Acetic acid-assisted copper methanesulfonate catalyst for chemoselective conversion of aldehydes to acylals

Article abstract of DOI:10.1080/00397910600633979

A new catalytic system has been discovered by combining copper methanesulfonate with acetic acid for chemoselective conversion of aldehydes to acylals in high yields at ambient temperature under solvent-free conditions. The efficiency of this system might result from the "double activation" of Bronsted-Lewis acid catalysis on aldehydes. Copyright Taylor & Francis Group, LLC.

Full text of DOI:10.1080/00397910600633979

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Acetic Acid-Assisted Copper
Methanesulfonate Catalyst for
Chemoselective Conversion of Aldehydes to
Acylals
a
b
b
a
Min Wang , Hong Gong , Heng Jiang & Zhichang Wang
a
Department of Chemistry , School of Sciences, Northeastern University ,
Shenyang, Liaoning, China
b
College of Petrochemical Engineering, Liaoning University of Petroleum
and Chemical Technology , Fushun, Liaoning, China
Published online: 20 Aug 2006.
To cite this article: Min Wang , Hong Gong , Heng Jiang & Zhichang Wang (2006) Acetic Acid-Assisted Copper
Methanesulfonate Catalyst for Chemoselective Conversion of Aldehydes to Acylals, Synthetic Communications:
An International Journal for Rapid Communication of Synthetic Organic Chemistry, 36:14, 1953-1960, DOI:
10.1080/00397910600633979
To link to this article: http://dx.doi.org/10.1080/00397910600633979
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Synthetic Communications , 36: 1953–1960, 2006
Copyright # Taylor & Francis Group, LLC
ISSN 0039-7911 print/1532-2432 online
DOI: 10.1080/00397910600633979
Acetic Acid-Assisted Copper
Methanesulfonate Catalyst for
Chemoselective Conversion of Aldehydes
to Acylals
Min Wang
Department of Chemistry, School of Sciences, Northeastern University,
Shenyang, Liaoning, China
Hong Gong and Heng Jiang
College of Petrochemical Engineering, Liaoning University of Petroleum
and Chemical Technology, Fushun, Liaoning, China
Zhichang Wang
Department of Chemistry, School of Sciences, Northeastern University,
Shenyang, Liaoning, China
Abstract: A new catalytic system has been discovered by combining copper methanesul-
fonate with acetic acid for chemoselective conversion of aldehydesto acylals in highyields
at ambient temperature under solvent-free conditions. The efficiency of this system
might result from the “double activation” of Brønsted–Lewis acid catalysis on aldehydes.
Keywords: Acylals, Brønsted acidity, copper methanesulfonate, Lewis acidity
INTRODUCTION
Superacidic systems that may be formed by mixing appropriate Lewis and
Brønsted acids have been the subject of much academic and industrial
research. Such complexation (ideally between an inactive Lewis acid and an
Received December 12, 2005
Address correspondence to Heng Jiang, College of Petrochemical Engineering,
Liaoning University of Petroleum and Chemical Technology, Fushun, Liaoning
113001, China. Tel.: þ 86-413-6860790; E-mail: hjiang78@hotmail.com
1
953

1
954
M. Wang et al.
inactive organic acid toward a specific reaction) has been used efficiently in
many types of electrophilic substitutions.
[1,2]
In this article, the mixture of
copper methanesulfonate (CMS) with acetic acid as a new catalytic system
is applied in synthesizing acylals.
Acylals (geminal diacetates) have been used as suitable protecting
groups for aldehydes because of their stability in mildly acidic and basic
media. The preparation of acylals has been achieved by the reaction of
aldehydes with Ac O under the catalysis of strong protic acids such as H SO ,
2
2
4
[
3]
[4]
[5]
H PO , CH SO H,
3
and H NSO H ; Lewis acids such as InCl ,
3
3
4
3
2
3
[
6]
[7]
[8]
[9]
.
.
Bi(OTf) xH O,
[13]
.
Bi(NO3)3 5H O,
2
Fe (SO ) xH O,
2
Cu(OTf) ,
2
[12]
4 3
2
3
2
[
10]
[11]
Sc(OTf)3, LiBF4, PCl3, and I2 ; and other types of catalysts such as
2
4
2 [14]
[15]
Amberlyst-15, and HY-zeolite. Although some improve-
[16]
ZrO /SO
,
2
ments have been observed in these methods, there are still some limitations, such
as the long reaction time, the highly corrosive and expensive catalysts, strictly
reactive condition (N atmosphere), and the large excess of Ac O (5–10
2
2
equiv.). Therefore, the development of an efficient, inexpensive, and green
protocol is of interest. For the CMS-HOAc catalytic system, methanesulfonates
[17,18]
that act as water-tolerant Lewis acid catalysts have been well documented.
Properties such as low toxicity and cost, air tolerance, and their recyclability
make them superior to common Lewis acids. Moreover, HOAc is a cheap
protonic acid, environmentally benign, commercially available, and easy to
handle. The systemovercomesthe problemsencountered with classical catalysts.
RESULTS AND DISCUSSION
The catalytic activities of CMS, HOAc, and CMS-HOAc in synthesizing
acylal from benzaldehyde are compared in Table 1. In a model reaction of
Table 1. Comparison of CMS, HOAc, and CMS-HOAc catalysts in
synthesizing acylal from benzaldehyde
Catalyst
a
CMS (0.6 mmol)
HOAc (24 mmol)
Time (h)
Yield (%)
Without
With
With
Without
Without
Without
Without
With
72
1.3
3
0
9
42
89
94
0
With
With
7
1.3
51
Without
With
a
Isolated yields.

Acetic Acid-Assisted Copper Methanesulfonate Catalyst
1955
benzaldehyde (30 mmol) and Ac O (60 mmol), a controlled experiment was
2
carried out first and the results show no reaction occurred even after 3 days.
After 0.6 mmol of CMS was added, the reaction proceeded slowly, and the
conversion of benzaldehyde reached 89% after 7 h. Under the same
condition, the conversion of benzaldehyde increased to 94% only after
1.3 h, when CMS was mixed with HOAc. However, acetic acid does not
induce the reaction in the absence of CMS.
Under the condition of aldehyde (30 mmol) and Ac O (60 mmol), a series
2
of aldehydes were subjected to a diacetoxylation reaction under catalysts of
CMS or CMS-HOAc. The results are summarized in Table 2. As can be
seen from Table 2, the reactions catalyzed by CMS proceeded with difficulty.
In most cases, CMS alone failed to catalyze the reactions. It is noteworthy that
the combination of CMS with HOAc is very efficient for the diacetoxylation
reaction of aldehydes. The aromatic aldehydes, a,b-unsaturated and aliphatic
aldehydes, afforded the corresponding 1,1-diacetates in high yields and purity.
Aromatic aldehydes with electron-withdrawing group such as nitro required
comparatively longer reaction time (12 h, entry 9). Phenol groups were also
protected for the acetylation of the hydroxyl groups in vanillin, 4-hydroxyben-
zaldehyde, and salicylal, which afforded the corresponding triacetates (entries
1
0, 11, and 12). Additionally, p-dimethylaminobenzaldehyde failed to give the
corresponding 1,1-diacetate because of the strong electron-donating ability of
dimethylin and a degree of tautomerization between a quinoid structure and
the aldehyde. Another reason may be the coordination of nitrogen in dimethy-
2
þ
lin with the Cu in CMS (entry 13). The chemoselectivity of this method was
demonstrated using equimolar benzaldehyde and acetophenone in the
presence of a catalytic amount CMS-HOAc for 9 h; exclusively diacetate of
benzaldehyde was obtained, and the quantitative acetophenone remained
because of the electron-donating ability and the steric effect of the methyl
group (entry 15).
A series of experiments confirmed that both CMS and HOAc are indis-
pensable for the conversion of aldehydes to acylals. The observed investi-
gations demonstrated a Brønsted acid–assisted Lewis acid catalysis. The
efficiency of this system might result from the “double activation” of
Brønsted–Lewis acid catalysis on aldehydes.
CONCLUSION
In conclusion, this is the first example of using a cooperative catalysis system,
combining a Lewis acid and a Brønsted acid, for synthesizing acylals.
Although the exact mechanism is uncertain, the use of an inexpensive
catalyst, the solvent-free condition, and excellent yields of the products
make this procedure more economic and environmentally friendly. Further
investigation of the mechanism and applications of the new catalytic system
is ongoing in our laboratory.

Table 2. Comparison of CMS and CMS-HOAc systems catalyzed conversion of aldehydes to acylals
a
Yield (%)
MP (8C) or bp (Torr)
Observed Reported
Time
(h)
b
Entry
R
Product
Cat: CMS
9
Cat: CMS þ HOAc
[
4]
1
PhCHO
1.3
PhCH(OAc)
2
94
44–45
79–80
44–46
79–80
[
13]
2
3
4
5
4
0
0
0
7
85
85
88
78
[
3]
8
55–56
97–99
60–62
56–57
[
4]
9
101–102
[
3]
2.5
64–65

[
[
13]
5]
6
4
8
79
50–52
52–53
84–86
7
8
2
4
6
0
67
92
80–82
[
16]
CH
3
CH 55 CHCHO
CH
3
CH 55 CHCH(OA)
2
88–90/15
89–90/15
[
5]
9
1
1
1
12
10
11
11
0
14
0
84
86
66
73
118–120
91–92
123–125
c
c
c
[19]
[19]
[6]
0
1
2
91–92
89–90
91–93
0
102–103
102–103
(
continued)

Table 2. Continued
a
Yield (%)
MP (8C) or bp (Torr)
Observed Reported
Time
(h)
b
Entry
R
Product
Cat: CMS
Cat: CMS þ HOAc
1
3
4
25
7
0
0
—
—
[
16]
1
CH
3
CH
2
CHO
CH
3
CH
2
CH(OAc)
2
0
0
73
95
70–73/18
72–74/20
d
5
1
9
—
—
0
0
a
b
c
1
Isolated yields and the purity of the products were determined by H NMR and GC.
CMS 0.6 mmol and HOAc 24 mmol.
Four equiv. of Ac O were used.
2
d
Equimolar mixture of benzaldehyde and acetophenone; the percentage of the products in the reaction mixtures was deterimined by GC analysis.

Acetic Acid-Assisted Copper Methanesulfonate Catalyst
EXPERIMENTAL
1959
Melting points were determined using RY-1 micromelting-point apparatus
and are uncorrected. Gas chromatographic analysis was carried out on a
Perkin-Elmer Auto System XL gas chromatograph. Infrared spectra were
recorded on a Perkin-Elmer Spectrum GX series Fourier Transform instru-
1
ment. H NMR spectra were recorded on a Bruker ARX-300 spectrometer
in CDCl3 using TMS as an internal standard. Elemental analyses were
carried out on an EA 2400II elemental analyzer (Perkin-Elmer).
General Procedure for the Preparation of Acylals
A mixture of aldehyde (30 mmol), Ac O (60 mmol), CMS (0.6 mmol), and
2
HOAc (24 mmol) were magnetically stirred at ambient temperature for an
appropriate time (monitored by GC). After the reaction, most of the mixture
solidified gradually, and 20 mL of CH Cl was added to dissolve the solid
2
2
product. The organic layer was washed twice with saturated Na CO
2
3
solution (20 mL), dried (Na SO ), and evaporated to yield the almost pure
2
4
product. The product was purified further by crystallization from cyclohexane
or by column chromatography on silica gel (ethyl acetate/hexane, 1:9 as the
1
eluent). All the acylals were characterized by IR, H NMR, and elemental
analysis.
Analytical Data for Selected Compounds
1
3
,1-Diacetoxy-1-propenyl methane (entry 8): colorless liquid, IR (KBr):
1
2
1
020, 1761, 1392, 1250, 1214 cm
.
H NMR (CDCl ): d ¼ 6.70
3
(
d, J ¼ 7 Hz, 1H), 6.04–5.11 (m, 2H). 2.13–1.90 (m, 9H). Anal. calcd. for
C H O : C, 55.81; H, 7.03. Found: C, 55.67; H, 7.17.
8 12 4
1
1
,1-Diacetoxy-1-ethyl methane (entry 14): colorless liquid, IR (KBr): 3000,
1
760, 1460, 1432, 1246, 1204, 1115, 1061, 1014, 966 cm
2
1
. H NMR
(
(
CDCl ): d ¼ 6.72 (t, J ¼ 6 Hz, 1H), 2.10 (s, 6H). 1.40–2.13 (m, 2H), 1.02
3
t, 3H). Anal. calcd. for C H O : C, 52.49; H, 7.55. Found: C, 52.81; H, 7.25.
7 12 4
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