1,3-Dibromo-5,5-dimethylhydantoin as catalyst for the conversion of aldehydes to their 1,1-diacetates (acetylals) under solvent-free and neutral conditions

Article abstract of DOI:10.1070/MC2005v015n05ABEH002124

Highly efficient and mild acetylation of aldehydes with acetic anhydride catalysed by 1,3-dibromo-5,5-dimethylhydantoin (DBH) was performed under neutral conditions to produce corresponding 1,1-diacetates (acetylals) in good to excellent yields.

Full text of DOI:10.1070/MC2005v015n05ABEH002124

, 2005, 15(5), 209–210
1,3-Dibromo-5,5-dimethylhydantoin as catalyst for the conversion of aldehydes
to their 1,1-diacetates (acetylals) under solvent-free and neutral conditions
Davood Azarifar,*a Hassan Ghasemnejada and Farhad Ramzanian-lehmalib
a Department of Chemistry, Faculty of Science, Bu-Ali-Sina University, 65174 Hamadan, Iran.
Fax: +98 811 827 2404; e-mail: azarifar@basu.ac.ir
b Department of Chemistry, Faculty of Science, Mazandaran University, Babolsar, Iran
DOI: 10.1070/MC2005v015n05ABEH002124
Highly efficient and mild acetylation of aldehydes with acetic anhydride catalysed by 1,3-dibromo-5,5-dimethylhydantoin (DBH)
was performed under neutral conditions to produce corresponding 1,1-diacetates (acetylals) in good to excellent yields.
The conversion of aldehydes and ketones to acetals and ketals,
Step 1:
O
respectively, by treatment with alcohols in the presence of acid
catalysts has long been used as a useful method of protection of
aldehyde or ketone functions from an attack by bases.1 It is also
known that aldehydes can be converted to acylals by treatment
with an anhydride in the presence of Lewis acids (e.g., BF3),2
proton acids (e.g., H2SO4 or Nafion-H)3 or PCl3.4 This reaction
cannot normally be applied to ketones, presumably, for steric
reasons, except when the reagent is trichloroacetic anhydride,
which gives acylals with ketones without a catalyst.5 More
recently, zeolites6 and iodine7 have also been reported as efficient
catalysts employed in these reactions. Ishihara and his co-workers
have used scandium triflates [e.g., Sc(OTf)3 and Sc(NTf2)3] to
catalyse the acylation of carbonyl compounds.8
O
O
Br
Br
N
N
2
2
Me
Me
O
Me
O
O
O
O
O
N
N
O–Br+
O
I
II
O–Br+
Step 2:
Step 3:
O
O
O
O
R
OAc
DBH (10 mol%)
O–Br+
Ar
H
Me
O
Ar
H
+ Ac2O
Me
room temperature,
solvent-free
H
OAc
R
H
II
1a–m
III
1a–m
2a–m
Scheme 1
O
N
O
O
To avoid the strong acids used in the previously reported
protocols and long reaction times, here, we report on the catalytic
activity of 1,3-dibromo-5,5-dimethylhydantoin (DBH) in the
reaction of aldehydes 1a–m with acetic anhydride under solvent-
free and neutral conditions to afford corresponding acetylals
O
O–Br+
Ar
N
Me
O
H
O
III
I
Table 1 DBH-catalysed conversion of aldehydes 1a–m to 1,1-diacetals
(acetylals) 2a–m with Ac2O under solvent-free conditions.
O
O
OAc
Ar
Br
N
N
Producta
Substrate RCHO
Time/h
Mp/°C
AcO
[yield (%)b]
H
O
1a
1b
1c
1d
1e
1f
1g
1h
1i
PhCHO
1.5
2.0
0.7
1.0
2.2
0.9
1.2
1.0
0.8
2a (98)
2b (92)
2c (99)
2d (98)
2e (98)
2f (99)
2g (98)
2h (95)
2i (92)
43–45
116–118
76–78
70–72
110–112
79–81
75–77
58–60
78–80
2a–m
IV
4-NO2C6H4CHO
4-ClC6H4CHO
2-ClC6H4CHO
3-NO2C6H4CHO
4-MeC6H4CHO
2-MeC6H4CHO
2-MeOC6H4CHO
PhCH=CHCHO
O
O
Step 4:
O
O–Br+
Ar
Br
N
N
Me
O
H
O
IV
III
O
1j
2.5
2j (90)
65–67
85–87
CHO
S
OAc
Ar
Br
Br
N
N
O
AcO
H
1k
3.0
2k (95)
O
CHO
2a–m
Scheme 2
O
CHO
2a–m in good to excellent yields (87–99%) (Scheme 1, Table 1).†
We also investigated the chemoselective conversion of alde-
hydes to acetylals in the presence of ketones such as benzo-
phenone, acetophenone and cyclohexanone. It was shown that,
when a 1:1 mixture of benzaldehyde and acetophenone was
allowed to react with acetic anhydride (10 mmol) in the pres-
ence of DBH (10 mol%), and the reaction mixture worked up
after 2 h, the crude products consisted of a 1:1 mixture of
benzaldehyde acetylal and unreacted acetophenone as indicated
1l
1.2
2.8
2l (87)
158–160
–35
Ph
O
O
O
1m
2m (98)
CHO
aAll products were characterised by IR and 1H NMR spectroscopy and mass
spectrometry. bIsolated yields.
Mendeleev Commun. 2005 209

, 2005, 15(5), 209–210
by NMR analysis. This provides chemoselective conversion of
an aldehyde to acetylal without affecting the ketone functionality.
In addition, the chemoselectivity of the method was observed by
the use of keto aldehydes 1k–m, which reacted merely at the
aldehyde group to produce corresponding 1,1-diacetates in high
yields without the ketone functionality being affected. This
suggests that DBH can be selectively used to convert the alde-
hyde groups into their acetylal form in the presence of ketone
functionalities. A possible mechanism suggested for this reac-
tion is shown in Scheme 2. In step 1, the N,N-diacetylation
of DBH occurs to afford 1,3-diacetyl-5,5-dimethylhydantoin I
and bromium acetate II, which undergoes a nucleophilic addi-
tion to the aldehyde in step 2 to yield an anionic acetyloxy
adduct III. The nucleophilic substitution of both N-acetyl groups
of I by adduct III in two successive steps 3 and 4 results in
the production of acetylal compounds 2a–m and regeneration
of the DBH catalyst.
In conclusion, we found that the use of Ac2O/DBH offers a
simple and convenient method for the conversion of aliphatic
and aromatic aldehydes to their acetylals in excellent yields
(87–99%). The neutrality of the reaction media that avoids
strong acids, high selectivity of the reaction between aldehyde
and ketone carbonyl groups, easy work-up procedure, high yields,
mildness of the reactions and solvent-free conditions make this
method suitable for the direct conversion of aldehydes into
acetylals.
This work was supported by the Bu-Ali-Sina University
(Hamadan, Iran) and Babol Free University (Mazandaran, Iran).
References
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†
Chemicals were obtained from Merck and Fluka. IR spectra were
recorded using a Shimadzu 435-U-04 spectrophotometer (KBr pellets),
and NMR spectra were measured in CDCl3 using a 90 MHz Jeol FT
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by DBH. General procedure. To a magnetically stirred solution of
aldehyde 1a–m (5 mmol) and freshly distilled acetic anhydride (1.02 g,
10 mmol), DBH (0.143 g, 0.5 mmol) was added at room temperature
and the resulting mixture was stirred for 1–3 h. After the complete
disappearance of starting materials, as monitored by TLC using EtOAc/
petroleum ether (1:9) or by 1H NMR analysis, the mixture was extracted
with diethyl ether (2×20 ml). The organic layer was then separated,
washed with distilled water (15 ml) and dried over anhydrous Na2SO4.
Evaporation of the solvent under reduced pressure resulted in highly
pure products 2a–m in 87–99% yields (Table 1) and no further purifica-
tion was necessary. The structures of the products were established on
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1
the basis of their H NMR, 13C NMR and mass spectra and by a direct
comparison with the authentic compounds.7,9
Received: 18th January 2005; Com. 05/2447
210 Mendeleev Commun. 2005