A new insight into the oxidation of cyclododecane with hydrogen peroxide in the presence of iron-substituted polyoxotungstates

Article abstract of DOI:10.1055/s-2008-1077876

The catalytic homogeneous oxidation of cyclododecane with hydrogen peroxide in the presence of tetrabutylammonium salts of iron-substituted Keggin-type polyoxotungstates of general formula (TBA)⁴H_zXW ₁₁Fe(H₂O)O₃₉-nH₂O (where X = P, Si, B, and z = 0-2) is described. In the reaction conditions reported, the corresponding alcohol, ketone, and hydroperoxide are obtained as the main reaction products. The catalytic activity of the anions with phosphorous, silicon, and boron is compared in different reaction conditions. These catalytic oxidation reactions seem to be radical processes, since they are totally inhibited in the presence of I₂, a well-known radical scavenger. Thieme Stuttgart.

Full text of DOI:10.1055/s-2008-1077876

LETTER
1623
A New Insight into the Oxidation of Cyclododecane with Hydrogen Peroxide
in the Presence of Iron-Substituted Polyoxotungstates
O
xidatio
s
n
of Cyclo
a
dodecane
w
b
ith Hydroge
e
n Peroxidel C. M. S. Santos,a Mário M. Q. Simões,a M. Salete S. Balula,a,b M. Graça P. M. S. Neves,a José A. S.
Cavaleiro,a Ana M. V. Cavaleiro*a,b
a
Chemistry Department, University of Aveiro, Universidade de Aveiro, 3810-193 Aveiro, Portugal
CICECO, University of Aveiro, 3810-193 Aveiro, Portugal
b
Fax +351(234)370084; E-mail: anacavaleiro@ua.pt
Received 28 January 2008
oxides could be formed in the presence of several
Abstract: The catalytic homogeneous oxidation of cyclododecane
with hydrogen peroxide in the presence of tetrabutylammonium
salts of iron-substituted Keggin-type polyoxotungstates of general
formula (TBA)4HzXW11Fe(H2O)O39·nH2O (where X = P, Si, B, and
iron(III)-substituted heteropolytungstates.14–19 The forma-
tion of alkyl hydroperoxides may be explored in several
ways, as they may be selectively decomposed to ke-
z = 0–2) is described. In the reaction conditions reported, the corre- tone(aldehyde)–alcohol mixtures20,21 or reduced to the
sponding alcohol, ketone, and hydroperoxide are obtained as the
corresponding alcohols.
main reaction products. The catalytic activity of the anions with
phosphorous, silicon, and boron is compared in different reaction
The oxidation of cyclododecane is important from the in-
dustrial point of view as it provides precursors for the syn-
thesis of nylon-12 polymers.22 This reaction is usually
carried out using O2 and Co(II) acetate in the presence of
boric acid until attaining 30–35% conversion, originating
80% of cyclododecanol, 10% of cyclododecanone, and
10% of byproducts. The catalytic dehydrogenation of the
cyclododecanol–cyclododecanone mixture at 220 °C
originates pure cyclododecanone.23 Despite its signifi-
cance, the catalytic oxidation of cyclododecane is much
less studied than related processes concerning cyclohex-
ane. Cyclododecane is much less reactive than cyclohex-
ane and its oxidation with Co(II) acetate in acetic acid is
not selective.23
conditions. These catalytic oxidation reactions seem to be radical
processes, since they are totally inhibited in the presence of I2, a
well-known radical scavenger.
Key words: iron, polyoxometalates, polyoxotungstates, oxidation,
hydrogen peroxide, cyclododecane
Selective oxidation of saturated hydrocarbons is one of
the most challenging subjects in oxidation chemistry and
many catalytic systems for these transformations have
been extensively investigated over the last decades.1–5
Among others, some systems have been described in
which the well-known transition-metal-monosubsti-
tuted Keggin-type polyoxometalates,6 like a-
[XW11M(H2O)O39]n– (X = P, Si, M = first-row transition-
metal ion), have been employed as catalysts or catalyst
precursors for several types of alkane oxidative reactions,
using different oxidants (e.g. PhIO, O2, TBHP) and di-
verse reaction conditions.7–10 Alcohols and ketones have
been the prominent reaction products obtained in these
studies.
To our knowledge, only a small number of publications
have reported the catalytic oxidation of cyclododecane
with H2O2. Barton and collaborators employed the Gif-
type system, oxidizing this compound with aqueous 30%
H2O2 at room temperature, and obtained mainly cy-
clododecanone, at moderate substrate conversions.24 Sev-
eral other studies have been carried out in heterogeneous
conditions. These include studies with V-MCM-41 and
other micro- or mesoporous materials containing transi-
tion metals (e.g. Ti, V, Cr, Mn, Fe, Co, or Cu).22,25,26 Mod-
erate to high conversions of the substrate are usually
obtained, giving cyclododecanol as the main product, be-
sides cyclododecanone and other minor products. Follow-
ing our interest on cycloalkane oxidation catalysed by
polyoxometalates, we now report new results on cy-
clododecane oxidation. Some preliminary results have
been published by us.17
The use of aqueous H2O2 in the oxidation of organic sub-
strates is very attractive from the point of view of synthet-
ic organic chemistry, since aqueous H2O2 is considered an
inexpensive, environmentally clean and easy to handle re-
agent.11,12 To our knowledge, the first report concerning
the oxidation of alkanes with H2O2 in the presence of a
transition-metal-substituted polyoxotungstate was pub-
lished by Mizuno et al.,13 who used {g-
SiW10[Fe(OH2)]2O38}6– in the catalytic oxidation of cy-
clohexane, n-hexane, n-pentane, and adamantane. Our
studies on cyclohexane and cyclooctane oxidation with
hydrogen peroxide in acetonitrile have shown that consid-
erable amounts of the corresponding alkyl hydroper-
In the present study, cyclododecane oxidation with H2O2
in acetonitrile was examined under different reaction con-
ditions. Tetrabutylammonium (TBA) salts of iron-substi-
tuted Keggin-type polyoxotungstates of general formula
(TBA)4HzXW11Fe(H2O)O39·nH2O, X = P, Si, B, z = 0–2
(abbreviated XW11Fe) were used as catalysts. Those com-
pounds were prepared by known procedures14,16,27 and
characterised as described elsewhere.14,16,27,28 The reac-
SYNLETT 2008, No. 11, pp 1623–1626
x
x
.x
x
.2
0
0
8
Advanced online publication: 11.06.2008
DOI: 10.1055/s-2008-1077876; Art ID: D02408ST
© Georg Thieme Verlag Stuttgart · New York

1624
I. C. M. S. Santos et al.
LETTER
OH
OOH
O
+
+
+
O
10
1
5
2
3
4
Scheme 1
tions were performed in refluxing MeCN, using 1.0 mmol observed. Conversion values were lower in more diluted
of substrate and 1.5 mmol of catalyst for a substrate/cata- solutions (conditions D).
lyst ratio = 667.29 The amounts of solvent and of aqueous
Cyclododecanone was the major product after 12 hours of
hydrogen peroxide added are presented in Table 1. De-
pending on the conditions, the alcohol 2, the ketone 3, and
the hydroperoxide 4 were obtained as the main products,
along with lower amounts of dodecanal (5) (Scheme 1,
reaction, giving rise to ketone selectivities always above
40% (Table 1). However, for all catalysts, cyclododecyl
hydroperoxide was the main product after three hours of
reaction under conditions C, which corresponded to the
Table 1).
highest H2O2 concentration in solution. Selectivity chang-
In the absence of catalyst no cyclododecane oxidation es during the course of reaction are exemplified in
products were found, for all the conditions presented here, Figure 2 for BW11Fe. The cyclododecyl hydroperoxide
and only the decomposition of hydrogen peroxide was de- subsequent decrease is accompanied by an increase on the
tected (usually around 13% after 12 h of reaction). The re- amount of cyclododecanone. For the other conditions
sults summarised in Table 1 include turnover numbers, studied, cyclododecanone is formed as the major product
product selectivity, and hydrogen peroxide consumption.
from the beginning of the reaction whenever BW11Fe and
SiW11Fe were used. In the case of PW11Fe, all the experi-
ments had the hydroperoxide as the main product during
the first hours of reaction.
The most efficient catalysts were BW11Fe and PW11Fe, at-
taining conversions near 70% under conditions B and C
(Table 1), corresponding to TON higher than 445, after 12
hours of reaction (entries 2, 3, 6, and 7). The SiW11Fe The efficiency of use of hydrogen peroxide30 is rela-
showed also the best efficiency under conditions B (32%, tively small when H2O2/substrate molar ratios are high-
entry 10) and C (41%, entry 11). Figure 1 illustrates the er than 2, with values of efficiency around 25–30% for
typical reaction profile observed for cyclododecane oxi- H2O2/substrate = 4 and even below these values for
dation in the presence of BW11Fe, in conditions A, B, and H2O2/substrate = 6, independently of the amounts of
C, where a regular increase of conversion with time was solvent and catalyst used (Table 1). For the ratio
Table 1 Oxidation of Cyclododecane with Hydrogen Peroxide Catalysed by XW11Fe, after 12 Hours of Reactiona
H2O2
Selectivity (%)
Entry
(conditions) (mL)
MeCN
Catalyst
Conversion TONc
(%)b
Added
(mmol)
Consumed Efficiency 2
3
4
5
(mmol)
1.91
3.95
5.03
4.91
1.48
3.45
4.53
4.43
0.64
2.07
2.88
2.67
(%)
51
28
24
11
59
31
27
21
63
25
24
13
1 (A)
2 (B)
3 (C)
4 (D)
5 (A)
6 (B)
7 (C)
8 (D)
9 (A)
10 (B)
11 (C)
12 (D)
3
3
3
5
3
3
3
5
3
3
3
5
BW11Fe
BW11Fe
BW11Fe
BW11Fe
PW11Fe
PW11Fe
PW11Fe
PW11Fe
SiW11Fe
SiW11Fe
SiW11Fe
SiW11Fe
61
67
70
31
57
67
73
54
25
32
41
20
407
447
467
207
380
447
487
360
167
213
273
133
2
4
6
6
2
4
6
6
2
4
6
6
32
29
22
23
38
31
18
16
32
33
25
23
58
59
52
50
42
45
44
47
56
50
52
50
3
4
7
8
9
6
17
21
10
17
25
29
4
10
7
13
8
8
9
8
18
19
5
8
a Conditions: 1.0 mmol of substrate; 1.5 mmol of catalyst; substrate/catalyst ratio = 667.
b Determined by gas chromatography.
c Calculated as mol of products/mol of catalyst.
Synlett 2008, No. 11, 1623–1626 © Thieme Stuttgart · New York

LETTER
Oxidation of Cyclododecane with Hydrogen Peroxide
1625
(R = C12H23).18,31 The generation of molecular oxygen in
situ from H2O2 (reactions 1 and 2) would favour the hy-
droperoxidation in the presence of excess of H2O2. The
amount of hydroperoxide in the products is probably de-
pendent on the existence of several possible concurrent re-
actions, namely the hydroperoxidation or hydroxylation
of cyclododecane, the decomposition of cyclododecyl hy-
droperoxide and the dismutation of H2O2. Variables such
as H2O2/substrate molar ratio, concentration of solutions
and amount and type of catalyst used will, thus, influence
the outcome of reaction at a given time.
XW11FeIII + H2O2 → XW11FeII + HOO• + H+
(1)
XW11FeIII + HOO• → XW11FeII + O2 + H+
Figure 1 Cyclododecane oxidation reactions profile in experimen-
(2)
tal conditions A, B, and C of Table 1, catalyzed by BW11Fe
XW11FeII + HOOH → XW11FeIII + HO• + OH–
(3)
H2O2/substrate = 2, the efficiency of use of H2O2 is al-
ways above 50%, with the higher value obtained with
SiW11Fe (64%). It is necessary to use a lower H2O2/sub-
strate molar ratio in order to get higher values of efficien-
cy of use of H2O2 (Table 1, entries 1, 5, 9). The higher
turnover numbers are always obtained in conditions C
(entries 3, 7, 11). However, under these conditions, the ef-
ficiency of use of hydrogen peroxide is less than 30%.
Therefore, low values of conversion are not necessarily
attached to H2O2 dismutation, since the low yields ob-
tained in the cyclododecane oxidation do not seem to be
related to the competition between the two reactions.
RH + HO• → R• + H2O
(4)
R• + O2 → ROO•
(5)
ROO• + RH → ROOH + R•
(6)
ROO• + XW11FeII + H+ → XW11FeIII + ROOH
(7)
Scheme 2
Dodecanal is obtained with identical selectivity with all
the three Fe-substituted compounds tested as catalysts (5–
13%). This ring-opened product may be formed via b-
cleavage of the intermediate cycloalkoxy radical
(Scheme 3), which may be formed via metal-catalysed de-
composition of the corresponding hydroperoxide.5,32 The
formation of carbonyl compounds via b-cleavage is a
well-known reaction of alkoxy radicals, which in the case
of cyclic alkoxy radicals leads to ring opening. Alkoxy
radicals normally cleave to give a ketone (or an aldehyde)
and the most stable possible alkyl radical.33–35
For all polyoxotungstates tested, a complete reaction inhi-
bition is observed in the presence of I2, suggesting that the
oxidation of cyclododecane is a radical process as already
registered for the oxidation of other cycloalkanes in simi-
lar conditions.19
O
O
O
RH
+
R +
Scheme 3
These results indicate that cyclododecane may be oxi-
dised with aqueous H2O2 in acetonitrile, using Fe-substi-
tuted polyoxotungstates as catalysts, and that, besides
cyclododecanol and cyclododecanone (the most common
products with other systems), other interesting products
(cyclododecyl hydroperoxide and dodecanal) may be
obtained. Under the conditions described here, higher
conversions of cyclododecane for molar ratio substrate/
catalyst = 667 have been observed for the H2O2/
substrate = 6, using 1 mmol of substrate and 3 mL of ace-
tonitrile. The best catalyst is PW11Fe, with 487 TON after
12 hours of reaction. It is also clear that all catalysts stud-
ied here in the oxidation of cyclododecane present an
Figure 2 Selectivity for cyclododecane oxidation in conditions C of
Table 1 catalyzed by BW11Fe
The fact that a significant amount of cyclododecyl hydro-
peroxide was detected during the reaction course when-
ever an excess of H2O2 was used may be accounted by the
mechanism summarised in Scheme 2, reactions 1–7
Synlett 2008, No. 11, 1623–1626 © Thieme Stuttgart · New York

1626
I. C. M. S. Santos et al.
LETTER
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identical behaviour, with the attainment of cyclodode-
canone as the main product after 12 hours of reaction.
However, it is possible to obtain the cyclododecyl hydro-
peroxide as the main product finishing the reaction after 3
hours. As far as we know, the system reported here is the
first to present the production of cyclododecyl hydroper-
oxide and dodecanal in the catalytic oxidation of cy-
clododecane. The higher efficiency of use of H2O2
corresponds to a molar ratio H2O2/substrate = 2 (Table 1,
entries 1, 5, 9), possibly corresponding to the most prom-
ising results.
Acknowledgment
Thanks are due to the University of Aveiro, FCT, and FEDER for
funding. I.C.M.S. Santos and M.S.S. Balula are also grateful to FCT
and FSE for their grants.
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Synlett 2008, No. 11, 1623–1626 © Thieme Stuttgart · New York