Ytterbium perfluorooctanesulfonate catalyzed synthesis of acylals from aldehydes and acetic anhydride in a fluorous biphasic system

Article abstract of DOI:10.1139/V06-162

Ytterbium perfluorooctanesulfonate [Yb(OPf)₃] catalyses the highly efficient synthesis of acylals from aldehydes and acetic anhydride in a fluorous biphase system (FBS) composed of toluene and perfluorodecalin. The fluorous phase containing the fluorous catalyst is easily separated and can be reused several times without significant loss of catalytic activity.

Full text of DOI:10.1139/V06-162

1
563
Ytterbium perfluorooctanesulfonate catalyzed
synthesis of acylals from aldehydes and acetic
anhydride in a fluorous biphasic system
Wen-Bin Yi, Ya-Qing Yin, and Chun Cai
Abstract: Ytterbium perfluorooctanesulfonate [Yb(OPf) ] catalyses the highly efficient synthesis of acylals from alde-
3
hydes and acetic anhydride in a fluorous biphase system (FBS) composed of toluene and perfluorodecalin. The fluorous
phase containing the fluorous catalyst is easily separated and can be reused several times without significant loss of
catalytic activity.
Key words: acylals, ytterbium perfluorooctanesulfonate, fluorous biphasic catalysis.
Résumé : Le perfluorooctanesulfonate d’ytterbium [Yb(OPf) ] catalyse d’une façon très efficace la synthèse d’acylals à
3
partir d’aldéhydes et d’anhydride acétique dans un système fluoré biphasique (SFB) formé de toluène et de perfluoro-
décaline. La phase fluorée qui contient le catalyseur fluoré se sépare facilement et elle peut être réutilisée plusieurs
fois sans perte significative de son activité catalytique.
Mots clés : acylals, perfluorooctanesulfonate d’ytterbium, catalyseur fluoré biphasique.
[
Traduit par la Rédaction] Yi et al. 1566
Introduction
acylation (13) and alkylation (6), aldol condensation (8), ni-
tration (14), Mannich reaction (15), and Diels–Alder reac-
tions (10). The catalyst immobilized in the fluorous phase
was recovered by simple phase separation and reused di-
rectly for the next reaction without a significant loss of cata-
lytic activity.
Recently, transition metal-catalyzed reactions in
a
fluorous biphasic system (FBS) have become among the
most important methods for facile catalyst separation from
the reaction mixture and recycling of the catalyst (1–3). In
this catalytic system, the metal catalyst coordinated by
perfluoroalkylated ligands can dissolve into the fluorous
phase containing the product after the reaction (1). Further-
more, FBS has the advantage of possible use for water-
sensitive reactions unlike aqueous biphase systems (4, 5). In
our continued interest in the utility of lanthanide perfluoro-
octanesulfonates-catalyzed reactions in fluorous media (6–
The acylals were prepared from aldehydes and acetic an-
hydride employing fluorous phase technology. The reaction
of benzaldehyde with acetic anhydride in a FBS was exam-
ined with various Lewis acid catalysts (Scheme 1) and the
results are summarized in Table 1. We found that the
Yb(OPf) and Sc(OPf) complexes had similar activity for
3
3
the reaction with a quantitive conversion to acylal product in
the presence of 0.3 mol% catalyst at 80 °C over 2 h, while
other lanthanide perflates such as Eu(OPf) , Tm(OPf) , or
8
), we report herein our newest results on a successful
approach to FBS, i.e., perfluorodecalin–toluene system for
the synthesis of 1,1-diacetates (acylals), efficient protecting
groups for aldehydes (9) catalyzed by a ytterbium perfluoro-
3
3
La(OPf) , yielded desired product in 62%, 51%, or 39%, re-
3
spectively. This could be ascribed to the higher Lewis acid-
ity of Yb(OPf) and Sc(OPf) relative to other lanthanide
octanesulfonate [Yb(OPf) ] complex.
Rare earth (III) perfluorooctanesulfonates (RE(OPf) , RE =
Sc,Y, La, Lu), a novel series of Lewis acids, have been of
special interest because of their characteristic features such
as low hygroscopicity, ease of handling, robustness for recy-
cling, and high solubility in fluorous solvent (10–12). A cat-
3
3
3
3
perflates (10). Heptadecafluorooctanesulfonic acid (C F SO H,
8
17
3
PfOH) can itself promote the reaction, but it was found to be
less effective than RE(OPf) . When the reaction was fin-
3
ished, the reaction mixture was cooled to RT. The fluorous
phase with RE(OPf) catalysts can separate from the organic
alytic amount of RE(OPf) successfully promotes several
synthetically useful reactions such as Friedel–Crafts
3
3
layer and return to the bottom layer. In addition, based on
1
9
F NMR, UV–vis, and GC-MS data, no loss of catalyst or
fluorous solvent to the organic phase was detected and no
organic compounds were found in the fluorous phase. There
were no significant differences between the UV–vis spectra
of the initial and recovered fluorous phases, which may indi-
cate that the fluorous lanthanide catalyst is recovered unal-
tered after the reaction process has taken place. RE(OPf)3
was completely (>99%) recovered, as determined by atomic
emission spectrometry (ICP) and was then reused (Table 1,
entries 3 and 4). The separated fluorous phase could be re-
Received 3 September 2006. Accepted 16 October 2006.
Published on the NRC Research Press Web site at
http://canjchem.nrc.ca on 23 November 2006.
1
W.-B. Yi, Y.-Q. Yin, and C. Cai. Chemical Engineering
College, Nanjing University of Science & Technology,
Nanjing 210094, China.
1Corresponding author (e-mail address:
c.cai@mail.njust.edu.cn).
Can. J. Chem. 84: 1563–1566 (2006)
doi:10.1139/V06-162
© 2006 NRC Canada

1
564
Can. J. Chem. Vol. 84, 2006
Scheme 1. Yb(OPf) -catalysed synthesis of acylals in FBS.
ent procedure is a selective preparation of acylals of
aldehydes in the presence of ketones.
3
O
H
COCH3
R
H
OCOCH3
It was reported that the mechanism of the title reaction
could involve either intermolecular or intramolecular trans-
fer of the second acetate group after initial attack by acetic
anhydride (Scheme 3) (19). To test this aspect of the conver-
sion in an FBS, we prepared acetic propionic anhydride for
the geminal acylation reaction (Scheme 4). Analysis of the
products by GC-MS showed a nearly 1:1:2 ratio of geminal
diacetate, geminal dipropionate, and mixed acetate–propio-
nate, respectively. This result supports the intermolecular
pathway of Scheme 3.
Yb(OPf)3
FBS
R
C
+ O
C
COCH3
OCOCH3
Table 1. Reaction of benzaldehyde with acetic
anhydride in FBS.
Yield (%)a
Entry
Catalyst
1
2
3
4
5
Sc(OPf)3
Eu(OPf)3
99
In summary, the efficiency of Yb(OPf) as a Lewis acid
3
62
catalyst in FBS as an ecofriendly reaction system was dem-
onstrated for the conversion of aldehydes into geminal
diacetates. The versatility of this reaction system benefits
greatly from both the efficient Lewis acidity of Yb(OPf)3
and the unique solution properties of FBS. We believe that
the following properties endow the catalytic approach with
great potential for synthetic applications: (i) the amount of
catalyst used is minute, (ii) the catalyst, which is completely
immobilized in the fluorous phase, can be recovered and re-
used, and (iii) fluorous biphasic catalytic technique allows
high yields and purities of the resultant 1,1-diacetates and
indeed they can be used directly, without purification, for
many reactions.
Tm(OPf) b
51, 50, 51, 52, 50
3
Yb(OPf) b
99, 99, 98, 96, 97
3
La(OPf)3
PfOH
39
27
6
a
Isolated yield.
b
The fluorous phase was run for five consecutive
cycles.
used for the next reaction without any specific treatment and
the recycling work-up procedure was accomplished by sim-
ple phase separation. For example, in the Yb(OPf) -cata-
3
lyzed reaction of aldehydes and acetic anhydride, the yields
from the first run to the fifth run were 99%, 99%, 98%,
Experimental section
9
6%, and 97%, respectively.
We decided to use the relatively cheap and similarly ac-
Melting points were obtained with a Shimadzu DSC-50
thermal analyzer. IR spectra were recorded on a Bomem
MB154S IR analyzer. UV–vis spectra were obtained with a
UV-1601 apparatus. H NMR and F NMR spectra were re-
corded with Bruker Advance RX300. Mass spectra were re-
corded on a Saturn 2000GC-MS instrument. Inductively
coupled plasma (ICP) spectra were measured on an
Ultima2C apparatus. Elemental analyses were performed on
a Yanagimoto MT3CHN recorder. The perfluorodecalin
tive catalyst Yb(OPf)3 for the synthesis of other acylals
Scheme 1). The results are summarized in Table 2. All iso-
(
1
lated pure products gave satisfactory spectral analysis for H
NMR, IR, and MS. The aromatic aldehydes, cinnamalde-
hyde and 1-naphthylaldehyde that have an electron-
withdrawing group, are worth mentioning as they almost
gave quantitative yields in the presence of 0.3 mol% cata-
lyst, which is less than those of the reported methods (21–
1
19
2
7). The nature of the substituents on the aromatic ring
(
octadecafluorodecahydronaphthalene) and rare earth (III)
seems to have no effect on the reaction system. It is worth
noting that hydroxyl groups in 2-hydroxy- and 4-hydroxy-
benzaldehyde were also acetylised to afford the correspond-
ing triacetates under these conditions. Notably, in the cases
of 4-hydroxy- and 4-(dimethylamino)-benzaldehyde, only
salts were purchased from Sigma-Aldrich Co. Heptadeca-
fluorooctanesulfonic acid was commercially obtained from
ARCOS Co. Commercially available reagents were used
without further purification.
4
1% of 4-hydroxy-benzaldehyde was converted to acylals,
Typical procedure for the preparation of rare earth
while 4-(dimethylamino)-benzaldehyde failed to give the ex-
pected acylals even though the reaction mixtures were
stirred for 24 h. Such reactions may have suffered from the
tautomerization that occurred in the catalysis of Yb(OPf)3,
which decreased the reactivity of the aldehyde groups (17).
However, such tautomerization does not happen in 2-
hydroxy-benzaldehyde because the intramolecular hydrogen
bond between hydroxyl and aldehyde groups prevents the
formation of a quinoid structure (20). The reaction of
heterocyclic aldehydes such as furyl aldehyde also gave
quantitive conversion to products. Greater reaction times
were required for aliphatic aldehydes than aromatic alde-
hydes to obtain excellent yields, while ketones such as buta-
none, acetophenone, and benzophenone were practically
unchanged in the reaction (Scheme 2). Therefore, the pres-
(
III) perfluorooctanesulfonates
RE(OPf)3 compounds were prepared according to litera-
ture methods (6). In method A, a mixture of a solution of
heptadecafluorooctanesulfonic acid (PfOH, 0.77 g,
1.5 mmol) in water (5 mL) and a solution of YbCl3·6H2O
(0.2 g, 0.5 mmol) in water (0.8 mL) was stirred at RT for
18 h. In method B, a mixture of a solution of PfOH (0.77 g,
1.5 mmol) in water (5 mL) and of Yb2O3 (0.12 g,
0.25 mmol) powder was stirred at boiling for 3 h.
In both methods, the resulting gelatinlike solid was col-
lected, washed, and dried at 150 °C in vacuum to give a
white solid that shrinks at around 380–450 °C. IR (KBr, cm )
–1
υ: 1237 (CF ), 1152 (CF ), 1081 (SO ), 1059 (SO ), 747 (S-
3
2
2
2
O), 652 (C-S). ICP (%) calcd. for C O F S Yb·H O: Yb
2
4
9
51
3
2
©
2006 NRC Canada

Yi et al.
1565
Table 2. Yb(OPf) -catalyzed synthesis of acylals in FBS.
3
R1
Time (h)
Yield (%)a
Mp or bp (XXX mmHg) (°C)
b
Entry
1
2
3
4
4-ClPh
3-ClPh
2-ClPh
2
2
2
2
98
97
97
99
81~82 [Lit. value (16) 80-81]
65~66 [Lit. value (17) 65-66]
55~56 [Lit. value (18) 56-57]
126~127 [Lit. value (17) 125-126]
4-NO Ph
2
5
6
3-NO Ph
2
2
99
98
64~66 [Lit. value (17) 65-66]
90~91 [Lit. value (17) 91-9]
2
2-NO Ph
2
7
8
9
1
1
4-MePh
4-MeOPh
4-HOPh
2-HOPh
3
4
12
4
92
90
41
91
—
76~78 [Lit. value (19) 78-80]
64~65 [Lit. value (20) 64-65]
88~90 [Lit. value (20) 89-90]
103~104 [Lit. value (20) 104-105]
—
c
c
0
1
4-(Me) NPh
24
2
1
1
1
1
2
3
4
5
Cinnamyl
1-Naphthyl
Furyl
2
2
3
8
99
97
98
60
84~85 [Lit. value (17) 83-84]
105~106 [Lit. value (18) 105-106]
52~53 [Lit. value (17) 50-51]
116~118 (8.0) [Lit. value (19) 115-118 (8.0)]
129~130 (2.1) [Lit. value (19) 128-129 (2.0)]
186~190 (0.7) [Lit. value (19) 185-190 (0.7)]
C H
2
5
1
1
6
7
n-C H
8
61
64
5
11
PhCH CH2
12
2
Note: All reactions were carried out at 80 °C in the presence of 0.3 mol% catalyst.
a
Isolated yields.
mmHg = 133. 322 4 Pa.
equiv. of acetic anhydride was used.
b
1
5
c
Scheme 2. Selective preparation of acylals of aldehydes in the
Scheme 3. The possible mechanism of the conversion of alde-
presence of ketones.
hydes into acylals.
_
O
OYb(OPf)3
OAc
CH
C
O
C
R
CH
O
COCH3
COCH3
Yb(OPf)
3
intramolecular
-Yb(OPf)3
H
RCHO
R
Ac
2
O
+
O
+
+ O
R1
R2
OAc
OAc
CH
CH3
O
CH3
1
equiv.
1
equiv.
3
equiv.
2O
R
intermolecular
-Yb(OPf)3
+
O
O
H
OCOCH3
C
CH3
O
CH3
O
C
Yb(OPf)
3
AcO-
OCOCH3
+
FBS
R1
R2
Scheme 4. Reaction of benzaldehyde with acetic propionic anhy-
dride.
1
2
R = Ph, Et; R = Ph, Me
O
C
H
OCOCH3
1
0.24; found: Yb 9.99. Anal. calcd. for C O F S Yb·H O:
C
2
4
9
51
3
2
COCH CH
2
3
H
Yb(OPf)
FBS
3
OCOCH3
C 17.21, H 0.10; found: C 17.03, H 0.18.
+
O
+
COCH3
Typical procedure for synthesis of acylals in FBS
The aldehyde (10 mmol) or ketone (10 mmol) was slowly
26%
C
added into a mixture of Yb(OPf) (50 mg, 0.03 mmol), tolu-
ene (2 mL), and perfluorodecalin (C F , cis and trans mix-
3
1
0 18
H
OCOCH CH
H
OCOCH3
OCOCH CH
3
2
3
ture, 2 mL) containing acetic anhydride (20 mmol). The
mixture was stirred at 80 °C for the time indicated in Ta-
ble 2. The fluorous layer on the bottom was then separated
for the next reaction. The reaction mixture (organic phase)
was washed subsequently with brine (10 mL), 5% NaHCO3
solution (10 mL), and water (2 × 10 mL), and dried over
C
OCOCH CH
2
3
2
+
2
5%
49%
MgSO . The solvent was removed under reduced pressure
4
and the residue was purified by a silica gel column chroma-
©
2006 NRC Canada

1
566
Can. J. Chem. Vol. 84, 2006
tography or by recrystallization from cyclohexane solution
to give the acylals.
3. K. Mikami (Editor). Green reaction media in organic synthe-
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4
. B. Cornils and W.A. Herrmann. Aqueous-phase organometallic
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The acylals of the following selected aldehydes show the
1
IR, H NMR, and MS (EI) spectra as given below.
5
6
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–
1
1
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8
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7
3
+
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1
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–
1
1
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3
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1
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4
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(
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1
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–
1
1
3
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1
Hexanal
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1
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1
1
7
2
750. H NMR (500 MHz, TMS, CDCl ) δ: 2.13 (s, 6H),
3
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Acknowledgements
2
(
We thank the National Defense Committee of Science and
Technology (40406020103) for financial support. We also
thank the Zhen-Ya Rare Earth Co. Ltd. for providing several
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©
2006 NRC Canada