Polymer supported reagents: Oxidative selection between benzylic alcohols

Article abstract of DOI:10.1081/SCC-100105658

Dowex 1-X8, as a quaternary ammonium resin, on which CI^- ? Cl^- is replaced by dichromate and bisulfate ions (DDB), can be used as a stable and efficient oxidizing reagent for oxidative selection between benzylic alcohols according to their structures.

Full text of DOI:10.1081/SCC-100105658

SYNTHETIC COMMUNICATIONS, 31(19), 2885–2889 (2001)
POLYMER SUPPORTED REAGENTS:
OXIDATIVE SELECTION BETWEEN
BENZYLIC ALCOHOLS
F. Shirini,* H. Tajik, and F. Jalili
Department of Chemistry, Faculty of Science,
Guilan University, Rasht-Iran
ABSTRACT
Dowex 1-X8, as a quaternary ammonium resin, on which
ꢀ
ꢀ
CI ) Cl is replaced by dichromate and bisulfate ions
(
DDB), can be used as a stable and efficient oxidizing reagent
for oxidative selection between benzylic alcohols according to
their structures.
There has been an incessant interest in the development of new Cr(VI)
reagents for the effective oxidation of organic substrates, especially under
mild aprotic conditions. That is why a number of monomeric and polymer-
1
supported Cr(VI) reagents have been reported in the literature, of
ꢀ
2
which Jones’ reagent, CrO /N Bu HSO , n-butyltriphenylphosphonium
þ
3
3
4
4
5
4
dichromate, pyridinium chlorochromate, pyrazinium dichromate, ferric
6
7
8
dichromate, polyvinylpyridinium chlorochromate and dichromate, poly-
9
1
vinylpyridine-supported ferric dichromate, polyethyleneimine-supported
0
*
Corresponding author.
2
885
Copyright & 2001 by Marcel Dekker, Inc.
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SHIRINI, TAJIK, AND JALILI
1
silver dichromate and supported quaternary ammonium complex chro-
1
3
mates are examples.
However, most of the monomeric and some of the polymeric reagents
mentioned above suffer from disadvantages such as instability, hygroscopi-
city, low selectivity, long reaction time, difficulty of preparation and the
need for a large excess of the reagent. Thus a milder, more selective and
inexpensive reagent is still desirable.
In this communication we report that Dowex 1-X8, in which
2ꢀ
ꢀ
ꢀ
ꢀ
CI ) Cl is replaced by Cr O ; and HSO4 (DDB), is able to oxidize
2
7
benzyl alcohols directly to their corresponding carbonyl compounds in
high yields (Table 1). Overoxidation of the product was not observed by
this method.
Sterically hindered benzyl alcohols do not undergo oxidation with this
reagent. Therefore, this methodology shows selectivity and is suitable for
oxidative selection amongst benzyl alcohols according to their structures.
This is exemplified by the competitive reaction between benzyl alcohol and
benzhydrol (Scheme 1).
Table 1. Oxidation of Benzylic Alcohols with DDB in Refluxing Acetonitrile
Oxidant
Substrate
Reaction Yield
Time (h) (%)
Entry
Substrate
Benzyl alcohol
4-Chlorobenzyl alcohol
4-Bromobenzyl alcohol
4-Methylbenzyl alcohol
1-Phenyl ethanol
Product
Benzaldehyde
1
2
3
4
5
6
7
4
4
4
4
4
4
6
1
90
85
80
92
90
90
82
4-Chlorobenzaldehyde
4-Bromobenzaldehyde
4-Methylbenzaldehyde
Acetophenone
1.2
1.5
1.5
3.5
9
0
1-Naphthyl ethanol
4-Phenoxybenzyl alcohol
1 -Acetonaphthone
4-Phenoxybenzaldehyde
7.5
Scheme 1.

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Scheme 2.
Table 2. Comparison of Some of the Results Obtained by the Oxidation with
DDB(1), with Some of Those Reported by Chromic Acid on Amberlyst A-26(2),
Crosslinked Polyvinyl-Pyridine-Supported Ferric Dichromate (3) and Pyrazinium
Dichromate (4)
(
Oxidant/Substrate) (h) (Yield (%))
(2) (3) (4)
Entry
Substrate
(1)
(4)(1)(90)
1-Phenyl Ethanol (4)(3.5)(90)
1
2
Benzyl Alcohol
(13)(1)(98) (1)(1.5)(35) (1)(3.5)(73)
(1)(1)(35) (1)(3.5)(91)
–
It should be noted that the progress of the reaction depends on
ꢀ
the presence of HSO in the reagent. This can be shown through comparing
4
the oxidation of benzyl alcohol by bisulfate-containing reagent (DDB)
with the oxidation of benzyl alcohol in the absence of bisulfate (DD) by
ꢀ
the reagent (Scheme 2). These results show the key role of HSO4 in the
oxidation process, which may be related to the requirement for acidic media,
needed in the oxidation of different alcohols with sodium or potassium
1
2,13
dichromates.
In order to show the oxidizing ability of this reagent we have com-
5
,10,14
pared some of the results with some of those reported in the literature
Table 2).
Recycling of DDB can be accomplished by using a simple washing-
reactivation procedure in which the spent reagent is washed repeatedly with
M sodium hydroxide and 2 M hydrochloric acid solution. After regenera-
(
2
tion, Dowex 1-X8 in chloride form is added to the solution of CrO in
3
H SO in order to reactivate the reagent. A sample of the reagent passed
4
2
through three reaction cycles with no loss of oxidizing capacity and only a
slight decrease in reaction rate.

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SHIRINI, TAJIK, AND JALILI
EXPERIMENTAL
All products were characterized through comparison of their spectral
1
5
and physical data with those of the known samples. The purity determina-
tion of the products was accomplished by TLC on silica gel polygram SIL
ꢀ
G/UV 254 plates. Dowex 1-X8 (Cl form, 20–50 mesh) and other chemicals
were purchased from Merck. Products were separated and purified by
different chromatography techniques, and were also identified by the
comparison of their mp, IR and NMR spectra, bp, refractive index with
those reported for the authentic samples. The capacity of the reagent
was determined by atomic absorption technique and titration method.
Preparation of DDB: To a solution of CrO (0.35 g, 3.5 mmol) in 0.5 M
3
1
H SO (40 mL), Dowex1-X8(5 g)wasaddedandthemixturestirredfor0.5 h
6
2
4
at room temperature. The resulting dark-orange resin was filtered and
washed with acetone (2 ꢁ 20 mL) and diethylether (2 ꢁ 5 mL) and was finally
ꢂ
dried in vacuo at 50 C for 0.5 h. The infrared spectrum of the dry reagent
ꢀ
1
showedbandsat930 and765 cm , characteristic ofdichromateion andbands
ꢀ
1
at 820, 847, 877, 1045, 1180, 1220, 1420 and 3250 cm , characteristic of
1
7
bisulfate ion.
2
7
ꢀ
The capacity of the reagent was determined to be 0.25 mmol Cr O
2
ꢀ
and 1.5 mmol H SO4 per gram of the resin.
2
General procedure for the oxidation of benzylic alcohols with DDB: To a
solution of benzylic alcohol (1 mmol) in acetonitrile (8 mL), DDB (4–6 mmol
based on capacities) was added and refluxed while stirred for 1–9 h. Progress
of the reaction was monitored by TLC. The reaction mixture was cooled to
room temperature and filtered being followed by repeated washing with
acetonitrile (2 ꢁ 5 mL). Evaporation of the solvent followed by column
chromatography on silica gel gave the corresponding carbonyl compound
from good to high yields (Table 1).
R
Regeneration of Dowex 1-X8 in its CI form: To an aqueous solution
of NaOH (2 M, 20 mL) the spent reagent (2 g) was added and the mixture
was stirred for 12 h. The suspension was filtered and washed repeatedly with
a solution of hydrochloric acid (2 M) and finally with water. The precipitate
ꢂ
was dried in vacuo at 80 C for 6 h to give the original quaternary ammonium
resin characterized by its IR spectrum.
ACKNOWLEDGMENT
We are thankful to Guilan University Research Council for the partial
support of this work.

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Received in the UK December 8, 1999

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