Iron(III) tosylate catalyzed acylation of alcohols, phenols, and aldehydes

Article abstract of DOI:10.1016/j.tetlet.2012.10.033

Iron(III) p-toluenesulfonate (tosylate) is an efficient catalyst for acetylation of alcohols, phenols, and aldehydes. The acetylation of 1° and 2° alcohols, diols, and phenols proceeded smoothly with 2.0 mol % of catalyst. However, the reaction worked well with only a few 3° alcohols. The methodology was also applicable to the synthesis of a few benzoate esters but required the use of 5.0 mol % catalyst. Aldehydes could also be converted into the corresponding 1,1-diesters (acylals) under the reaction conditions. Iron(III) tosylate is an inexpensive, and easy to handle, commercially available catalyst.

Full text of DOI:10.1016/j.tetlet.2012.10.033

Tetrahedron Letters 53 (2012) 6946–6949
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Tetrahedron Letters
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Iron(III) tosylate catalyzed acylation of alcohols, phenols, and aldehydes
⇑
Neil J. Baldwin, Anna N. Nord, Brendan D. O’Donnell, Ram S. Mohan
Laboratory for Environmentally Friendly Organic Synthesis, Department of Chemistry, Illinois Wesleyan University, Bloomington, IL 61701, USA
a r t i c l e i n f o
a b s t r a c t
Article history:
Iron(III) p-toluenesulfonate (tosylate) is an efficient catalyst for acetylation of alcohols, phenols, and
aldehydes. The acetylation of 1° and 2° alcohols, diols, and phenols proceeded smoothly with 2.0 mol %
of catalyst. However, the reaction worked well with only a few 3° alcohols. The methodology was also
applicable to the synthesis of a few benzoate esters but required the use of 5.0 mol % catalyst. Aldehydes
could also be converted into the corresponding 1,1-diesters (acylals) under the reaction conditions.
Iron(III) tosylate is an inexpensive, and easy to handle, commercially available catalyst.
Ó 2012 Elsevier Ltd. All rights reserved.
Received 21 September 2012
Revised 4 October 2012
Accepted 8 October 2012
Available online 18 October 2012
Keywords:
Iron and its compounds
Acylation
Alcohols
Diols
Esters
Green chemistry
Phenols
The conversion of alcohols to esters is an important synthetic
transformation that has received considerable attention.1 Conver-
sion of an alcohol to the corresponding acetate is typically carried
out using acetic anhydride or acetyl chloride in the presence of
pyridine or triethylamine as a catalyst.2 4-(Dimethylamino)pyri-
dine (DMAP) is known to cause a remarkable rate acceleration
in this reaction.3 One problem with tertiary amines is that they
are corrosive, toxic, and often highly unpleasant to work with. Le-
wis acids have also been reported to catalyze the acetylation of
alcohols. These include Bi(OTf)3,4 Sc(OTf)3,5,6 CoCl2,7 and I2.8 Many
of these catalysts are either corrosive (such as I2) or very expen-
sive (scandium salts). Some examples of acylation catalysts from
the recent literature include pentafluorophenylammonium tri-
flate,9 silica magnesium oxide,10 polyvinylpolypyrrolidone-bound
boron trifluoride,11 and N-acyl 1,5-diazabicyclo[4.3.0]non-5-ene
tetraphenylborate salts.12 Acylation methods that utilize enzymes
as catalysts have also been developed.13 With increasing environ-
mental concerns, it is imperative that new ‘environmentally
friendly’ reagents be developed. Our continued interest in devel-
oping environmentally friendly synthetic methodology prompted
us to investigate a mild and catalytic method for the acylation
of alcohols, phenols, diols, and aldehydes utilizing inexpensive,
commercially available reagents. Herein we wish to report that
iron(III) tosylate14 is a mild catalyst for the acylation of a variety
of alcohols, phenols, and diols (Table 1) as well as aldehydes
(Table 2). As can be seen from Table 1, the reaction worked well
with 1° and 2° alcohols (entries 1–11), and phenols (entries 15–
17 and 20). When acetic anhydride was used as the acylating
agent the reaction could be carried out under solvent-free condi-
tions. With allylic alcohols (entries 3 and 5), the use of solvent
(CH3CN) gave fewer side products. Solvent was also necessary
when the acylating agent (benzoic anhydride) was a solid, the
reaction mixture solidified without solvent, or if the starting alco-
hol was poorly soluble in acetic anhydride. In most cases, the
crude product was found to be P98% pure by 1H and 13C NMR
spectroscopy and further purification was deemed unnecessary.
For solubility reasons, CH2Cl2 was used as a solvent in case of ben-
zil (entry 11). Although the methodology was not broadly applica-
ble to tertiary alcohols, we were able to successfully acetylate
some 3° alcohols. For example, 1-ethynylcyclohexanol (entry 12)
gave a moderate yield of the corresponding acetate. When 1-
methylcyclohexanol (entry 13) was subjected to the reaction con-
ditions (in CH3CN), the crude product although colored was found
to contain mostly (80%) the 3° acetate. However, chromatography
yielded the pure acetate in only a low yield (38%). Any 1-methyl-
cyclohexene that may have formed is likely to have been lost dur-
ing removal of the solvent on a rotary evaporator, and hence was
not seen in the 1H spectrum of the crude product.
The hindered 3° alcohol, triphenylmethanol (entry 14), failed to
yield the acetate even under reflux conditions, and the starting
material was recovered unchanged. When 2-phenyl-2-propanol 1
(Scheme 1) was subjected to the reaction conditions, none of the
corresponding acetate was isolated. GC analysis of the crude prod-
uct, which was obtained as a dark red–brown liquid showed that it
mostly (88%) contained product 3. However, product 3 was
⇑
Corresponding author. Fax: +1 309 556 3844.
E-mail address: rmohan@iwu.edu (R.S. Mohan).
0040-4039/$ - see front matter Ó 2012 Elsevier Ltd. All rights reserved.
http://dx.doi.org/10.1016/j.tetlet.2012.10.033

N. J. Baldwin et al. / Tetrahedron Letters 53 (2012) 6946–6949
6947
Table 1
Iron(III) tosylate catalyzed acylation of alcohols, phenols, and diols
Entry
Alcohol
Anhydride R1
Solventa
Time & Tempb
Product
Yieldc (%)
1a
1b
1c
2
CH3
Phd
—
10 min
9415
73e,16
8517
9218
95f,15
CH3CN
24 h, 70 °C
1 h
n-Pr
CH3
CH3
—
—
—
1 h, 0 °C
15 min, 0 °C
3
4a
4b
CH3
Phd
—
25 min, 0 °C
27 h, 70 °C
66e,19
66e,20
CH3CN
d
5
6
CH3
CH3CN
CH3CN
22.5 h, 0 °C
82g,18
9815
p-NO2C6H4CH2OH
CH3
CH3
45 min
p-NO2C6H4CH2OAc
7
CH3CN
10 min
9215
8
CH3
CH3CN
1 h 50 min
9321
9
Phd
CH3CN
CH3CN
44 h, 70 °C
30 h, 50 °C
6122,e
9218
10
CH3
11
12
CH3
CH3
CH2Cl2
CH3CN
18 h
9815
5 h, 0 °C
9523
385,e
13
14
CH3
CH3
CH3CN
CH3CN
21 h, 0 °C to rt
49 h, rt to 70 °C
Ph3COH
NRh
15
CH3
—
2 h
7715
16
17
CH3
CH3
CH3CN
CH3CN
24.5 h, 50 °C
9515
1.5 h
7024,e
i
18
19
CH3
CH3CN
CH3CN
3 h
8425
8926
i
CH3
50 min
i
20
21
CH3
CH3CN
—
50 min
9915
96j
i
CH3
2 h, 0 °C
a
b
c
Reagent grade acetonitrile was used.
All reactions were run at room temperature unless otherwise mentioned, and reaction progress was monitored by GC or TLC.
Refers to yield of isolated product that was deemed to be sufficiently pure (> 98%) by 1H & 13C NMR spectroscopy, unless otherwise mentioned. All products have been
previously reported in the literature or are commercially available. The superscript next to yield refers to literature reference for spectral data of the product.
d
Reaction was carried out using 5.0 mol % catalyst.
Yield of product after purification by flash chromatography.
Product was determined to be 96% pure by GC.
Reaction was carried out using 0.5 mol % catalyst.
No reaction was observed even when the mixture was heated at reflux for 29 h.
Reaction was carried out with 2.6 equiv of acetic anhydride.
Product is commercially available (CAS # 6963-44-6).
e
f
g
h
i
j
isolated only in 26% yield after chromatographic purification of the
crude product. Its identity was confirmed by 1H and 13C NMR spec-
troscopy, and HRMS.27 Based on the fact that the same product was
obtained in the absence of acetic anhydride but at a much slower

6948
N. J. Baldwin et al. / Tetrahedron Letters 53 (2012) 6946–6949
Table 2
Formation of acylals from aldehydes using Fe(OTs)3ꢀ6H2O
Entry
Substrate
Solventa
Tempa (°C)
Timeb
Productc
Yieldd (%)
1
2
3
4
p-ClC6H4CHO
—
—
—
—
rt
50
rt
4 h
2 h
6.5 h
3 h 40 min
p-ClC6H4CH(OAc)2
9739
p-CH3OC6H4CHO
p-CH3C6H4CHO
m-CH3OC6H4CHO
p-CH3OC6H4CH(OAc)2
p-CH3C6H4CH(OAc)2
m-CH3OC6H4CH(OAc)2
50d,e,39
9440
rt
92d,39
5
—
3.5 h
72f,40
6
7
p-NO2C6H4CHO
CH3(CH2)8CHO
CH3CN
—
rt
rt
4 h
26 h
p-NO2C6H4CH(OAc)2
CH3(CH2)8CH(OAc)2
90e,29
71d,f,41
8
—
rt
27 h
NR
a
b
c
Reagent grade acetonitrile was used.
All reactions were run at room temperature unless otherwise mentioned, and reaction progress was monitored by 1H NMR or TLC.
Refers to yield of isolated product that was deemed to be sufficiently pure (>98%) by 1H & 13C NMR spectroscopy, unless otherwise mentioned. Superscript next to yield
refers to literature reference for the product.
d
Yield of product after purification by flash chromatography.
Reaction was carried out with 5.0 equiv of acetic anhydride.
Crude product was purified by trituration with pentane/methanol (9:1).
e
f
Scheme 1.
rate, we propose that product 3 arises via the initially formed acetate
vent-free conditions in most cases. Acetophenone failed to yield
any acylal.
2, which subsequently eliminates and dimerizes via
a 3°
carbocation.
Although detailed mechanistic studies were not carried out, a
few points merit comment. The observation that a solution of iron
tosylate in water is acidic (pH ꢁ2) suggests that the p-TsOH might
be an active catalyst in these reactions. Not surprisingly, p-TsOH
was found to catalyze the acylation of menthol (Table 1, entry 8)
under similar reaction conditions. Furthermore, the acylation of
menthol with Fe(OTs)3 (2.0 mol %) in the presence of proton-
Attempts to make the monoacetate from a symmetrical diol 1,
5-pentanediol (entry 21) using 1 equiv of acetic anhydride were
not successful. When 1,5-pentanediol was reacted with 1.05 equiv
of acetic anhydride, the product was a mixture of the monoacetate,
diacetate, and unreacted starting material. However, formation of
the diacetate proceeded smoothly in the presence of 2.6 equiv of
acetic anhydride. With diols containing a 1° and a 2° hydroxy
group, a noticeable difference in the rate of acetylation of the 1°
vs 2° OH was observed in THF as the solvent. When 1-phene-
thane-1,2-diol (entry 19) was reacted with 1.0 equiv of acetic
anhydride in THF as the solvent in the presence of 0.5 mol % of
Fe(OTs)3ꢀ6H2O, the crude product was found to be (by 1H NMR) a
mixture of the 1° monoacetate (44%), 2° monoacetate (14%), the
diacetate (7%) and unreacted starting material (35%). Again, the
use of 2.6 equiv of acetic anhydride afforded the diacetate in good
yield. We have previously reported that aliphatic TBDMS groups
can be cleaved with iron(III) tosylate in the presence of a phenolic
TBDMS ether.14c Consistent with this observation is the fact that
we were able to acetylate a phenol in the presence of a phenolic
TBDMS group (entry 17).
Acylals (geminal diacetates) have often been used as protecting
groups for carbonyl compounds because they are stable to neutral
and basic conditions.1 Acylals can also be converted into other
functional groups, adding to their synthetic utility.28 Some recent
examples of catalysts used for their synthesis include Al(OTf)3,29
silica chloride,30 Fe(NO3)3ꢀ9H2O,31 SnCl2ꢀ2H2O,32 CoCl2,33 boric
acid,34 ferrous methanesulfonate,35 Er(OTf)3,36 Bi(NO3)3ꢀ5H2O,37
and Bi(OTf)3.38 Herein we report that iron(III) tosylate is also an
efficient catalyst for the acylation of a range of aldehydes under
mild conditions (Table 2). The reaction was carried out under sol-
spongeÒ
(N,N,N0,N0-tetramethyl-1,8-naphthalenediamine)Ò42
(6.0 mol %) was not successful, and the starting material was
recovered. This evidence suggest that p-TsOH could be the true cat-
alyst though the role of Fe3+ as a Lewis acid cannot be completely
ruled out because it is possible that the Fe3+ loses its activity by
coordinating to the nitrogen atom in the proton sponge. From a
practical standpoint, the use of Fe(OTs)3 is still preferable to p-
TsOH because the latter compound is highly toxic and its handling
poses a health hazard.43
Representative procedures are given here
Method A (synthesis of an acetate under solvent-free condi-
tions): A homogenous mixture of cinnamyl alcohol (0.998 g,
7.44 mmol) and acetic anhydride (0.987 g, 0.91 mL, 9.67 mmol)
was stirred as Fe(OTs)3ꢀ6H2O (0.101 g, 0.1487 mmol, 2.0 mol %)
was added. The progress of the reaction was followed by GC. After
15 min, aqueous 10% Na2CO3 (10 mL) was added and the mixture
was stirred for 10 min. The reaction mixture was extracted with
ethyl acetate (2 ꢂ 20 mL). The combined organic layers were
washed with saturated aqueous NaCl (15 mL), dried (Na2SO4),
and concentrated on a rotary evaporator to yield 1.248 g (95%) of
a clear, slightly yellow liquid that was identified as cinnamyl

N. J. Baldwin et al. / Tetrahedron Letters 53 (2012) 6946–6949
6949
acetate and was determined to be >98% pure by 1H & 13C NMR
spectroscopy, and 96% pure by GC.
Organic Synthesis, 1st ed.; Blackwell Science, Inc.: Malden, MA, 1999; (c)
Kocienski, P. J. Protecting Groups, 1st ed.; Georg Thieme Verlag: Stuttgart, 1994.
2. Stork, G.; Takahashi, T.; Kawamoto, I.; Suzuki, T. J. Am. Chem. Soc. 1978, 100,
8272.
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2877; (b) Carrigan, M. D.; Freiberg, D. A.; Smith, R. C.; Zerth, H. M.; Mohan, R. S.
Synthesis 2001, 2091.
Method B (synthesis of an acetate in solvent): A suspension of p-
nitrobenzyl alcohol (0.486 g, 3.18 mmol) and acetic anhydride
(0.421 g, 0.390 mL, 4.13 mmol) in CH3CN (5 mL) was stirred at
room temperature as Fe(OTs)3ꢀ6H2O (43.1 mg, 0.0636 mmol,
2.0 mol %) was added. The progress of the reaction was followed
by gas chromatography. After 45 min, CH3CN was removed on a ro-
tary evaporator, and then aqueous 10% Na2CO3 (5 mL) was added
to the residue and stirred for 10 min. The reaction mixture was ex-
tracted with ethyl acetate (2 ꢂ 20 mL). The combined organic lay-
ers were washed with saturated NaCl (15 mL), dried (Na2SO4) and
concentrated on a rotary evaporator to yield 0.61 g (98%) of a yel-
low solid that was identified as p-nitrobenzyl acetate and was
determined to be 97% pure by GC, 1H NMR, and 13C spectroscopy.
Method C (synthesis of a benzoate): A heterogeneous mixture of
phenethyl alcohol (0.544 g, 4.45 mmol) and benzoic anhydride
(1.51 g, 6.67 mmol) was dissolved in CH3CN (5 mL) as Fe(OTs)3ꢀ6
H2O (151.2 mg, 0.223 mmol, 5.0 mol %) was added. The reaction
mixture was heated at 70 °C (temperature controlled hot plate)
and the progress of the reaction was followed by TLC (EtOAc/hep-
tane, 30/70). After 24 h, the mixture was cooled, acetonitrile was
removed on a rotary evaporator and ethyl acetate (20 mL) was
added to the residue. The resulting solution was washed with
Na2CO3 (2 ꢂ 15 mL) and saturated aqueous NaCl (15 mL). The or-
ganic layer was dried (Na2SO4) and concentrated on a rotary evap-
orator to yield 1.56 g of a yellow orange liquid. NMR spectroscopy
analysis showed that benzoic anhydride was still present in the
crude product. The crude product was purified by flash chromatog-
raphy on silica gel (70 g). A solvent gradient of EtOAc/heptane (10/
90, then 20/80) was used for elution. A total of 45 fractions (8 mL-
size) were collected, and fractions 15–21 were combined to yield
0.74 g (73%) of a very pale yellow clear liquid that was identified
as phenethyl benzoate and was determined to be >98% pure by
1H and 13C NMR spectroscopy.
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reported. Hence it is given here. 1H NMR (270 MHz, CDCl3) d 0.87 (t, 3 H, J =
7.2 Hz), 1.42 (m, 2 H), 2.00 (s, 3 H), 2.03 (m, 2 H), 2.41 (m, 2 H), 4.04 (t, 2 H, J =
6.9 Hz);13C NMR (67.5 MHz, CDCl3, 9 peaks): 13.2, 19.1, 20.5, 20.7, 22.1, 62.8,
75.5, 81.7, 170.7.
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H), 2.08 (m, 2 H), 2.59 (m, 2 H), 4.38 (t, 2 H, J = 7.2 Hz), 7.3 – 8.0 (m, 5 H); 13C
NMR (67.5 MHz, CDCl3, 12 peaks)13.2, 19.2, 20.5, 22.1, 63.1, 75.5, 81.8, 128.1,
129.5, 130.0, 132.8, 166.1
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Method D (synthesis of an acylal): A homogeneous mixture of
m-anisaldehyde (0.500 g, 3.67 mmol) and acetic anhydride
(0.75 g, 0.694 mL, 7.35 mmol, 2.0 equiv) was stirred at room tem-
perature as Fe(OTs)3ꢀ6H2O (0.0448 g, 0.0735 mmol, 2.0 mol %)
was added. (Caution: an exothermic reaction occurs and hence
due care must be exercised when scaling up this reaction). The
reaction progress was followed by 1H NMR spectroscopy. After
3 h 40 min, aqueous 10% NaHCO3 (10 mL) was added to the reac-
tion mixture and stirred for 10 min. The reaction mixture was ex-
tracted with EtOAc (2 ꢂ 20 mL) and the combined organic layers
were washed with saturated NaCl (15 mL), dried (Na2SO4) and con-
centrated on a rotary evaporator to yield 0.869 g of an orange li-
quid. The crude product was purified by flash chromatography
(5 g silica) using EtOAc/heptane (20/80) as the eluent to yield
0.801 g (92%) of the acylal as a colorless liquid that was determined
to be >98% pure by 1H and 13C NMR spectroscopy.
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Acknowledgements
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We wish to acknowledge a grant from the American Chemical
Society-Petroleum Research Fund (ACS PRF 51036-UR1) awarded
to RM.
43. According to the MSDS sheets, iron(III) tosylate is safer to use than p-
toluenesulfonic acid. Iron(III) tosylate has
identification system) rating of 2 (moderate hazard) while p-toluenesulfonic
a HMIS (hazardous material
References and notes
acid has a HMIS rating of 3 (serious hazard).
1. (a) Greene, T. W.; Wots, P. G. M. Protective Groups in Organic Synthesis, 3rd ed.;
John Wiley and Sons, Inc.: New York, 1999; (b) Hanson, J. R. Protecting Groups in