NaIO4/LiBr-mediated direct conversion of benzylic alcohols and aromatic aldehydes to aromatic esters

Article abstract of DOI:10.1080/00397910701465032

Aromatic aldehydes or benzylic alcohols are directly converted to the corresponding aromatic esters in high yields on treatment with methanol or ethanol mediated by sodium metaperiodate (NaIO4)/LiBr in an acidic medium. Copyright Taylor & Francis Group, LLC.

Full text of DOI:10.1080/00397910701465032

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NaIO4/LiBr‐Mediated Direct Conversion of
Benzylic Alcohols and Aromatic Aldehydes to
Aromatic Esters
Tanveer Mohammad Ali Shaikh a , Lourdusamy Emmanuvel a & Arumugam
Sudalai a
a National Chemical Laboratory, Chemical Engineering and Process
Development Division, Pune, Maharastra, India
Published online: 14 Aug 2007.
To cite this article: Tanveer Mohammad Ali Shaikh , Lourdusamy Emmanuvel & Arumugam Sudalai (2007)
NaIO4/LiBr‐Mediated Direct Conversion of Benzylic Alcohols and Aromatic Aldehydes to Aromatic Esters,
Synthetic Communications: An International Journal for Rapid Communication of Synthetic Organic Chemistry,
37:16, 2641-2646, DOI: 10.1080/00397910701465032
To link to this article: http://dx.doi.org/10.1080/00397910701465032
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Synthetic Communicationsw, 37: 2641–2646, 2007
Copyright # Taylor & Francis Group, LLC
ISSN 0039-7911 print/1532-2432 online
DOI: 10.1080/00397910701465032
NaIO4/LiBr-Mediated Direct Conversion
of Benzylic Alcohols and Aromatic
Aldehydes to Aromatic Esters
Tanveer Mohammad Ali Shaikh, Lourdusamy Emmanuvel,
and Arumugam Sudalai
National Chemical Laboratory, Chemical Engineering and Process
Development Division, Pune, Maharastra, India
Abstract: Aromatic aldehydes or benzylic alcohols are directly converted to the
corresponding aromatic esters in high yields on treatment with methanol or ethanol
mediated by sodium metaperiodate (NaIO4)/LiBr in an acidic medium.
Keywords: aromatic ester, benazaldehyde, benzyl alcohol, lithium bromide, sodium
metaperiodate
The direct conversion of aldehydes or alcohols to the corresponding car-
boxylic esters is often required in organic synthesis, particularly in the
synthesis of natural products.[1] The conventional method for the synthesis
of carboxylic esters involves oxidation of aldehydes to carboxylic acids
followed by esterification with alcohols catalyzed by either acid or base. In
contrast, the direct method of conversion of alcohols or aldehydes to car-
boxylic esters holds promise in organic synthesis because it minimizes the
number of steps. Such transformation of aldehydes directly into esters has
Received May 1, 2007
Address correspondence to Arumugam Sudalai, National Chemical Laboratory,
Chemical Engineering and Process Development Division, Dr. Homi Bhabha Road,
Pune 411008, Maharastra, India. E-mail: a.sudalai@ncl.res.in
2641

2642
T. M. A. Shaikh, L. Emmanuvel, and A. Sudalai
been achieved using a variety of reagents such as V2O5/H2O2,[2] oxonew,[3]
DMSO/Ac2O,[4] Br2/NaHCO3,[5] O3/KOH,[6] pyridinium hydrobromide per-
bromide,[7] pyridinium dichromate,[8] electrochemical methods,[9] TMSOTf/
NBS/AIBN,[10] S SnO2/SBA-1-H2O2,[11] and more recently acetone cyano-
.
hydrin.[12] Also, the direct conversion of alcohols into esters has been
achieved using oxonew/NaCl,[13] Ca(OCl)2,[14] pyridinium hydrobromide
perbromide,[7] NaBrO3/NaHSO3,[15] NaBrO2/AcOH,[16] PhI(OAc)2,[17] and
H2O2/Br2.[18] Unfortunately, many of these procedures often require a large
excess of reagents, expensive catalysts, transition metals, dry solvents,
stringent reaction conditions, photochemical methods, and so on. Moreover,
the formation of carboxylic acids due to the over oxidation of aldehydes
further complicates the reaction and results in diminished yield of the esters.
Recently, we reported[19] that NaIO4-mediated oxidation of alkylarenes
led to the high-yield preparation of benzoic acids, formed via the oxidation
of their respective benzyl alcohols and aldehydes when the reaction was
carried out in water. In this regard, we envisioned that replacing water with
alcohol should result in the formation of esters. In this communication, we
report that the NaIO4-LiBr-Hþ combination oxidizes benzylic alcohols or
aromatic aldehydes directly to the corresponding aromatic esters in high
yields (Scheme 1).
To establish the scope of this reaction, a number of aromatic aldehydes
and benzylic alcohols were subjected to oxidation, and the results are given
in Table 1.
As can be seen from Table 1, both aromatic aldehydes and benzylic
alcohols with electron-donating as well as electron-withdrawing substituents
on the nucleus underwent oxidative esterification smoothly to give
their corresponding aromatic esters in 71–95% yields. In the case of sub-
strates with electron-donating groups, the reaction proceeded at 258C
whereas the substrates with electron-withdrawing groups, except
4-chloro aromatics (entry 3), required higher temperatures (658C) to
achieve excellent conversions. For 4-methoxy-benzyl alcohol, the
oxidative esterification occurred along with nuclear bromination when 2
molar equivalents of LiBr were used (entry 4). We also observed that
use of substoichiometric amounts of NaIO4 (25–40 mol%) resulted in
poor yields of esters (,40%).
Scheme 1. NaIO4/LiBr-mediated direct oxidative esterification of benzylic alcohol
and benzaldehydes.

NaIO4/LiBr-Mediated Direct Conversion of Benzylic Alcohols
2643
Table 1. Direct conversion of benzylic alcohols or aldehydes to aromatic estersa
Product yieldb
Entry
1
Substrate
Product
X ¼ OH
X ¼ O
89
98
2c
78
82
3
4
86
91
85
77d
5
80
87
6
73
77
78
85
83
80
7
8e
9e
71
73
78
79
10e
11e
80
82
aAlcohol or aldehyde (3 mmol), NaIO4 (3 mmol), LiBr (3 mmol), conc. H2SO4
(1 mL), methanol (9 mL), 258C, 18 h.
bAfter purification by column chromatography.
cEthanol was used as the solvent.
dYield of the methyl 3-bromo-4-methoxybenzoate when two equivalents of LiBr are
used.
eReaction done at 658C with aq. H2SO4 (0.85 N, 1 mL).

2644
T. M. A. Shaikh, L. Emmanuvel, and A. Sudalai
Among the several solvent combinations screened, a mixture of
MeOH/H2O (9:1, 10 ml) gave the highest yield of esters. Control exper-
iments have proved that both NaIO4 and LiBr are needed to produce
esters. Ethanol was also found to give ethyl esters (entry 2), whereas
other alcohols such as 2-propanol and benzyl alcohol failed to give their
respective esters, probably because of steric hindrance. Aliphatic alcohols
or aldehydes were found to be inactive under the reaction conditions,
which may be a limitation of this process. Replacing LiBr with other
halide sources such as tetrabutylammonium bromide or NaBr also
brought about this conversion with comparable yield, whereas KI and
NaCl failed.
Based on the observations that no traces of acetal or carboxylic acid were
identified in the present study as well as from our earlier work on NaIO4/LiBr-
mediated reactions,[19] a probable mechanism[18] for the oxidative esterifica-
tion of aromatic aldehydes and benzylic alcohols is given in Scheme 2.
Initially, benzylic alcohols are oxidized to the corresponding aromatic
aldehydes [Eq.(1)], which subsequently undergo oxidative esterification
with methanol via a radical pathway as shown in Eqs. (2–5).
Scheme 2. Proposed radical pathway for the direct esterification of aromatic alde-
hydes and benzylic alcohols.
In conclusion, we have developed a new oxidative procedure for the
direct conversion of both activated and deactivated benzylic alcohols or
aromatic aldehydes to the corresponding aromatic esters in high yields
using NaIO4/LiBr/Hþ as the reagent system.
EXPERIMENTAL
Reagents and solvents were commercially available and were used without
1
further purification. H and 13C NMR spectra were recorded on a Bruker
AC-200 spectrometer. Column chromatography was performed on silica gel
(60–120 mesh).

NaIO4/LiBr-Mediated Direct Conversion of Benzylic Alcohols
Typical Experimental Procedure
2645
Lithium bromide (3 mmol) to a 50-ml round-bottom flask charged with
aromatic aldehyde or benzylic alcohol (3 mmol), sodium metaperiodate
(3 mmol), and conc. H2SO4 (1 mL) in methanol (9 ml) was added at 258C.
The reaction mixture was stirred for 18 h, and then the solvent was
removed under reduced pressure. The residue was extracted with ethyl
acetate, and the organic phase was washed with water, saturated and
Na2S2O3, and brine; dried over anhydrous sodium sulphate; and concentrated
under reduced pressure. The crude ester was purified over silica gel (60–120
mesh) by column chromatography using hexane/EtOAc (19:1) as eluent.
Data
1
Methyl 4-fluorobenzoate: mp of methyl 4-fluorobenzoate ¼ 89–908C. H
NMR (CDCl3) d 3.91 (s, 3H), d 7.06–7.15 (dd, 2H, J ¼ 6 Hz), d 8.02–8.09
(dd, 2H, J ¼ 6 Hz). 13C NMR (CDCl3) 51.90, 115.07, 115.50, 126.27,
131.86, 132.04, 163.06, 165.82, 168.11.
Methyl 4-tert-butylbenzoate: 1H NMR (CDCl3) d 1.34 (s, 9H), d 3.90 (s, 3H),
d 7.43–7.47 (dd, 2H, J ¼ 8 Hz), d 7.95–7.99 (dd, 2H, J ¼ 8 Hz). 13C NMR
(CDCl3) 31.09, 35.04, 51.89, 125.25, 127.31, 129.38, 156.42, 167.02
Methyl benzoate: 1H NMR (CDCl3) d 3.92 (s, 3H), d 7.39–7.56 (m, 3H), d
8.01–8.06 (dd, 2H, J ¼ 8 Hz). 13C NMR (CDCl3) 52, 128.30, 129.53, 130.21,
132.82, 166.99.
1
Methyl 4-nitrobenzoate: mp of methyl 4-fluorobenzoate ¼ 89–908C. H
NMR (CDCl3) d 3.99 (s, 3H), d 8.19–8.32 (m, 4H). 13C NMR (CDCl3)
52.79, 123.50, 130.68, 135.51, 150.51, 165.10.
ACKNOWLEDGMENTS
Tanveer M. S. and Emmanuvel L. thanks the Council of Scientific and Indus-
trial Research (CSIR), New Delhi, India, for the award of research fellowships.
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