Triphenylmethylphosphonium dichromate: An effective and chemoselective reagent for oxidation of benzylic alcohols to the corresponding aldehydes and ketones

Article abstract of DOI:10.1080/00397910902805218

An efficient and straightforward method for oxidation of the benzylic alcohols to the corresponding aldehydes and ketones has been accomplished using triphenylmethylphosphonium dichromate (MTPPD) under solvent-free conditions with high chemoselectivity. The reaction is fast with good yields and straightforward workup.

Full text of DOI:10.1080/00397910902805218

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Triphenylmethylphosphonium
Dichromate: An Effective
and Chemoselective Reagent
for Oxidation of Benzylic
Alcohols to the Corresponding
Aldehydes and Ketones
Abdol R. Hajipour a b , Somayeh Safaei b & Arnold E.
Ruoho a
a Department of Pharmacology , University of
Wisconsin, Medical School , Madison, Wisconsin, USA
b Pharmaceutical Research Laboratory, College
of Chemistry, Isfahan University of Technology ,
Isfahan, Iran
Published online: 08 Sep 2009.
To cite this article: Abdol R. Hajipour , Somayeh Safaei & Arnold E. Ruoho (2009)
Triphenylmethylphosphonium Dichromate: An Effective and Chemoselective Reagent
for Oxidation of Benzylic Alcohols to the Corresponding Aldehydes and Ketones,
Synthetic Communications: An International Journal for Rapid Communication of
Synthetic Organic Chemistry, 39:20, 3687-3692, DOI: 10.1080/00397910902805218
To link to this article: http://dx.doi.org/10.1080/00397910902805218
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Synthetic Communications1, 39: 3687–3692, 2009
Copyright # Taylor & Francis Group, LLC
ISSN: 0039-7911 print=1532-2432 online
DOI: 10.1080/00397910902805218
Triphenylmethylphosphonium Dichromate:
An Effective and Chemoselective Reagent
for Oxidation of Benzylic Alcohols
to the Corresponding Aldehydes and Ketones
Abdol R. Hajipour,1,2 Somayeh Safaei,2 and Arnold E. Ruoho1
1Department of Pharmacology, University of Wisconsin,
Medical School, Madison, Wisconsin, USA
2Pharmaceutical Research Laboratory, College of Chemistry,
Isfahan University of Technology, Isfahan, Iran
Abstract: An efficient and straightforward method for oxidation of the benzylic
alcohols to the corresponding aldehydes and ketones has been accomplished using
triphenylmethylphosphonium dichromate (MTPPD) under solvent-free condi-
tions with high chemoselectivity. The reaction is fast with good yields and
straightforward workup.
Keywords: Alcohols, aldehydes, ketones, oxidation, triphenylmethylphospho-
nium dichromate
INTRODUCTION
Oxidation in modern organic chemistry is one of the most important
reactions.[1] Classic reagents such as manganese dioxide (MnO2), potas-
sium permanganate (KMnO4), chromium trioxide (CrO3), potassium
chromate (K2CrO4), and potassium dichromate have been introduced.[1]
Some new methods have also been reported.[2–10] Although some of these
methods have convenient protocols with good to high yields, most of
Received October 18, 2008.
Address correspondence to Abdol R. Hajipour, Pharmaceutical Research
Laboratory, College of Chemistry, Isfahan University of Technology, Isfahan
84156, Iran. E-mail: haji@cc.iut.ac.ir
3687

3688
A. R. Hajipour, S. Safaei, and A. E. Ruoho
them suffer at least from one of the following disadvantages: (1) high cost
of preparation, (2) long reaction time, (3) hygroscopicity, (4) high acidity,
(5) instability, (6) no selectivity, (7) photosensivity, (8) dangerous
procedures for their preparation, and (9) tedious workup procedures.
Reactions under solvent-free conditions have recently attracted
attention.[11,12] The advantage of these methods over conventional classi-
cal methods is that they are cleaner reactions and have decreased reaction
time and straightforward workup.
RESULTS AND DISCUSSION
In continuation of our ongoing program to develop environmentally
benign methods under solvent-free conditions,[13–16] we report an extre-
mely convenient method for oxidation of alcohols to their corresponding
aldehydes or ketones using triphenylmethylphosphonium dichromate
(MTPPD, 1). This reagent was easily prepared by adding an aqueous
solution of CrO3 in HCl (3N) to a stirring aqueous solution of
MePh3PþBrꢀ. After 15 min, MTPPD (1) was obtained as an orange pre-
cipitate in good yield (Scheme 1).
This reagent is soluble in acetonitrile, dimethylformamide (DMF),
dimethylsulfoxide (DMSO), and N-methylpyrrolidone (NMP) and slightly
soluble in ethyl acetate. By using this oxidant, various alcohols were
converted to their corresponding carbonyl compounds in good to high
yields within short reaction times (Scheme 2, Table 1).
In comparison to benzylic alcohols, oxidation of aliphatic ones with
this reagent do not occur at all, even when using an excess amount of
reagent and grinding for a longer time. The electron-donor functional
groups on the phenyl ring of benzylic alcohols accelerate the reaction rate
and the electron-acceptor ones decrease it. The rates for oxidation of pri-
mary alcohols in comparison to the secondary alcohols are faster and
also the reaction times are shorter (Scheme 2, Table 1).
To evaluate the selectivity of the reagent, the competitive reactions
shown in Eqs. (1–3) were carried out. When equimolar amounts of
2ꢀphenylethyl alcohol and benzyl alcohol were treated with MTPPD
(one molar ratio), only benzyl alcohol was selectively oxidized [Eq. (1)].
Treatment of benzyl alcohol with MTPPD (one molar ratio) in the
Scheme 1. Preparation of triphenylphosphonium dichromate (MTPPD).

Triphenylmethylphosphonium Dichromate
3689
Scheme 2. Oxidation of alcohols with MTPPD.
Table 1. Oxidation of alcohols by (MePh3P)2Cr2O7 (1) under solvent-free
conditionsa,b
Time Yield
(min)
Entry
Substrate (1)
(%) Mp or bp (ꢁC)=torr (lit.[15,16]
)
1
2
3
Benzyl alcohol
3.0
10.0
4.0
95
90
94
176–178=760 (178–179=760)
103–105 (103–106)
280–282=760 (281=760)
4-Nitrobenzyl alcohol
3,4-Dimethoxybenzyl
alcohol
4
5
6
7
8
9
4-Methoxybenzyl alcohol
2-Methoxybenzyl alcohol
1,1-Diphenylmethanol
3-Methoxybenzyl alcohol
4-Chloroybenzyl alcohol
3-Chloroybenzyl alcohol
3.0
3.0
6.0
3.0
5.0
5.0
6.0
5.0
3.0
10.0
6.0
93
95
95
93
95
93
141–143=50 (141–143=50)
37–39 (37–39)
46–48 (47–49)
141–143=50 (143=50)
45–48 (45–47)
211–213=760 (213–214=760)
10 1-Phenylethanol
11 1-(p-bromophenylethanol)
12 1-(p-chlorophenyl)ethanol
13 Benzoin
14 1-(p-Bromophenyl)-
2-bromoethanol
95 200–2002=760 (200–2002=760)
93
90
0
49–52 (49–52)
231–232=760 (232=760)
—
93
107–110 (108–110)
15 Cyclohexanol
16 1-Tetralol
17 n-Heptanol
18 n-Pentanol
19 L-Menthol
30.0
7.0
20.0
20.0
20.0
5.0
7.0
20.0
4.0
0
90
0
0
0
82
82
0
92
0
—
126–127=23 (127=23)
—
—
—
242–244=760 (243–245=760)
20 1-Indanol
21 9-Fluorenol
81–83 (80–83)
—
80–82 (79–91)
22 4-t-Decylcyclohexanol
23 2-naphthalene-methanol
24 2-Phenylethanol
25 3-Methylcyclohexanol
26 4-Methylphenylethyl
alcohol
30.0
30.0
30.0
—
—
—
0
0
aConfirmed by comparison with authentic samples (IR, TLC, and NMR).[15,16]
bYield of isolated products.

3690
A. R. Hajipour, S. Safaei, and A. E. Ruoho
presence of diphenylmethanol (1mmol) led to exclusive oxidation of
diphenylmethanol [Eq. (2)]. Interestingly, in the oxidation of alcohols,
the overoxidation of products to the corresponding carboxylic acids
was not observed. When we treated benzyl alcohol (1mmol) with
MTPPD (one molar ratio) in the presence of thioanisol (1mmol), only
the benzyl alcohol was selectively oxidized [Eq. (3)].
Solidꢀstate
10 min :; grinding
PhCH2CH2OH þ PhCH2OH
ꢀ!
PhCH2CH2OH þ PhCHO
unchanged
95%
ð1Þ
ð2Þ
Solidꢀstate
ꢀ!
PhCHOHPh þ PhCH2OH
PhSMe þ PhCH2OH
PhCOPh þ PhCH2OH
unchanged
5 min :; grinding
100%
Solidꢀstate
10 min :; grinding unchanged
ꢀ!
PhSMe þ PhCHO
ð3Þ
95%
EXPERIMENTAL
General
Yields refer to isolated pure products. The products were characterized
1
by comparison of their spectral (IR, H NMR) and physical data with
those of authentic samples.[15,16] All 1H NMR spectra were recorded
at 300 and 500 MHz in CDCl3 relative to tetramethylsilane (TMS)
(0.00 ppm), and IR spectra were recorded on Shimadzu 435 IR spectro-
meter. All reactions were carried out under solvent-free conditions at
room temperature in a hood with strong ventilation. The reaction is safe,
and we did not observe any dangers using this procedure under solvent-
free conditions.
Preparation of Methyltriphenylphosphonium Dichromate (MTPPD)
A solution of CrO3 (10 mmol, 1.0 g) in HCl 3 N (10 mL) was added to a
solution of methylriphenylphosphonium bromide (10 mmol, 3.57 g) in
water (25 mL) under stirring at room temperature. After 30 min of stir-
ring, an orange precipitate was formed. The mixture was filtered, washed
with water (2 ꢂ 15 mL), and dried at room temperature (3.85 g, 96%
yield), which decomposed at 220–222ꢁC to a dark-brown material. IR
(KBr) 3100, 2980, 1600, 1495, 1480, 1260, 1190, 1050, 860, 750,
690 cmꢀ1. 1H NMR (DMSO-d6) d, ppm: 7.9–7.7 (m, 15 H), 3.19

Triphenylmethylphosphonium Dichromate
3691
(d, J ¼ 14.6, P-CH3). 13CNMR: d 134.5, 134.4, 132.5, 132.3, 129.7, 129.3,
116.4. Anal. calcd. for C38H36Cr2O7P2: C, 63.16; H, 4.99%. Found: C,
63.25; H, 4.90%.
Oxidation of Benzyl Alcohol with MTPPD to Benzaldehyde
In a mortar, 0.36 g of MTPPD (0.5 mmol) and the alcohol (1 mmol) were
ground with a pestle for the time specified in Table 1. The progress of the
reaction was monitored by thin-layer chromatography (TLC; silica gel,
EtOAc=n-hexane ¼ 20=80) until the alcohol disappeared. When the reac-
tion was completed, the product was extracted with Et2O (2 ꢂ 10 mL) and
dried (MgSO4), and the solvent was evaporated to yield the pure product
without any further need for purification.
CONCLUSION
In conclusion, we report an efficient and versatile method for converting
alcohols to their corresponding carbonyl compounds with the following
advantages: (a) our reagent is inexpensive and easily handed and can
be stored on the bench for months without losing its activity; (b) the
procedure is simple and occurs under solvent-free conditions at room
temperature; (c) the reaction is environmentally green; (d) the reaction
time is short; and (e) the isolation of the product is straightforward.
ACKNOWLEDGMENTS
We gratefully acknowledge the funding support received for this project
from the Isfahan University of Technology (IUT), Iran (A. R. H.), and
Grants GM 033138, MH 065503, and NS 033650 (A. E. R.) from the
National Institutes of Health. Further financial support from the Center
of Excellency in Sensor and Green Chemistry Research (IUT) is grate-
fully acknowledged.
REFERENCES
1. Trost, B. M. Comprehensive Organic Synthesis–Oxidation; Pergamon:
New York, 1991; vol. 7.
2. Sheidon, R. A.; Kkochi, J. K. Metal-Catalysed Oxidation of Organic
Compounds; Academic Press: New York, 1981.
3. Hudlicky, M. Oxidation in Organic Chemistry; American Chemical Society:
Washington, DC, 1990.

3692
A. R. Hajipour, S. Safaei, and A. E. Ruoho
4. Ley, S. V.; Norman, J.; Griffith, W. P.; Marsden, S. P. Tetrapropylammo-
nium perruthenate, PR4NþRuOꢀ4 ,TPAP—A catalytic oxidant for organic
synthesis. Synthesis 1994, 639.
5. Stevens, R. V.; Chapman, K. T.; Waiter, H. N. Convenient and inexpensive
procedure for oxidation of secondary alcohols to ketones. J. Org. Chem.
1980, 45, 2030.
6. Hajipour, A. R.; Rafiee, F.; Ruoho, A. E. Facile and selective oxidation of
benzylic alcohols to their corresponding carbonyl compounds with sodium
nitrate in the presence of Brønsted acidic ionic liquids. Synlett 2007, 1118.
7. Lee, D. G. Aqueous dichromate oxidation of primary alcohols. J. Org. Chem.
1970, 35, 3589.
8. Hajipour, A. R.; Bgheri, H. R.; Ruoho, A. E. Solid state oxidation of
alcohols using 1-butyl-4-aza-1-azoniabicyclo[2.2.2]octane chlorchromate. B.
Kor. Chem. Soc. 2004, 25, 1238.
9. Hajipour, A. R.; Mohammadpoor-Baltork, I., Kianfar, G. Bis(1-benzyl-4-
aza-1-azoniabicyclo[2.2.2]octane) peroxodisulfate: A mild and efficient oxi-
dant for cleavage of nitrogen double bonds and oxidation of alcohols under
anhydrous conditions. J. Chem. Soc. Jpn. 1998, 71, 2655.
10. Larock, R. C. Comprehensive Organic Transformations, 2nd ed.; John Wiley &
Sons: New York, 1999.
11. Hajipour, A. R.; Zarei, A.; Ruoho, A. E. P2O5=Al2O3 as an efficient hetero-
geneous catalyst for chemoselective synthesis of 1,1-diacetates under
solvent-free conditions. Tetrahedron Lett. 2007, 48, 2881.
12. Hajipour, A. R.; Mostafavi, M.; Ruoho, A. E. Deprotection of thioacetals
using k2S2O8=[bmim]Br as a mild and efficient reagent under solvent-free
conditions. Monatsh. Chem. 2007, 138, 569.
13. (a) Hajipour, A. R.; Arbabian, M.; Ruoho, A. E. Tetramethylammonium
dichloroiodate: An efficient and environmentally friendly iodination reagent
for iodination of aromatic compounds under mild and solvent-free conditions.
J. Org. Chem. 2002, 67, 8622; (b) Hajipour, A. R.; Ruoho, A. E. Iodination of
aromatic compounds under mild and solvent-free conditions. Org. Prep. Proced.
Int. 2002, 34, 647; (c) Hajipour, A. R.; Mazloumi, G. An efficient and simple
procedure for preparation of esters and anhydrides from acid chlorides in the
presence of 1,4-diazabicyclo[2.2.2]octane (DABCO) under solvent-free condi-
tions. Synth. Commun. 2002, 32, 23; (d) Hajipour, A. R.; Ruoho, A. E. Oxidation
of thiols to the corresponding disulfides with tetramethylammonium chloro-
chromate under non aqueous conditions. J. Chem. Res., Synop. 2002, 547.
14. Hajipour, A. R.; Mirjalili, B. F.; Zarei, A.; Khazdooz, L.; Ruoho, A. E. A
novel method for sulfonation of aromatic rings with silica sulfuric acid.
Tetrahedron Lett. 2004, 45, 6607.
15. Hajipour, A. R.; Ruoho, A. E. Solid state deprotection of thioacetals and
thioketals using 1-benzyl-4-aza-1-azoniabicyclo[2.2.2]octane periodate and
aluminum chloride. Org. Prep. Proced. Int. 2005, 37, 279.
16. Hajipour, A. R.; Adibi, H.; Ruoho, A. E. Wet silica-supported permanganate
for the cleavage of semicarbazones and phenylhydrazones under solvent-free
conditions. J. Org. Chem. 2003, 68, 4553.