RuCl3·xH2O: A new efficient catalyst for facile preparation of 1,1-diacetates from aldehydes

Article abstract of DOI:10.1080/00397910701650831

An efficient, facile preparation of aldehyde 1,1-diacetates (acylals) in excellent yields catalyzed by RuCl3·xH2O is described. Ketones do not react under these conditions. Copyright Taylor & Francis Group, LLC.

Full text of DOI:10.1080/00397910701650831

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RuCl · xH O: A New Efficient Catalyst for
3
2
Facile Preparation of 1,1‐Diacetates from
Aldehydes
a
a
a
Anil Saini , Sanjay Kumar & Jagir. S. Sandhu
a
Department of Chemistry , Punjabi University , Patiala, Punjab, India
Published online: 25 Jan 2008.
To cite this article: Anil Saini , Sanjay Kumar & Jagir. S. Sandhu (2007) RuCl · xH O: A New Efficient Catalyst
3
2
for Facile Preparation of 1,1‐Diacetates from Aldehydes, Synthetic Communications: An International Journal
for Rapid Communication of Synthetic Organic Chemistry, 38:1, 106-113, DOI: 10.1080/00397910701650831
To link to this article: http://dx.doi.org/10.1080/00397910701650831
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Synthetic Communications , 38: 106–113, 2008
Copyright # Taylor & Francis Group, LLC
ISSN 0039-7911 print/1532-2432 online
DOI: 10.1080/00397910701650831
.
RuCl xH O: A New Efficient Catalyst for
3
2
Facile Preparation of 1,1-Diacetates from
Aldehydes
Anil Saini, Sanjay Kumar, and Jagir. S. Sandhu
Department of Chemistry, Punjabi University, Patiala, Punjab, India
Abstract: An efficient, facile preparation of aldehyde 1,1-diacetates (acylals) in
.
excellent yields catalyzed by RuCl
these conditions.
3 2
xH O is described. Ketones do not react under
Keywords: acetic anhydride, aldehydes, gem-diacetates, ruthenium(III) chloride,
solvent-free
Selective protection of carbonyl function as 1,1-diacetates (acylals) or gem-
[1]
bis(acyloxy)-alkanes is an important transformation in organic chemistry.
[1b]
Protective group acylals are fairly stable under neutral and basic conditions
as well as under critically controlled acidic conditions. Apart from mere pro-
tective groups, acylals are important synthons and useful precursors. The
acylals derived from a, b-unsaturated aldehydes are important starting
[2]
materials in the Diels–Alder reaction. These 1,1-diacetates have several
synthetic and industrial applications and are used as cross-linking
[
3]
reagents in cellulose and the cotton industry and are also used as stain-
bleaching agents. As synthons, their usefulness has been exploited in well-
[4]
[4b]
known reactions of organic chemistry, such as the Grignard, Barbier,
[
and Prins reactions, and condensation reactions, such as the Knoevenagel
5]
[6]
[7]
and benzoins reactions. Because of their unique properties as protective
Received in India May 15, 2007
Address correspondence to Jagir. S. Sandhu, Department of Chemistry, Punjabi
University, Patiala 147 002 Punjab, India. E-mail: j_sandhu2002@yahoo.com
1
06

.
RuCl3 xH O
107
2
groups as well as important synthons, search for efficient, mild, and facile
preparation of acylals is of current interest. Apart from other methods,
1
and a catalyst (viz. strong Bronsted acids such as H SO and H PO , and
,1-diacetates are conventionally prepared from aldehyde, acetic anhydride,
[8]
2
4
3
4
[
super acids such as Nafion-H and heteropolyacids). Use of strong Lewis
9]
[
10]
acids
Zn(ClO4)2 6H O, have also been reported. Apart from these catalysts,
such as BF , PCl , ZnCl , LiBF , ZrCl , FeCl , FeCl /SiO , and
3 3 2 4 4 3 3 2
.
11]
2
[
expensive
graphite, zeolite, tungstosilicic acid, and zirconium sulfophenyl
phosphonate have also been employed in this protection process. Some of
these methods still suffer from drawbacks such as prolonged reaction time
(viz. up to 120 h in the case of 2-furyl aldehyde with PCl ), low yields in
the case of 4-nitrobenzaldehyde (4%) and cinnamaldehyde (30%) when
3
PCl is used, and in some cases the requirement of elevated temperature.
3
Several of these catalysts are not very safe to handle, such as metal perchlor-
ates and BF . Consequently, it seems desirable and necessary to develop
3
a simple, safe, efficient, and facile method for the preparation of these gem-
diacetates.
Development of novel methodologies utilizing ruthenium salts and
.
complexes in organic synthesis is developing fast and use of RuCl3 xH O
2
as a mild Lewis acid is of current interest. Ruthenium(III) chloride is
catalyze Prins
[12]
reported to cleave epoxides to yield trans-diols;
[
13]
[14]
[15]
reaction,
and very recently has been reported for deoxygenation of organic
well-known Biginelli reaction,
and aldol condensation;
[
16]
N-oxides.
H O and NaIO , a plethora of oxidative transformation has been reported,
In addition to this, when employed with oxidants such as
2
2
4
notably N-alkyl pyrrolidines to pyrrolidin-2-ones, Hantzsch 1,4-dihydropyri-
dines to pyridines (here only aerial oxygen is used), double bond cleavage,
and oxidation of alcohols to aldehydes and ethers to lactones.
[16]
Because ruthenium has a good affinity for oxygen
and has never been
used for this transformation, it was of extreme interest to see its utility for the
preparation of 1,1-diacetates from aldehyde and acetic anhydride. Indeed, we
were delighted to see success in this (see Scheme 1), and there was no
evidence of any oxidation of aldehydes to corresponding carboxylic acids,
which was our major concern.
In some initial trial experiments, we used varying quantities of catalyst
viz. from 1 mol% to 10 mol% (entries 3 and 4, Table 1)]. Indeed, we could
[
Scheme 1.

1
08
A. Saini, S. Kumar, and J. S. Sandhu
O-promoted preparation of aldehyde 1,1-diacetates
Time (min) Yield (%)
Method Method Method Method
.
Table 1. RuCl
3
xH
2
b
a
c
d
Entry
1
Substrates
Product
A
B
A
B
2a
10
6
97
98
2
2b
2b
2b
2c
2d
10
12
10
12
20
6
7
98
89
98
97
96
98
87
98
96
97
e
3
f
4
6
5
6
6
10
7
8
9
2e
2f
25
35
13
16
96
92
96
94
2g
2h
15
30
8
90
94
91
91
1
0
13
1
1
2i
15
7
91
90
1
2
3
2j
15
15
8
8
90
87
92
88
1
2k
1
4
2l
20
14
80
82
(
continued)

.
RuCl3 xH O
109
2
Table 1. Continued
b
Yield (%)
Time (min)
Method Method Method Method
d
a
c
A
Entry
Substrates
Product
B
A
B
15
2m
15
8
90
91
1
6
2n
12
15
6
93
96
1
7
8
2o
9
82
—
83
—
g
6
g
6
1
—
g
6
g
6
19
—
—
—
a
All the products were characterized by comparison of their spectral and physical
data with those of authentic samples.
b
Isolated yields of corresponding gem-diacetates.
Method A: Aldehyde (2 mmol), acetic anhydride (6 mmol), RuCl3 xH O
c
.
2
(
(
2 2
5 mol%) in CH Cl stirred at room temperature.
d
.
3 2
xH O
Method B: Aldehyde (2 mmol), acetic anhydride (6 mmol), RuCl
5 mol%) in solvent-free conditions stirred at room temperature.
e
.
0
.01 equivalent of RuCl
3
xH
2
O is used.
.
3 2
xH O is used.
f
0
.10 equivalent of RuCl
g
Time in hours.
establish the optimum quantity of the catalyst to 5 mol%. In a pilot exper-
.
iment, p-tolualdehyde, acetic anhydride, and RuCl3 xH O (1:3:0.05) in
2
dichloromethane (method A) were stirred at room temperature for 10 min to
obtain 1,1-diacetate in 98% yield.
To see the wide application of this procedure, other aldehydes were
reacted following a similar procedure to afford gem-diacetates in 80–98%
yields. Under these conditions, the reaction time was reduced dramatically
and reaction completed within 10–35 min (method A, Table 1). After this

1
10
A. Saini, S. Kumar, and J. S. Sandhu
success, we looked for further improvements in this process, namely to carry
this reaction out under solvent-free conditions at room temperature (method
B). Under solvent-free conditions, equivalent results were obtained, and
reaction time was shortened to 6–14 min, affording gem-diacetates in
82–98% yields.
A variety of aromatic, aliphatic, and heterocyclic aldehydes were
converted to corresponding 1,1-diacetates using acetic anhydride in the
.
presence of RuCl3 xH O in excellent yields at room temperature and in
2
short reaction times. All aromatic aldehydes carrying electron-donating or
electron-withdrawing substituents reacted well; however, as one can see
from Table 1, yields are slightly lower in aromatic aldehydes with electron-
withdrawing groups and in cases of conjugated aldehydes, particularly croto-
naldehyde and acrolein (entries 13 and 14, Table 1). This fall in yields may be
due to the formation of polymeric materials as side products, which is well
expected. To see selectivity/chemoselectivity when a mixture of aldehyde
and ketones was employed, only aldehyde diacetates were obtained; ketones
remained unaffected, such as acetophenone and benzophenone (entries 18
and 19, Table 1). Further continuing this observation, when aldehyde and
ketone groups are in the same molecule, only aldehyde diacetate was
obtained; the ketone part remained intact (see entry 16, Table 1)
(
Scheme 2). This reaction was further extended to naturally occurring
terpine 7-hydroxycitronellal (entry 15, Table 1). In this reaction sequence,
the alcoholic group is also acylated apart from acylal formation (Scheme 3).
.
In conclusion, the present method, which employs RuCl3 xH O, is mild,
2
facile, and efficient for the preparation of 1,1-diacetates from aldehydes. In
addition, this protocol has advantages in terms of short reaction times, high
yields, high selectivity, and fairly wide scope and does not need rigorous
reaction conditions. Also this research discloses the first-ever use of
Scheme 2.

.
RuCl3 xH O
111
2
Scheme 3.
.
RuCl3 xH O in acylal formation and adds further merits to the utility of this
mild Lewis acid catalysis in organic chemistry.
2
EXPERIMENTAL
Melting points were determined in open capillaries and are uncorrected.
Reagent-grade chemicals were purchased from a commercial source and
used without further purification. IR spectra were recorded in KBr discs on
1
a Perkin-Elmer 24OC analyzer. H NMR spectra were recorded on Varian
Gemini 300 (300-MHz) spectrometer using TMS as internal standard. The
progress of the reaction was monitored by thin-layer chromatography (TLC)
using silica gel G (Merck). Acetic anhydride was distilled prior to use.
General Procedure for Preparation of 1,1-Diacetates: Method A
To a stirred solution of aldehyde (2 mmol) and acetic anhydride (6 mmol) in
.
dichloromethane (10 mL), RuCl3 xH O (5 mol%) was added, and the
2
mixture was stirred at room temperature for time indicated in Table 1. After
completion (monitored via TLC), the reaction mixture was diluted with
dichloromethane and washed with saturated NaHCO solution (3 Â 15 mL)
3
and then with saturated brine. The organic layer was dried over anhydrous
Na SO and concentrated in vacuo to afford the pure corresponding gem-
diacetates.
2
4
General Procedure for Preparation of 1,1-Diacetates under Solvent-
Free Conditions; Method B
To a stirred solution of aldehyde (2 mmol) and acetic anhydride (6 mmol) in
.
solvent-free conditions, RuCl3 xH O (5 mol%) was added, and the mixture
was stirred at room temperature for the time indicated in Table 1. After
2

1
12
A. Saini, S. Kumar, and J. S. Sandhu
completion of reaction (monitored via TLC), the reaction mixture was
extracted with dichloromethane and washed with saturated NaHCO3
solution (3 Â 15 mL) and then with saturated brine. The organic layer was
dried over anhydrous Na SO and concentrated in vacuo to afford the pure
2
4
corresponding gem-diacetates.
Representative Spectral Data
[9]
Compound 2b: Mp 81–828C, Lit. Mp 81–82; IR (KBr): 2950, 1771, 1742,
2
1 1
1
510, 1398, 1250, 1010, 960 cm ; H NMR (300 MHz, CDCl ): d ¼7.66
3
(
s, 1H), 7.38 (d, J ¼ 8.2 Hz, 2H), 7.21 (d, J ¼ 8.2 Hz, 2H), 2.40 (s, 3H),
2.14 (s, 6H). Anal. calcd. for C H O : C, 64.86; H, 6.30. Found: C, 64.96;
H, 6.21.
1
2
14
4
[9]
Compound 2f: Mp 124–1258C, lit. Mp 124–125; IR (KBr): 2952, 1760,
21 1
619, 1525, 1351, 1239, 1200, 995 cm ; H NMR (300 MHz, CDCl ): d
1
3
¼
8.30 (d, J ¼ 7.9 Hz, 2H), 7.70 (d, J ¼ 7.9 Hz, 2H), 7.52 (s, 1H), 2.15
(
s, 6H). Anal. calcd. for C H NO : C, 52.17; H, 4.34; N, 5.53. Found: C,
6
1
1
11
52.10; H, 4.25; N, 5.45.
[9]
Compound 2g: Mp 83–858C, lit. Mp 84–86; IR (KBr): 2945, 1758, 1604,
2
1 1
1
501, 1402, 1249, 1198, 1009, 940 cm ; H NMR (300 MHz, CDCl ): d ¼
3
7.29–7.38 (m, 6H), 7.01 (d, 1H), 6.20–6.72 (m, 1H), 2.11 (s, 6H). Anal. calcd.
for C H O : C, 66.67; H, 5.98. Found: C, 66.90; H, 6.17.
1
3 14 4
21 1
Compound 2k: Liquid IR (KBr): 2950, 1755, 1398, 1246, 1216 cm ; H
NMR (300 MHz, CDCl ): d ¼ 6.80 (d, J ¼ 6.1 Hz, 1H), 5.20–6.10
3
(
Found: C, 55.99; H, 6.79.
m, 2H), 1.86–2.11 (m, 9H). Anal. calcd. for C H O : C, 55.81; H, 6.97.
8 12 4
ACKNOWLEDGMENT
The Council of Scientific and Industrial Research, New Delhi, India, is
thanked for liberal support and the award of stipends to A. Saini and S. Kumar.
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