2
K. Ahmed et al. / Tetrahedron 75 (2019) 130605
cyclopentadienylesilsesquioxane titanium complex as a highly
active catalyst for the selective oxidation of sulfide to sulfoxide and
sulfone in reactions conducted in MeOH as solvent [72]. In fact, as
revealed by a survey of literature, majority of existing procedures
still rely upon the use of hazardous and volatile organic solvents as
reaction medium [17,68,71e79] due to which the important crite-
rion of environmental sustainability remains a challenging issue to
address. Difficulty in catalyst regeneration is another limitation
with respect to many of the otherwise efficient catalytic sulfox-
idations, particularly in case of homogeneous systems
ready availability and in particular, due to the presence of pendant
carboxylate groups in the polymer matrices which are capable of
forming facile attachment with the Ti(IV) centers. To the best of our
knowledge, this is the first report dealing with synthesis of well
defined pTi complexes in macroligand environment comprising of
poly(acrylate) based WSP and their application as catalysts for
organic oxidations.
2. Experimental section
[
69,73,74,80e82]. It is notable in this context that, we have come
2.1. Materials
across only few reports dealing with titanium based catalytic
oxidation of sulfide that have used water as reaction medium
Titanium(IV) chloride solution, 0.09 M in 20% HCl, poly(sodium
[
83e85].
Water has already been widely recognized as a natural solvent
acrylate) (Mw ¼ 2100), poly(sodium methacrylate) (Mw ¼ 4000),
methyl phenyl sulfide (MPS), methyl p-tolyl sulfide (MpTS), ethyl
phenyl sulfide (EPS), dimethyl sulfide (DMS), dibutyl sulfide (DBS),
with obvious benefits of cost efficiency, nontoxicity, nonflamma-
bility, abundance and environmental compatibility, of vital impor-
tance for organic synthesis [86e95]. Apart from reducing the
environmental impact, use of water as a reaction medium often
facilitates simple separation and quantitative recovery of a water
soluble catalyst through easy phase separation due to poor solu-
bility of organic products in water [94,95]. A variety of commer-
2
-(phenylthio)ethanol (PTE), dihexyl sulfide (DHS), diphenyl sul-
fide (DPS) and allyl phenyl sulfide (APS), dibenzothiophene (DBT)
were purchased from SigmaeAldrich Chemical Company, Mil-
waukee, USA. Acetone, methanol, ethylacetate, hydrogen peroxide,
petroleum ether, silica gel (60e120 mesh), sodium hydroxide
RANKEM), diethyl ether, sodium sulfate (E. Merck, India). The
water used for solution preparation was deionized and distilled.
(
cially
important
processes,
including
hydroformylation,
carbonylation, hydrogenation, olefin metathe-sis, polymerization
etc. have already been carried out in aqueous medium at industrial
scale [91e93]. The recent upsurge in activity in the area of water-
centered organic synthesis has stimulated a concomitant rise in
the demand for water-tolerant and water compatible catalysts to
support aqueous phase organic reactions [86,95].
2
.2. Synthesis of water soluble peroxotitanium complexes,
[
[
Ti
Ti
2
(O
(O
2
)
)
2
O
O
2
2
(OH)
(OH)
2
(carboxylate)]-PA (PATi) or
(carboxylate)]-PMA (PMATi)
2
2
2
2
Our group has previously developed a number of polymer
supported heterogeneous as well as homogeneous catalysts
employing peroxo derivatives of d transition metals viz., vanadium
TiCl
4
(1.72 mL, 10 mmol) in 20% HCl solution was placed in a
ꢁ
1
00 mL beaker maintaining temperature below 4 C in an ice bath.
0
To this, 8 M NaOH solution was added dropwise with stirring until
no further precipitation occurred. The titanic acid thus obtained
was filtered and washed repeatedly with water to remove chloride
as well as excess NaOH. The absence of chloride in the filtrate was
confirmed by silver nitrate test. To the obtained precipitate, 30%
[
45,96e98], Niobium [99], molybdenum [49e51,100,101] and
tungsten [101e104], which displayed excellent activity in a variety
of organic oxidative transformations. Recently, we have reported
the remarkable performance of a set of peroxo compounds of
molybdenum [51], immobilized on linear water soluble polymers
2 2
H O (2.26 mL, 20 mmol) was added dropwise with constant stir-
(
WSP) viz., poly(sodium acrylate) (PA) and poly(sodium vinyl sul-
fonate) (PS) as highly efficient water-tolerant recyclable catalysts to
achieve H induced sulfoxidation in aqueous medium. It is
ring until a clear yellow solution was obtained. The pH of the so-
lution was ca. 2. To this yellowish solution, 1 g (for PATi) or 2 g (for
PMATi) of the soluble polymer was then added in portions with
continuous stirring. The pH of the system was recorded to be ca. 4 at
this stage. The resulting solution was allowed to stand in an ice bath
for 12 h. Subsequently, 50 mL of acetone was added to the mixture
under stirring and the system was kept as such for another 2 h
2 2
O
somewhat surprising that the idea of using linear WSP as support to
generate catalytically active metal complexes appears to have
received scant attention, although Merrifiled and Letsinger utilized
soluble polymers during their pioneering work on peptide syn-
thesis [105e107], which paved the way towards preparing immo-
bilized homogeneous catalysts [108,109].
Encouraged by the afore mentioned observations in the present
study, we focused on developing convenient, non-toxic and stable
peroxotitanium compounds that can be used as environmentally
safe, water-compatible recyclable catalysts to accomplish organic
oxidation in aqueous medium. Titanium has been considered a
good choice for our study because it is cheap, naturally abundant
and non-toxic [15]. Significantly, majority of the Ti mediated sul-
foxidation reactions have been achieved via in situ generated per-
oxotitanium species in the employing Ti complexes as pre-catalysts
or under “Ligand Assisted Catalysis” (LAC) [35,75]. There is a
paucity of information on activity of well-defined synthetic per-
oxotitanium complexes (PTC) in sulfide oxidation [76,83].
ꢁ
below 4 C. A pasty mass separated out on adding pre-cooled
acetone to this mixture under vigorous stirring. The supernatant
liquid was decanted off and pale yellow residue was treated
repeatedly with acetone under scratching. The microcrystalline
product obtained was separated by centrifugation and dried in
vacuo over concentrated sulfuric acid.
2.3. Elemental analysis
For the compounds PATi and PMATi, the C and H elemental
analyses were performed on an elemental analyzer (Perkin-Elmer
2400 series II). The content of titanium in the compounds was
estimated with inductively coupled plasma optical emission spec-
trophotometer (ICP-OES) and atomic absorption spectroscopy
(AAS). The weight percentage of the elements Ti, C and Na present
in the compounds were also obtained from the EDX analysis.
Peroxide content of the compounds was estimated by adding a
weighted amount of the PATi or PMATi to a cold solution of 1.5%
We describe herein the preparation and characterization of a
pair of peroxo-Ti(IV) compounds anchored to water soluble poly-
mer matrices, poly(sodium acrylate) (PA) and poly(sodium meth-
acrylate) (PMA) and their activity in controlled oxidation of sulfides
2 2
with H O , with respect to selectivity, TOF, reusability and eco-
compatibility. The polymers, PA and PMA were chosen for the
purpose of this study mainly owing to their chemical stability,
boric acid(w/v) in 0.7 M H
then titrated with a standard solution of cerium(IV) [110].
2 4
SO (100 mL). The whole system was