Indium tribromide as a highly efficient and versatile catalyst for chemoselective synthesis of acylals from aldehydes under solvent-free conditions

Article abstract of DOI:10.1055/s-2004-829549

A mild and efficient method has been developed for the chemoselective preparation of acylals from aldehydes in the presence of catalytic amounts (0.01-1.0 mol%) of InBr₃ under solvent-free conditions in very good to excellent yields.

Full text of DOI:10.1055/s-2004-829549

LETTER
1727
Indium Tribromide as a Highly Efficient and Versatile Catalyst for
Chemoselective Synthesis of Acylals from Aldehydes under Solvent-Free
Conditions
I
ndium Tribrom
i
ide as
a
a
H
ighlyEf
n
f
icient andV
g
ersatile Catalyst Yin, Zhan-Hui Zhang, Yong-Mei Wang,* Mei-Li Pang
Department of Chemistry, the State Key Laboratory of Functional Polymer Materials for Absorption and Separation, Nankai University,
Tianjin 300071, P. R. China
Fax +86(22)23502654; E-mail: zhanhui@nankai.edu.cn
Received 14 April 2004
do not make triflates good contenders for use in large
Abstract: A mild and efficient method has been developed for the
scale synthesis. Therefore, mild reaction conditions that
can overcome the shortcomings of previous methods are
necessary.
chemoselective preparation of acylals from aldehydes in the pres-
ence of catalytic amounts (0.01–1.0 mol%) of InBr3 under solvent-
free conditions in very good to excellent yields.
Key words: acylals, aldehydes, chemoselective, indium tribromide, Since Li and Chan initiated and applied indium-mediated
reactions in aqueous media in 1991,15 indium complexes
solvent-free conditions
have emerged as mild and water-tolerant Lewis acid cata-
lysts for organic synthesis with high chemo-, regio- and
stereoselectivity.16 Particularly, indium tribromide has
Selective protection and deprotection is the heart and soul
been found to be a more effective catalyst than conven-
tional Lewis acids in promoting various transformations
including thioacetalization, Biginelli reaction, sulfonation
of indoles, stereoselective synthesis of alkynyl sugars, nu-
cleophilic addition, intramolecular Michael addition, syn-
thesis of 1,3-dioxane derivatives, Ferrier rearrangement,
regioselective ring opening of epoxides, conversion of ox-
iranes to thiiranes, and conjugate addition.17 In this paper,
we wish to report the results of the chemoselective con-
version of aldehydes to acylals using a catalytic amount of
InBr3 (0.01–1.0 mol%) under solvent-free conditions
(Scheme 1).
of multistep organic synthesis. Acylals (geminal diace-
tates) of aldehydes have already been recognized as im-
portant protecting group alternatives to acetals1 because
of their stability in neutral and basic medium2 and their
easy conversion into the parent aldehydes.3 In addition,
acylals are useful starting materials and intermediates in
organic synthesis4 and have been used as cross linking re-
agents for cellulose in cotton.5 Smonou and Angelis have
reported the synthesis of optical aldehydes by lipase-cata-
lyzed resolution of the corresponding acylals.6 As a result,
a variety of methods have been examined for their forma-
tion. Generally, acylals are prepared from aldehydes and
acetic anhydride using strong protonic acids, such as sul-
furic, phosphoric, methanesulfonic or perchloric acids,7
and Lewis acids, such as zinc chloride, ferric chloride, fer-
rous sulfate, lithium bromide, phosphorus trichloride,
tungsten hexachloride, indium chloride, ceric ammonium
nitrate, lithium tetrafluoroborate, zinc tetrafluoroborate,
Fe2(SO4)3·xH2O, zirconium chloride, copper(II) tetrafluo-
roborate, bismuth nitrate,8 heteropolyacid Well–Dawson
(H6P2W18O62·24H2O) and aluminum dodecatungstophos-
phate (AlPW12O40),9 and heterogeneous catalysts like
Nafion-H, expansive graphite, zeolites, clay and support-
ed reagents.10 Other catalysts, such as iodine,11 N-bromo-
succinimide,12 trimethylchlorosilane/sodium iodide,13
have also been used for this transformation. But these pro-
cedures are often accompanied by longer reaction times,
low product yields, stringent conditions, high catalyst
loading, corrosive reagents, high temperature and require
the use of toxic metal ions and solvents. Although recently
the triflates like Cu(OTf)2, Sc(OTf)3 and LiOTf have
emerged as the most effective catalysts,14 the high cost
and susceptibility to aqueous medium of the metal triflates
OAc
Ac2O, InBr3 (0.01–1.0 mol%)
RCHO
R
neat, r.t.
OAc
1
2
Scheme 1
As shown in Table 1, a series of aliphatic and aromatic al-
dehydes reacted with acetic anhydride smoothly to afford
the corresponding acylals in the presence of 0.1 mol% cat-
alyst at ambient temperature in very good to excellent
yields. In the solvent-free condition, the acetylations
could be successfully performed with a ten-fold increase
of benzaldehyde (1a) in the presence of lower catalyst
loading (0.01 mol%) with only a marginal drop in reaction
rate (entry 3). The presence of electron-donating and elec-
tron-withdrawing groups on the aromatic ring of alde-
hydes, irrespective of their positions in the ring, did not
make any obvious difference in the acylation. Aliphatic
(1b–f) as well as a,b-unsaturated (1g and 1t) aldehydes
also gave the corresponding acylals in good yields under
similar reaction conditions. 2,4-Dinitrobenzaldehyde (1y)
and 4-(dimethylamino) benzaldehyde (1z), however, due
to deactivation of the carbonyl group, remained unaffect-
ed even when the reaction mixtures were stirred at room
SYNLETT 2004, No. 10, pp 1727–1730
0
9
.0
8
.2
0
0
4
Advanced online publication: 15.07.2004
DOI: 10.1055/s-2004-829549; Art ID: U10604ST
© Georg Thieme Verlag Stuttgart · New York

1728
L. Yin et al.
LETTER
Table 1 InBr3-Catalyzed Conversion of Aldehydes to Acylals
Entry
R
Catalyst load
(mol%)
Time (min)
Yield (%)a
Mp (°C) or bp (°C)/Torr
Found
44–45
Reported
44–4510g
1
2
Ph (1a)
1.0
0.1
0.01
0.1
0.1
0.1
0.1
0.1
0.1
0.1
0.1
0.1
0.1
0.1
0.1
0.1
0.1
0.1
0.1
0.1
0.1
0.1
0.1
0.1
0.1
0.1
0.1
0.1
1.0
5
10
30
60
20
40
30
30
30
35
6
98
Ph (1a)
96
3
Ph (1a)
95b
4
Ph (1a)
91c
5
CH3(1b)
84
87–88/30
111–113/10
128–130/10
128–130/2
141–143/10
81–83/10
58–59
73–75/158f
85–87/510a
145/2010f
6
CCl3 (1c)
76
7
CH3(CH2)3 (1d)
CH3(CH2)4 (1e)
CH3(CH2)8 (1f)
CH3-CH=CH (1g)
2-MeC6H4 (1h)
4-MeC6H4 (1i)
2-MeOC6H4 (1j)
4-MeOC6H4 (1k)
2-ClC6H4 (1l)
3-ClC6H4 (1m)
4-ClC6H4 (1n)
3,4-(OCH2O)C6H3 (1o)
2-NO2C6H4 (1p)
3-NO2C6H4 (1q)
4-NO2C6H4 (1r)
4-FC6H4 (1s)
89
8
90
127–129/28h
9
92
10
11
12
13
14
15
16
17
18
19
20
21
22
23
24
25
26
27
28
29
83
89–90/158f
97
8
98
82–83
80.5–81.510g
74–757c
6
94
75–76
20
8
96
64–65
64–6510g
97
57–58
56–577c
6
96
65–66
65–6610g
15
3
97
80–81
81–8210g
98
78–79
78–7910g
10
12
15
20
20
30
12 h
14 h
10 h
24 h
24 h
98d
86–87
85.5–86.57c
64–6510g
96d
66–67
99d
127–128
50–51
125.5–126.510g
50.5–51.57c
83.5–84510g
50.5–51.510g
89–907c
98
(E)-PhCH=CH (1t)
Furfural (1u)
96
84–85
88
50–51
4-OHC6H4 (1v)
2-OH-4-BrC6H3 (1w)
3-MeO-4-OHC6H3(1x)
2,4-(NO2)2C6H3 (1y)
4-Me2NC6H4 (1z)
96d,e
89–91
95d,e
90–91
94d,e
90–91
90–9110g
No reaction
No reaction
a Isolated yield of the corresponding acylals.
b Ten-fold benzaldehyde was used.
c InCl3 as catalyst.
d 6 Equiv of acetic anhydride (with respect to substrate) was used.
e Yield refers to pure triacetate.
Synlett 2004, No. 10, 1727–1730 © Thieme Stuttgart · New York

LETTER
Indium Tribromide as a Highly Efficient and Versatile Catalyst
1729
temperature for one day, and the starting materials were presence of InCl3 required longer reaction times and
quantitatively recovered. The reaction conditions are mild higher catalyst loading and led to poorer yields of prod-
enough not to induce any isomerization for conjugated ucts than those obtained with InBr3. This result clearly
aldehydes (1g and 1t) or damage to moieties such as shows the strong catalytic activity of InBr3 in comparison
methoxy (1j and 1k) and methylenedioxy (1o), which with InCl3.
often undergo cleavage in strongly acidic reaction media.
In conclusion, InBr3 was found to be a novel and highly
Side product formation was not observed in the reactions
efficient Lewis acid catalyst for the conversion of alde-
we studied.
hydes into acylals under mild conditions. The advantages
It is worth noting that hydroxy groups in 4-hydroxy-, 2- include the low loading of catalyst, operation at room
hydroxy-4-bromo-benzaldehydes (1v and 1w) and va- temperature, high yields, excellent chemoselectivity, and
niline (1x) were also acetylated to afford the correspond- applicability to large-scale reactions.
ing triacetates under similar conditions, but the
acetylation of phenolic aldehydes required a longer reac-
tion period.
General Procedure for the InBr3-Catalyzed Conversion of
Aldehydes into Acylals:
The selectivity of the present method is demonstrated by
a competition experiment for the acylation of aldehydes in
the presence of ketones under solvent-free conditions at
room temperature. When a 1:1 mixture of benzaldehyde
and acetophenone was allowed to react with acetic anhy-
A mixture of aldehydes (10 mmol), freshly distilled Ac2O (30
mmol) and InBr3 (3.6 mg, 0.01 mmol, 0.1 mol%) was stirred at r.t.
The progress of the reaction was monitored by TLC or GC. After
completion of the reaction, EtOAc (100 mL) was added and the
mixture was washed successively with 1 M NaOH solution (2 × 20
dride in the presence of 0.1 mol% InBr3 for 15 minutes, mL), brine (10 mL), and H2O (10 mL). The organic layer was sep-
arated and dried (Na2SO4). Evaporation of the solvent under re-
TLC analysis of reaction mixture indicated complete dis-
duced pressure gave almost pure product. Further purification was
appearance of benzaldehyde, while acetophenone was
achieved by column chromatography on silica gel or recrystalliza-
tion from EtOAc–hexane to give pure product in good to excellent
yield (Table 1). The products were characterized by IR and H
NMR and by comparison of the spectral data with those of authentic
samples.
still intact, even when the mixture continued to stir for 24
hours, due to the steric effects of methyl group in ace-
tophenone.
1
In comparison with other catalysts such as InCl3, CAN,
LiOTf, Cu(OTf)2, Sc(OTf)3, ZrCl4, AlPW12O40, LiBF4
and Cu(BF4)2·xH2O, which were recently reported in the
acylation of 4-nitrobenzylaldehyde, InBr3 employed here
shows more effective catalytic reactivity than InCl3,
CAN, LiOTf, Cu(OTf)2, ZrCl4 and LiBF4 in terms of the
amount of catalyst, yields and reaction times (Table 2). It
should be pointed out that acylation of aldehydes in the
Selected spectroscopic data:
2c: IR (neat): 797, 1029, 1219, 1375, 1431, 178, 2941, 2985 cm–1.
1H NMR (300 MHz, CDCl3): d = 2.21 (s, 6 H), 7.25 (s, 1 H) ppm.
2f: IR (neat): 1004, 1201, 1240, 1371, 1760, 2860, 2921 cm–1. H
NMR (300 MHz, CDCl3): d = 0.88 (t, J = 6.6 Hz, 3 H), 1.26–1.35
(m, 14 H), 1.72–1.77 (m, 2 H), 2.08 (s, 6 H), 6.77 (t, J = 5.4 Hz, 1
H) ppm.
1
2h: IR (KBr): 683, 746, 840, 944, 105, 1120, 1220, 1373, 1492,
1583, 1604, 1767, 3014, 3063, 3106 cm–1. H NMR (300 MHz,
CDCl3): d = 2.13 (s, 6 H), 2.46 (s, 3 H), 7.19–7.31 (m, 3 H), 7.51
(dd, J = 7.8, 2 Hz, 1 H), 7.83 (s, 1 H) ppm.
1
Table 2 Comparison of the Effect of Catalysts in the Acylation of 4-
Nitrobenzylaldehyde
2w: IR (KBr): 731, 768, 848, 928, 969, 1011, 1045, 1213, 1247,
Catalyst
InCl3
Catalyst load (mol%) Time
Yield (%)
888h
1
1374, 1468, 1606, 1751, 2936, 2965, 3034 cm–1. H NMR (300
10
10
20
4 h
MHz, CDCl3): d = 2.12 (s, 6 H), 2.34 (s, 3 H), 7.02 (d, J = 8.7 Hz, 1
H), 7.54 (dd, J = 7.8, 2.7 Hz, 1 H), 7.77 (d, J = 2.7 Hz, 1 H), 7.85 (s,
1 H) ppm.
CAN
24 h
968i
LiOTf
15 h
9414c
9414a
9914b
928m
899b
Acknowledgment
Cu(OTf)2
Sc(OTf)3
ZrCl4
2.5
4 h
We thank Tianjin Natural Science Foundation (0236127711), the
State Key Laboratory of Functional Polymer Materials for Absorp-
tion and Separation, the State Key Laboratory of Elemento-Organic
Chemistry and Nankai University for financial support.
2
10 min
30 min
45 min
7.5 h
3 min
15 min
5
AlPW12O40
LiBF4
0.1
100
808j
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Synlett 2004, No. 10, 1727–1730 © Thieme Stuttgart · New York