Transition metal free one pot synthesis of aryl carboxylic acids via dehomologative oxidation of styrenes

Article abstract of DOI:10.1016/j.tetlet.2018.10.061

Iodine/NaOH-catalyzed one-pot dehomologative oxidation of styrenes to aryl carboxylic acids has been reported. A wide range of carboxylic acids are obtained using iodine (I₂) as a catalyst, tert-butyl hydroperoxide (TBHP) as an oxidant and sodium hydroxide (NaOH) as a base. This reliable conversion involves dehomologation of styrene to aromatic aldehyde which on subsequent oxidation affords aryl carboxylic acid. This protocol was used for gram-scale synthesis as it is free from chromatographic purification. This is the first report for the oxidative transformation of styrenes into aryl carboxylic acids under transition metal-free conditions.

Full text of DOI:10.1016/j.tetlet.2018.10.061

Accepted Manuscript
Transition Metal Free One Pot Synthesis of Aryl Carboxylic Acids via Deho-
mologative Oxidation of Styrenes
Bhausaheb N. Patil, Pratima A. Sathe, Babasao S. Parade, Kamlesh S.
Vadagaonkar, Atul C. Chaskar
PII:
S0040-4039(18)31293-0
https://doi.org/10.1016/j.tetlet.2018.10.061
TETL 50371
DOI:
Reference:
Tetrahedron Letters
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Revised Date:
Accepted Date:
14 August 2018
23 October 2018
26 October 2018
Please cite this article as: Patil, B.N., Sathe, P.A., Parade, B.S., Vadagaonkar, K.S., Chaskar, A.C., Transition Metal
Free One Pot Synthesis of Aryl Carboxylic Acids via Dehomologative Oxidation of Styrenes, Tetrahedron
Letters (2018), doi: https://doi.org/10.1016/j.tetlet.2018.10.061
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Transition Metal-free One-Pot Synthesis of Aryl Carboxylic
Acids via Dehomologative Oxidation of Styrenes
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Bhausaheb N. Patil,†a Pratima A. Sathe,a Babasao S. Parade,a Kamlesh S. Vadagaonkar,†*b and Atul C.
Chaskar*a

Tetrahedron Letters
Transition Metal Free One Pot Synthesis of Aryl Carboxylic Acids via
Dehomologative Oxidation of Styrenes
Bhausaheb N. Patila,c, Pratima A. Sathea, Babasao S. Paradea, Kamlesh S. Vadagaonkarb,c,*, and Atul C.
Chaskara,*
aNational Centre for Nanosciences and Nanotechnology, University of Mumbai, Mumbai 400098, India
bDepartment of Dyestuff Technology, Institute of Chemical Technology, Mumbai 400019, India
c These authors contributed equally to this work.
Tel.: +91-7507375261; E-mail: achaskar25@gmail.com, vadgaonkarks@gmail.com
ARTICLE INFO
ABSTRACT
Article history:
Received
Received in revised form
Accepted
Iodine/NaOH-catalyzed one-pot dehomologative oxidation of styrenes to aryl carboxylic acids
has been reported. A wide range of carboxylic acids are obtained using iodine (I2) as a catalyst,
tert-butyl hydroperoxide (TBHP) as an oxidant and sodium hydroxide (NaOH) as a base. This
reliable conversion involves dehomologation of styrene to aromatic aldehyde which on
subsequent oxidation affords aryl carboxylic acid. This protocol was used for gram-scale
synthesis as it is free from chromatographic purification. This is the first report for the oxidative
transformation of styrenes into aryl carboxylic acids under transition metal-free conditions.
Available online
Keywords:
Styrenes
2009 Elsevier Ltd. All rights reserved.
Aryl carboxylic acids
Transition metal-free
Dehomologative oxidation
Iodine catalyzed reaction
Introduction
manganese-catalyzed acceptorless dehydrogenative coupling of
alcohols with hydroxides generating H2 as the sole by-product
(Scheme 1).13b Moreover, our group recently reported a novel
iodine(I2)-promoted direct conversion of arylacetic acids into aryl
carboxylic acids under metal-free conditions.14
Carboxylic acid is one of the most important functional groups
in organic chemistry owing to its elaboration into esters, acid
halides, amides, anhydrides and so on. The importance of
carboxylic acid group in biology, organic chemistry, natural
products, agriculture, pharmaceutical etc. makes it very crucial
and remarkable.1-2 The global production capacity of aryl
carboxylic acids have been estimated to go beyond 620000 tons
by 2023 with increase of over 3.2% from 2016 to 2023.3
Generally, carboxylic acids are synthesized from olefins or
alkynes by ozonolysis4a-4c as well as dihydroxylation and
oxidative cleavage of diols with NaIO4 and oxidants.4d–4e Aryl
olefins, aryl alkynes, aryl halides, benzylic alcohols, aromatic
aldehydes have also been used as starting materials for the
synthesis of aryl carboxylic acids.5 However, oxidation of
aldehydes and alcohols to carboxylic acids is carried out with
stoichiometric quantity of metal oxidants such as permanganate
and chromate in the presence of unsafe solvents along with
harmful byproducts.6
Figure 1. Methods for the synthesis of aryl carboxylic acid from
different sources.
So far, many metal-catalyzed reactions have been
reported for the synthesis of aryl carboxylic acids from alkenes
and alkynes using oxides of Au,7a-7b Os,7c-7e Mn,7f-7j Re,8a Ru8b-8n
and transition metal species such as Cr,9 Co,10 Cu,11 and V.12 But,
most of these methods involve the use of harsh reaction
conditions. Hence, there is a need to develop a practically
efficient, convenient and non-hazardous method for the synthesis
of aryl carboxylic acids from olefins. Furthermore, Hong et al.
developed FeCl3.6H2O-catalyzed synthesis of carboxylic acids
from olefins and alkynes by using aqueous tert-butyl
hydroperoxide(TBHP) as an oxidant.13a Recently, Liu and co-
workers reported the synthesis of carboxylic acids via
Hitherto, very few reports are known in the literature for the
direct synthesis of aryl carboxylic acids from alkenes. Hence, the
development of a new method for the synthesis of aryl carboxylic
acids which involves the use of safe, non-toxic and inexpensive
reagents is of significant interest. In this context, molecular
iodine has attracted great attention as a non-metal, harmless,
affordable and readily available catalyst for various organic
conversions. Thus, herein we report a convenient and transition
metal-free method for the synthesis of carboxylic acids from
alkenes, using inexpensive iodine and TBHP. This conversion
can be done by simple operation and goes through liberal
environmental way. This is the first report for transition metal-
 Corresponding author. Tel.: +91-7507375261; e-mail: achaskar25@gmail.com, vadgaonkarks@gmail.com

2
aReaction conditions: styrene 1a (1 equiv.), catalyst (0.05-0.2 equiv.), 70%
free oxidative transformation of alkenes into aryl carboxylic
acids.
aq. TBHP (2-10 equiv.) and base (0.1-0.4 equiv.) were heated in a sealed tube
at 105 oC for 16 h. bIsolated yields. cReaction in the absence of aq. TBHP.
dReaction in the absence of I2 and NaOH.
Inspired from our recently reported work involving iodine-
mediated dehomologative oxidation of arylacetic acids, we began
our investigation by examining the dehomologative oxidation of
styrene 1a. Initially, the reaction of styrene 1a was carried out in
the presence of I2 (0.1 equiv.) as a catalyst, aq. 70% TBHP (2
equiv.) as an oxidant as well as solvent and sodium
nr = No Result
nd= Not Detected.
In order to explore the generality of this protocol for the
synthesis of aryl carboxylic acids, various substituted styrenes
were examined under the optimized reaction conditions. The
protocol was found to be robust and could tolerate a variety of
o
hydroxide(NaOH) (0.2 equiv.) as a base at 105 C in a sealed
tube for 16 h (Table 1, entry 1). But, the reaction failed to
produce benzoic acid 2a. Similar result was obtained when the
same reaction was performed using 4 equiv. of aq. TBHP (Table
1, entry 2). In order to get benzoic acid, the concentration of aq.
TBHP was further increased to 6 equiv. surprisingly, we
observed the formation of benzoic acid 2a in 33% yield (Table 1,
entry 3). Encouraged by this result, the reactions with 8 and 10
equiv. aq. TBHP were performed. These reactions afforded the
desired product 2a in 57% and 60% yields, respectively (Table 1,
entries 4 and 5). With a view to improve the yield of the product,
the reactions with lower as well as higher concentrations of
iodine and NaOH in the presence of 10 equiv. aq. TBHP were
carried out (Table 1, entries 6 and 7). However, these reactions
offered the desired product 2a in lower yields.
electron-donating,
substituents. Styrene 1a afforded benzoic acid 2a in 60% yield
under the optimized reaction conditions.
Table 2 Substrate scope with various styrenesa
electron-withdrawing
and
halogen
Scheme 1. General reaction condition.
The reaction in the absence of aq. TBHP failed to give benzoic
acid 2a (Table 1, entry 8), which highlighted the important role
of aq. TBHP as an oxidant and a solvent in this reaction. Next,
the reaction in the absence of I2 and NaOH was also attempted. It
also failed to produce benzoic acid 2a (Table 1, entry 9), which
indicated the key role played by I2 as a catalyst and NaOH as a
base in this reaction. The reaction was also performed by using
N-chlorosuccinimide (NCS) and N-bromosuccinimide (NBS) as
the catalysts instead of I2. But, these reactions did not form
benzoic acid 2a (Table 1, entries 10 and 11). The reaction by
using KOH as a base furnished benzoic acid 2a in lower yields
(50%, Table 1, entry 12). While in absence of iodine catalyst the
reaction did not formed product 2a. These results suggested that
the use of 0.1 equiv. I2 as a catalyst, 0.2 equiv. NaOH as a base
and 10 equiv. aq. 70% TBHP as an oxidant as well as a solvent at
105oC for 16 h is the best reaction condition for the conversion of
styrene 1a into benzoic acid 2a.
aReaction conditions: substituted styrene 1 (1 equiv.), I2 (0.1 equiv.), 70% aq.
TBHP (10 equiv.) and NaOH (0.2 equiv.) were heated in a sealed tube at 105
oC for 16 h.
The reaction of styrenes bearing halogen substituents (2-Cl, 2-I, 4-F,
4-Cl and 4-Br) proceeded smoothly to offer corresponding benzoic
acids 2b-2f in good yields (52-63%). styrenes with electron-donating
substituents (2-CH3, 4-CH3, 4-OMe, 2,6-dimethoxy and 3,4-
dimethoxy) furnished respective benzoic acids 2g-2k in yields
ranging from 45-56%. The reaction of styrenes bearing electron-
withdrawing substituents (3-NO2 and 4-NO2) proceeded efficiently
to give corresponding benzoic acids 2l and 2m in 65% and 68%
yields, respectively. 1-Vinylnaphthalene was also easily converted
into 1-naphthoic acid 2n in 57% yield. Further the scope of this
protocol was extended by using heterocyclic styrenes such as 2-
vinylfuran and 2-vinylthiophene. The reaction of 2-vinylfuran and 2-
vinylthiophene produced corresponding aryl carboxylic acids 2o and
2p in 43% and 47% yields, respectively. In view of the broad
substrate scope and environmentally benign approach for synthesis
of aryl carboxylic acids, this protocol was also evaluated for the
gram-scale synthesis of benzoic acid 2a. Notably, 5.0 g of styrene
produced 3.5 g of benzoic acid (60%).
Table 1 Optimization of reaction conditions.a
Entry
Catalyst
(equiv.)
Oxidant
(equiv.)
Base (equiv.)
Yield (%)b
1
I2 (0.1)
TBHP (2)
TBHP (4)
TBHP (6)
TBHP (8)
TBHP (10)
TBHP (10)
TBHP (10)
-
NaOH (0.2)
NaOH (0.2)
NaOH (0.2)
NaOH (0.2)
NaOH (0.2)
NaOH (0.1)
NaOH (0.4)
NaOH (0.2)
-
nd
nd
33
57
60
52
20
nr
2
I2 (0.1)
I2 (0.1)
I2 (0.1)
I2 (0.1)
I2 (0.05)
I2 (0.2)
I2 (0.1)
-
3
4
5
6
7
8c
9d
10
11
12
13
TBHP (10)
TBHP (10)
TBHP (10)
TBHP (10)
TBHP (10)
nd
nd
nd
50
nd
NCS (0.1)
NBS (0.1)
I2 (0.1)
-
NaOH (0.2)
NaOH (0.2)
KOH (0.2)
NaOH (0.2)

3
Supporting information for this article is available online at
http://dx.doi.org/.
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