The synthesis and application of 3,6-bis(triphenylphosphonium) cyclohexene dichromate: An efficient oxidizing agent

Article abstract of DOI:10.1080/10426500600634715

The synthesis of 3,6-bis(triphenylphosphonium) cyclohexene dichromate is described. This reagent oxidizes primary and secondary alcohols to their corresponding carbonyl compounds thiols to disulfides, and amines to their azo compounds. Copyright Taylor & Francis Group, LLC.

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Phosphorus, Sulfur, and Silicon
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The Synthesis and
Application of 3,6-
Bis(triphenylphosphonium)
Cyclohexene Dichromate: An
Efficient Oxidizing Agent
a
a
Rashid Badri & Azar Mostoufi
a
Chemistry Department , Chamran University ,
Ahwaz, Iran
Published online: 01 Feb 2007.
To cite this article: Rashid Badri & Azar Mostoufi (2006) The Synthesis and Application
of 3,6-Bis(triphenylphosphonium) Cyclohexene Dichromate: An Efficient Oxidizing
Agent, Phosphorus, Sulfur, and Silicon and the Related Elements, 181:7, 1513-1519,
DOI: 10.1080/10426500600634715
To link to this article: http://dx.doi.org/10.1080/10426500600634715
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Phosphorus, Sulfur, and Silicon, 181:1513–1519, 2006
Copyright © Taylor & Francis Group, LLC
ISSN: 1042-6507 print / 1563-5325 online
DOI: 10.1080/10426500600634715
The Synthesis and Application of
3
,6-Bis(triphenylphosphonium) Cyclohexene
Dichromate: An Efficient Oxidizing Agent
Rashid Badri
Azar Mostoufi
Chemistry Department, Chamran University, Ahwaz, Iran
The synthesis of 3,6-bis(triphenylphosphonium) cyclohexene dichromate is de-
scribed. This reagent oxidizes primary and secondary alcohols to their correspond-
ing carbonyl compounds thiols to disulfides, and amines to their azo compounds.
Keywords Alcohol; aromatic amines; azo compounds; BTPCD; carbonyl compounds;
dichromate anion; disulfides; MeCN; oxidation; thio; triphenylphosphine
INTRODUCTION
The oxidation of organic substrates with chromium (VI)-based ox-
idants such as dichromate anion (Cr2O2 ) and chromium trioxide,
−
7
which are used alone or with an organic ligand, is an important
reaction in synthetic organic chemistry.1
−6
Most procedures using
these oxidants are usually conducted under aqueous acidic condi-
tions. Methods using these kind of reagents are faced with limita-
tions: The low pH of the medium is not suitable for acid-sensitive
compounds; long reaction times and low selectivity are observed with
some reactions; some reagents are hygroscopic and photochemically
unstable; and a large excess of the reagent and an unsatisfactory
yield are usually accompanied with reactions. However, for the ox-
idation of organic substrates under nonaqueous and aprotic condi-
tions, a condition that is very useful in modern organic synthesis,
a number of different chromium (VI)-based oxidants are reported
7
in the literature, such as n-butyltriphenylphosphoniumdichromate,
benzyltriphenylphosphonium dichromate,8 potassium dichromate in
Received February 8, 2006; accepted February 9, 2006.
We are thankful to Shahid Chamran University for support of this work.
Address correspondence to Rashid Badri, Chemistry Department, Chamran Univer-
sity, Ahwaz 65355-141, Iran. E-mail: rashidbadri@yahoo.com
1513

1
514
R. Badri and A. Mostoufi
dimethylformamide,9 zincchlorochro-mate, and potassium dichro-
10
mate in a benzene-water system.11
In this article, we wish to report the synthesis and application of
,6-bis-(triphenylphosphonium) cyclohexene dichromate (BTPCD) as a
3
mild and selective oxidant for the effective oxidation under neutral and
nonaqueous conditions.
RESULTS AND DISCUSSION
The synthesis of the BTPCD is shown in Scheme 1. The required start-
ing material, 3,6-dibromocyclohexene 1, was prepared via the reaction
of cyclohexene with N-bromosuccinimide (NBS) according to the pro-
1
2
cedure reported by Engman and Bystrom. Dibromide 1 was stirred
with triphenylphosphine in refluxing CHCl3 for 2.5 h to produce, on
cooling and treatment first with diethylether and then with acetone,
3
,6-bis(triphenyl-phosphonium) cyclohexene dibromaide 2 as a white
solid in an 80% yield. The reaction of 2 with K2Cr2O7 in water at
an ambient temperature for 1 h yielded, after filtration and washing
with acetone and drying, BTPCD 3 as a yellow solid in an 78% yield
(
Scheme 1).
SCHEME 1
The structures of 2 and 3 were confirmed by IR, H NMR, 13C NMR,
1
and elemental analysis. Compound 3 is a yellow solid, which decom-
◦
poses above 130 C and is stable for long periods without losing its ac-
tivity. This reagent is insoluble and stable in solvents, such as CHCI3,
CCl4, CH2Cl2, CH3CN, CH2C1CH2C1, THF, benzene, toluene, and ace-
tone. In this study, we found that BTPCD is able to oxidize primary and
secondary alcohols to their corresponding carbonyl compounds, amines
to their azo compounds, and thiols to disulfides.
The Oxidation of Alcohols
The oxidation of primary and secondary alcohols with BTPCD was per-
formed in refluxing acetonitril. Benzilic alcohols were converted to their
corresponding aldehydes and ketones in high yields (Table I, entries 1–
11). Other primary and secondary alcohols were also oxidized with this

An Efficient Oxidizing Agent
1515
TABLE I The Oxidation of Alcohols with BTPCD in Refluxing MeCN
Entry
Substrate
Time (h)
Yield(%)a
Productb
1
2
3
4
5
6
7
8
9
PhCH2OH
1.5
2
2
1.5
2.5
2
2.5
2.5
2
94
84
76
90
81
80
87
81
86
92
87
PhCHO
p-ClC6H4CH2OH
PhCH(OH)CH CHCO2Et
o-MeOC6H4CH2OH
p-NO2C6H4CH2OH
PhCH(OH)COPh
p-ClC6H4CHO
PhCOCH CHCO2Et
o-MeOC6H4CHO
p-NO2C8H4CHO
PhCOCOPh
PhCHOHPh
PhCOPh
p-BrC6H4CHOHCH3
o-CIC6H4CH2OH
p-CH3C6H4CH2OH
p-BrC6H4COCH3
o-CIC6H4CHO
p-CH3C6H4CHO
1
1
0
1
2.5
2
12
13
14
15
16
17
PhCH CHCH2OH
PhCH CHCH(OH)CH3
PhCH(CH3)OH
Ph(CH2)3OH
Ph(CH2)2OH
2
77
72
75
70
82
93
PhCH CHCHO
PhCH CHCOCH3
PhCOCH3
Ph(CH2)2CHO
PhCH2CHO
2.5
2.5
2.5
2.5
2
CH3(CH2)6CH2OH
CH3(CH2)6CHO
a
b
Yields refer to isolated yields.
Products were characterized by a comparison of their physical and spectral data
with authentic samples.
oxidant and afforded carbonyl compounds in good yields (Table I, en-
tries 14–17). Under the same reaction conditions, allylic alcohols were
selectively converted to the corresponding α,β-unsaturated carbonyl
compounds. Carbon–carbon double bounds remained intact during the
oxidation process (Table I, entries 3, 12, and 13). It is noteworthy that
the further oxidation of aldehydes to their carboxylic acids were not
observed in this investigation.
The effect of various solvents, such as CCl4, CH2Cl2, CHCl3,
DMSO, DMF, CH3CN, THF, and Et2O, on the oxidation of p-
methoxybenzylalcohol to p-methoxybenzaldehyde with BTPCD were
also investigated. The experimental results show that CH3CN is the
best choice for this purpose (Table II).
The Oxidation of Amines and Thiols
The conversion of primary amines to their corresponding azo com-
pounds under nonaqueous and aprotic conditions is an interesting
reaction in organic synthesis.7
,12,13
We found that BTPCD is an

1
516
R. Badri and A. Mostoufi
TABLE II Data for the p-Methoxybenzyl Alcohol
Oxidation With BTPCD in Various Solvents
Entry
Solvent
Time (h)
Yield (%)a
1
2
3
4
5
6
7
8
CCl4
8
8
8
2.5
2
1.5
8
8
Trace
20
20
70
54
90
—
—
CH2Cl2
CHCl3
DMSO
DMF
CH3CN
THF
Et2O
aTLC yields.
effective oxidant for the transformation of aromatic amines to their
corresponding azo compounds under neutral conditions in good yields
(
Table III, entries 1–6). New methods for the oxidation of thiols to
7
,14−16
their disulfides are reported in the literature.
BTPCD, however, is
shown to be an effective reagent for the conversion of thiols to disulfides,
a reaction which demonstrates the mildness of this oxidant (Table III,
entries 7–12).
TABLE III The Oxidation of Amines and Thiols withs BTPCD in
Dry CH
3
CN
Entry
Substrate
Time (h) Yield (%)a
Productb
1
2
3
4
5
6
7
8
9
0
1
C6H5NH2
3
3
3.5
3
3
3.5
1.5
1.5
1
1
1
75
81
70
76
74
60
93
86
83
90
92
PhN NPh
p- MeC6H4NH2
p-ClC6H4NH2
p-MeOC6H4NH2
o-MeC6H4NH2
o-BrC6H4NH2
C6H5SH
p-NO2C6H4SH
p-ClC6H4SH
n-C4H9SH
p-MeC6H4N NC6H4Me-p
p-ClC6H4N NC6H4Cl-p
p-MeOC6H4N NC6H4MeO-p
o-MeC6H4N NC6H4Me-o
o-BrC6H4N NC6H4Br-o
C6H5S SC6H5
p-NO2C6H4S SC6H4NO2-p
p-ClC6H4S SC6H4Cl-p
n-C4H9S SC4H9-n
1
1
12
1
40
a
b
Yields refer to isolated yields.
Products were characterized by comparison of their physical and spectral data with
authentic samples.

An Efficient Oxidizing Agent
1517
CONCLUSION
BTPCD is an oxidant that could be synthesized very easily from com-
mercially available materials. It is a stable, mild, and selective oxidant
that can be used under neutral and nonaqueous conditions for the oxi-
dation of different substrates.
EXPERIMENTAL
General
1H NMR spectra were recorded on a Bruker Advance DPX-250 MHz
NMR spectrometer in CDCl3 and d6-DMSO. IR spectra were recorded
−
1
on a Shimadzu 470 spectrophotometer (4000–400 cm region). The C,H
analysis was performed by the microanalysis service of the research
institute of petroleum industry (NIOC). Melting points were measured
on a Mettler FP5 apparatus. TLC confirmed the purity of the substrates,
and reactions were monitored on silica gel plates. All yields refer to
isolated products. Products were characterized by comparison of their
physical and spectral data with those of authentic samples.
The Preparation of
3
,6-Bis(triphenylphosphonium)-cyclohexene Dibromide (2)
To a solution of 3,6-dibromocyclohexene 1 (1.1 g, 5 mmol) in CHCl3
(
10 mL) in a 50 mL round-bottomed flask equipped with a magnetic stir-
rer and a reflux condenser, triphenylphosphine (2.62 g, 10 mmol) was
added. The reaction mixture was refluxed on a water bath for 2.5 h.
The solution was cooled to r.t. and then, while vigorously stirring, di-
ethylether was added dropwise until an oily product was separated. The
ether was removed by decantation, and acetone (40 mL) was added. Stir-
ring the acetone solution for 40 min afforded a white precipitate, which
was filtered, washed with acetone, and then dried. Yield (80%), m.p.:
◦
2
76–277 C.
IR (KBr): 3053, 2755, 1613, 1575, 1475, 1446, 1108, 750, 723, 692,
−
1 1
5
37 cm
. H NMR (250 MHz), δh (CDC13): 2.37–2.85 (m, 4H), 5.78 (d,
2
H), 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%.
The Preparation of 3,6-Bis(triphenylphosphonium)-
cyclohexene Dichromate (3)
To a stirring suspension of powdered 2 (1.14 g, 1.5 mmol) in wa-
ter (20 mL), a solution of K2Cr2O7 (0.45 g, 1.5 mmol) in water (5 mL)
was added dropwise. A yellow precipitate was formed. The mixture was

1
518
R. Badri and A. Mostoufi
stirred for 1 additional h. The resulting yellow solid was separated by
filtering and then poured into acetone (20 mL) and stirred for 15 min; it
◦
was separated by a Buchner funnel and dried. Yield (78%), m.p.: 130 C
(
dec).
IR(KBr): 3445, 3030, 2910, 1633, 1470, 1425, 1101, 990,933, 770, 745,
−1 1
7
6
21, 690, 528 cm . H NMR (250 MHz), δ (d6-DMSO): 2.46 (m, 4H),
1
3
.38 (d, 2H), 6.68 (m, 2H), 7.39–7.94 (m, 30). C NMR (250 MHz), δ
(
d6 DMSO): 22.63, 31.12, 116.38, 130.91, 132.11, 134.78, 136.17. Anal.
calc.: C, 61.46; H, 4.63%; found: C, 61.32; H, 4.57%).
The General Procedure for the Oxidation of Alcohols, Amines,
and Thiols
To a refluxing solution of substrate (0.8 mmol) in dry acetoni-
tril (20 mL) in a 50 mL round-bottomed flask equipped with a con-
denser and a magnetic stirrer was added 3,6-bis (triphynylphospho-
nium)cyclohexene dichromate (0.8 mmol) in small portions over a period
of 1–3.5 h. At the end of the reaction, a greenish solution was obtained.
The progress of the reaction, was monitored by TLC. The reaction mix-
tures were filtered and the solid was washed with ether (15 mL). The
filterates were evaporated and the residue was purified on a silica-gel
plate (eluent: CC14/Et2O, 4/1). Pure products were obtained. Yields:
carbonyl compounds 70–94% (Table I), azo compounds and sulfides 40–
93% (Table III).
The Oxidation of Benzyl Alcohol: A Typical Procedure
A solution of benzyl alcohol (87.3 mg, 0.8 mmol) in dry acetonitril
20 mL) in a 50 mL round-bottomed flask equipped with a condenser,
(
a magnetic stirrer is prepared. To this solution under reflux condition
was added BTPCD (663 mg, 0.8 mmol) in 10–12 portions over a period of
1.5 h. After completion of the reaction, the solution was filtered, and the
solid was washed with acetonitril (2 × 15 mL). The solvent was evapo-
rated under reduced pressure at an ambient temperature. The result-
ing crude material was purified on a silica-gel plate (eluent: CC14/Et2O,
4/1). Evaporation of the solvent afforded pure benzaldehyde. Yield 94%.
The Oxidation of p-Methoxyaniline: A Typical Procedure
In a 50 mL round-bottomed flask equipped with a condenser and a
magnetic stirrer, a solution of p-methoxy aniline (98.4 mg, 0.8 mmol)
acetonitril was prepared. To this solution, while stirred magnetically
under reflux condition, was added BTPCD (656 mg, 0.8 mmol) in 10–
1
2 portions over a period of 3 h. After completion of the reaction, the
solution was filtered, and the solid was washed with acetonitril (2 ×
5 mL). The solution was evaporated, and the residue was purified on
1

An Efficient Oxidizing Agent
1519
a silica-gel plate (eluent: CCl4/Et2O, 6/1). Evaporation of the solvent
afforded pure azo compounds. Yield 76%.
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