DABCO-bromine complex: A novel oxidizing agent for oxidative deprotection of THP and silyl ethers and semicarbazones to corresponding carbonyl compounds

Article abstract of DOI:10.1080/00397910701473234

A tetrameric DABCO-bromine complex was synthesized, characterized, and utilized as a novel active bromine complex for the oxidative deprotection of THP and silyl ethers and semicarbazones to carbonyl compounds. Copyright Taylor & Francis Group, LLC.

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DABCO–Bromine Complex: A Novel Oxidizing
Agent for Oxidative Deprotection of THP
and Silyl Ethers and Semicarbazones to
Corresponding Carbonyl Compounds
Mahmood Tajbakhsh a , Majid M. Heravi b & Setareh Habibzadeh a
a Department of Chemistry, School of Sciences , Mazandaean University ,
Babolsar, Iran
b Department of Chemistry, School of Sciences , Azzahra University , Vanak,
Tehran, Iran
Published online: 30 Aug 2007.
To cite this article: Mahmood Tajbakhsh , Majid M. Heravi & Setareh Habibzadeh (2007) DABCO–Bromine
Complex: A Novel Oxidizing Agent for Oxidative Deprotection of THP and Silyl Ethers and Semicarbazones
to Corresponding Carbonyl Compounds, Synthetic Communications: An International Journal for Rapid
Communication of Synthetic Organic Chemistry, 37:17, 2967-2973, DOI: 10.1080/00397910701473234
To link to this article: http://dx.doi.org/10.1080/00397910701473234
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Synthetic Communicationsw, 37: 2967–2973, 2007
Copyright # Taylor & Francis Group, LLC
ISSN 0039-7911 print/1532-2432 online
DOI: 10.1080/00397910701473234
DABCO–Bromine Complex: A Novel
Oxidizing Agent for Oxidative Deprotection
of THP and Silyl Ethers and
Semicarbazones to Corresponding Carbonyl
Compounds
Mahmood Tajbakhsh
Department of Chemistry, School of Sciences, Mazandaean University,
Babolsar, Iran
Majid M. Heravi
Department of Chemistry, School of Sciences, Azzahra University,
Vanak, Tehran, Iran
Setareh Habibzadeh
Department of Chemistry, School of Sciences, Mazandaean University,
Babolsar, Iran
Abstract: A tetrameric DABCO–bromine complex was synthesized, characterized,
and utilized as a novel active bromine complex for the oxidative deprotection of
THP and silyl ethers and semicarbazones to carbonyl compounds.
Keywords: DABCO–bromine complex, deprotection, oxidation, semicarbazones,
silyl ethers, Tetrahydropyranyl ethers
INTRODUCTION
Silyl and THP (tetrahydropyranyl) ethers are extensively used as protective
groups for alcohols in synthetic chemistry because of their low cost, efficiency
Received in the U.K. October 20, 2006
Address correspondence to Mahmood Tajbakhsh, Department of Chemistry, School
of Sciences, Mazandaean University, Babolsar, Iran. E-mail: tajbaksh@umz.ac.ir
2967

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M. Tajbakhsh, M. M. Heravi, and S. Habibzadeh
of preparation, stability under the intended reaction conditions, and easy and
selective removal.[1] A variety of methods for the selective deprotection of
these protective groups have been developed, but the direct synthesis of
carbonyl compounds from THP or silyl ethers is not widespread in the
literature.[2]
Semicarbazones also serve as important synthetic intermediates, and they
can be used for isolation, purification, and characterization of aldehydes and
ketones.[3]
Although there are many methods for deprotection of silyl and THP ethers
and semicarbazones to the corresponding carbonyl compounds, many of these
methods have severe limitations such as tedious workup, long reaction times,
low yields, high temperatures, and also the use of organic solvents, Lewis acid
catalysts, expensive reagents, etc. Thus, the introduction of new methods,
inexpensive reagents, and environmentally friendly reaction conditions for
such functional group transformations is still in demand. It is also interesting
that the use of water as a solvent makes the reaction interesting from both an
[4]
economical and environmental point of view.
We have recently reported the oxidation of alcohols,[5] deoximation,[6] dese-
micarbonization,[7] and oxidative deprotection of silyl[8] and THP ethers[9] using
hexamethylenetetramine bromine and used DABCO as the catalyst for the
synthesis of N-arylphthalimide[10] and the desilylation of silyl ethers.[11]
1,4-Diazabicyclo[2,2,2]octane (DABCO)–bromine complex is easily
prepared from bromination of DABCO with liquid bromine at room temperature.
It has been used as an oxidant for conversion of alcohols to their carbonyl
compounds.[12] In this letter, we describe another use of DABCO–bromine
complex in the transformation of silyl and THP ethers and semicarbazones to
their carbonyl compounds in water at 80 8C.
RESULTS AND DISCUSSION
DABCO–bromine complex is a yellowish solid that is stable in exposure to
light, air, and water. Its structure was characterized by X-ray crystallogra-
phy[13] [Eq. (1)].
CHCl3
4DABCO þ 14Br2 ꢀꢀꢀꢀꢀꢀꢀꢀꢀꢀ!½ðH2DABCOÞ2ðHDABCOÞ2ðBrÞ2ðBr3Þ4ꢁ
DABCO-bromine
ð1Þ
It is a homogeneous nonhygroscopic solid, which can be stored for months
without losing its activity. It had no offensive of bromine or amine odor.
To find the optimum reaction conditions for oxidative deprotection of
THP and silyl ethers, we carried out several reactions, changing the molar
ratio of substrate to reagent, temperature, and solvents. Our results revealed
that the highest yield is obtained with a 1:1/6 molar ratio of substrate to
reagent in water at 808C (Scheme 1).

DABCO–Bromine Complex
2969
Scheme 1.
The results are shown in Table 1. As seem in Table 1, THP and silyl ether
of 1-pentanol converted to the corresponding aldehyde in 90% and 95% yield
respectively after 20 min. To show the generality of this procedure, we carried
out the cleavage of a variety of THP ethers. As indicated in Table 1, aliphatic
Table 1. Oxidative deprotection of tetrahydropyranyl and silyl ethers with DABCO–
bromine complex in water
Time
(min)
Yield
(%)b
Entry
1
Substrate
Producta
20
25
90
92
2
3
25
30
90
93
4
5
30
89
6
7
35
35
87
88
(continued)

2970
M. Tajbakhsh, M. M. Heravi, and S. Habibzadeh
Table 1. Continued
Time
(min)
Yield
(%)b
Entry
8
Substrate
Producta
35
84
9
30
74
10
30
91
11
12
20
30
95
95
13
14
30
25
90
89
15
35
86
16
17
35
40
90
87
(continued)

DABCO–Bromine Complex
2971
Table 1. Continued
Time
(min)
Yield
(%)b
Entry
18
Substrate
Producta
40
85
19
55
77
aAll products were characterized spectroscopically (1H NMR, IR) and showed
physical and spectral data in accordance with their expected structure in comparison
with authentic samples.
bThe yields refer to isolated products.
THP ethers (Table 1, entries 1 and 5), aromatic THP ethers (Table 1, entries
2–4 and 6–9), and a,b-unsaturated THP ether (Table 1, entry 10) were effi-
ciently cleaved to the corresponding carbonyl compounds in good yields. The
yields were diminished when an NO2 group was present on the aromatic ring
(Table 1, entry 9) and longer reaction times had no impact on the yield. THP
ethers containing a double bond (Table 1, entry 10) were also oxidized to the
parent carbonyl compounds. It is worth mentioning that no bromination of the
double bond took place. Similarly, secondary silyl ether (Table 1, entries 15,
16, and 18) were cleaved to the corresponding carbonyl compounds. The
products were characterized by recording IR, 1H NMR spectra, and
elemental analyses and comparing them with authentic samples. No overoxi-
dation products were detected in the case of aldehydes.
As indicated in Table 2, we regenerated some semicarbazones to their
corresponding carbonyl compound with DABCO–bromine complex in
water with good yields.
The positive features of the present method are summarized as follows: 1)
ease of operation; 2) excellent yields, and 3) environmental consciousness: no
organic solvent is used in the reaction, and only a small amount is needed in
workup.
EXPERIMENTAL
Preparation of the Tetrameric DABCO–Bromine Complex
A solution of bromine (20.0 g, 125 mmol) in chloroform (100 mL) was added
dropwise with stirring to a solution of DABCO (6.72 g, 60 mmol) in

2972
M. Tajbakhsh, M. M. Heravi, and S. Habibzadeh
Table 2. Regeneration of carbonyl compounds from semicarbazone with DABCO–
bromine complex
Time Yield
(%)b
Entry
1
Substrate
Producta
(min)
20
90
2
3
4
15
20
20
92
89
95
5
25
87
6
15
92
aAll products were characterized spectroscopically (1H NMR, IR) and showed
physical and spectral data in accordance with their expected structure in comparison
with authentic samples.
bThe yields refer to isolated products.
chloroform (100 mL). A yellow solid appeared as the bromine was taken up.
The mixture was stirred for an additional 1 h, and then the yellow solid was
collected by vacuum filtration. Yield 23.12 g (98%), decomp. 160–1658C.
General Procedure
To a suspension of DABCO–Br2 (0.262 g, 0.166 mol) in water (15 mL) in a
round-bottomed flask, an appropriate amount of the substrate (1 mmol) was
added. The reaction mixture was stirred and warmed to 808C until the
yellow color of the complex disappeared. The progress of the reaction was

DABCO–Bromine Complex
2973
monitored by thin-layer chromatography (TLC) using petroleum ether/ethyl
acetate (8:2) as eluent. Upon completion of the reaction, ether was added to
reaction mixture and was washed with solution of 1% HCl. The aqueous
layer was separated, and the organic layer was washed with 3% sodium bi-
carbonate and water respectively. The organic solution was dried over
MgSO4, filtered, and evaporated to dryness under reduced pressure to afford
the pure corresponding carbonyl compound.
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