Recyclable and selective Lewis acid catalysts for transesterification and direct esterification in a fluorous biphase system: Tin(IV) and hafnium(IV) bis(perfluorooctanesulfonyl)amide complexes

Article abstract of DOI:10.1016/j.tetlet.2003.11.035

Tin(IV) and hafnium(IV) bis(perfluorooctanesulfonyl)amide complexes were shown to give excellent yield and selectivity for highly practical transesterification and direct esterification, respectively, with an equimolar ratio of the reactants in a fluorous biphase system. It was found that these metal complexes were completely recovered and reused in the immobilized fluorous phase without loss of their catalytic activities.

Full text of DOI:10.1016/j.tetlet.2003.11.035

Tetrahedron
Letters
Tetrahedron Letters 45 (2004) 781–785
Recyclable and selective Lewis acid catalysts for transesterification
and direct esterification in a fluorous biphase system: tin(IV) and
hafnium(IV) bis(perfluorooctanesulfonyl)amide complexes
Xiuhua Hao, Akihiro Yoshida and Joji Nishikido*
The Noguchi Institute, 1-8-1 Kaga, Itabashi-ku, Tokyo 173-0003, Japan
Received 8 October 2003; revised 4 November 2003; accepted 7 November 2003
Abstract—Tin(IV) and hafnium(IV) bis(perfluorooctanesulfonyl)amide complexes were shown to give excellent yield and selectivity
for highly practical transesterification and direct esterification, respectively, with an equimolar ratio of the reactants in a fluorous
biphase system. It was found that these metal complexes were completely recovered and reused in the immobilized fluorous phase
without loss of their catalytic activities.
Ó 2003 Elsevier Ltd. All rights reserved.
4
It is well known that transesterification of ester/alcohol
and direct esterification of carboxylic acid/alcohol play
important roles in the production of organic esters,
especially for some important products or intermediates
mild reaction conditions. Fluorous biphase system
(FBS), as a phase-separation and catalyst immobiliza-
tion technique, was shown to be a potential technical
5
candidate. Our recent reports have shown that metal
1;2
in the chemical and pharmaceutical industries. How-
ever, either of the reactants in excess and/or the removal
of the formed alcohol (or water) from the products
are generally required to bias the innate equilibrium
complexes (e.g., Sn, Sc, and Yb) with bis(perflu-
orooctanesulfonyl)amide and tris(perfluorooctanesulfo-
nyl)methide ponytails are highly active and recyclable
catalysts in the fluorous immobilized phase for Baeyer–
Villiger oxidation, alcohol acylation, and Friedel–Crafts
acylation, which have been approved satisfactorily in a
7
bench-scale continuous-flow system. Here we will re-
port our successful approach to FBS for transesterifi-
cation and direct esterification in an equimolar ratio of
the reactants, under mild operating conditions without
recourse to any technique to remove the formed alcohol
3
process in favor of the product side (Eqs. 1 and 2).
6
1
2
2
1
RCO
2
R þ R OH ! RCO
2
R þ R OH
ð1Þ
ð2Þ
3
3
RCO
2
H þ R OH ! RCO
2
R þ H
2
O
(
or water).
In view of the rapidly increasing demands of green and
sustainable chemistry, the practical transesterification
and direct esterification should be improved at best to
meet the following requirements: (1) the ester (or car-
boxylic acid) and alcohol reactants to be mixed in the
ratio of 1:1; (2) no need of special technology to remove
the liberated alcohol (or water); (3) easy separation of
the catalyst and its recycled use for the next reaction; (4)
The catalytic activities of various catalysts (3 mol %)
were first investigated in FBS with a model reaction of
methyl butyrate (1 mmol) and n-octanol (1 mmol) under
the mild conditions of 80 °C for 15 h, as described in
Scheme 1. During the reaction period, the methanol
Cat. (3 mol%)
CO
2
CH
3
+
n-C H17OH
2 8 17
CO -n-C H
8
perfluorodecalin 1.0 mL
toluene 0.4 mL
80 oC, 15 h
1
mmol
1 mmol
Keywords: Lewis acid catalyst; Fluorous biphase system; Perfluori-
anted ligand; Transesterification; Direct esterification.
*
Corresponding author. Tel.: +81-3-5248-3596; fax: +81-3-5248-3597;
e-mail: nishikido.jb@om.asahi-kasei.co.jp
Scheme 1. Transesterification of methyl butyrate with n-octanol in
FBS.
0
040-4039/$ - see front matter Ó 2003 Elsevier Ltd. All rights reserved.
doi:10.1016/j.tetlet.2003.11.035

782
X. Hao et al. / Tetrahedron Letters 45 (2004) 781–785
Table 1. Transesterification of methyl butyrate with n-octanol in FBS
2 8 17 2 4
Table 2. Recycles of Sn[N(SO C F ) ] catalyst in transesterification
under the conditions in Scheme 1
of methyl butyrate with n-octanol under the conditions in Scheme 1
a
a
b
c
Entry
Catalyst
Yield (%)
Selectivity
b
Cycle
Yield (%)
Selectivity (%)
(
%)
1
2
3
4
5
89
88
87
84
84
99
97
97
97
96
c
1
2
3
4
5
6
7
8
9
Sn[N(SO
2
2
C
C
8
8
F
17
)
)
2
2
]
]
4
2
89 (88 )
99
50
96
96
89
87
86
87
85
48
53
0
SnCl
Sn[N(SO
Sn(OSO
Hf[N(SO
Hf(OSO
Sc[N(SO
Sc(OSO
Yb[N(SO
4
9
84
83
76
65
74
59
55
12
7
F
17
2
2
3
CF )
2
C
8
F
17
)
2
]
4
a
b
c
Cycles of the lower fluorous catalyst phase.
GC yield (internal standard: n-nonane).
Selectivity: mmol product/mmol converted methyl butyrate.
2
3
CF )
4
2
8 17 2 3
C F ) ]
2
CF
3
)
3
2
C
CF
8
F
17
3
)
2
]
3
10
11
12
Yb(OSO
â
2
3
)
2 8 17 2 4
The recyclable performance of Sn[N(SO C F ) ] was
also investigated (Table 2). The operation was quite sim-
ilar to those we have reported for Baeyer–Villiger oxida-
tion, that is, the catalyst was completely immobilized in
the fluorous phase for the transesterification and the flu-
orous solution was directly used in the subsequent reac-
Nafion SAC-13
None
0
6a
a
b
c
GC yield (internal standard: n-nonane).
Selectivity: mmol product/mmol converted methyl butyrate.
Isolated yield.
11
2 8 17 2 4
tion. As shown in Table 2, even after Sn[N(SO C F ) ]
ꢀ
catalyst was recycled five times, the GC yield was still
higher than 84%, which confirmed that there was not only
no loss of the catalyst amount but also no depression of
catalytic activity during its subsequent reuse.
by-product was not removed. As summarized in
Table 1, no product was obtained in the absence of the
catalyst (entry 12). On the other hand, all of the metal
II
IV
III
III
complexes (Sn , Hf , Sc , Yb ) with –N(SO C F )
17 2
2
8
8
ligand (entries 3, 5, 7, and 9) generally gave better yields
than those with –OSO CF ligand (entries 4, 6, 8, and
0), but there was no significant difference in the selec-
tivity except for Yb complex. These results can be
attributed to the presence of the strong electron-with-
drawing –N(SO C F ) ligand bearing a higher fluorine
load on the metal center. It can also be seen that almost
the same results were obtained with Sn(OSO CF and
Sn[N(SO C F ) ] in this FBS (entries 3 and 4), but
To
Sn[N(SO
explore
the
C F ) ] -catalyzed transesterification, this
generality
of
the
above
2
3
1
2
8 17 2 4
III
reaction system was also examined with some other
esters and n-octanol (Table 3). It was found that, either
with aliphatic or with aromatic esters, good yields were
obtained under the simple and mild operating condi-
tions, for example, at 80 °C for 15 h without any pres-
sure devices. This can be viewed as more practical than
2
8
17 2
9
2
3 2
)
2
8
17 2 2
4c;12
most of the reported catalytic systems
the 1:1 reactant ratio and technically easy handling.
on account of
Sn(OSO CF ) could not be simply and directly recycled
3 2
due to its distribution in the upper toluene phase (unlike
Sn[N(SO immobilized in the lower perfluoro-
2
2
8 17 2 2
C F ) ]
Furthermore, the fluorous catalytic phase was directly
reused in the next reaction by combining with another
mixture of reactants more than five times without loss of
catalytic activity for all of the reactions in Table 3
except the transesterification of methyl butyrate/cyclo-
hexanol (entry 6) due to its lower yield of 58%. Further
experiments are still under progress.
decalin phase and directly reused for the next reaction),
which could lie in the difference of their partition
coefficients (perfluorodecalin/toluene), (<0.1)/(>99.9) for
Sn(OSO CF ) and (>96.0)/(<4.0) for Sn[N(SO C F ) ]
13
2
3 2
2
8
17 2 2
determined by atomic emission spectrometry. As for
M[N(SO
(n ¼ 3, 4) complexes, the yield and
selectivity were in the order of Sn >Hf >Sc >Yb
entries 1, 5, 7, and 9). It also can be found that the yield
of 89% and ester selectivity of 99% were obtained with
2
8 17 2 n
C F ) ]
IV
IV
III
III
(
Table 3. Fluorous biphase transesterifications catalyzed by
10
2 8 17 2 4
Sn[N(SO C F ) ]
2 8 17 2 4 4
Sn[N(SO C F ) ] (entry 1), whereas SnCl gave quite
â
1
2
low results (entry 2). In addition, Nafion SAC-13 is a
well known strong Brønsted acid, but its catalytic
activity for transesterification was significantly low in
the present reaction system (entry 11).
Entry
RCO R
2
R OH
Yield
a
(%)
Selectivity
b
(%)
1
2
CO CH
2
3
n-C
n-C
8
8
H
17OH
17OH
89
99
2 3
CO CH
Ph
H
91
98
ꢀ
Typical transesterification procedure (Table 1, entry 1): A 10-mL test
tube equipped with a Teflon-coated magnetic stirring bar was
charged with methyl butyrate (102 mg, 1 mmol), n-octanol (130 mg,
CO
2
CH
3
n-C
8
H17OH
90
98
3
c
4
PhCO
PhCO
2
CH
3
n-C
n-C
8
8
H
17OH
17OH
91
90
98
98
1
2 8 17 2 4
mmol), Sn[N(SO C F ) ] (121 mg, 0.03 mmol), toluene (0.4 mL),
c
5
2
2 5
C H
H
and perfluorodecalin (1.0 mL). The tube was placed in an organic
synthesizer equipped with a magnetic stirrer and heated at 80 °C for
2 3
CO CH
6
OH
58
98
15 h. The reaction mixture was cooled to room temperature and then
toluene (0.6 mL) was added. After being stirred for 10 min, this
mixture was separated into two liquid phases within 10 s. The upper
toluene phase afforded pure n-octyl butyrate (176 mg, 0.88 mmol) by
silica gel chromatography, while the lower fluorous phase was used in
subsequent reactions.
ꢀ
Reaction conditions: see Scheme 1 and footnote .
a
GC yield (internal standard: n-nonane).
Selectivity: mmol product/mmol converted ester.
b
c
2 8 17 2 4
5 mol % Sn[N(SO C F ) ] for 24 h.

X. Hao et al. / Tetrahedron Letters 45 (2004) 781–785
783
Table 5. Recycles of Hf[N(SO
2
C
8
F
17
)
2
]
4
catalyst in direct esterification
Cat. (5 mol%)
AcOH
mmol
+
OH
OAc
of acetic acid with cyclohexanol under the conditions in Scheme 2
perfluoro(methylcyclohexane) 3.0 mL
1
,2-dichloroethane
3.0 mL
a
b
c
1
1 mmol
Cycle
Yield (%)
Selectivity (%)
o
5
0 C, 8 h
1
2
3
4
5
82
83
81
83
82
99
97
97
97
98
Scheme 2. Direct esterification of acetic acid with cyclohexanol in
FBS.
a
b
c
Cycles of the lower fluorous catalyst phase.
GC yield (internal standard: n-nonane).
Selectivity: mmol product/mmol converted acetic acid.
Table 4. Direct esterification of acetic acid with cyclohexanol in FBS
under the conditions in Scheme 2
a
Entry
Catalyst
Yield (%)
Selectivity
b
(
%)
c
In addition, the recyclabilty of Hf[N(SO
2
C
8
F
17
)
2
]
4
cat-
1
2
3
4
5
6
7
8
9
Hf[N(SO
2
C
CF
8
F
17
)
2
]
4
82 (81 )
99
88
61
60
98
45
89
77
45
19
0
15
alyst in the direct esterification of acetic acid with
cyclohexanol was investigated (Table 5). It was found
that there was almost no reduction of the catalytic
activity (yield: 81–83%) and no other by-products were
detected except for the desired ester formation (selec-
tivity: >97%).
Hf(OSO
2
3
)
4
70
22
29
78
16
73
14
28
8
HfCl
HfCl
4
4
Æ2THF
Sn[N(SO
2
C
8
F
17
)
2
]
4
SnCl
Zr[N(SO
ZrCl
4
2
C
8
F
17
)
2
]
4
4
Yb[N(SO
â
2
C
8
F
17
)
2
]
3
2 8 17 2 4
The effectiveness of Hf[N(SO C F ) ] for esterification
10
1
Nafion SAC-13
None
was examined with some substrates involving bulky
carboxylic acids and alcohols (Table 6). It can be seen
that cyclohexanecarboxylic acid (entries 2, 3, and 4)
gave better yield than benzoic acid (entries 5, 6, and 7),
when reacted with either n-butanol, benzyl alcohol, or
cyclohexanol. Meanwhile, benzyl alcohol (entries 3 and
1
0
a
b
c
GC yield (internal standard: n-nonane).
Selectivity: mmol product/mmol converted acetic acid.
Isolated yield.
6) was found to be more easily esterified than cyclo-
hexanol (entries 4 and 7, respectively), which may be
ascribed to cyclohexanol being liable to dehydration. In
addition, a,b-unsaturated methacrylic acid (entry 8) was
shown to be converted to the desired methyl metha-
crylate (a very important polymer material) in a good
yield (86%) and selectivity (98%). Furthermore, the
This catalytic FBS system was also evaluated with the
direct esterification of acetic acid/cyclohexanol under
the mild conditions of 50 °C for 8 h (Scheme 2). Com-
pared with the above-described transesterification of
methyl butyrate/n-octanol, Sn[N(SO C F ) ] (entry 5)
2 8 17 2 4
could no longer give the best yield, which was just
16
1
4
recyclable performance
of Hf[N(SO C F ) ] for
2 8 17 2 4
below Hf[N(SO C F ) ] (entry 1) as summarized in
2
8
17 2 4
ꢁ
IV
esterification of cyclohexanecarboxylic acid/n-butanol,
cyclohexanecarboxylic acid/benzyl alcohol, benzoic
acid/n-butanol, and methacrylic acid/methanol (entries
Table 4. With respect to Hf -based catalysts,
Hf[N(SO showed significantly higher activity
than Hf(OSO CF ) , HfCl , and HfCl Æ2THF (entries 1,
2
8 17 2 4
C F ) ]
2
3 4
4
4
2, 3, 5, and 8), was quite similar to the above acetic acid/
2
, 3, and 4), even though the latter two are well known
3b
cyclohexanol esterification (Table 5), that is, reused
more than five times without depression of the catalytic
activity.
efficient catalysts for direct esterification. In addition,
Zr[N(SO can also be thought to be an effective
catalyst, but ZrCl was much less active (entries 7 and
2
8 17 2 4
C F ) ]
4
8
). These results indicated that the unique complexation
of –N(SO ligand with metals (e.g., Hf, Sn, and
Zr) is highly significant for the present esterification
The transesterification and direct esterification catalyzed
by Sn[N(SO C F ) ] and Hf[N(SO C F ) ] catalysts
2
8 17 2
C F )
2
8
17 2 4
2
8
17 2 4
8
in FBS were proved to be highly efficient on the inves-
tigated substrates. These experimental results have great
potential for large-scale operation in terms of the fol-
lowing: (1) equimolar ratio of ester (or carboxylic acid)/
alcohol; (2) no requirement for removal of the formed
alcohol (or water); (3) complete immobilization of the
2 8 17 2 4
system. Accordingly, 5 mol % of Hf[N(SO C F ) ]
was considered to be the best catalyst in view of a yield
of 82% and excellent selectivity of 99%.
Sn[N(SO
fluorous phase and their reuse in subsequent reactions;
2 8 17 2 4 2 8 17 2 4
C F ) ] or Hf[N(SO C F ) ] catalyst in the
ꢁ
Typical esterification procedure (Table 4, entry 1): As the above-
described transesterification of methyl butyrate/n-octanol, acetic acid
(4) high yield and selectivity under mild conditions.
(
(
(
2 8 17 2 4
60 mg, 1 mmol), cyclohexanol (100 mg, 1 mmol), Hf[N(SO C F ) ]
205 mg, 0.05 mmol), and a mixture of perfluoro(methylcyclohexane)
3 mL) and 1,2-dichloroethane (3 mL) were added to a 20-mL test
tube equipped with a Teflon-coated magnetic stirring. The reactant
mixture was stirred continuously at 50 °C for 8 h and settled down for
Acknowledgements
10 s. Then this reacted mixture was turned into an upper 1,2-
dichoroethane phase for pure cyclohexyl acetate (115 mg, 0.81 mmol)
by silica gel chromatography and a lower fluorous phase for the
subsequent reaction.
This work was supported by New Energy and Industrial
Technology Development Organization (NEDO) and

784
X. Hao et al. / Tetrahedron Letters 45 (2004) 781–785
2 8 17 2 4
Table 6. Fluorous biphase esterifications catalyzed by Hf[N(SO C F ) ]
3
a
b
Entry
RCO
2
H
R OH
Conditions
Yield (%)
82
Selectivity (%)
1
AcOH
OH
98
5
0 °C, 8 h
7
0 °C, 15 h
2
3
4
CO
CO
CO
2
2
2
H
H
H
n-C
4
H
9
OH
OH
OH
92
89
55
97
98
98
5
0 °C, 24 h
PhCH
2
50 °C, 24 h
c
5
PhCO
PhCO
2
H
n-C
4
H
9
OH
OH
90 °C, 15 h
50 °C, 24 h
0 °C, 24 h
85
21
96
98
6
2
H
PhCH
2
5
7
PhCO
2
H
OH
12
86
98
98
d
8
CH
3
OH
60 °C, 8 h
2
CO H
ꢁ
Reaction conditions: see Scheme 2 and footnote .
a
GC yield (internal standard: n-nonane).
Selectivity: mmol product/mmol converted carboxylic acid.
b
c
In perfluorodecalin (3.0 mL)/1,2-dichloroethane (3 mL) biphase system.
5
d
mol equiv of methanol.
Japan Chemical Innovation Institute (JCII) through the
R&D program for Process Utilizing Multi-Phase Cata-
lytic Systems.
7. Yoshida, A.; Hao, X.; Nishikido, J. Green Chem. 2003, 5,
54–557.
8
5
. The catalytic activity was investigated in FBS for trans-
esterification of methyl butyrate/n-octanol with 12 mol %
HN(SO
anol with 20 mol % HN(SO
2
C
8
F
17
)
2
and esterification of acetic acid/cyclohex-
, respectively. The
2
8 17 2
C F )
obtained results from these reactions (55% yield and 73%
selectivity from the former transesterification; 65% yield
and 65% selectivity from the latter esterification) were
considerably lower than those with metal complexes (89%
yield and 99% selectivity from the former transesterifica-
tion with Sn[N(SO C F ) ] ; 82% yield and 99% selectiv-
References and notes
1
. (a) Otera, J. Esterification; Wiley-VCH: Weinheim, 2003;
b) Otera, J. Chem. Rev. 1993, 93, 1449–1470; (c) Greene,
(
2
8
17 2 4
T. W.; Wuts, P. G. M. Protective Groups in Organic
Synthesis. 3rd ed. John Wiley: New York, 1999; pp 269–
2 8 17 2 4
ity from the latter esterification with Hf[N(SO C F ) ] ).
Additionally, HN(SO C F ) was found to be difficult for
17 2
2
8
4
53.
. Larock, R. C. Comprehensive Organic Transformations.
nd ed. VCH: New York, 1999; pp 1932–1941.
efficient phase separation and recycle due to its weight loss
(>53%) in the lower fluorous phase and distribution in the
2
3
1
9
2
upper organic phase (confirmed by F NMR) after the
reaction was completed either with the former transeste-
rification or the latter esterification. These results verified
that the efficient and recyclable catalysis requires the
. (a) Trost, B. M. Science 1991, 254, 1471–1476; (b)
Ishihara, K.; Nakayama, M.; Ohara, S.; Yamamoto, H.
Tetrahedron 2002, 58, 8179–8188.
. (a) Wakasugi, K.; Misaki, T.; Yamada, K.; Tanabe, Y.
Tetrahedron Lett. 2000, 41, 5249–5252; (b) Xiang, J.;
Toyoshima, S.; Orita, A.; Otera, J. Angew. Chem., Int. Ed.
IV
IV
4
unique complexation of metal (Sn or Hf ) with –
N(SO C F ) .
2
8
17 2
9. (a) Nishikido, J.; Yamamoto, F.; Nakajima, H.; Mikami,
Y.; Matsumoto, Y.; Mikami, K. Synlett 1999, 10, 1990–
1992; (b) Koppel, I. A.; Taft, R. W.; Anvia, F.; Zhu, S.-Z.;
Hu, L.-Q.; Sung, K.-S.; DesMarteau, D. D.; Yagupolskii,
L. M.; Yagupolskii, Y. L.; Ignatꢂev, N. V.; Kondratenko,
N. V.; Volkonskii, A. Y.; Vlasov, V. M.; Notario, R.;
Maria, P.-C. J. Am. Chem. Soc. 1994, 116, 3047–3057.
10. Synthesis of Sn[N(SO C F ) ] : same as the description in
2001, 40, 3670–3672; (c) Xiang, J.; Orita, A.; Otera, J. Adv.
Synth. Catal. 2002, 344, 84–90; (d) Xiang, J.; Orita, A.;
Otera, J. Angew. Chem., Int. Ed. 2002, 41, 4117–4119; (e)
Gacem, B.; Jenner, G. Tetrahedron Lett. 2003, 44, 1391–
1394.
5
6
. Horv ꢀa th, I. T.; R ꢀa bai, J. Science 1994, 266, 72–75.
. (a) Hao, X.; Yamazaki, O.; Yoshida, A.; Nishikido, J.
Tetrahedron Lett. 2003, 44, 4977–4980; (b) Yamazaki, O.;
Hao, X.; Yoshida, A.; Nishikido, J. Tetrahedron Lett.
2
8
17 2 4
Ref. 6a.
11. The catalyst was recovered from the fluorous phase
without apparent weight loss, that is, 119 mg of catalyst
was recovered after the original 121 mg was recycled five
times. The recovered Sn[N(SO C F ) ] catalyst was
2
003, 44, 8791–8795; (c) Nishikido, J.; Kamishima, M.;
Matsuzawa, H.; Mikami, K. Tetrahedron 2002, 58, 8345–
349; (d) Mikami, K.; Mikami, Y.; Matsumoto, Y.;
8
2
8
17 2 4
Nishikido, J.; Yamamoto, F.; Nakajima, H. Tetrahedron
Lett. 2001, 42, 289–292.
characterized by elemental analysis: Calcd C, 19.03; Sn,
2.94; Found: C, 19.41; Sn, 2.92.

X. Hao et al. / Tetrahedron Letters 45 (2004) 781–785
785
1
2. Baumhof, P.; Mazitschek, R.; Giannis, A. Angew. Chem.,
Int. Ed. 2001, 40, 3672–3674.
3. Representative results in the subsequent four cycles:
methyl cyclohexanecarboxylate/n-octanol obtained yields
of 92%, 90%, 91%, and 92% (selectivity of 98%, 96%,
powder in 97% yield (5.96 g). Hf[N(SO
analysis: Calcd C, 18.75; Hf, 4.35; Found: C, 18.65; Hf,
4.33; F NMR d )126.2, )121.1, )114.2, )81.5.
15. The catalyst was recovered from the fluorous phase
without apparent weight loss, that is, 203 mg modified
catalyst was recovered after the original 205 mg was
2
C
8
F
17
)
2
]
4
elemental
1
9
1
1
00%, and 97%, respectively); ethyl benzoate/n-octanol
obtained yields of 92%, 90%, 93%, and 92% (selectivity of
8%, 98%, 98%, and 100%, respectively).
2 8 17 2 4
recycled five times. The recovered Hf[N(SO C F ) ]
catalyst was characterized by elemental analysis: Calcd
C, 18.75; Hf, 4.35; Found: C, 18.72; Hf, 4.33.
9
1
4. Hf[N(SO
added
2
C
HN(SO
8
F
17
)
2
]
4
: To anhydrous methanol (10 mL) was
(6.00 mmol) and HfCl
2
C
8
F
17
)
2
4
16. Representative results in the subsequent four cycles:
cyclohexanecarboxylic acid/n-butanol obtained yields of
92%, 93%, 93%, and 92% (selectivity of 98%, 98%, 98%,
and 100%, respectively); benzoic acid/n-butanol obtained
yields of 86%, 85%, 86%, and 87% (selectivity of 100%,
98%, 97%, and 97%, respectively).
(
1.50 mmol), which was stirred continuously at 50 °C for
5 h. After being cooled to the room temperature, the
reaction mixture was evaporated and dried at 80 °C/
.01 mmHg for 16 h to give hafnium(IV) bis(perfluoro-
octanesulfonyl)amide complex as a hygroscopic white
1
0