Lanthanide(III) tosylates as new acylation catalysts

Article abstract of DOI:10.1002/ejoc.200400788

Lanthanide(III) complexes of p-toluenesulfonic acid (Ln(TOS)₃) were prepared, characterized, and examined as catalysts for the acetylation of various alcohols in acetic acid solution. Examination of a series of Ln(TOS)₃ catalysts in the acetylation of 2-phenylethanol revealed a clear correlation between the ionic radius of the lanthanide(III) ion and the yield of the reaction, with the heavier lanthanides being more effective. In the presence of 5 mol-% of Yb(TOS)₃, a quantitative conversion of phenethyl alcohol to phenethyl acetate was achieved within 18 hours at 50°C. Faster reaction was obtained under reflux conditions, in which case acetylation was complete within 30 minutes and in the presence of only 2 mol-% of the Yb(TOS)₃ catalyst. The Yb(TOS)₃ catalyst was effective for acetylation of a range of primary, secondary, and tertiary alcohols. The Yb(TOS)₃ catalyst was also effective for acylation of phenethyl alcohol with propionic acid and cyclohexanecarboxylic acid. The catalysts could be easily recovered and reused for further acetylation, with no apparent change in selectivity or efficiency. Wiley-VCH Verlag GmbH & Co. KGaA, 69451 Weinheim, Germany, 2005.

Full text of DOI:10.1002/ejoc.200400788

FULL PAPER
Lanthanide(III) Tosylates as New Acylation Catalysts
Tatjana N. Parac-Vogt,[a] Karen Deleersnyder,[a] and Koen Binnemans*[a]
Keywords: Homogeneous catalysis / Lanthanides / Lewis acids / Acylation / Rare earths
Lanthanide(III) complexes of p-toluenesulfonic acid (Ln(TOS)3) was complete within 30 minutes and in the presence of only
were prepared, characterized, and examined as catalysts
for the acetylation of various alcohols in acetic acid solution.
Examination of a series of Ln(TOS)3 catalysts in the acety-
lation of 2-phenylethanol revealed a clear correlation be-
tween the ionic radius of the lanthanide(III) ion and the yield
of the reaction, with the heavier lanthanides being more ef-
fective. In the presence of 5 mol-% of Yb(TOS)3, a quantita-
tive conversion of phenethyl alcohol to phenethyl acetate
was achieved within 18 hours at 50 °C. Faster reaction was
obtained under reflux conditions, in which case acetylation
2 mol-% of the Yb(TOS)3 catalyst. The Yb(TOS)3 catalyst was
effective for acetylation of a range of primary, secondary, and
tertiary alcohols. The Yb(TOS)3 catalyst was also effective for
acylation of phenethyl alcohol with propionic acid and cyclo-
hexanecarboxylic acid. The catalysts could be easily reco-
vered and reused for further acetylation, with no apparent
change in selectivity or efficiency.
(© Wiley-VCH Verlag GmbH & Co. KGaA, 69451 Weinheim,
Germany, 2005)
Introduction
described as efficient catalysts for the acetylation of various
alcohols in the presence of acetic acid, acetic anhydride, or
acetyl chloride. An important property of triflate salts is
their compatibility with water and other protic solvents. Af-
ter workup, the catalysts can be easily recycled and reused
without significant loss of activity. However, the disadvan-
tages of triflate salts include their relatively high cost and
the fact that one needs to handle the corrosive triflic acid
for their preparation.
In pursuit of lanthanide(iii) catalysts that would not re-
quire the use of hazardous or costly compounds, we re-
cently examined lanthanide(iii) salts of noncorrosive aro-
matic sulfonic acids, such as p-toluenesulfonic acid and
nitrobenzenesulfonic acids, as catalysts for the nitration of
aromatic compounds.[18–20] The aromatic sulfonic acids are
much weaker acids than triflic acid, (p-toluenesulfonic acid,
for example, is about 106 times weaker acid than triflic
acid),[21] and their complexes with lanthanides would there-
fore be expected to be significantly weaker Lewis acids than
the lanthanide(iii) triflates. Surprisingly, we found that their
catalytic efficiency was comparable to that of the triflate
salts.[18–20] This prompted us to exploit the advantages of
lanthanide(iii) p-toluenesulfonate complexes (usually re-
ferred to as lanthanide(iii) tosylates) over lanthanide(iii) tri-
flates, which include much cheaper costs and easier hand-
ling, and to explore lanthanide(iii) tosylates further as cata-
lysts for other types of organic reactions. In this study we
demonstrate that lanthanide(iii) tosylates can be used as ef-
ficient, inexpensive, recyclable, and environmentally friendly
catalysts for the acetylation (and acylation in general) of
various alcohols.
The acetylation (or acylation in general) of hydroxyl
groups is one of the most frequently used synthetic pro-
cedures in organic chemistry.[1] Acetylation of alcohols is
frequently performed with acid anhydride or acetyl chlo-
ride, in the presence of a basic catalyst such as triethyl-
amine, pyridine, 4-(dimethylamino)pyridine, 4-pyrrolidino-
pyridine, or N,N,NЈ,NЈ-tetramethylethylenediamine.[2–4]
Alternatively, various Lewis acid catalysts including zinc
chloride, magnesium bromide, cobalt(ii) chloride, chloro-
titanium(iv) tris(trifluoromethanesulfonate), diorganotin
dichloride, and tantalum(v) chloride, among others, could
be applied.[5–9] Although the Lewis acids are usually very
efficient acylation catalysts, they have some major draw-
backs: they need to be used in stoichiometric amounts, they
are easily deactivated by water, and they cannot be recycled
or reused.
An alternative approach is the use of ionic liquid solvents
as media for Lewis acid-catalyzed acetylation. The develop-
ment of “recyclable” ionic catalyst solutions offers new pos-
sibilities for “green” chemistry in the search for better alter-
natives to acid-/base-catalyzed reactions.[10]
In recent years, various trifluoromethanesulfonates (so-
called triflates) such as ytterbium(iii) triflate,[11] lantha-
num(iii) triflate,[11] cerium(iii) triflate,[12] copper(ii) tri-
flate,[13] scandium(iii) triflate,[11a,14] erbium(iii) triflate,[15]
indium(iii) triflate,[16] and bismuth(iii) triflate[17] have been
[a] Katholieke Universiteit Leuven, Department of Chemistry,
Celestijnenlaan 200F, 3001 Leuven, Belgium
Fax: +32-16-327992
E-mail: Koen.Binnemans@chem.kuleuven.ac.be
1810
© 2005 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
DOI: 10.1002/ejoc.200400788
Eur. J. Org. Chem. 2005, 1810–1815

Lanthanide(iii) Tosylates as New Acylation Catalysts
FULL PAPER
Results and Discussion
Synthesis and Characterization of the Catalysts
Structural and thermal studies of lanthanide(iii) tosylates
(Figure 1) showed that complexes of this type initially crys-
tallize with nine water molecules in the first coordination
sphere, and that seven water molecules are lost upon heat-
ing to circa 55 °C.[22] Since the coordination number of the
trivalent lanthanide ion is commonly eight or nine, it has
been suggested that the coordination mode of the sulfonate
anions changes upon removal of the hydrated water mole-
cules from the crystal, so that they act as multidentate li-
gands forming more metal–oxygen bonds in order to re-
place the removed water oxygen atoms in the first coordina-
tion sphere of the lanthanide(iii) ion. The elemental analysis
results of lanthanide(iii) tosylate catalysts (see Experimental
Section) indicated that some water of crystallization is lost
upon drying of the crystalline solid at 50 °C in vacuo, but
that the complexes still contain one to three molecules of
water. Consequently, infrared spectra of the complexes
showed strong and broad absorption in the 3200–3600 cm–1
region, corresponding to the –OH stretching frequency of
water molecules.
Scheme 1.
As can be seen in Figure 2, all complexes exhibited cata-
lytic activity, but noticeable differences in reactivity were
observed. While lanthanum(iii) tosylate gave only 26% con-
version of phenethyl alcohol to phenethyl acetate after 18
hours of reaction at 50 °C, ytterbium(iii) tosylate and er-
bium(iii) tosylate yielded quantitative conversion under the
same conditions.
Figure 1. Chemical structure of Ln(TOS)3 complexes.
Because it is known that p-toluenesulfonic acid is itself
capable of promoting acylation reactions[23] it is important
to demonstrate the absence of trace amounts of free p-tolu-
enesulfonic acid in the lanthanide(iii) catalyst. The infrared
spectra of lanthanide(iii) complexes showed that the broad
absorption between 2400 and 3100 cm–1, typical of the OH
Figure 2. Effect of the lanthanide(iii) ion in the tosylate salt on the
stretching mode of protonated p-toluenesulfonic acid,[24] catalyzed conversion of phenethyl alcohol into phenethyl acetate in
the presence of acetic acid. The ionic radii of the lanthanide(iii)
ions are shown for comparison.
was absent. Furthermore, aqueous solutions of lantha-
nide(iii) complexes always had close to neutral pH, which
further confirms the absence of free p-toluenesulfonic acid
in the isolated catalysts.
Apart from some scattered data points, there is a clear
inverse correlation between the ionic radii of the lantha-
nide(iii) ion and the yield of the acetylation reaction. Pre-
vious studies involving reactions catalyzed by lantha-
nide(iii) triflates have revealed a similar trend.[25] The
smaller ionic radius of trivalent lanthanide ion leads to a
greater charge to size ratio (Z/r), which in turn results in
Screening of the Lanthanide(III) Catalysts
greater polarizing power of the lanthanide(iii) center.
In order to optimize the rate of the acylation reaction, a
range of lanthanide(iii) tosylates were examined as potential
catalysts. The reaction we chose for the initial screening of
Effects of Reaction Time, Temperature, and Amount of
the catalytic activity of the lanthanide(iii) tosylates was the
Catalyst on the Extent of Acetylation
acetylation of phenethyl alcohol (2-phenylethanol)
(Scheme 1) in acetic acid as the solvent. The use of acetic
acid rather than acetyl chloride or acetic anhydride is both first examined the effects of temperature and reaction time
economically and environmentally beneficial. on the acylation reaction. Since the screening of the series
In order to optimize the catalytic reaction further, we
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T. N. Parac-Vogt, K. Deleersnyder, K. Binnemans
FULL PAPER
of lanthanide(iii) catalysts had shown that ytterbium(iii) to- Acylation of Alcohols Catalyzed by Yb(TOS)3
sylate was the most effective, all further acetylation experi-
The applicability of the acetylation reaction catalyzed by
ments were performed with this catalyst. As can be seen in
Yb(TOS)3 was tested for different alcohols as the substrate.
Table 1, acetylation of phenethyl alcohol was rather slow at
All acetylation reactions were performed with acetic acid
room temperature, most probably due to the low solubility
acting both as the substrate and as the solvent. Treatment
of the catalysts in acetic acid. Upon increasing the tempera-
of an alcohol with acetic acid was performed at reflux tem-
ture to 50 °C, nearly quantitative conversion was achieved
perature, in the presence of 5 mol-% of the catalyst. After
within 18 hours. The fastest reaction was obtained under
the reaction mixture had been brought to reflux (approxi-
reflux conditions, in which case acetylation was essentially
mately 30 minutes), the reaction was quenched by the ad-
complete in the time taken to bring the reaction mixture to
dition of water and dichloromethane and the products were
reflux.
analyzed by gas chromatography or by 1H NMR spec-
troscopy. As can be seen in Table 2, the acetylation of pri-
Table 1. Effects of temperature and reaction time on the acetylation
mary alcohols proceeded quantitatively under these condi-
tions. As would be expected, the acetylation of secondary
and tertiary alcohols was slower, and required longer reac-
tion times for completion. It is also worth mentioning that
no elimination products were observed.
of phenethyl alcohol with acetic acid in the presence of ytter-
bium(iii) tosylate.[a]
Yb(TOS)3
(mol-%)
Time
(h)
T
(°C)
Conversion
(%)[b]
5
–
5
–
5
–
5
–
24
24
18
18
1
1
0.5
0.5
room temp.
room temp.
35
3
97
23
64
5
50
50
80
80
reflux
reflux
Table 2. Acetylation of alcohols with acetic acid catalyzed by ytter-
bium(iii) tosylate.[a]
Alcohol or phenol
Yb(TOS)3
(mol-%)
Conversion
(%)[b]
Ͼ98
39
1-phenylethanol
5
–
5
–
5
–
5
–
5
–
5
–
5
–
5
–
5
–
5
–
5
–
5
–
Ͼ98
26
Ͼ98
39
Ͼ98
35
Ͼ98
28
94
42
51
14
Ͼ98
33
62
12
95
[a] 1 mmol of phenethyl alcohol in 2 mL of acetic acid. [b] Deter-
mined by GC.
2-phenylethanol
tetrahydropyran-2-methanol
3-bromo-3-buten-1-ol
benzyl alcohol
cyclohexanol
The acetylation reaction in the presence and in the ab-
sence of the catalyst was followed as a function of time, and
the results shown in Figure 3 are consistent with pseudo-
first order kinetics with one of the reactants (i.e., acetic
acid) present in large excess. Fitting of the data revealed
that the catalyzed reaction was at least an order of magni-
tude faster than the reaction run in the absence of the cata-
lyst.
octan-1-ol
octan-2-ol
butan-1-ol
37
74[c]
29[c]
76[d]
trace[d]
43[d]
2[d]
butan-2-ol
adamantan-1-ol
phenol
[a] 1 mmol of alcohol in 2 mL of acetic acid, at reflux for 30 min-
utes. [b] Determined by GC and/or 1H NMR. [c] At reflux for 1
hour. [d] At reflux for 18 hours.
Besides acetylation in acetic acid solutions, Yb(TOS)3
was also an effective catalyst for the acylation of phenethyl
alcohol in propionic acid and in cyclohexanecarboxylic acid
solutions, although the latter reactions were somewhat slow,
but could be completed within one or two hours (Table 3).
The effect of catalyst concentration on the reaction yields
was examined for the acetylation reaction of phenethyl
alcohol. Only 2 mol-% of the Yb(TOS)3 catalyst in acetic
acid solution were sufficient for the quantitative conversion
of phenethyl alcohol into phenethyl acetate within 30 min-
utes. (Table 4).
Figure 3. Conversion of phenethyl alcohol into phenethyl acetate
in the presence of 5 mol-% of Yb(TOS)3 (black squares) and in the
absence of the catalyst (open circles). The fitting of the results for
the catalyzed reaction yielded pseudo-first-order rate constant of
3.0×10–3 min–1.
1812
© 2005 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
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Eur. J. Org. Chem. 2005, 1810–1815

Lanthanide(iii) Tosylates as New Acylation Catalysts
FULL PAPER
Table 3. Acylation of phenethyl alcohol with carboxylic acids cata- studies we discovered that the lanthanum(iii) complexes, de-
lyzed by ytterbium(iii) tosylate.[a]
spite their much smaller polarizabilities, were more efficient
Acid
Yb(TOS)3
(mol-%)
Conversion (%)[b]
catalysts than the corresponding ytterbium(iii) com-
plexes.[18,20] Intriguingly, the reactivity trend observed in the
acylation reactions is exactly the opposite: Yb(TOS)3 is a
more effective catalyst than La(TOS)3. How can these find-
ings, which on the first glance may seem contradictory, be
rationalized?
50 °C, 18 h
120 °C, 30 min
acetic acid
5
–
5
–
5
–
Ͼ98
23
67
11
49
9
Ͼ98
39
86
20
59
propionic acid
cyclohexane-car-
boxylic acid
The X-ray single-crystal structures of most of the lantha-
nide(iii) tosylates have been reported, and they indicate dis-
tinct structural differences across the lanthanide(iii)
series.[22,26] For example, in the La(TOS)3·9H2O complex,
nine water molecules occupy the first coordination sphere
and the tosylate counterions are merely spectator ions lo-
cated in the second coordination sphere.[26] In contrast, the
structure of Yb(TOS)3 is markedly different: the coordina-
tion environment around the ytterbium(iii) is occupied by
two oxygen atoms from the sulfonate group of TOS and six
oxygen atoms from water molecules.[22] The third TOS
anion is not located in the first coordination sphere, but
forms hydrogen bonds with one of the coordinated water
molecules. Solution studies of Ln(TOS)3 complexes have
indicated that the solid-state structure is preserved in solu-
tion in noncoordinating or weakly coordinating solvents.[27]
This suggests that, in noncoordinating solvents, the interac-
tion between the ytterbium(iii) and the incoming substrate
may be hindered through the steric hindrance associated
with the bulky coordinated TOS ligands. Therefore, for re-
actions performed in noncoordinating solvents, (such as, for
example, nitration of arenes), Yb(TOS)3 is not the best cat-
alyst. In the case of La(TOS)3 this steric hindrance is absent
and the interaction between the metal center and the sub-
strate is facilitated. However, in strongly coordinating sol-
vents such as water, the structures of all lanthanide(iii)
tosylates are the same, since the coordinating solvent re-
places TOS ligand from the first coordination sphere.[27]
The question, then, is what the structures of Ln(TOS)3
complexes in acetic acid solutions are. In order to elucidate
this problem, we recorded the 1H NMR spectrum of
Eu(TOS)3 in deuterated acetic acid solution. X-ray single-
crystal structures of Eu(TOS)3 shows that this complex is
isostructural with Yb(TOS)3, so the coordination environ-
ment around the europium(iii) is occupied by two oxygen
atoms from the sulfonate group of TOS and six oxygen
atoms from water molecules.[28] Previous NMR studies of
europium(iii) complexes have revealed that this structure is
preserved in weakly coordinating solvents so that the two
sulfonate ligands bound to europium(iii) each exhibited a
large paramagnetic shift, while the third ligand appeared at
the same chemical shifts as the fully dissociated sulfonic
acid, implying that there was no binding to the lantha-
nide(iii) ion.[20] In acetic acid solution, however, only one
set of proton resonances appeared in the region between 6.8
and 7.2 ppm typical of the fully dissociated p-toluenesul-
fonic acid (Figure 4). The 1H NMR spectrum was taken
12
[a] 1 mmol of phenethyl alcohol in 2 mL of acid. [b] Determined by
GC and/or H NMR spectroscopy.
1
Table 4. The effect of the ytterbium(iii) tosylate loading on the
acetylation of phenethyl alcohol with acetic acid.[a]
Catalyst load (mol-%)
Conversion (%)[b]
None
0.1
0.2
0.5
1
2
5
39
73
74
81
89
Ͼ98
Ͼ98
[a] 1 mmol of phenethyl alcohol in 2 mL of acetic acid, at reflux
for 30 minutes. [b] Determined by GC.
Recovery and Reuse of the Yb(TOS)3 Catalyst
Upon quenching of the reaction by addition of dichloro-
methane and water, the catalysts could easily be recovered
from the aqueous phase by evaporation of the solvent under
reduced pressure. The FT-IR spectra of the recycled cata-
lysts were identical to spectra of freshly prepared Yb(TOS)3.
In addition, in the experiments performed with diamagnetic
1
La(TOS)3, the H NMR spectra of the recovered catalysts
were identical to the spectrum of freshly prepared La(TOS)3.
The recovered catalysts were shown to be active for further
acetylation reaction with no apparent change in selectivity
or in efficiency (Table 5). Because the catalyst was very ef-
fective for acetylation at a loading of only 2 mol-%, it is not
surprising that the reaction goes to completion even after
some loss of the catalyst during recycling.
Table 5. Recovery of ytterbium(iii) tosylate after acetylation of
phenethyl alcohol.[a]
Run
Conversion (%)[b]
Recovery of catalyst (%)
1
2
3
Ͼ98
Ͼ98
Ͼ98
91
84
71
[a] 1 mmol of phenethyl alcohol in 2 mL of acetic acid, at reflux
for 30 minutes, with 5 mol-% of Yb(TOS)3 in the first run.
Postulated Mechanism for the Yb(TOS)3-Catalyzed
Acylation
Our initial work on lanthanide(iii) tosylate and lantha- over a large proton resonance range (from –30 to +30 ppm),
nide(iii) nosylate catalysts was mostly focused on nitrations and no presence of paramagnetically shifted protons was
of aromatic compounds.[18–20] During the course of these detected. This indicates that in acetic acid solution the tos-
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1813

T. N. Parac-Vogt, K. Deleersnyder, K. Binnemans
FULL PAPER
ylate ligands are removed to the second coordination sphere
and that the first coordination sphere around europium(iii)
is occupied by acetic acid ligands. It is reasonable to assume
that other Ln(TOS)3 complexes also undergo this ligand
substitution upon being dissolved in acetic acid.
Experimental Section
Materials and Methods: Reagents were obtained from Aldrich
Chemical Co. Inc., Acros Organics, or Rhodia Electronics and Ca-
talysis, and were used without further purification. 1H NMR spec-
tra were run on a Bruker Avance 300, operating at 300 MHz. GC
analyses were performed at ThermoFinnigan Trace GC. Elemental
analysis was performed on a CE Instruments EA-1110 elemental
analyzer.
General Procedure for the Synthesis of Ln(TOS)3 Complexes: The
tosylate complexes were synthesized by the reaction of 1.1 equiva-
lent of the corresponding lanthanide(iii) oxide with six equivalents
of p-toluenesulfonic acid in water. After the solution had been
stirred in boiling water for 30 minutes, the excess of oxide was re-
moved by filtration. The filtered solution was either left to crys-
tallize or was evaporated to dryness and the resulting solid was
dried in a vacuum oven at 50 °C overnight.
Elemental Analysis Results for Ln(TOS)3·xH2O: La(TOS)3·3H2O
(C21H27S3O12La) calcd. (found): C 35.70 (35.68), H 3.85 (3.76) %.
Nd(TOS)3·2H2O, (C21H25S3O11Nd) calcd. (found):
C 36.47
(36.76), H 3.65 (3.36) %. Sm(TOS)3·3H2O, (C21H27S3O12Sm) calcd.
(found): C 35.05 (34.79), H 3.78 (3.73) %. Eu(TOS)3·3H2O,
(C21H27S3O12Eu) calcd. (found): C 35.00 (35.35), H 3.78 (3.74) %.
Gd(TOS)3·3H2O, (C21H27S3O12Gd) calcd. (found):
C 34.76
Figure 4. Proton NMR spectrum of Eu(TOS)3 in [D3]acetic acid
solution at 80 °C. The large peak at δ = 2.08 ppm belongs to the
solvent [D3]acetic acid.
(34.60), H 3.75 (3.70)%. Dy(TOS)3·2H2O, (C21H25S3O11Dy) calcd.
(found): C 35.34 (35.54), H 3.53 (3.30) %. Ho(TOS)3·3H2O,
(C21H27S3O12Ho) calcd. (found): C 34.43 (34.45), H 3.72 (3.62) %.
Er(TOS)3·2H2O, (C21H25S3O11Er) calcd. (found): C 35.25 (35.50),
Since the acylation experiments have shown that there is
a correlation between the charge to size ratio (Z/r) and the
catalytic activity, there is obviously an interplay between the
lanthanide(iii) ion and acetic acid, in which an increasing
electrostatic interaction results in greater reactivity. The
mechanism can be postulated as follows: upon displace-
ment of TOS and water molecules from the inner coordina-
tion sphere by acetic acid, the resulting strong polarization
due to the lanthanide(iii) ion results in polarization of the
carbonyl group. This polarization increases the electrophi-
licity of the carbonyl carbon, making it more susceptible to
nucleophilic attack by the oxygen atom of the alcohol. In
this way an oxonium ion is formed, and this further reacts
in classical manner to produce an ester. Thus, the experi-
mentally observed correlation between the increasing
charge to size ratios and the extent of esterification can be
rationalized by noting that the nucleophilic attack of the
alcohol becomes more facile as the metal becomes more
polarizing.
H
3.52 (3.87) %. Tm(TOS)3·2H2O, (C21H25S3O11Tm) calcd.
(found): 35.10 (35.13), 3.51 (3.30) %. Yb(TOS)3·H2O
(C21H23S3O10Yb) calcd. (found): C 35.75 (35.63), H 3.29 (3.69) %.
C
H
General Procedure for the Ln(TOS)3-Catalyzed Acylation of
Alcohols: The alcohol (1 mmol) was added to a solution of the
respective lanthanide(iii) catalyst (5 mol-%, 0.05 mmol) in glacial
acetic acid (2 mL). The mixture was stirred and heated at reflux.
After a given period of time the solution was cooled and diluted
with water and with dichloromethane. The organic phase was dried
with MgSO4, the solvent was evaporated, and the residue was ana-
1
lyzed by H NMR or gas chromatography.
Acknowledgments
T. N. P. V. and K. B. are Postdoctoral Fellows of the FWO-Flanders
(Belgium). Financial support has been provided by the K. U.
Leuven (VIS/01/006.01/2002/-06/2004 and GOA 03/03) and by the
FWO-Flanders (Krediet aan Navorsers to K. B.). The authors
thank Rhodia Electronics and Catalysis for a gift of rare-earth ox-
ides.
Conclusions
[1] T. W. Green, P. G. Wuts, Protective Groups in Organic Synthe-
sis, 3rd ed., Wiley, New York, 1999, p. 150.
In summary, we have demonstrated that efficient lantha- [2] a) D. Horton, in Organic Synthesis, Coll. Vol., John Wiley &
Sons, New York, 1991, pp. 1–6; b) R. I. Zhdanov, S. M. Zheno-
nide(iii) acylation catalysts can be easily prepared from
noncorrosive and relatively inexpensive p-toluenesulfonic
darova, Synthesis 1975, 4, 222–245.
[3] a) W. Steglich, G. Höfle, Angew. Chem. Int. Ed. Engl. 1969, 8,
acid. Although the acidity of p-toluenesulfonic acid is about
981–985; b) G. Höfle, W. Steglich, H. Vorbrüggen, Angew.
106 times weaker than that of triflic acid, its complexes with
Chem. Int. Ed. Engl. 1978, 17, 569–583; c) E. F. V. Scriven,
Chem. Soc. Rev. 1983, 12, 129–161.
[4] T. Sano, K. Ohashi, T. Oriyama, Synthesis 1999, 7, 1141–1144.
[5] R. H. Baker, F. G. Bordwell, Org. Synth. 1995, 3, 141–142.
[6] a) E. Vedejs, O. Daugulis, J. Org. Chem. 1996, 61, 5702–5703;
b) S. V. Pansare, M. G. Malusare, A. N. Rai, Synth. Commun.
2000, 30, 2587–2592.
lanthanide(iii) ions are strong Lewis acids able to catalyze
the acylation of alcohols with comparable efficiency. The
use of p-toluenesulfonic acid over triflic acid for the prepa-
ration of lanthanide(iii) catalysts is both environmentally
and economically advantageous.
1814
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Lanthanide(iii) Tosylates as New Acylation Catalysts
FULL PAPER
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Received: November 03, 2004
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