3,6-bis(triphenylphosphonium)cyclohexene peroxodisulfate: A highly efficient oxidant for the selective oxidation of benzylic alcohols

Article abstract of DOI:10.1081/SCC-100106207

The synthesis of 3,6-bis(triphenylphosphonium)cyclohexene peroxodisulfate and its application for the selective oxidation of benzylic alcohols is reported.

Full text of DOI:10.1081/SCC-100106207

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3,6-BIS(TRIPHENYLPHOSPHONIUM)-CYCLOHEXENE
PEROXODISULFATE: A HIGHLY EFFICIENT OXIDANT
FOR THE SELECTIVE OXIDATION OF BENZYLIC
ALCOHOLS
R. Badri a , H. Shalbaf a & M. A. Heidary a
a Chemistry Department , Shahid-chamran University , Ahvaz, Iran
Published online: 22 Aug 2006.
To cite this article: R. Badri , H. Shalbaf & M. A. Heidary (2001) 3,6-BIS(TRIPHENYLPHOSPHONIUM)-CYCLOHEXENE
PEROXODISULFATE: A HIGHLY EFFICIENT OXIDANT FOR THE SELECTIVE OXIDATION OF BENZYLIC ALCOHOLS, Synthetic
Communications: An International Journal for Rapid Communication of Synthetic Organic Chemistry, 31:22, 3473-3479,
DOI: 10.1081/SCC-100106207
To link to this article: http://dx.doi.org/10.1081/SCC-100106207
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SYNTHETIC COMMUNICATIONS, 31(22), 3473–3479 (2001)
3,6-BIS(TRIPHENYLPHOSPHONIUM)-
CYCLOHEXENE PEROXODISULFATE:
A HIGHLY EFFICIENT OXIDANT
FOR THE SELECTIVE OXIDATION
OF BENZYLIC ALCOHOLS
R. Badri,* H. Shalbaf, andM. A. Heidary
Chemistry Department, Shahid-chamran
University, Ahvaz, Iran
ABSTRACT
The synthesis of 3,6-bis(triphenylphosphonium)cyclohexene
peroxodisulfate and its application for the selective oxidation
of benzylic alcohols is reported.
Peroxodisulfate is a well-known strong oxidizing agent for a variety of
organic compounds. Some reported successful examples of oxidation with
this oxidant are: oxidation decarboxylation of acids,1 aromatic hydroxyla-
tion,2–4 oxidation of carbon-nitrogen double bonds,5,6 oxidation of alkyl
benzenes,7 alkenes,8–10 amines11 and alcohols.12–15 Although the standard
redox potential of peroxodisulfate ion is high (about 2.01 V in aqueous
solution), its reaction at ambient temperature is considerably slow and
therefore, as reported, is accelerated in the presence of Ag(I),16,17 Cu(I)17
*Corresponding author.
3473
Copyright & 2001 by Marcel Dekker, Inc.
www.dekker.com

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3474
BADRI, SHALBAF, AND HEIDARY
and Ni(II)18 under reflux condition. In the case of alcohols, the oxidation
reaction carried out at acidic condition and is accompanied by fragmenta-
tion.13 Therefore, introduction of a new Peroxodisulfate oxidizing agent
which works under neutral and aprotic condition for the selective oxidation
of alcohols is an important process in organic synthesis.
In this paper we wish to report the synthesis of new Peroxodisulfate
reagent and its application in selective oxidation of benzylic alcohols in
neutral solution and in the absence of the metal ions as catalysts. Thus,
3,6-dibromocyclohexene19 I was stirred with triphenylphosphine in refluxing
CHCl3 for 2 h to produce, upon cooling and treatment first with Et2O and
then with acetone, 3,6-bis(triphenylphosphonium)-cyclohexene dibromide 2
as a white solid in 80.5% yield. Stirring a mixture of 2 and K2S2O8 in H2O at
ambient temperature for 3 h, afforded after filtration and washing with H2O
and drying, 3,6-bis(triphenylphosphonium)cyclohexene peroxodisulfate 3 in
78% yield (Scheme 1). Structures of 2 and 3 were confirmed by elemental
1
analysis, IR and H NMR. Compound 3 is a white solid which is stable for
long periods without losing its activity.
Scheme 1.
As shown in Table 1, primary and secondary benzylic alcohols are
oxidized to their corresponding aldehydes and ketones in high yields (entries
1–12). Further oxidation of aldehydes to their carboxylic acids and also
fragmentation products were not observed in this method. It is noteworthy
that under the same reaction conditions, primary and secondary saturated
and allylic alcohols were resistant toward oxidation and the corresponding
carbonyl compounds were obtained in non or poor yields. It is also import-
ant to note, since the reaction medium was almost neutral, some sensitive
functionalities such as carbon-carbon double bond remained intact (entries
11, 12, Scheme 2).
In order to show the better chemoselectivity of the described method
an equimolar mixture of benzyl, allyl and alkyl alcohols was treated with
this reagent in refluxing MeCN for 2 h. We found that p-methoxybenzyl

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PEROXODISULFATE OXIDATION OF BENZYLIC ALCOHOLS
3475
alcohol is oxidized selectively in the presence of allyl and alkyl alcohols
(Scheme 2). This selectivity is of vital importance in modern synthetic
methodologies.
The effect of various solvents such as DMSO, DMF, MeCN, CCl4,
CHCl3 and CH2Cl2 were also investigated. The experimental results show
that MeCN is the best choice for this purpose (Table 2).
Table 1. Oxidation of Alcohols with BTPCP in Refluxing MeCN
t
Yield
Entry
Substrate
(min) (%)a
Productb
1
2
3
4
5
6
7
8
PhCH2OH
p-ClPhCH2OH
O-NO2PhCH2OH
P-NO2PhCH2OH
P-MeOPhCH2OH
PhCHOHMe
PhCHOHPh
PhCHOH(CH2)2CO2Me
p-MePhCH2OH
9
11
20
15
7
8
7
15
8
92 PhCHO
93 p-ClPhCHO
57 O-NO2PhCHO
86 P-NO2PhCHO
95 p-MeOPhCHO
94 PhCOMe
95 PhCOPh
87 PhCO(CH2)2CO2Me
94 p-MePhCHO
9
10
11
12
p-MeOPhCHOH(CH2)2CO2Me 14
PhCHOHCH¼CHCH2OH 10
PhCHOH(CH2)2CO2(CH2)2OH 11
96 p-MeOPhCO(CH2)2CO2Me
89 PhCOCH¼CHCH2OH
89 PHCO(CH2)2CO2(CH2)2OH
aYields refers to isolated yields.
bProducts were characterized by comparison of their physical and spectra data with
authentic samples.
Scheme 2.

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BADRI, SHALBAF, AND HEIDARY
Table 2. Data for the p-MeOPhCH2OH Oxidation with BTPCP
in Various Solvents
Entry
Solvent
t. (min)
Yield (%)*
1
2
3
4
5
6
DMSO
DMF
MeCN
CCl4
CHCl3
CH2Cl2
9
13
7
20
15
20
88
80
95
40
70
50
*GLC yields
Table 3. Reusability of Oxidant*
Experimental Yield (%)
1st
2nd
3rd
95
92
89
*All reactions are carried out under similar
conditions.
The noteworthy advantages of this reagent lies in its reusability. Thus
Table 3 demonstrates the reusability of the oxidant after 3 replicate oxida-
tions of p-methoxy benzyl alcohol to p-methoxy benzaldehyde. The oxidant
was recovered by stirring of the white solid, which obtained after oxidation
of alcohols, with K2S2O8 in H2O for 3 h.
In conclusion, 3,6-bis(triphenylphosphonium)cyclohexene peroxodi-
sulfate is a stable, mild, efficient, easily prepared and reusable reagent for
the selective oxidation of benzylic alcohol.
EXPERIMENTAL
1
General: H NMR was recorded on a Bruker advance DPX-250 MHz
NMR spectrometer in CDCl3. The IR spectra were recorded on a Shimadzu
470 Spectrophotometer (4000–400 cmꢀ1 region). GLC analysis was per-
formed with a Gas Chromatography 6A Shimadzuwith a flame ionization
detector and Carbowax 20 M/Chromosorb-W60–80-mesh column. The C,

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PEROXODISULFATE OXIDATION OF BENZYLIC ALCOHOLS
3477
H, S, analysis were performed by the microanalytical service of research
institute of petroleum industry (N.I.O.C). Melting points were measured
on a mettler FP5 apparatus. TLC accomplished the purity of the substrates
and reactions monitored on silica gel plates. All yields refer to isolated
products.
Preparation of 3,6-bis(Triphenylphosphonium)-
cyclohexene Dibromide (2)
To a solution of 3,6-dibromocyclohexene I (2.4 g, 10 mmol) in CHCl3
(10 ml) in a 50 ml round-bottomed flask equipped with a magnetic stirrer
and a reflux condenser was added triphenylphosphine (5.24 g, 20 mmol). The
reaction mixture was refluxed for 2 h. The solution was cooled to room
temperature and then Et2O was added drop wise with stirring until an
oily product was separated. The ether was removed by decantation and
acetone (40 ml) was added. Stirring the acetone solution for 40 min afforded
a white precipitate that was filtered, washed with acetone and then dried.
Yield (80.5%), m.p. ¼ 276–7ꢁC.
IR (KBr): 3053, 2755, 1613, 1575, 1475, 1446, 1108, 750, 723, 692, 537
1
(cmꢀ1). H NMR (250 MHz), dH (CDCl3): 2.37–2.85 (m, 4H), 5.78 (d, 2H),
6.47 (m, 2H), 7.12–7.96 (m, 30H).
Anal. (Calc. C% 65.98; H% 5.01; found C% 66.02; H% 4.95).
Preparation of 3,6-bis(Triphenylphosphonium)-
cyclohexene Peroxodisulfate (3)
In a 25 ml round-bottomed flask with a magnetic stirrer a solution of
K2S2O8 (0.135 g, 2 mmol) in H2O (5 ml) was prepared. Compound 2 (3.82 g,
2 mmol) was added to this solution and the reaction mixture was stirred at
ambient temperature for 3 h. The resulting white precipitate was filtered,
washed with distilled water (10 ml) and dried in vacuo; yield (78%)
m.p. ¼ 177–8ꢁC.
IR (KBr): 3050, 1625, 1576, 1478, 1437, 1258, 1108, 1059, 754, 724,
693, 556, 525 (cmꢀ1).
1H NMR (250 MHz), d (d6-DMSO): 2.46 (m, 4H), 5.12 (d, 2H), 6.10
(m, 2H), 7.91 (m, 30 H). 13C NMR (250 MHz), d (d6-DMSO): 22.51(t),
31.05(dd), 117.58, 130.80(d), 131.00, 134.95, 135.52.
Anal. (Calc. C% 63.31; H% 4.81; S% 8.05; found C% 63.29; H%
4.82; S% 8.52).

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BADRI, SHALBAF, AND HEIDARY
General Procedure for Oxidation of Alcohols
To a solution of alcohol (1 mmol) in solvent CH3CN (5 ml) in a 50 ml
round-bottomed flask-equipped with a condenser and a magnetic stirrer was
added 3,6-bis(triphenylphosphonium)cyclohexene peroxodisulfate (1 mmol)
in three portions. The reaction mixture was stirred magnetically under reflux
condition for the appropriate time indicated in Table 1. The progress of the
reaction was monitored by TLC or GLC. The reaction mixture was filtered
and the solid was washed with ether (3 ꢂ 15 ml). The filtrates were evapo-
rated and the residue was purified on a silica-gel plate with appropriate
eluent. Pure carbonyl compounds were obtained in 57–96% yields (Table 1).
ACKNOWLEDGMENT
We were thankful to Shahid Chamran University for the partial sup-
port of this work.
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Received in the UK June 19, 2000

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