496
Chemistry Letters Vol.37, No.5 (2008)
Alcohol Oxidation in Ionic Liquid with UHP
and Recyclable Amberlite IR-120 Acidic Resin: A Green Approach
Nishi Bhati, Kuladip Sarma, and Amrit GoswamiÃ
Synthetic Organic Chemistry Division, North-East Institute of Science and Technology,
Jorhat, Assam 785 006, India
(Received February 5, 2008; CL-080137; E-mail: amritg 2007@rediffmail.com)
O
OH
Amberlite IR-120 acidic resin, an age old polymer matrix,
R1
R2
plays a significant role as catalyst in the oxidation of aromatic
alcohols with urea–hydrogen peroxide in ionic liquid at 70 ꢀC
to disproportionate into phenols and carbonyl compounds in
30–60 min following two pathways. Under similar condition
tertiary aromatic alcohol gave exclusively monohydroxyphenol
accompanied by acetone. However, aliphatic and alicyclic
alcohols yielded carbonyl compounds in moderate to good yields
in 1–5 h.
R1
1a−1h
R2
2a−2h
PS−SO3H, UHP, BMIMBr, 70 oC
Aliphatic or alicyclic alcohol
OH
O
OH
R3
+
R3
R4
X
X
Only when R4=H
X
1i−1r
3i−3q
2i−2r
Benzylic alcohol
g
h
i
j
k
l
R1= R2= −[(Me)CH(CH2)3CH(Me)]−
R1= R2= −(CH2)4−
R3= R4= X= H
R3= Me, R4= X= H
R3= Me, R4= H, X= 2-OH
R3= Et, R4= X= H
m
n
o
p
q
r
R3= C6H5, R4= X= H
R3= C6H5(CH2)2-, R4= X= H
R3= R4= H, X= 4-Cl
R3= R4= H, X= 2-OMe
R3= R4= H, X= 4-NO2
R3= R4= Me, X= H
a
R1= sec-butyl, R2= H
R1= n-pentyl, R1= H
R1= n-hexyl, R2= Me
R1= n-heptyl, R2= H
R1= n-undecyl, R2= H
R1= R2= -(CH2)5-
b
c
d
e
f
The development of new catalytic methods without use
of any metal catalyst and toxic organic solvents with efficient
recycling of catalyst avoiding stringent reaction conditions is a
conditio sine quo non in chemical research for economically
and environmentally attractive processes world wide. Oxidative
transformation of alcohols to carbonyl and other industrially
important molecules such as phenols is one such attractive area
due to the wide ranging utility of these products as precursors
and intermediates for many drugs, resins, vitamins, fragrances,
plasticizers, pharmaceuticals, disinfectant, bis-phenol A, and
other uses. Apart from the conventional methods,1–5 there have
been several reports for preparation of such oxidative products
using oxygen or hydrogen peroxide as the oxidants under the
influence of different metal catalysts.6–10 On the other hand,
phenol is generally prepared by indirect three steps cumene
process in which benzene and propylene are first reacted to form
cumene which is oxidized with oxygen to hydroperoxide under
high temperature and pressure. Hydroperoxide thus formed
was then treated with sulfuric acid to form phenol and acetone.
Boger and Coleman11 have however reported preparation of
phenol through rearrangement of benzyl hydroperoxide as an al-
ternative to the Baeyer–Villiger rearrangement. Incessant efforts
have been reported,12,13 for direct hydroxylation of benzene
either through liquid phase using hydrogen peroxide,14,15 or
through gas phase using oxygen,16 in presence of different cata-
lyst. However, success rate with considerable yield is very poor.
One of the strategic developments in organic synthesis is the
use of reagents and catalysts which are bound to polymeric ma-
trix insoluble in the reaction media so that it can be recycled.17
Organic reactions in green solvents such as water or ionic liquid
using insoluble, recoverable, and reusable catalyst system are of
high current interest.18,19 For polymer-supported catalysts that
work efficiently in water or in ionic liquids, hydrophobic interac-
tions between the polymer and the organic substrate play an
important role in the catalytic activity.20 Having encouraged
by such ideas it was deemed desirable to use a hydrophobic
polymer-supported catalyst in water or in an ionic liquid to carry
out the transformation of alcohol to carbonyl compounds.
In the course of our study on the oxidation of 1-phenyletha-
Scheme 1. Alcohol oxidation with UHP in the presence of
Amberlite sulphonic acid resin in ionic liquid (BMIMBr).
nol to carbonyl compounds with 50% hydrogen peroxide as an
oxidizing agent in water in presence of polystyrene-supported
sulphonic acid resin (Amberlite IRA-120), we found that phenol
was formed in 66% isolated yield accompanied by minor
amounts of acetophenone (10%) at 90 ꢀC in 45 min (Scheme 1).
However, the resin simultaneously underwent complete decom-
position to form a clear aqueous solution inhibiting catalyst
recycling. The degradation of the resin in 50% H2O2 solution
also occurred in the absence of the alcohol when a blank reaction
was carried out. We could not determine the fate of the decom-
posed resin in aqueous solution.
The reaction was then shifted to carry out with urea–hydro-
gen peroxide adduct instead of aqueous hydrogen peroxide. The
reaction was studied in four different solvents viz. 1-butyl-3-
methylimidazolium bromide, 1-butyl-3-methylimidazolium
tetraflouroborate, toluene, and 1,2-dichloroethane taking 1-
phenylethanol as a substrate. The maximum yield was obtained
when the reaction was carried out in the ionic liquid [BMIMBr]
in which the resin remained intact. After completion of the
reaction the insoluble resin catalyst and the ionic liquid were re-
covered by filtration and the resin was washed with dichloro-
methane (3 times). The recovered resin and the ionic liquid were
reused for 5 times (Table 1) without any appreciable loss of
activity to get similar yields. The reaction was then repeated with
various aromatic, aliphatic, and alicyclic alcohols (Table 2). It
was observed that both phenols and carbonyl compounds were
formed from all the 1-phenyl-substituted alcohols 1i–1q as the
major and minor products respectively accompanied by 2–6%
formaldehyde leaving behind the resin intact except for 1r in
which exclusively phenol and minor amount of acetone were
formed. In case of alicyclic alcohols 1f–1h, it was found the
oxidized products to be exclusively corresponding ketones with
good yields (75–81%). However, both primary and secondary
Copyright Ó 2008 The Chemical Society of Japan