A molybdenum based metallomicellar catalyst for controlled and chemoselective oxidation of activated alcohols in aqueous medium

Article abstract of DOI:10.1016/j.jcat.2019.06.013

A surfactant based oxodiperoxo molybdenum complex, which could activate molecular oxygen, has been employed as a catalyst for controlled oxidation of benzylic alcohols to corresponding carbonyls. The oxidation reactions were carried out under aqueous environment, however, in the absence of any extraneous base or co-catalyst. Sensitive/oxidizable functional groups like cyano, sulfide, hydroxyl, aryl-hydroxyl, alkene (internal/terminal), alkyne (internal/terminal), and acetal were tolerated during the transformations. Such selectivity is attributed to the mild nature of the catalyst. The methodology could also be scaled-up for multi-gram synthesis and the protocol is likely to find practical use since it requires an inexpensive recyclable-catalyst and easily available oxidant (under green conditions). A plausible mechanism is proposed with the help of preliminary computational study.

Full text of DOI:10.1016/j.jcat.2019.06.013

Journal of Catalysis 376 (2019) 123–133
Contents lists available at ScienceDirect
Journal of Catalysis
journal homepage: www.elsevier.com/locate/jcat
A molybdenum based metallomicellar catalyst for controlled and
chemoselective oxidation of activated alcohols in aqueous medium
⇑
Prabaharan Thiruvengetam, Rajan Deepan Chakravarthy, Dillip Kumar Chand
Department of Chemistry, Indian Institute of Technology Madras, Chennai 600036, India
a r t i c l e i n f o
a b s t r a c t
Article history:
A surfactant based oxodiperoxo molybdenum complex, which could activate molecular oxygen, has been
employed as a catalyst for controlled oxidation of benzylic alcohols to corresponding carbonyls. The oxi-
dation reactions were carried out under aqueous environment, however, in the absence of any extraneous
base or co-catalyst. Sensitive/oxidizable functional groups like cyano, sulfide, hydroxyl, aryl-hydroxyl,
alkene (internal/terminal), alkyne (internal/terminal), and acetal were tolerated during the transforma-
tions. Such selectivity is attributed to the mild nature of the catalyst. The methodology could also be
scaled-up for multi-gram synthesis and the protocol is likely to find practical use since it requires an inex-
pensive recyclable-catalyst and easily available oxidant (under green conditions). A plausible mechanism
is proposed with the help of preliminary computational study.
Received 10 January 2019
Revised 4 June 2019
Accepted 10 June 2019
Keywords:
Molybdenum
Oxodiperoxo
Metallomicellar
Molecular oxygen
Oxidation
Ó 2019 Elsevier Inc. All rights reserved.
Unsaturated alcohols
Aqueous environment
Multi-gram synthesis
1
. Introduction
Selective and controlled oxidation of primary and secondary
inexpensive and ‘green’ solvent. However, in-solubility of many
non-polar organic substrates in water has essentially limited its
usage as a solvent for many organic transformations. The solubility
issues are addressed by the addition of an external surfactant
which helps to solubilize organic substrates in aqueous medium
by forming micelles [24–27]. A suitable micellar medium accom-
modates the relevant organic substrates in its hydrophobic core
that also enhances the reactivity of the intended reactions [28–
30]. It is known that organic substrates are distributed between
bulk water and micelles depending on their polarity. Preferentially
organic non-polar substrates are hosted within the hydrophobic
core of micelles and experience a high local concentration in the
supramolecular aggregates. Similarly, the dissolved oxygen con-
tent in micellar medium is known to be much higher than non-
micellar medium. We anticipated the possibility of higher solubil-
ity of molecular oxygen inside of the hydrocarbon core of nanomi-
celles in line with literature precedence [24]. Instead of the
addition of external surfactant, it has been considered advanta-
geous to integrate the micelle forming and the metal-based cat-
alytically active entities within the same molecule. This would
provide the required micellar medium to encourage the proximity
of substrate and the catalyst. Reports on such metallo-surfactant
based catalysts are scarce in literature [31–36].
alcohols to their corresponding carbonyl compounds is a key trans-
formation in organic synthesis and it has immense industrial as
well as academic interest [1,2]. Traditional methods for such trans-
formations involve the addition of stoichiometric amount of a cho-
sen organic/inorganic oxidant. These methods are very efficient,
however, concomitant generation of equal amount of hazardous
by-products is the related undesirable feature [3,4]. To address this
issue, a variety of transition metal based compounds and benign
molecular oxygen have been developed as catalyst and oxidant,
respectively [5–23]. The modified protocols developed in recent
times need further improvement with respect to one or more of
the associated disadvantageous aspects. To make the methodolo-
gies attractive, one should avoid certain parameters e.g. the use
of organic solvents, specialized bimetallic catalytic systems [10–
1
3], external bases [14–17], elaborated ligands [18], and radicals
(
e.g. nitroxyl) [19–23].
Thus, development of easily accessible yet effective oxidant-
catalyst combinations for oxidation reactions in aqueous medium
in the absence of other adducts is highly desirable. Water is an
A number of well-defined oxomolybdenum complexes are
known for their efficient catalytic behaviors in a variety of key
⇑
Corresponding author.
E-mail address: dillip@iitm.ac.in (D.K. Chand).
https://doi.org/10.1016/j.jcat.2019.06.013
021-9517/Ó 2019 Elsevier Inc. All rights reserved.
0

1
24
P. Thiruvengetam et al. / Journal of Catalysis 376 (2019) 123–133
used was 632.8 nm with the scattering angle of 90°. TEM images
were obtained on PHILIPS CM12 transmission electron microscope
at an acceleration voltage of 120 kV. DFT study was performed
with the Gaussian 09 software package. The B3LYP (Becke’s three
parameter hybrid functional using the LYP correlation) functional
was used for geometry optimizations and frequencies with
LANL2DZ for Mo atom, and the 6-31G* basis set for carbon, nitro-
gen, oxygen and hydrogen. Frequency calculations were performed
for the optimized structures to confirm the absence of any imagi-
nary frequencies. The calculations were performed at gaseous
phase without any consideration of solvent effect.
2 2 6 4 2
Scheme 1. Synthesis of the catalyst (C19H42N)[MoO(O ) (C H NO )], Mo1.
2 2 6 4 2
2.2. Synthesis of (C19H42N)[MoO(O ) (C H NO )], Mo1
organic transformations [37–62]. We are interested in the develop-
ment of oxomolybdenum complexes as mild and selective cata-
lysts. Specially, we care for sensitive functional groups in the
substrates [36,63–66]. Certain oxidation protocols developed by
us [64], are actually useful in organic synthesis, largely due to
the functional group tolerance feature, and adopted by other
researchers in their works [67–71]. Hence, we have taken the chal-
lenge of making green protocols [36], in view of the environmental
concerns.
The complex Mo1 was synthesized using a simple protocol
36,57]. Two solutions (namely Solution-A and Solution-B) were
prepared then the solutions were mixed to prepare the targeted
complex as described below.
Solution-A: Sodium molybdate dihydrate (0.193 g, 0.8 mmol)
was dissolved in 5 mL of water and the acidity of the solution
was adjusted to pH 2 using 0.1 M sulfuric acid. Then 1 mL of 40%
[
(
w/v) hydrogen peroxide was added and the resulting solution
was diluted up to 20 mL with water. Solution-B: Picolinic acid
0.102 g, 0.83 mmol) was added to a solution of cetyltrimethylam-
The present work reports the use of a molybdenum based met-
(
allomicellar
catalyst
(C19
H
42N)[MoO(O
2
)
(C
2 6
4 2
H NO )],
Mo1
monium bromide (0.302 g, 0.83 mmol) prepared in 2.5 mL of water
and the resulting solution was diluted up to 5 mL with water.
Solution-A was slowly added in a drop wise manner to the
Solution-B under ice-cold condition with vigorous stirring and the
acidity of the solution was maintain at pH 2 by adding 0.1 M sulfu-
ric acid. After 5 min the formation of a pale yellow precipitate was
observed. The reaction mixture was centrifuged and the precipitate
was separated, then washed with cold water followed by drying
under vacuum (Yield 78%). mp. 248–250 °C (decomposed). Single
crystals suitable for X-ray diffraction were obtained by slow evap-
oration of a solution of the complex prepared in 3:1 ethyl acetate-
hexane.
(
Scheme 1) for controlled and selective oxidation of unsaturated
alcohols under aqueous conditions using molecular oxygen as the
sole oxidant. This work introduces the activation of molecular oxy-
gen in aqueous medium using a molybdenum complex. Certain
sensitive/oxidizable functional groups are also tolerated during
the catalyzed transformations that could be considered as an
added advantage of the protocol for further uses.
2
. Experimental section
2
.1. Materials and methods
IR (KBr pellet)
m
= 943, 860, 584, 528 cmꢀ1.
H NMR (400 MHz, CDCl , 24 °C): d 8.16 (d, J = 4.89 Hz, 1H), 8.03
1
Some of the alcohols were purchased from commercial sources
3
and others were prepared by following or adopting literature pro-
(d, J = 7.77 Hz, 1H), 7.86–7.82 (m, 1H), 7.39–7.36 (m, 1H), 3.45–
tocols as suitable and noted below. The alcohols 1a–14a, 20a, 29a–
3.41 (m, 2H), 3.27 (s, 9H), 1.80–1.74 (m, 3H), 1.34–1.30 (m, 4H),
1
3
3
2a, 1c, 5c, 6c and the deuterated solvents were purchased from
1.29–1.23 (m, 22H), 0.88–0.85 (m, 3H) ppm; C NMR (100 MHz,
CDCl , 24 °C): d 169.37, 147.53, 145.25, 139.17, 127.40, 124.54,
Sigma-Aldrich whereas 15a, and 16a were purchased from TCI
Chemicals (India Pvt. Ltd.). The alcohols (17a, 18a) [72], (19a)
3
67.45, 53.48, 32.05, 29.84, 29.80, 29.76, 29.65, 29.53, 29.49,
[
73], (21a, 23a–28a, 35a, 15c–16c and 18c) [74], (33a–34a) [75],
new (22a) [74] and new (36a) [75] were prepared. The carbonyls
d–4d, 7d–9d, 11d–13d were purchased from Sigma-Aldrich Com-
29.31, 26.31, 23.38, 22.82, 14.25 ppm.
2 7
Anal. Calcd. for C25H46MoN O : C, 51.54; H, 7.96; N, 4.81. Found
98
2
C, 51.87; H, 8.34; N, 4.79. MS (ESI): m/z = 299.89 for [ Mo1-
ꢀ
pany whereas 10d was obtained from Spectrochem. These car-
bonyls were reduced by standard methods to afford the
corresponding alcohols. Sodium molybdate dihydrate, picolinic
acid, CTAB, diethyl ether and ethyl acetate were purchased from
SISCO research laboratories Pvt. Ltd., India and used without fur-
ther purification. Aqueous hydrogen peroxide was purchased from
Merck specialities Pvt. Ltd., and the concentration of hydrogen per-
(CTA)] ; calculated 299.90.
2.3. General procedure for the catalytic oxidation of alcohols to
aldehydes
A mixture of alcohol (0.75 mmol), and catalyst Mo1 (13 mg,
3.0 mol%) taken in 0.5 mL of water was stirred at 100 °C under oxy-
gen atmosphere (O bladder) and the stirring was continued for
2
1
oxide was determined by permanganate titration method. H and
1
3
C NMR spectra were recorded on Bruker Avance-400 instrument.
16–24 h as per requirement. The progress of reaction was moni-
tored by TLC. After completion of the reaction, ethyl acetate was
added to the mixture. The aqueous phase was extracted with ethyl
acetate 2–3 times. Then the combined organic extracts were dried
over anhydrous sodium sulfate and the solvent was removed under
reduced pressure. The crude product so obtained was purified by
column chromatography using hexane-ethyl acetate as eluent.
While the known products were characterized by spectroscopic
techniques and compared with reported data and the new prod-
ucts 22b and 36b were characterized completely. The characteriza-
tion detail is provided in supporting information section.
IR spectrum of the complexes was measured on Jasco FT/IR 4100
instrument by KBr pellet method. Single crystal X-ray analyses
were carried out using Bruker X8 Kappa XRD instrument. Powder
XRD was recorded in Bruker D8 advance instrument. ESI-Mass
spectrum of the complex was recorded on Micromass Q-Tof mass
spectrometer in a negative ion mode. Data obtained from a
SYSTRONICS digital bench top conductivity meter (Model 306)
was used for the determination of critical micellar concentration.
Dynamic light scattering (DLS) experiments were performed on a
Malvern Zetasizer nano-series at 25 °C. The wavelength of the laser

P. Thiruvengetam et al. / Journal of Catalysis 376 (2019) 123–133
125
3
. Results and discussion
3.1. Synthesis and characterization of the catalyst
Synthesis of the complex Mo1 following a literature protocol
[
76,77], failed in our hand even after several attempts. The analysis
1
of the obtained material by H NMR and ESI-MS techniques
revealed the formation of a mixture of products, however, the tar-
geted complex was not detected in the mixture. It is well known
that pH can dramatically alter the nature of oxomolybdenum spe-
cies. In order to achieve the targeted complex Mo1, the synthesis
was carried out under acidic pH following another protocol
[
36,57], and it was successful. Combination of an acidified aqueous
2 2 6 4 2
Fig. 2. TEM image of (C19H42N)[MoO(O ) (C H NO )], Mo1.
solution of Na MoO containing H with another aqueous solu-
2
4
2 2
O
tion containing picolinic acid and cetyltrimethylammonium bro-
mide (CTABr) precipitated the desired complex (Mo1) in pure
lished a narrow size distribution of particles with the average
hydrodynamic diameter of about 10 nm. Thus, the TEM images
and DLS (See ESI, Figure S8) data supported the aggregation behav-
ior of Mo1 in water.
state as a pale yellow solid (Scheme 1). Molecular structure of
the complex was unambiguously ascertained by IR, 1H NMR,
13
C
NMR, ESI-MS, melting point, elemental analysis, powder XRD
PXRD), and single crystal XRD techniques. The critical micellar
(
concentration (CMC) was determined by conductivity method
and particle sizes were approximated by TEM and DLS studies.
The IR spectrum of Mo1 (See ESI, Figure S1) showed character-
3.2. Scope of the catalyst
ꢀ1
istic signals [57]. Sharp and intense peaks at 943 and 860 cm
accounts for the stretching of (Mo@O) and (OAO) bonds. The bands
In order to evaluate the catalytic ability of Mo1, we set to exam-
ine oxidation reactions of benzyl alcohol derivatives under aque-
ous conditions using molecular oxygen. To begin with, 4-
methoxybenzyl alcohol, 1a was chosen as the model substrate.
The reaction condition was optimized by varying the catalyst load-
ing, and temperature as summarized in the Table 1. When oxida-
tion of 1a (103 mg, 0.75 mmol) was carried out in the absence or
presence of catalyst Mo1 (1 or 5 mol%) at room temperature the
substrate remained unchanged (Table 1, entries 1, 3 and 4). How-
ever, when the temperature was increased to 100 °C, 4-
methoxybenzaldehyde, 1b was produced in 22% yield only when
the catalyst (1 mol%) was employed (Table 1, entry 5). The yield
increased with catalyst loading and afforded 49 or 96% of 1b upon
using 2 or 3 mol% of Mo1, respectively (Table 1, entries 6–7). The
aldehyde, 1b was observed in 86% when reaction was carried out
in open atmosphere. However, the yield dropped due to over-
oxidation when 5 mol% of catalyst was used. Based on the above
trials, the optimized condition for oxidation of 0.75 mmol of the
benzylic alcohol includes 3 mol% the catalyst Mo1 in 0.5 mL of
refluxing water maintained under molecular oxygen from a blad-
der. Under the optimized reaction condition, the concentration of
the catalyst (ca. 45 mM) remained well above the CMC.
ꢀ
1
at 1680 and 1330 cm were assigned to the asymmetric and sym-
metric stretching of ACOO moiety. The presence of [Mo(O )] group
was confirmed from the vibrational stretches at 584 and 528 cm
2
ꢀ
1
.
1
13
The H and C NMR spectra of Mo1 recorded in deuterated chlo-
roform, established the presence of a single species in solution.
The integral values of peaks in 1H NMR spectrum supported the
presence of CTA and picolinate at 1:1 ratio. ESI-MS data (negative
ion mode) confirmed the anionic part of the complex. The isotopic
peak patterns at m/z = 299.89 (See ESI, Figure S5) corresponds to
ꢀ
the monoanionic species [Mo1 ꢀ CTA] and it matches with the
simulated pattern (m/z = 299.90).
Single crystals of Mo1 suitable for X-ray diffraction were
obtained by the slow evaporation of a solution of Mo1 in ethyl
acetate-hexane (3:1). The asymmetric unit of the structure con-
tains a molybdenum-based complexed anion and a CTA counter
cation. The molybdenum center is coordinated by one oxo, two
peroxo and one picolinate units creating distorted pentagonal
bipyramidal geometry. The axial positions of the coordination
sphere are occupied by the oxo group and nitrogen atom of the
picolinate ligand, while the equatorial positions by four oxygen
atoms of two peroxo groups and one oxygen atom of picolinate
moiety (Fig. 1). The sample was further characterized by powder
X-ray diffraction (PXRD) technique. The observed 2h peaks were
in excellent agreement with the simulated data suggesting homo-
geneity and purity of the bulk sample (See ESI, Figure S6).
Surfactants are known to form micelles above a particular con-
centration called CMC. The CMC of Mo1 was found to be 0.378 mM
that was obtained by conductivity measurement method (See ESI,
Figure S7). Near spherical congregates with an average diameter of
The reactivity was compromised when EtOH was chosen as the
solvent (Table 1, Entry 9). However, high yield of 1b was observed
in toluene medium (not a green solvent). This is attributed to the
formation of reverse micelle in toluene that could be conducive
for the progress of the reaction. An analogues complex ((CH
3
)
4
N)
0
[MoO(O
2
)
2
(C H
6 4
2
NO )], Mo1 that is devoid of the cationic surfac-
tant [57], hence not capable of micelle formation was employed
to further probe the necessity of a micellar catalyst under aqueous
conditions. Poor yield of 1b was observed when 3 mol% of Mo1
0
3
± 1 nm were observed using TEM (Fig. 2). The DLS data estab-
was employed as the catalyst (Table 1, entry 11). As per the obser-
vation it is assumed that either micelle or reverse micelle is
required to bring the catalyst and substrate together to effectively
promote the oxidation reactions.
Controlled oxidation of 1a to form the aldehyde 1b inspired us
to further investigate the versatility of the metallomicellar cat-
alytic system under aqueous conditions. Thus we explored the sub-
strate scope and the result (conversion of the alcohols 1a–30a to
the aldehydes 1b–30b) is summarized in Table 2. Typical methoxy,
bromo and benzyloxy benzyl alcohols were oxidized to corre-
sponding aldehydes (Table 2, entries 1–8/20–22) in high isolated
yields (88–96%). Substrates containing electron withdrawing func-
2 2 6 4 2
Fig. 1. Crystal structure of (C19H42N)[MoO(O ) (C H NO )], Mo1.

1
26
P. Thiruvengetam et al. / Journal of Catalysis 376 (2019) 123–133
Table 1
Optimization of controlled oxidation of 4-methoxybenzyl alcohol.
a
1a
1b
Entry
Cat. Mo1 Solvent Temp. Time
Yield (%)b
Conv.
(%)
TONc
(mol%)
(°C)
(h)
Alcohol
1a)
Aldehyde
(1b)
Acid
(1b')
(
1
2
3
4
5
6
7
8
9
-
-
H
H
H
H
H
H
2
2
2
2
2
2
O
O
O
O
O
O
rt
24
24d
24
24
24
24
24
24
24
24
24
98
97
98
98
77
44
-
-
-
-
-
100
rt
-
-
-
-
1
5
1
2
3
5
3
3
3g
-
-
2
2
rt
-
-
2
0.4
23
28
33
20
13
32
10
100
100
100e
100
80
22
49
96f
83
37
90
23
-
23
56
>99
>99
38
95
29
2
2
10
-
H
2
O
H
2
O
-
EtOH
62
5
1
0
1
Toluene 110
100
3
-
1
H
2
O
71
a
Reaction condition: 4-methoxybenzyl alcohol (0.75 mmol) in 0.5 mL of water and O
2
-bladder. bIsolated yield.
c
f
d
e
TON: [Reacted mol of alcohol]/[Total mol of catalyst]. No change even after 48 h. At 80 °C only 18% aldehyde.
Yield of 1b is 86% (11% unreacted 1a) when reaction carried out in open atmosphere.
g
(
3 4 2 2 6 4 2
(CH ) N)[MoO(O ) (C H NO )], Mo1' was used as catalyst.
tionalities like nitro and cyano afforded their corresponding alde-
hydes (Table 2, entries 9–11) in moderate to good yields. Sub-
strates containing sensitive/oxidizable functional groups such as
cyano, sulfide, aryl-hydroxyl, tertiary amine, internal/terminal
alkene, internal/terminal alkyne and acetal were employed to
examine the chemoselectivity where upon the corresponding alde-
hydes were afforded (Table 2, entries 11–19 and 23–29). Cyano
compounds are known to undergo hydrolysis to form the corre-
sponding acid whereas the sulfides afford the sulfoxides/sulfones
due to oxidation even under mild conditions. Similarly, alkene/
alkyne groups can result epoxide or hydrolytically cleaved prod-
ucts in presence of oxidizing agents. Oxidation of phenols (aryl-
hydroxyls) to quinones is a common variety of oxidation reaction.
Further, de-protection of acetal group is well known in presence of
acidic catalysts. The catalyst and reaction conditions established in
this work allowed selective oxidation of the substituted alcohols to
corresponding aldehydes (Table 2, entries 11–19, 23–29) in good to
excellent yields. Thus, all the above mentioned functional groups
were found to be tolerated. However, extension of the substrate
scope for the oxidation of aliphatic alcohol was not successful. Oxi-
dation of n-decanol, 30a under the optimized condition employed
for activated alcohols, afforded miniscule yield of corresponding
decanal, 30b (Table 2, entry 30). The difficulty in the activation
of CAH bonds of the methylene group located adjacent to the
hydroxyl group could be the reason for this low yield. Some pre-
liminary mechanistic study is provided in a later section.
pyridine-2-methanol, 32a; triazolylmethanol and triazolylmethyl
derivative of benzyl alcohol (33a and 34a) could be performed
smoothly to afford the corresponding aldehydes (Table 3 entries
1–4)). Thus, heteroaromatics were found to be compatible under
the reaction condition employed.
Subsequently, we tried our hands on oxidation of secondary
alcohols to prepare ketones under optimized conditions. The reac-
tions proceeded as planned, also in chemoselective manners as
enumerated in Table 4. A range of substrates (1c–17c) could be
smoothly oxidized to the corresponding ketones (1d–17d). Pres-
ence of sensitive functional groups such as sulfide, aryl-hydroxyl,
alkene and alkyne were probed and found to be tolerated during
the oxidation.
Since primary aliphatic alcohols tend to undergo oxidation at a
slow rate we considered to perform selective oxidation of repre-
sentative activated primary benzylic alcohols (35a and 36a) having
un-activated alcohol as substituent. The corresponding carbonyls
(35b and 36b) were afforded successfully as shown in Scheme 2.
Selective oxidation of a model secondary benzylic alcohol over
un-activated alcohol when present in the same molecule was also
demonstrated as shown in Scheme 3.
3.3. Multi-gram synthesis and recyclability test
It was considered to probe the feasibility of multi-gram scale
synthesis of carbonyls under the optimized reaction conditions.
Oxidation of 4-methoxybenzyl alcohol, 1a at 3 and 10 g scales were
performed to prepare the aldehyde 1b. High isolated yields (87 and
72%) were noted for the reactions performed at above mentioned
Overall the metallomicellar catalyst facilitated controlled and
chemoselective oxidation of benzylic alcohols under aqueous con-
ditions. Controlled oxidation of pyridine-3-methanol, 31a;

P. Thiruvengetam et al. / Journal of Catalysis 376 (2019) 123–133
127
scales, respectively (Scheme 4). This data demonstrated the robust-
ness of the catalyst. Thus, the possibility of using micellar catalyst
for practical uses could be achieved [78,79].
controlled regime (43% conversion for 4 h, Fig. 3) and using diethyl
ether as extracting solvent for better phase separation (see sup-
porting information for recycling procedure). The catalyst in aque-
ous medium shows good catalytic activity for 8 cycles without any
significant loss of yield. Moreover the state of catalyst is evaluated
by hot filtration test and found to be heterogeneous in nature (see
ESI for hot filtration test procedure). We also performed the oxida-
tion reaction by taking 4-methoxybenzyl alcohol, 1a as model sub-
strate in solvent free condition under optimized reaction condition.
After completion of reaction we obtained 4-methoxy benzalde-
hyde, 1b in 91% with 4% of corresponding acid as over oxidized
product.
Recycling of the catalyst was examined using 1a as a model
substrate (Scheme 5). Following a simple ‘‘in-flask” workup (see
ESI for recycling procedure), the product could be isolated in good
yields after every cycle. Diethyl ether was added to the reaction
mixture, after completion of a cycle, and the aqueous and organic
layers were then separated by a pipetting-dropper. The organic
layer was evaporated to get the product whereas the aqueous layer
was reused for the next run. Eight catalytic runs were successfully
demonstrated and the yields are noted in the range of 92–97% indi-
cating effective recyclability of the surfactant based molybdenum
catalyst without any significant loss of yield. However, at the end
of each reaction cycle in 1.5 mmol scale, we realized that we lose
incremental amount of catalyst due to ineffective phase separation
by manual pipetting (see supporting information). For avoiding
this loss, we performed recyclability test of reaction in kinetically
3.4. Mechanistic study of Mo1 for alcohol oxidation: Computational
study
As mentioned in introduction section, the micelle forming unit
i.e. CTA moiety of the catalyst Mo1 is suitable for solubilization of
Table 2
a
Oxidation of primary benzylic alcohols to aldehydes.
Cat. Mo1 (3 mol%)
2
H O
1a-30a
O
2
1
-bladder
1b-30b
00 °C
b
c
b
c
Entry
1
Aldehydes
Time
Yields
(%)
Conv.
(%)
TON
Entry
Aldehydes
Time
(h)
Yields
(%)
Conv.
(%)
TON
(
h)
d
1b
2b
24
96
98
33
16
17
16b
17b
18
18
92
96
32
2
3
18
98
>99
33
33
96
>99
33
33
3b
20
94
98
18
18b
20
98
>99
4
5
4b
5b
18
18
88
98
92
31
33
19
20
19b
20b
18
24
63
96
72
24
33
>99
>99
6
7
6b
7b
20
18
91
97
96
98
32
33
21
21b
22b
18
18
89
83
>99
92
33
31
22
(
continued on next page)

1
28
P. Thiruvengetam et al. / Journal of Catalysis 376 (2019) 123–133
8
9
8b
9b
20
24
88
72
96
78
32
26
23
24
23b
24b
18
20
83
86
86
92
29
31
1
1
0
1
10b 20
91
85
96
89
32
30
25
26
25b
26b
20
24
91
79
96
86
32
29
11b 24
1
1
2
3
12b 24
13b 24
96
94
98
96
33
32
27
28
27b
28b
18
20
78
81
84
86
28
29
1
1
4
5
14b 16
86
77
89
84
30
28
29
30
29b
16
24
87
92
16
31
5
e
15b 24
30b
14
a
b
c
Reaction condition: Alcohol (0.75 mmol), catalyst (3 mol%) in 0.5 mL H
2
O at 100°C and O
2
-bladder. Isolated yield. TON: [Reacted mol of
d
-1
e
alcohol]/(Total mol of catalyst). TOF calculated in kinetically controlled regime for 4 hours and found to be 3.6 (h ). Yield and conversion
were monitored by GC-MS analysis using mesitylene as internal standard.
Table 3
Oxidations of heteroaromatic benzylic-like alcohols to aldehydes.a
Entry
1
Aldehydes
Time (h)
18
Yieldsb (%)
91
Conv. (%)
96
TONc
32
31b
32b
33b
2
3
20
18
79
86
89
94
30
31
4
34b
18
98
>99
33
a
2 2
Reaction condition: Alcohol (0.75 mmol), catalyst (3 mol%) in 0.5 mL H O at 100 °C and O
-bladder. b Isolated yield. c TON: [Reacted mol of alcohol]/[Total mol of catalyst].
the alcoholic substrates as well as molecular oxygen in water med-
ium. As the result, the rate of oxidation reaction would increase in
micellar environment. The role of oxodiperoxo-molybdenum(VI)
core of Mo1 in the oxidation of benzyl alcohols was probed by pre-
liminary experiments and DFT calculations. Maeda et al studied
some mechanistic aspects of oxidation of alcohols to aldehydes
using an oxovanadium complex as catalyst and suggested the con-
version of metal bound oxo species as the molybdenum-based
intermediates [80]. In similar line, we also carried out few experi-
ments (see supporting information) to gain insights to the reaction
kinetics and mechanism. The reaction was performed in presence
of a radical inhibitor (2,6-di-tert-butyl-4-methylphenol). No signif-

P. Thiruvengetam et al. / Journal of Catalysis 376 (2019) 123–133
129
Table 4
a
Oxidation of secondary benzylic alcohols to ketones.
Entry
Ketones
Time (h)
Yieldsb (%)
Conv. (%)
TONc
Cat. Mo1 (3 mol%)
2
H O
2
O -bladder
1c-17c
1d-17d
1
00 °C
1
2
3
1d
2d
3d
24
98
>99
96
33
32
31
24
20
92
89
92
4
5
6
7
4d
5d
6d
7d
24
18
20
24
84
93
87
77
86
98
91
82
29
33
30
27
8
9
8d
24
91
96
32
9d
24
20
20
20
20
20
73
93
93
81
94
79
77
98
96
86
96
86
26
33
32
29
32
29
1
1
1
1
1
0
1
2
3
4
10d
11d
12d
13d
14d
1
1
5
6
15d
16d
20
24
98
86
>99
92
33
31
1
7
17d
24
92
96
32
a
2 2
Reaction condition: Alcohol (0.75 mmol), Catalyst (3 mol%) in 0.5 mL H O at 100 °C, O
-bladder. b Isolated yield. c TON: [Reacted mol of alcohol]/[Total mol of catalyst].
icant change in reaction yield was noticed, thus an ionic mecha-
nism for the catalytic reaction is proposed. Stoichiometric reaction
between Mo1 and 4-methoxybenzyl alcohol (1:2 equiv.) was car-
ried out where in the anionic part of trioxo molybdenum complex
3 6 4 2
(C19H42N)[MoO (C H NO )], Mo2 was detected in-situ as con-
firmed by ESI-MS study (negative ion mode) and 4-methoxy ben-
zaldehyde was isolated in 92% yield as the oxidized product (see
supporting information). The mechanism for the formation of oxo-

1
30
P. Thiruvengetam et al. / Journal of Catalysis 376 (2019) 123–133
4
4%
43%
44%
4
3%
43%
4
2%
42%
42%
4
3
2
1
0
0
0
0
0
Scheme 2. Selective oxidations of activated primary alcohol over un-activated
alcohol.
1
2
3
4
5
6
7
8
Catalytic cycle
Fig. 3. Recyclability test in kinetically controlled regime.
Scheme 3. Selective oxidation of activated secondary alcohol over unactivated
alcohol.
molybdenum species from peroxo compound is already explained
by Jacobson et al. [81]. Taking into account the experimentally
observed intermediacy of Mo2, DFT optimization for the various
putative intermediates was carried out in the gas phase. (The
energy optimized structures and related information are provided
in supporting information).
As depicted in Scheme 6, the benzyl alcohol 1a reacts with the
pre-catalyst Mo1 giving actual catalyst Mo2 via cleavage of the
Mo-O bond of peroxo groups [81] and produced two equivalence
of benzaldehyde 1b. The reaction is thermodynamically feasible
with computed to be exergonic by 101.87 kcal/mol. The intermedi-
ate complex Mo3 is then obtained by the reacting alcohol 1a with
trioxo molybdenum complex Mo2 [80]. The formation of Mo3 from
Mo2 is calculated to be endergonic value of 19.49 kcal/mol. The
intermediary complex Mo3 can further possibly lead to another
intermediary molybdenum complex Mo4 by releasing aldehyde
Scheme 4. Multi-gram scale synthesis of 4-methoxybenzaldehyde.
[
80]. The formation of molybdenum complex Mo4 from Mo3 was
found to be highly endergonic with the calculated value of
8.38 kcal/mol and which is determine the rate of reaction. Fur-
6
thermore, the formation of actual catalyst i.e. trioxo molybdenum
complex Mo2 from Mo4 is achieved by oxidation of Mo4 with half
equivalence of molecular oxygen [80]. This reaction was found to
be highly exergonic (ꢀ147.18 kcal/mol) reiterating the thermody-
namical feasibility for the reformation of Mo2. The conversion of
Mo4–Mo2 shows catalytic cycle can be potentially closed with
the reaction of oxygen molecule to Mo4. From the above prelimi-
nary DFT study, the overall energetics seems to favor the suggested
mechanism. From the proposed mechanism, the alcohol oxidation
using oxomolybdenum based catalyst involves two important con-
secutive steps which are believed to be pivotal. First step involves
the formation of bond between hydroxyl of alcohol and the molyb-
denum center. The second step involves a b-hydrogen transfer
from benzylic CAH to oxo group of molybdenum. In the case of
benzylic alcohols, the b-hydrogen transfer is easier since it is acti-
vated by the adjacent phenyl group, whereas aliphatic alcohols
demand more energy for b-hydrogen transfer. Similarly the forma-
tion of acid is controlled by aforementioned two steps. Although
the experimental results supported the above explained mecha-
nism, we don’t have evidence to rule out the radical path way of
catalytic cycle (see supporting information scheme S5 for a radical
pathway).
1
00
96%
97% 96%
9
5%
94%
93% 94%
9
2%
80
60
40
20
0
1
2
3
4
5
6
7
8
Catalytic cycle
Scheme 5. Recycling test of the catalyst Mo1.

P. Thiruvengetam et al. / Journal of Catalysis 376 (2019) 123–133
131
2 2 6 4 2
Scheme 6. Proposed mechanistic cycle of catalyst (C19H42N)[MoO(O ) (C H NO )], Mo1 (the counter cations are not shown for clarity).
4
. Conclusion
In conclusion, a methodology for the controlled and chemose-
5. Accession codes
CCDC 981316 contains the supplementary crystallographic data
for this paper. These data can be obtained free of charge via www.
ccdc.cam.ac.uk/data_request/cif, or by emailing data_re-
quest@ccdc.cam.ac.uk, or by contacting The Cambridge Crystallo-
graphic Data Centre, 12 Union Road, Cambridge CB2 1EZ, UK;
fax: +44 1223 336033.
lective oxidation of unsaturated alcohols to their corresponding
carbonyls has been developed under green conditions. The reac-
tions were performed using a well-defined molybdenum based
metallomicellar complex, Mo1 as catalyst and molecular oxygen
as the oxidant under aqueous environment. The micellar portion
of the catalyst is presumed to have significant role in enhancing
the catalytic activity by solubilizing the substrate inside the
hydrophobic pocket of the micelle and bringing the substrate and
catalyst in close proximity. A variety of sensitive/oxidizable func-
tional groups are tolerated during the oxidation reactions. The rea-
sonable recyclability of catalyst further augments its consideration
as potential ‘green’ and robust eco-friendly oxidation catalyst. The
multi-gram scale preparation of a model aldehyde shows the capa-
bility of the micellar catalyst for practical use. A mechanistic cycle
for the oxidation of benzyl alcohols catalyzed by Mo1 was pro-
posed with the help of preliminary DFT calculations.
6
. Notes
The authors declare no competing financial interest.
Acknowledgements
We thank Mr. Suvendu Karak, Dr. Rajamony Jagan, and Dr. Jai
Anand Garg for rendering help during synthesis of Mo1, structure
solution of Mo1 and scientific discussion, respectively. The authors

1
32
P. Thiruvengetam et al. / Journal of Catalysis 376 (2019) 123–133
thank IIT Madras for financial support. D.K.C. acknowledges the
financial support provided by IIT Madras under the Mid-Career
Institute Research and Development Award (IRDA-2019).
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