Efficient and highly selective aqueous oxidation of alcohols and sulfides catalyzed by reusable hydrophobic copper (II) phthalocyanine

Article abstract of DOI:10.1016/j.inoche.2011.10.033

A practical innovative method for highly chemoselective oxidation of alcohols to the aldehyde and ketones and sulfides to the sulfones using tetra-n-butylammonium peroxomonosulfate (n-Bu₄NHSO₅) catalyzed by simple water-insoluble copper (II) phthalocyanine (CuPc) in neat water has been developed. Organic co-solvents, surfactants, co-catalyst and hydrophilic auxiliaries were completely missed in this heterogeneous catalytic strategy. The CuPc catalyst and by-product of oxidant (TBAHSO₄) could easily be recycled and reused without loss of activity providing readily scalability.

Full text of DOI:10.1016/j.inoche.2011.10.033

Inorganic Chemistry Communications 15 (2012) 230–234
Contents lists available at SciVerse ScienceDirect
Inorganic Chemistry Communications
journal homepage: www.elsevier.com/locate/inoche
Efficient and highly selective aqueous oxidation of alcohols and sulfides catalyzed by
reusable hydrophobic copper (II) phthalocyanine
⁎
⁎
Abdolreza Rezaeifard , Maasoumeh Jafarpour , Atena Naeimi, Mehri Salimi
Catalysis Research Laboratory, Department of Chemistry, Faculty of Science, University of Birjand, Birjand, 97179-414, Iran
a r t i c l e i n f o
a b s t r a c t
Article history:
A practical innovative method for highly chemoselective oxidation of alcohols to the aldehyde and ketones
and sulfides to the sulfones using tetra-n-butylammonium peroxomonosulfate (n-Bu4NHSO5) catalyzed by
simple water-insoluble copper (II) phthalocyanine (CuPc) in neat water has been developed. Organic co-
solvents, surfactants, co-catalyst and hydrophilic auxiliaries were completely missed in this heterogeneous
catalytic strategy. The CuPc catalyst and by-product of oxidant (TBAHSO4) could easily be recycled and
reused without loss of activity providing readily scalability.
Received 2 August 2011
Accepted 23 October 2011
Available online 3 November 2011
Keywords:
Phthalocyanine
Water
© 2011 Elsevier B.V. All rights reserved.
Oxidation
Peroxymonosulfate
Alcohol
Sulfide
The selective oxidation of organic compounds by metal com-
plexes has been extensively studied to develop new synthetic strat-
egies for industrial applications [1,2]. Often-used catalysts for
oxidation reactions are porphyrin and Schiff base complexes mim-
icking the heme-containing monooxygenase enzymes that show
high catalytic activity and selectivity under different oxidation
conditions [3]. However, a problem often encountered in the
metalloporphyrin- and metallosalen-catalyzed oxidation reactions
is the deactivation of the catalytic species by the oxidative degrada-
tion of ligands [4]. Moreover, the extensive use of toxic and volatile
organic solvents in these oxidation systems has reduced consider-
ably the practical importance of biologically inspired oxidation catal-
ysis. Discarding of harmful organic solvents is the major problem in
chemical industries which accounts around 80% of their wastes [5].
Thus, a new challenge is to make such processes by using non-toxic
solvents in particularly aqueous media [6–10].
Phthalocyanine transition metal complexes as important industri-
al pigments have been considered as potential oxidation catalysts be-
cause of their rather cheap and facile preparation in a large scale and
in particularly their chemical and thermal stabilities [11–14]. Howev-
er, the low solubility of metallophthalocyanines is perhaps the most
significant limitation in their application as catalysts. It may be over-
come by sulfonation and carboxylation at the periphery of the mole-
cule to give rise to water-soluble derivatives [14]. Nevertheless, the
separation of water-soluble catalysts from the aqueous phase is a
time consuming process making them out of recycled materials.
Tetra-n-butylammonium
peroxomonosulfate
(n-Bu4NHSO5,
TBAOX) was firstly introduced as a sluggish oxygen source for the ox-
idation of sulfides to sulfones [15]. For example, oxidation of thioani-
sole by 3 equivalents of TBAOX in CH2Cl2 took four days to produce
78% yield of the corresponding sulfone. A prolonged catalyst-free
method for aqueous oxidation of alcohols using this oxidant has
also been reported in which, low/moderate yields of the related car-
bonyl compounds were achieved [16]. Recently, efficient oxidation
of alcohols in the presence of immobilized TEMPO in ionic liquid
[bmim][PF6] has also been reported [17]. Our ongoing research on
the development of new applications of TBAOX in the oxidation of
organic compounds containing sulfides and alcohols [18–25] resulted
in a novel and innovative strategy for clean and selective oxygenation
of hydrocarbons in neat water catalyzed by water-insoluble Fe(III)
and Mn (III) porphyrins [26]. Now, we wish to report the catalytic
performance of water-insoluble copper (II) phthalocyanine (CuPc)
as a cheap heterogeneous catalyst in the selective oxidation of alco-
hols to carbonyl compounds and sulfides to sulfones using TBAOX in
neat water in the absence of any organic co-solvents and surfactants
(Scheme 1). The catalyst and the reduction product n-Bu4NHSO4
(TBAHSO4) from the oxidant TBAOX were easily recovered and
reused in the procedure.
The oxidation of cyclohexanol using two equivalents of TBAOX
(0.7 g) in water which did not proceed in the absence of catalyst
was used as a model reaction. Preliminary catalytic experiments
were addressed to the oxidation of cyclohexanol (1 mmol, 0.1 g)
with TBAOX (2 mmol, 0.7 g) in the presence of a minute amount of
cheap commercially available CuPc (0.5 mol%) in neat water at 25 °C
under air which led to the formation of cyclohexanone in 40% yield
after 10 h according to GC analysis. The addition of imidazole (35%)
⁎
Corresponding authors. Tel.: +98 561 2502516; fax: +98 561 2502515.
E-mail addresses: rrezaeifard@birjand.ac.ir, rrezaeifard@gmail.com (A. Rezaeifard),
mjafarpour@birjand.ac.ir (M. Jafarpour).
1387-7003/$ – see front matter © 2011 Elsevier B.V. All rights reserved.
doi:10.1016/j.inoche.2011.10.033

A. Rezaeifard et al. / Inorganic Chemistry Communications 15 (2012) 230–234
231
100
100
80
80
72
60
43
40
20
2
0
25
40
50
60
70
T/ C
Fig. 1. The effect of temperature on the aqueous oxidation of cyclohexanol using
TBAOX catalyzed by CuPc after 60 min.
In order to establish the general applicability of the present cata-
lytic system, a wide range of primary and secondary aliphatic and
benzylic alcohols was subjected to oxidation under the optimized
conditions in water. Secondary alcohols are generally excellent sub-
strates for this oxidation system (Table 2, entries1–7). Oxidation of
primary alcohols including benzylic and aliphatic ones proceeded
slowly and gave the low/moderate yields of the corresponding alde-
hydes (entries 8–10). However, the over oxidation of aldehydes to
carboxylic acid was controlled completely. Moreover, the chemos-
electivity of the procedure was prominent. Alcohol containing C–C
double bond oxidized absolutely to the corresponding α, β-
unsaturated ketone (entry 11). This was further supported by per-
forming a competitive oxidation reaction between cyclooctene and
cyclohexanol. Cyclohexanone was formed as sole product (>98%)
in the oxidation of an equimolar mixture of alcohol and olefin at 70 °C
after 15 min confirming the excellent chemoselectivity of CuPc towards
oxidation of hydroxyl group.
Encouraged by promising results obtained in the oxidation of alco-
hols, we evaluated the oxidation activity of this clean catalytic system
towards organic sulfides. Knowing that the oxidant has the ability to
trigger sulfide oxidation, we performed the blank experiment using
1 mmol thioanisole (0.12 g) and 3 mmol of TBAOX (1 g) in neat
water at 25 °C under air afforded a mixture of the related sulfoxide
(31%) and sulfone (39%) after 1 h according to GC analysis. Neverthe-
less, addition of a catalytic amount of CuPc (0.005 mmol, 0.5 mol %)
to the aqueous mixture led to complete conversion of thioanisole
with the formation of the corresponding sulfone as the sole product
at the same conditions. It should be noted that some attempts to con-
trol the reaction for selective production of sulfoxide in desired yield
were unsuccessful (Table S1 in supporting information). Next, differ-
ent sulfides were subjected to the reaction system in the presence of
CuPc and the results are listed in Table 3. All substrates could be
smoothly converted to sulfones with moderate/high yields and excel-
lent selectivities were obtained under mild conditions (entries 1–6).
Scheme 1. Oxidation of alcohols and sulfides with aqueous solution of TBAOX catalyzed
by CuPc.
and pyridine (42%) as common additives, the use of micellar solution
of sodium dodecyl sulfate at CMC (0.008 mol/L) condition (39%) and
also the use of copper (II) phthalocyanine-tetrasulfonic acid tetraso-
dium salt as a water-soluble catalyst (40%), did not affect the conver-
sion rate in this oxidation method. Also, performing the reactions at
controlled pH conditions using standard buffered solutions did not
improve the yield of cyclohexanone (15, 30, 40, 17 and 37% yield at
pH=2, 4, 7, 9 and 13 respectively). It is worthy to mention that the
addition of organic solvents such as ethanol, ethylacetate, dichloro-
methane, dichloroethane, n-hexane and acetone to the aqueous solu-
tion reduced the oxidation rate and yield of product (Table 1).
Then, the influence of temperature on the reaction rate was
examined. It was observed that, an increase in the temperature in
this oxidation system, promoted remarkably the effectiveness of the
catalytic system and full conversion of cyclohexanol was achieved at
70 °C within 30 min (Fig. 1). Under these conditions the oxidation
reaction proceeded smoothly (Fig. 2) and cyclohexanone was se-
cured in 95% yield after a simple extraction from aqueous phase. It
should be noted that blank experiments for oxidation of cyclohexa-
nol using TBAOX at 70 °C gave only 15 percentage of cyclohexanone
after 60 min.
The effect of catalyst loading has also been examined. Our exper-
iments on the oxidation of cyclohexanol with TBAOX by using 0.1,
0.2, 0.5 and 1 mol% of CuPc led to 28, 60, 100 and 100% yields of
cyclohexanone respectively. Thus, a low catalyst loading of 0.5 mol
% was established for complete conversion of cyclohexanol in the
desired time. Nevertheless, an increase in this ratio up to 5 mol%
did not improve the conversion rate. The effect of simple copper
salts for the activation of TBAOX and also other common
oxidants including H2O2 (and also UHP), t-BuOOH, NaIO4 and n-
Bu4NIO4 in combination with CuPc was examined for the oxidation
of cyclohexanol in water and found them virtually inactive at differ-
ent temperatures.
100
80
60
40
20
0
100
92
Table 1
80
The effect of organic co-solvents on the aqueous oxidation of cyclohexanol
using TBAOX catalyzed by CuPc.a.
Solvent
Cyclohexanone%b
44
Dichloromethane/water
Dichloroethane/water
n-Hexane/water
Acetone/water
Ethylacetate/water
Water
32
36
4
15
20
40
30
17
5
10
15
20
25
30
a
Time/min
The reactions were run under air at room temperature in 1:1 V/V mixture
of solvents for 10 h and the molar ratio of alcohol: TBAOX: catalyst was
200:400:1.
Fig. 2. The reaction profile for aqueous oxidation of cyclohexanol using TBAOX cata-
b
GC yields.
lyzed by CuPc at 70 °C.

232
A. Rezaeifard et al. / Inorganic Chemistry Communications 15 (2012) 230–234
Table 2
Table 3
Oxygenation of alcohols with aqueous solution of TBAOX catalyzed by CuPc.a.
Oxidation of sulfides and sulfoxides with aqueous solution of TBAOX catalyzed by
CuPc.a.
Entry Alcohol
Productb
Yield%c
Entry
1
Substrate
Sulfone yield%b
100c
1
95
2
100c
3
95
2
90
4
5
93
65
3
92
6
88
4
5
6
7
85
94
80
55
25
7
8
87
94
9
97
a
The reactions were run under air at room temperature for 1 h with a molar ratio of
substrate/TBAOX/catalyst, 200:600:1.
b
Yield of isolated sulfone products. All products were sulfones in 100% selectivity
[29] except for entry 7 which gave 6% of the related sulfoxide “4-(methylsulfinyl)
benzyl alcohol” [30].
c
GC yield.
A salient feature of the present oxidation system is its excellent selec-
tivity. Sulfones could be nearly stoichiometrically produced and no
sulfoxide was detected, which makes this process a good alternative
for sulfone production. Accordingly, sulfoxides oxidized easily to the
related sulfones with excellent yields (entries 7,8). Also sulfides hav-
ing a carbon–carbon double bond (entry 2, 4), or benzylic C–H bond
(entry 6) were cleanly transformed into the corresponding sulfones
in excellent yield without formation of any epoxide and benzylic
oxidation by-products. Moreover, oxidation of 4-(methylthio)benzyl
alcohol (entry 7) including sulfide and alcohol functionalities both
of which are susceptible to oxidation in this catalytic system, gave
4-(methylsulfonyl) benzyl alcohol in 94% selectivity (b6% 4-(methyl-
sulfinyl) benzyl alcohol), while the hydroxyl group remained intact. It
was further supported by competition reaction between thioanisole
and benzyl alcohol. Thioanisole converted selectively to the related
sulfone (>97%) in the presence of benzyl alcohol which was oxidized
slightly (0, 5% benzaldehyde at 25 °C and 70 °C respectively).
8
9
55
15
94
10
11
CH3(CH2)6CH2OH
CH3(CH2)6CHO
The comparison of non-catalytic oxidation of thioanisole and
methyl phenyl sulfoxide with aqueous solution of TBAO (Table 4, en-
tries 1 and 3) suggests a more facile oxidation of sulfide to sulfoxide
relative to the over oxidation of sulfoxide to sulfone. Moreover, our
outcome on catalytic oxidation of thioanisole and methyl phenyl sulf-
oxide giving the corresponding sulfone as sole product, clearly shows
the ability of catalyst to promote both steps in the oxidation of sulfide
a
The reactions were run under air at 70 °C for 30 min and the molar ratio of alcohol:
TBAOX:catalyst was 200:400:1.
b
All products were identified by comparison with commercial and authentic samples.
Yields of isolated products. No over oxidation products were observed for all
c
substrates.

A. Rezaeifard et al. / Inorganic Chemistry Communications 15 (2012) 230–234
233
Table 4
Oxidation of PhSMe and PhSOMe with aqueous solution of TBAOX.a.
Entry Substrate Oxidation system Conversion% Sulfoxide%b Sulfone%b
1
2
3
4
PhSMe
PhSMe
PhSOMe
PhSOMe
TBAOXc
70
100
10
43
0
0
27
100
10
TBAOX/CuPcd
TBAOXc
TBAOX/CuPcd
100
0
100
a
b
c
The reactions were run under air at room temperature for 1 h.
GC yields.
The molar ratio of substrate/TBAOX was 1/3.
The molar ratio of substrate/TBAOX/catalyst was 200:600:1.
d
Fig. 4. UV–Vis spectral changes of CuPc complex in H2SO4 during the aqueous oxidation
of cyclohexanol using TBAOX.
to the sulfone. The attachment of TBAO to the metal centre giving a
six coordinate adduct [PcCu–O–OHSO3] enhances the electrophilic-
ity of Cu-coordinated oxygen atom of the oxidant. Despite the easy
oxidation of sulfides and sulfoxides to sulfones using this species,
oxidation of alcohols to aldehydes and ketones required higher tem-
perature. Moreover, this less active oxidant does not have ability to
oxidize aldehydes and ketones produced in the oxidation of alcohols.
Presumably a high valent [Cu=O]+ species should be provided for
this over oxidation reaction [27] as well as oxidation of hydrocarbons
[28]. However, the possible involvement of a high valent [Cu=O]+
species as an intermediate cannot be excluded, due to its much
higher activity towards oxidation of organosulfur compounds than
the alcohols and hydrocarbons [26].
with aqueous solution of TBAOX does not change noticeably the elec-
tronic absorption spectra of catalyst after five run (Fig. 4). By compar-
ing the absorbance (λmax =792 nm) at the end of the fifth run
(A=1.1) with that observed for fresh catalyst (A=1.3) we found
that 85% of the catalyst remained intact. In addition, it was estab-
lished that the IR spectra were identical for the fresh and reused cat-
alysts demonstrating the stability of the Pc structure under the real
catalytic conditions (Fig. S1 in supporting information). Moreover,
oxidant's by-product (TBAHSO4) was separated by lyophilizing of
aqueous phase and reused in the preparation of TBAOX [31]. There-
fore, from the stand point of greener chemical processes, the use of
CuPc as catalyst in combination with aqueous solution of TBAOX
doesn't lead to three major sources of waste: organic solvents, cata-
lysts and harmful by-products. These advantages for this high yield-
ing oxidation method along with the low cost for the preparation of
CuPc offered ready scalability. For example the use of a semi scale-
up procedure (20 mmol) for the oxidation of thioanisole in the pres-
ence of CuPc led to the isolation of the related products in 93% yield.
The high/excellent yields of oxidation products obtained using
this novel oxidation method in desired times display the high catalyt-
ic activity and relative stability of CuPc in association with aqueous
solution of TBAOX. It was further supported by the impressive total
turnover numbers obtained for CuPc in the oxidation of cyclohexanol
(6500, with molar ratio of 10,000:20,000:1 for cyclohexanol/TBAOX/
CuPc) and thioanisole (9100, with molar ratio of 10,000:60,000:1
for thioanisole/TBAOX/CuPc) indicating well the high efficiency and
stability of the present catalytic system. These results encouraged us
to evaluate the reusability of the catalyst. After easy isolation of
hydrophobic products by using ethyl acetate as an environmentally
benign solvent, the solid catalyst was separated from the aqueous so-
lution by centrifuging and was reused for the subsequent reaction
under the similar reaction conditions. The averaged GC yields of
cyclohexanone and methylphenyl sulfone for five runs were 94%,
and 100% respectively, demonstrating well the high reusability of
the catalyst in this oxidation system (Fig. 3). The stability of CuPc in
this oxidation system was also established by spectrophotometry. It
was found that the mediation of CuPc in the oxidation of cyclohexanol
Conclusion
In conclusion, the catalytic performance of water-insoluble copper
(II) Phthalocyanine in the oxidation of alcohols and sulfides using
aqueous solution of TBAOX was established. The good/high yields and
excellent selectivity were achieved for carbonyl and sulfone products
in the absence of organic co-solvents, surfactants and nitrogenous
bases. The employment of neat water as a standard ‘green’ solvent in
this high yielding oxidation method as well as easy and safe work-up
procedure and reusability of catalyst and by-product providing ready
scalability, are strong points of this novel methodology making it
more attractive for practical goal.
100
98
98
100
90
80
70
60
Supplementary materials related to this article can be found on-
line at doi: 10.1016/j.inoche.2011.10.033.
91
91
90
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[31] General oxidation Procedure and recycling To a mixture of substrate (alcohol and
sulfide) (1 mmol) and CuPc complex (0.005 mmol) in distilled water (10 ml) was
added freshly prepared TBAOX (0.7 g for alcohol and 1 g for sulfide). The reaction
mixture was stirred under reaction conditions mentioned in Tables 2 and 3. After
completion of the reaction which was monitored by TLC and GC, the mixture was
washed with ethylacetate (3×10 ml), and the organic phase was separated and
evaporated. Then, the reaction mixture was centrifuged and the solid catalyst
was separated, washed with ethylacetate, dried and reused for the subsequent
reaction under the similar reaction conditions. The aqueous phase was lyophi-
lized to obtain TBAHSO4 which can be reused in the preparation of TBAOX [17].