
3
4
5
6
7
8
PhCHO (8a)
12a
12b
12c
12d
12e
12 f
12g
12h
92
91
89
93
95
92
94
90
13a
89
87
86
92
94
85
81
75
14a
14b
14c
14d
14e
14 f
14g
14h
78
75
69
83
90
80
68
65
p-MeC6H4CHO (8b)
p-MeOC6H4CHO (8c)
p-ClC6H4CHO (8d)
p-CF3C6H4CHO (8e)
PhCH=CHCHO (8 f)
PhCH2CH2CHO (8g)
C7H15CHO (8h)
13b
13c
13d
13e
13 f
13g
13h
[a] Substrates: 8, 1.0 mmol; 9, 0.3 mmol; catalyst: 0.01 mmol; solvent: CH3OH, 3.0 mL; RT, 6 h; yields are
given for the isolated products. [b] Substrates: 8, 1.0 mmol; 10, 1.2 mmol; catalyst: 0.05 mmol; solvent: Et2O,
3.0 mL; 08C!RT, 6 h; yields are given for the isolated products. [c] Substrates: 8, 1.0 mmol; 11, 1.5 mmol; cat-
ketone or b-hydroxy ester de- alyst: 0.05 mmol; solvent: Et2O, 3.0 mL; 08C!RT, 24 h; yields are given for the isolated products.
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S. Yin, X. Xu, D. An et al.
Table 7. Yields for the nucleophilic addition of benzaldehyde 8a with nu-
cleophiles 9–11[a] catalyzed by recovered catalyst 1·4H2O·2THF.
To show the advantages of hafnocene perfluorooctanesul-
fonate (1·4H2O·2THF) over [Cp2HfCl2] (2) and [Cp2Hf-
ACHTUNGTRENNUNG(OSO2CF3)2] (3), the catalytic activities of 1·4H2O·2THF
and 2 and 3, as control cationic species, were assessed in the
acetylation of 2-phenylethanol and anisole and in the allyla-
tion and Mukaiyama aldol reactions of benzaldehyde (see
Table 6 for the yields). High yields were constantly attained
Cycle
number [%][b] [%][c]
8a+9!12a
Yield Catalyst
Yield Catalyst
[%][b] [%][c]
8a+10!13a
Yield Catalyst
[%][b [%][c]
8a+11!14a
1
2
3
4
5
92
93
91
90
91
99
89
88
87
88
86
99
98
99
97
98
78
77
76
74
76
99
97
97
95
97
99
98
97
97
Table 6. Yield [%] for the reactions of carbonyl compounds with nucleo-
[a] Substrates: 8a, 10 mmol; 9, 0.3 equiv (or 10, 1.2 equiv or 11,
1.5 equiv); catalyst loading: 0.05 equiv; RT. [b] Yield of the desired iso-
lated product. [c] Yield of the recovered isolated catalyst.
philes catalyzed by 1·4H2O·2THF, 2, and 3.
Entry
RC(O)R’
Nu-R’’
Product
Catalyst
(1)[f]
2
3
1[a]
2[b]
3[c]
4[d]
5[e]
Ac2O
Ac2O
8a
8a
8a
4a
6a
9
10
11
5a
7a
12a
13a
14a
99
81
92
89
78
25
14
39
25
13
99
71[g]
90
acetic anhydride and aldehydes, over solid 1·4H2O·2THF in
a solution of an organic solvent is postulated (Scheme 2).
When solid 1·4H2O·2THF is added to the reaction solution,
75
53
[a] Substrates: 4a, 1.0 mmol; Ac2O, 1.2 mmol; catalyst: 0.01 mmol; RT,
5 min; yields are given for the isolated products. [b] Substrates: 6a,
1.0 mmol; solvent: Ac2O, 2.0 mmol; catalyst: 0.05 mmol; solvent:
CH3CN, 2 mL; RT, 8 h; yields are given for the isolated para-substitute
products. [c] Substrates: 8a, 1.0 mmol; 9, 0.3 mmol; catalyst: 0.01 mmol;
solvent: CH3OH, 3.0 mL; RT, 6 h; yields are given for the isolated prod-
ucts. [d] Substrates: 8a, 1.0 mmol; 10, 1.2 mmol; catalyst: 0.05 mmol;
Et2O, 3.0 mL; 08C!RT, 6 h; yields are given for the isolated products.
[e] Substrates: 8a, 1.0 mmol; 11, 1.5 mmol; catalyst: 0.05 mmol; solvent:
Et2O, 3.0 mL; 08C!RT, 24 h; yields are given for the isolated products.
[f] (1)=1·4H2O·2THF. [g] Mixture of ortho- and para-isomer products.
over 1·4H2O·2THF (up to 99%), whereas the other cata-
lysts resulted in much lower yields (down to 13%), plausibly
due to their lower Lewis acidity and/or greater air and mois-
ture sensitivity. However, CF3SO3H, considered to be a pow-
erful catalyst by Hollis and Bosnich, can be formed through
the facile hydrolysis of 3,[19] and an enhancement of catalytic
activity results. It is worth pointing out that only a 35%
yield of homoallyl alcohol 12a and no desired aldoate prod-
ucts 13a and 14a were obtained in the absence of a catalyst
under similar conditions, thus explicitly indicating the effec-
tiveness of the catalysts. With regards to metal sulfonate cat-
alysis, it must be taken into account that a sulfonic acid that
arises in situ during the reaction may possibly work as an
active species.[20] Thus, the reaction of 8a with 9, 10, or 11
could be conducted in the presence of C8F17SO3H
(5 mol%). The reaction was rather complex because many
spots were detected in TLC analysis and only 17–19% yield
of the desired products was reported.[7c]
To test the reusability of the catalyst and reproducibility
of the catalytic performance, 1·4H2O·2THF was subjected
to cycles of addition reactions of benzaldehyde with nucleo-
philes 9–11, respectively (Table 7). It was detected that the
change in product yield (i.e., 12a: 91–93%; 13a: 86–89%;
14a: 74–78%) was minimal in a trial of five cycles, thus indi-
cating that the catalyst is stable and suitable for reuse.
Therefore, 1·4H2O·2THF has the advantage of being high in
activity, selectivity, stability, and reusability.
Scheme 2. A proposed mechanism of reaction of carbonyl compounds
with various nucleophiles catalyzed by 1·4H2O·2THF.
the hafnium cationic groups dissociate and hydration occurs
immediately. At this stage, there are frequent intra- and in-
termolecular exchanges between water and the organic sol-
vent. The carbonyl compound coordinates with the metal
cations and is activated. Then nucleophiles such as alcohols
and alkyl aryl ethers attack the activated carbonyl com-
pounds (i.e., acetic anhydride) to produce the desired ester
and para-alkoxylarylketone derivatives, whereas nucleo-
philes such as tetraallyltin (9), ketene silyl acetals 10, and
enol silyl ethers 11 attack the activated carbonyl compounds
(aldehydes or ketones) to produce the corresponding homo-
allyl alcohols and b-hydroxy ester or b-hydroxy ketone de-
rivatives. The catalyst 1·4H2O·2THF is renewed with the
intake of one molecule of water or solvating ligand. At the
present stage of investigation, we are still not sure of the
transient intermediates that are possibly formed during the
reaction. Further work is still being conducted to understand
this aspect.
Conclusions
In summary, we have successfully synthesized and character-
ized a cationic m2-hydroxy-bridged binuclear hafnocene per-
fluorooctanesulfonate complex. This Lewis acid is strongly
According to the findings reported so far, the mechanism
of the nucleophilic reaction of carbonyl compounds, such as
6492
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Chem. Eur. J. 2009, 15, 6488 – 6494

A Binuclear Hafnocene Perfluorooctanesulfonate Complex as a Lewis Acid
FULL PAPER
lected/unique, 12065/8352, Rint =0.05791; final R indices [I>2s(I)], R1=
0.0463, wR2=0.1164; R indices (all data), R1=0.0521, wR2=0.1226;
GOF=1.019. CCDC-660034 (1·4H2O·2THF) 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.
acidic and is extremely air stable. It shows high catalytic ac-
tivity in the esterification of alcohols, phenol, thiol, and
amines; the Friedel–Crafts acylation of alkylaryl ethers; the
Mukaiyama aldol reaction; and the allylation of aldehydes
and could be reused. On account of its stability and storabil-
ity, the compound should find broad catalytic applications in
organic synthesis.
Typical procedure for the esterification of 2-phenylethanol (4a) with
acetic anhydride catalyzed by 1·4H2O·2THF: 2-Phenylethanol (4a;
122 mg, 1.0 mmol), acetic anhydride (122 mg, 1.2 mmol), and a catalytic
amount of 1·4H2O·2THF (14 mg, 0.01 mmol, 1.0 mol% based on the al-
cohol) were added to a 50-mL round-bottomed flask. The reaction mix-
ture was stirred at room temperature for 5 min and was monitored with
TLC analysis until the alcohol was consumed completely. The residue
was diluted with petroleum ether. The catalyst was precipitated and col-
lected by filtration. The filtrate was evaporated under vacuum and the
residue was subject to column chromatography on silica gel. A colorless
liquid (5a) was obtained (160 mg, 99% yield of the isolated product).
Experimental Section
General: The chemicals used were purchased from Aldrich. Co. Ltd and
other chemical companies and were used as received unless otherwise
specified. The preparation of the catalyst was carried out in a nitrogen at-
mosphere with freshly distilled solvents unless otherwise noted. THF was
distilled from sodium/benzophenone and CH3CN was distilled from
CaH2. Acetone was heated to reflux with KMnO4 for 4 h, distilled, dried
with K2CO3, distilled, and kept inside a dry box. The catalytic reactions
were carried out in air, and the solvents are used as received. The NMR
spectra were recorded at 258C on INOVA-400M (USA) calibrated with
tetramethylsilane (TMS) as an internal reference. Elemental analyses
were performed over VARIO EL III (Germany). Conductivity was mea-
sured on a REX conductivity meter DDS-307 (China). The IR spectra
were recorded on a NICOLET 6700 FTR spectrophotometer (Thermo
Typical procedure for the Friedel–Crafts acylation of anisole (6a) with
acetic anhydride catalyzed by 1·4H2O·2THF: Anisole (6a; 108 mg,
1.0 mmol), a solution of 1·4H2O·2THF (70 mg, 0.05 mmol) in CH3CN
(2.0 mL), and acetic anhydride (204 mg, 2.0 mmol) were added to a 50-
mL round-bottomed flask by a syringe. The reaction mixture was stirred
at room temperature for 8 h. The mixture was subjected to column chro-
matography on silica gel. The Friedel–Crafts acylation product (7a) was
obtained (121 mg, 81% yield of the isolated product).
Typical procedure for the allylation of benzaldehyde (8a) with tetraallyl-
tin (9) catalyzed by 1·4H2O·26THF: Complex 1·4H2O·2THF (70 mg,
0.05 mmol) was added to benzaldehyde (8a; 106 mg, 1.0 mmol) in metha-
nol (3.0 mL). Tetraallyltin (9) (85 mg, 0.3 mmol) was added to the reac-
tion mixture at room temperature and stirred for 6 h with monitoring by
TLC analysis. The reaction mixture was evaporated in vacuum at room
temperature and hexane (3ꢂ10 mL) was added to the residue. The cata-
lyst precipitated and was recovered by filtration for the next reaction
cycle. The combined hexane portions were concentrated, MeOH and
HCl (aq) were added, and the resulting mixture stirred for 15 min.
NaHCO3 (aq) was added for neutralization and the mixture was evapo-
rated. The as-obtained solids were dissolved in AcOEt and water, ex-
tracted with AcOEt (3ꢂ10 mL), and the organic layer was washed with
NaCl(aq) and dried over MgSO4. After evaporation, the yield was mea-
sured by GLC. Otherwise, the residue was subjected to column chroma-
tography on silica gel (petroleum ether/ethyl acetate=8:1). A colorless
oil (12a) was obtained (136 mg, 92% yield of isolated product).
Electron Corporation). The TG–DSC analysis was performed on
a
NETZSCH-STA-449C machine (operating conditions: O2 or Ar atmos-
phere; heating rate: 58Cminꢀ1). X-ray single-crystal diffraction analysis
was performed on SMART-APEX equipment at the Shanghai Institute
of Organic Chemistry (China Academy of Science). The acidity of the
catalyst was measured by using the Hammett indicator method. The indi-
cators employed included crystal violet, dimethyl yellow, and methyl red
(pKa =0.8, 3.3, and 4.8, respectively), as described previously.[11] The
strength of the acid was expressed by the Hammett acidity function (H0),
which was scaled by the pKa value of the indicators. The hydrate molecu-
lar structure and solubility were determined according to previous re-
ports.[7b]
[7b]
Preparation of 1·4H2O·2THF: A solution of AgOSO2C8F17
2.0 mmol) in THF (10 mL) was added to
(1.214 g,
a
solution of [Cp2HfCl2]
(0.379 g, 1.0 mmol) in THF (20 mL). After the mixture was stirred at
258C for 2 h in the absence of light, it was filtered. The filtrate was
placed in a small jar, which was put into a larger jar containing dry
hexane (40 mL). The larger jar was sealed and refrigerated for 24 h. Col-
orless crystals of complex 1·4H2O·2THF were obtained (1.222 g, 86%
yield of the isolated product). Recrystallization of this complex in THF/
hexane produced good crystals suitable for X-ray analysis. Moreover, the
treatment of 1·4H2O·2THF under vacuum for 1 week and exposure to
Typical procedure for the Mukaiyama aldol reaction of benzaldehyde
(8a) with ketene silyl acetal 10 catalyzed by 1·4H2O·2THF: Complex
1·4H2O·2THF (70 mg, 0.05 mmol) and (1-methoxy-2-methylprop-1-eny-
loxy)trimethylsilane (10; 209 mg, 1.2 mmol) were added to benzaldehyde
(8a; 106 mg, 1.0 mmol) in diethyl ether (3.0 mL) at 08C. The temperature
of the reaction mixture was slowly raised to room temperature and
stirred for 6 h with monitoring by TLC analysis. The reaction mixture
was evaporated in vacuum at room temperature and the residue was dis-
solved in hexane (3ꢂ10 mL). The catalyst was collected by means of fil-
tration for the next cycle of the reaction. MeOH and HCl (aq) were
added to the combined hexane portions, and the mixture was stirred for
15 min. NaHCO3 (aq) was added for neutralization and the reaction mix-
ture was evaporated. The solids thus obtained were dissolved in AcOEt
and water, extracted with AcOEt (3ꢂ10 mL), and the organic layer was
washed with NaCl (aq) and dried over MgSO4. After evaporation, the
yield was measured by GLC. Otherwise, the residue was subjected to
column chromatography on silica gel (petroleum ether/ethyl acetate=
10:1). Colorless crystals (13a) were obtained (185 mg, 89% yield of iso-
lated product).
air for 2 days yielded complexes
1
and 1·6H2O, respectively.
1
1·4H2O·2THF: H NMR (400 MHz, [D6]acetone, 258C, TMS): d=6.81 (s,
3H; Cp), 6.62 (s, 1H; Cp), 6.55 (s, 1H; Cp), 3.61 to 3.64 (m, 4H; THF),
3.39 (s, 10H; H2O), 1.78–1.85 ppm (m, 4H; THF); 19F NMR (376 MHz,
[D6]acetone, 258C): d=ꢀ121.12 to ꢀ121.21 (m, 2F; -CF2-), ꢀ117.69 (s,
2F; -CF2-), ꢀ116.54 to ꢀ116.85 (d, 6F; -(CF2)3-), ꢀ115.46 (s, 2F; -CF2-),
ꢀ109.40 (s, 2F; -CF2-), ꢀ76.07 to ꢀ76.12 ppm (m, 3F; CF3-); IR (KBr):
n˜ =3566, 3520, 3450, 3402, 3388, 2362, 1647, 1370, 1251, 1150, 107, 1040,
942, 846, 659, 625, 530, 477, 428 cmꢀ1; elemental analysis calcd (%) for
C50H48F68Hf2O26S4: C 21.13, H 1.70; found: C 21.19, H 1.72. Complex 1:
1H NMR (400 MHz, [D6]acetone, 258C, TMS): d=6.80 (s, 3H; Cp), 6.61
(s, 1H; Cp), 6.55 (s, 1H; Cp), 3.42 ppm (s, 6H; H2O); elemental analysis
calcd (%) for C42H24F68Hf2O20S4: C 19.21, H 0.92; found: C 19.25, H 0.92.
1
Typical procedure for the Mukaiyama aldol reaction of benzaldehyde
(8a) with enol silyl ether 11 catalyzed by 1·4H2O·2THF: Complex
1·4H2O·2THF (70 mg, 0.05 mmol) and (1-methoxy-2-methylprop-1-enyl-
oxy)trimethylsilane (11; 230 mg, 1.2 mmol) were added to benzaldehyde
(8a; 106 mg, 1.0 mmol) in diethyl ether (3.0 mL) at 08C. The temperature
of the reaction mixture was raised slowly to room temperature and
stirred for 24 h with monitoring by TLC analysis. The reaction mixture
Complex 1·6H2O: H NMR (400 MHz, [D6]acetone, 258C, TMS): d=6.81
(s, 3H; Cp), 6.62 (s, 1H; Cp), 6.55 (s, 1H; Cp), 3.42 ppm (s, 12H; H2O);
elemental analysis calcd (%) for C42H36F68Hf2O26S4: C 18.45, H 1.33;
found: C 18.48, H 1.33. Crystal data for 1·4H2O·2THF: C25H24F34HfO13S2;
Mr =1421.05; triclinic; space group: P-1; a=10.4394(10), b=11.0730(10),
c=20.2914 (19) ꢁ; a=97.945(2), b=98.005(2), g=97.246(2)8; V=
2275.0(4) ꢁ3; T=293(2) K; Z=2; Dcalcd =2.074 Mgmꢀ3; reflections col-
Chem. Eur. J. 2009, 15, 6488 – 6494
ꢀ 2009 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
www.chemeurj.org
6493

S. Yin, X. Xu, D. An et al.
was evaporated in vacuum at room temperature and the residue was dis-
solved in hexane (3ꢂ10 mL). The catalyst was collected by filtration for
the next cycle of the reaction. MeOH and HCl (aq) were added to the
combined hexane portions and the reaction mixture was stirred for
15 min. NaHCO3 (aq) was added for neutralization and the mixture was
evaporated. The solids thus obtained were dissolved in AcOEt and
water, extracted with AcOEt (3ꢂ10 mL), and the organic layer was
washed with NaCl (aq) and dried over MgSO4. After evaporation, the
yield was measured by GLC. Otherwise, the residue was subjected to
column chromatography on silica gel (petroleum ether/ethyl acetate=
8:1). Colorless crystals (14a) were obtained (176 mg, 78% yield of isolat-
ed product).
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Acknowledgements
This work was supported by the National Natural Science Foundation of
China (Grant No. 20372020, 20507005, and 20672033). The authors would
like to thank Professors Junzo Otera and Akihiro Orita (Okayama Uni-
versity of Science), and C. T. Au (Adjunct Professor of HNU) of the
Hong Kong Baptist University for valuable advice.
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Received: December 25, 2008
Published online: May 21, 2009
6494
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ꢀ 2009 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
Chem. Eur. J. 2009, 15, 6488 – 6494