Silica gel supported jones reagent (SJR): A simple and efficient reagent for oxidation of benzyl alcohols to benzaldehydes

Article abstract of DOI:10.1081/SCC-100104049

Jones reagent supported on silica gel (SJR) has been used for oxidation of benzyl alcohols to the corresponding benzaldehydes in high yields utilizing organic solvent. This newly developed reagent is easy to prepare and use. SJR is also safer than the traditional Jones reagent.

Full text of DOI:10.1081/SCC-100104049

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SILICA GEL SUPPORTED
JONES REAGENT (SJR):
A SIMPLE AND EFFICIENT
REAGENT FOR OXIDATION
OF BENZYL ALCOHOLS TO
BENZALDEHYDES
Mohammed Hashmat Ali a & Candace J. Wiggin a
a Department of Chemistry , Southeast Missouri
State University , Cape Girardeau, MO, 63701,
U.S.A.
Published online: 09 Nov 2006.
To cite this article: Mohammed Hashmat Ali & Candace J. Wiggin (2001) SILICA
GEL SUPPORTED JONES REAGENT (SJR): A SIMPLE AND EFFICIENT REAGENT FOR
OXIDATION OF BENZYL ALCOHOLS TO BENZALDEHYDES, Synthetic Communications:
An International Journal for Rapid Communication of Synthetic Organic Chemistry,
31:9, 1389-1397, DOI: 10.1081/SCC-100104049
To link to this article: http://dx.doi.org/10.1081/SCC-100104049
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SYNTHETIC COMMUNICATIONS, 31(9), 1389–1397 (2001)
SILICA GEL SUPPORTED JONES REAGENT
(SJR): A SIMPLE AND EFFICIENT REAGENT
FOR OXIDATION OF BENZYL ALCOHOLS
TO BENZALDEHYDES
Mohammed Hashmat Ali* and Candace J. Wiggin
Department of Chemistry, Southeast Missouri State
University Cape Girardeau, MO 63701
ABSTRACT
Jones reagent supported on silica gel (SJR) has been used for
oxidation of benzyl alcohols to the correspondingbenzalde-
hydes in high yields utilizing organic solvent. This newly
developed reagent is easy to prepare and use. SJR is also
safer than the traditional Jones reagent.
Oxidation of alcohols to carbonyl compounds is one of the most
important reactions in organic chemistry. Chromium (VI) reagents are
widely utilized in the oxidation of alcohols to carbonyl compounds. Jones
reagent is the best known of these Cr (VI) reagents for oxidation of second-
ary alcohols to ketones. However, it fails to produce satisfactory results for
oxidation of primary alcohols, includingbenzyl alcohols to the correspond-
1
ingaldehydes. Difficulty in the above oxidation reaction arises due to the
fact that aldehydes react with water and alcohols under acidic aqueous
media employed in the Jones oxidation procedure, producinghydrates
* Correspondingauthor.
1389
Copyright & 2001 by Marcel Dekker, Inc.
www.dekker.com

1390
ALI AND WIGGIN
and hemiacetals, which are then oxidized to carboxylic acids,12 and esters3
respectively. A number of Cr (VI) based reagents or procedures have been
developed to avoid these problems.11–15 Most of these reported reagents are
complexes of an amine and Cr (VI) oxide. Unfortunately, these procedures
produce slow reactions,5,8,9,11,14 and require meticulous and time-consuming
preparative methods8,11,13 Recently, employment of solid supported
chromium (VI) reagents in the oxidation of alcohols to carbonyl compounds
has generated interest. In most cases, solid-supported reagents have been
found superior to the non supported reagents. Solid supports modify
activity of the reagent, improve selectivity, and, more importantly, make
product isolation easier. There are some isolated reports on oxidation of
benzyl, alcohols to benzaldehydes with solid supported chromium (VI)
reagents.9,15–22 However, these solid supported chromium reagents require
difficult and laborious preparation,9,16,17,21 activation by subjectingthe sup-
ported reagents to high temperature for prolonged periods of time,20,21 have
a longer reaction time,20,21 produce low yields,17,18 and require elevated
temperature for the reaction to work.15–17 Herein, we report a systematic
study of the oxidation of benzyl alcohols to benzaldehydes with a new
reagent prepared by supporting Jones reagent on silica gel (SJR). It is
noteworthy that SJR does not suffer from many of the undesired properties
associated with previously reported solid supported chromium reagents.
Scheme 1.
As a part of our on going study of organic reactions on solid sup-
ports,22–25 we are reportingthe first successful use of Jones reaegnt
supported on silica gel (SJR) for the oxidation of benzyl alcohols to the
correspondingbenzaldehydes. The SJR can be prepared in few minutes by
mixing a known amount of Jones reagent and chromatographic grade silica
gel. Activation of the reagent prior to use is not required to effect oxidation
of alcohols. One of the advantages of this reagent is that the amount of
chromium (VI) oxidant present on the silica gel is known precisely. For other
reported solid supported chromium reagents, the amount of chromium (VI)

SILICA GEL SUPPORTED JONES REAGENT
1391
loaded on the support needs to be determined by a method such as titration.
We have found SJR is capable of oxidizinga wide range of benzyl alcohols
efficiently and selectively to the correspondingbenzaldehydes (see the Tab.).
Accordingto this method, stirringa heterogeneous mixture of a benzyl
alcohol and SJR in methylene chloride at room temperature carries out
oxidation of benzyl alcohols. The reaction takes less than 15 min to com-
plete. Products are isolated by a simple filtration of the reaction mixture and
washingthe solid residue with methylene chloride. Evaporation of methy-
lene chloride from the filtrate under vacuum produces product that is often
pure by NMR and TLC. Byproducts of the Jones reagent are adsorbed on
the silica gel and are removed during the filtration. The reaction is very fast
and does not require any special care or conditions to prevent overoxida-
tion. The procedure produces an excellent yield of benzaldehydes. No sig-
nificant amount of carboxylic acid or ester is detected in the product.
However, we have found that if the product is not separated from the
excess reagent after the reaction is over, the aldehyde is slowly oxidized to
the correspondingcarboxylic acid. Products containingimpurities were
purified by radial chromatography using a Chromatotron instrument.
The Cr (VI) and Cr (III) compounds remain on the silica gel support
and never leach into the methylene chloride solvent duringthe reaction or
duringwashingof the solid residue with methylene chloride after the reaction
is complete. We observed that when the magnetic stirrer is turned off, the
orange silica gel supported Jones reagent (SJR) settles, producinga colorless
liquid layer on the top of an orange solid at the bottom of the flask. Also dark
green color, a typical color of Cr (III) salts produced in the Jones reaction,
was absent in the filtered solution. In addition, proton NMR produced
sharp and narrow peak, and chemical shift values were as expected. The
presence of paramagnetic chromium compounds in the products would
broaden and shift the positions of the NMR peaks. The above observations
indicated the absence of Cr (III) and/or Cr (VI) compounds in the product.
The electronic nature of the group present on aromatic ring has no
detrimental effect on this oxidation procedure. The presence of electron-
donating( 1b, 1c, 1d, 1f, and 1h) as well as electron-withdrawing( 1g, 1i,
and 1j) groups on the benzene ring produced very similar results.
However, oxidation of 2,3-dimethoxybenzyl alcohol 2c produced a lower
yield of the correspondingaldehyde, and 2,5-dimethoxybenzyl alcohol 2e
produced a mixture of unidentified inseparable products. We anticipated
complication in these oxidation reactions, since both 1,2- and 1,4-
dimethoxybenzes are known to undergo oxidation to benzoquinone with
Jones reagent.1
Preparation and utilization of SJR in the oxidation reaction requires
very little handlingof the toxic chromium reagent. Therefore, the possibility

1392
ALI AND WIGGIN
Table. Oxidation of Benzyl Alcohols 1 to Benzaldehydes 2
Benzyl alcohol (1)
Benzaldehyde (2)
(Yield %)a
a.
b.
85
100
c.
82
98
d.
e.
Mixture of products
f.
99
99
98
g.
h.
i.
j.
90
99
aUnoptimized isolated yield; Products were identified by comparison of the NMR
and IR data with those of the authentic benzaldehyde products.

SILICA GEL SUPPORTED JONES REAGENT
1393
for workers to come into contact with the reagent is minimum. The amount
of toxic waste produced in this procedure is significantly smaller than the
acetone-aqueous media employed in the traditional Jones oxidation pro-
cedure. Moreover, the solid nature of the waste makes for easy disposal.
Overall, this procedure is safer to carry out and also environmentally safer.
We have found that, in addition to methylene chloride, chloroform and
hexane can also be used as the solvent in this reaction. However, if
hexane is employed as the solvent, the solid residue should be washed
with a polar solvent such as methylene chloride or ethyl acetate to obtain
higher yields. Flexibility in the choice of solvents makes SJR more attractive
than the traditional Jones reagent. Also the higher yields, greater selectivity,
26
ease of preparation, ease of use, safety, and longlife
of SJR make this
reagent clearly superior to all previously reported Cr (VI) reagents utilized
in the oxidation of benzyl alcohols to the correspondingbenzaldehydes.
The results of our studies in progress indicated that SJR, in addition to
the oxidation of benzyl alcohols to benzaldehydes, can be utilized in the
followingtransformations; oxidation of secondary alcohols to ketones oxi-
dation of primary alcohols to aldehydes with some limitations: oxidative
regeneration of carbonyl compounds from oximes; oxidation of aldehydes
to carboxylic acids; oxidation of hydroquinones to quinines and oxidation of
sulfides to sulfones. We have also found that the above reactions can be
promoted by a catalytic amount of Jones reagent utilizing a stoichiometric
oxidant to regenerate Cr (VI) both supported on silica gel. The results we
have so far show that all of the above SJR-promoted reactions, except oxi-
dation of primary alcohols to aldhydes, produce excellent results. Oxidation
of primary alcohols to aldehydes produced moderate to good yields due to
overoxidation of alcohols to carboxylic acids that we have not been able to
control sofar. Upon completion, resultsofthese later studies willbepublished.
EXPERIMENTAL
All reaction mixtures were stirred with a Teflon coated magnet.
Methylene chloride was used as received from the supplier without any
further purification. Benzyl alcohols were purchased from Aldrich, Acros,
and Lancaster Chemical Companies in the USA and were used without
further purification. Jones reagent (8M) was prepared according to the
literature method.1 Silica gel used in the oxidation reactions, as solid sup-
port, was MN-Kieselgel 60 (0.04–0.063 mm mesh size) supplied by Fisher
Scientific. NMR spectra were recorded on a Bruker DPX-300 NMR instru-
1
ment H at 300 MHz, 13C at 75 MHz). Samples for NMR were dissolved in
CDCl3. Proton chemical shifts are expressed as ppm relative to tetramethyl-

1394
ALI AND WIGGIN
silane, and 13C chemical shifts were referenced to the solvent signal. IR
spectra were recorded on a Perkin Elmer Spectrum-1000 FT-IR instrument
and are reported in wavenumbers (cm–1). Chromatographic separations
were carried out by PCTLC (preparative centrifugal thin-layer chromato-
graphy) with silica gel (Merck #7749) on a Chromatotron Model 7924T.
Analytical thin-layer chromatography was done on pre-coated silica gel
plates with 254 nm fluorescent indicator (Merck #5715) and developed in
a 1:9 mixture of ethyl acetate-hexane solvent system. Compounds were
visualized by UV lamp and/or by stainingeither with a p-anisaldehyde/
sulfuric acid or phosphomolybdic acid. All products were identified by
comparingspectroscopic data with values reported in the literature and
those of the authentic samples.
Oxidation of Benzyl Alcohol 1a to Benzaldehyde 2a: General
Procedure for Oxidation of Benzyl Alcohols(1a–j) with SJR
The followingoxidation procedure for benzyl alcohol is representative
of all alcohols presented in the Table.
Dry silica gel (5 g) was placed in a 100-mL round-bottom flask con-
taininga mangetic stirringbar and fitted with a rubber septum. Jones
reagent (1.5 mL, 12 mmol) was added slowly from a syringe to the vigor-
ously stirred silica gel. After complete addition of the reagent, stirring con-
tinued until a free-flowingorange powder was obtained (less than 5 min).
Methylene chloride (25 mL) was added to the above flask. A solution of
benzyl alcohol 1a (396 mg, 3.66 mmol) in methylene chloride (5 mL) was
added slowly from a syringe through the rubber septum to the stirred het-
erogeneous mixture. The orange SJR turned dark green/brown immedi-
ately. After complete disappearance of the startingbenzyl alcohol as
indicated by TLC (10 min), the reaction mixture was filtered through a
sintered glass funnel, the solid residue was washed with methylene chloride
(75 mL) and the washings were added to the filtrate. Solvent was removed
from the solution under vacuum to produce a colorless oil (331.6 mg, 85.3%
yield) which was pure benzaldehyde accordingto NMR and IR data.
Spectral Data for Compounds (2a–j)
Benzaldehyde (2a)27
1H NMR (300 MHz, CDCl3) 7.40–7.46 (m, 2H), 7.50–7.63 (m, 1H),
7.80–7.88 (dt, J ¼ 6.83 Hz, J ¼ 1.5 Hz , 1H, 9.96 (s, 1H) ppm; 13C NMR
(75 MHz, CDCl3) 192.39, 136.46, 134.44, 129.68, 129.02 ppm. IR (film) 650,

SILICA GEL SUPPORTED JONES REAGENT
1395
688, 739, 828, 1023, 1167, 1266, 1310, 1390, 1455, 1584, 1597, 1654, 1702,
2696, 2739, 2821, 2852, 3031, 3060 cm–1.
O-Anisaldehyde (2b)27
1H NMR (300 MHz, CDCl3)3.88 (s, 3H), 6.97 (m, 2H),7.52 (m, 1H),
7.80 (dd, J ¼ 7.44, 1.44, 1H), 10.44 (s, 1H) ppm; 13C NMR (75 MHz, CDCl3)
189.72, 161.83, 136.04, 128.33, 124.72, 120.60, 111.69, 55.58 ppm; IR (film)
647, 759, 835, 1022, 1043, 1104, 1162, 1190, 1247, 1287, 1396, 1439, 1468,
1484, 1600, 1687, 2761, 2845, 3010, 3077 cm–1.
2,3-Dimethoxybenzaldehyde (2c)27
1H NMR (300 MHz, CDCl3) 3.90 (s, 3H), 3.98(s, 3H), 7 10–7.18 (m,
2H), 7.35–7.42 (dd, J ¼ 7, 2.5 Hz, 1H), 10.42 (s, 1H) ppm; 13C NMR
(75 MHz, CDCl3) 190.21, 153.04, 152.74, 129.75, 124.19, 119.08, 118.15,
62.33, 56.02 ppm; IR (film) 749, 787, 805, 911,1000, 1067, 1083, 1174,
1228, 1266, 1311, 1386, 1431, 1461, 1484, 1585, 1691, 2746, 2838, 2940,
2976, 3030, 3070 cm–1.
2-Methylbenzaldehyde (2d)28
1H NMR (300 MHz, CDCl3) 2.64 (s, 3H), 7.22 (m, 1H), 7.31 (m, 1H),
7.43 (m, 1H), 7.75 (dd, J ¼ 7.6, 1.4 Hz), 10.21 ppm; NMR (75 MHz, CDCl3)
19.45, 126.30, 131.74, 132.02, 133.57, 134.14, 140.45, 192.61 ppm; IR (film)
662, 730, 862, 908, 1124, 1195, 1213, 1285, 1304, 1381, 1405, 1439, 1458,
1489, 1573, 1601, 1697, 2740, 2859, 2929, 2966, 3072 cm–1.
P-Anisaldehyde (2f)27
1H NMR (300 MHz, CDCl3) 3.87 (s, 3H), 7.0 (d, J ¼ 8.7 Hz, 2H), 7.83
(d, J ¼ 8.7 Hz, 2H), 9.87 (s, 1H) ppm; 13C NMR (75 MHz, CDCl3) 55.59,
114.33, 129.96, 131.99, 164.63, 190.81 ppm; IR (film) 833, 1024, 1109, 1160,
1216, 1260, 1315, 1426, 1460, 1511, 1578, 1600, 1683, 2740, 2806, 2841, 2937,
2968, 3010 3076 cm–1.
4-Nitrobenzaldehyde (2g)27
1H NMR (300 MHz, CDCl3) 8.10 (d, J ¼ 8.7 Hz, 2H),8.42 (d, J ¼ 8.7 Hz,
2K), 10.20 (s, 1H) ppm; 13C NMR (75 MHz, CDCl3) 190.44, 151.32, 140,09,

1396
ALI AND WIGGIN
130.55, 124.35 ppm; IR (film) 737, 813, 848, 1007, 1103, 1197, 1323, 1346,
1537, 1605, 1707, 2729, 2778, 2848, 3010, 3060, 3107 cm–1.
P-Tolualdehyde (2h)27
1H NMR (300 MHz, CDCl3) 2.41 (s, 3H), 7.30 (d, J ¼ 7.7 Hz, 2H),
7.75 (d, J ¼ 7.7 Hz, 2H), 9.94 (s, 1H) ppm; 13C NMR (75 MHz, CDCl3)
21.85, 129.71, 129.83, 134.20, 145.54, 191.98 ppm; IR (film) 758, 809, 847,
1018, 1039, 1105, 1169, 1209, 1298, 1307, 1387, 1415, 1450, 1577, 1605, 1702,
2734, 2827 2922, 3030 cm–1.
2-Chlorobenzaldehyde (2i)27
1H NMR (300 MHz, CDCl3) 7.29–7.40 (m, 2H), 7.45–7.53 (m, 1H),
7.82–7.86 (dd, J ¼ 1.5, 7.7 Hz 1H), 10.4 (s, 1H); 13C NMR (75 MHz, CDCl3)
127.30, 129.26, 130.56, 132.36, 135.12, 137.73, 189.40 ppm; IR (film) 633, 740,
825, 896, 1053, 1200, 1265, 1443, 1480, 1592, 1698, 2872, 2986, 3054 cm–1.
4-Chlorobenzaldehyde (2j)27
1H NMR (300 MHz, CDCl3) 7.51 (d, J ¼ 7.8 Hz, 2H), 7.82 (d,
J ¼ 7.8 Hz, 2H), 9.98 (s, 1H) ppm; 13C NMR (75 MHz, CDCl3) 129.45,
130.92, 134.70, 140.94, 190.89 ppm; IR (film) 650, 742, 821, 837, 911, 1014,
1093, 1165, 1206, 1265, 1421, 1487, 1576, 1596, 1702, 2840, 2986, 3054 cm–1.
ACKNOWLEDGMENTS
We gratefully acknowledge the Grants and Research Funding
Committee (GRFC) for financial support and National Science
Foundation (ILI-IP grant #96-50399) for providing grant for the purchase
of a Bruker DPX-300 NMR instrument. M.H.A is also grateful to Dr. Bjorn
Olesen, Dr. David Ritter, and Dr. Phil Crawford for helpful suggestions
duringthe preparation of this manuscript.
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Received in the USA June 14, 2000