Baeyer-Villiger oxidation of ketones catalysed by rhenium complexes bearing N- or oxo-ligands

Article abstract of DOI:10.1016/j.apcata.2012.07.007

Rhenium (I, III-V or VII) complexes bearing N-donor or oxo-ligands catalyse the Baeyer-Villiger oxidation of cyclic and linear ketones (e.g. 2-methylcyclohexanone, 2-methylcyclopentanone, cyclohexanone, cyclopentanone, cyclobutanone and 3,3-dimethyl-2-butanone) into the corresponding lactones or esters, in the presence of aqueous H₂O₂ (30%). The effects of various reaction parameters are studied allowing to achieve yields up to 54%.

Full text of DOI:10.1016/j.apcata.2012.07.007

Applied Catalysis A: General 443–444 (2012) 27–32
Contents lists available at SciVerse ScienceDirect
Applied Catalysis A: General
journal homepage: www.elsevier.com/locate/apcata
Baeyer–Villiger oxidation of ketones catalysed by rhenium complexes bearing
N- or oxo-ligands
Elisabete C.B.A. Alegriaa,b, Luísa M.D.R.S. Martinsa,b, Marina V. Kirillovab, Armando J.L. Pombeirob,∗
a Área Departamental de Engenharia Química, ISEL, R. Conselheiro Emídio Navarro, 1959-007 Lisboa, Portugal
b Centro de Química Estrutural, Complexo I, Instituto Superior Técnico, Technical University of Lisbon, Av. Rovisco Pais, 1049-001 Lisboa, Portugal
a r t i c l e i n f o
a b s t r a c t
Article history:
Rhenium (I, III–V or VII) complexes bearing N-donor or oxo-ligands catalyse the Baeyer–Villiger oxida-
tion of cyclic and linear ketones (e.g. 2-methylcyclohexanone, 2-methylcyclopentanone, cyclohexanone,
cyclopentanone, cyclobutanone and 3,3-dimethyl-2-butanone) into the corresponding lactones or esters,
in the presence of aqueous H2O2 (30%). The effects of various reaction parameters are studied allowing
to achieve yields up to 54%.
Received 23 March 2012
Received in revised form 28 June 2012
Accepted 8 July 2012
Available online 17 July 2012
© 2012 Elsevier B.V. All rights reserved.
Keywords:
Baeyer–Villiger oxidation
Hydrogen peroxide
Rhenium complexes
Homogeneous catalysis
Ketones
Lactones
C-scorpionates
Pyrazole
on d8 metal centres such as PdII [42,43] and PtII [3,11–13,19] have
been successfully explored in recent years.
1. Introduction
Transition-metal-catalysed Baeyer–Villiger (BV) oxidations, viz.
the transformation of cyclic and acyclic ketones into the lactones
and esters, has become an important research topic in the last years
[1–30] due to the wide applications of the products, e.g. in the gas
and oil industry, for the production of gasoline additives [31], per-
fume components [32] and pharmaceutical materials (e.g. antiviral,
antiproliferative and imunosupressor agents [33,34]).
Owing to economic and environmental reasons, a growing
attention has been paid to the replacement of organic peroxy
acids, traditionally used as stoichiometric oxidants in the BV
oxidation, by more atom efficient and environmentally friendly
oxidants, such as molecular oxygen [35–37] or hydrogen peroxide
[3,11,13,17,23,38,39].
The application of transition metal catalysts for the BV reactions
was started by Mares and co-workers, using MoVI picolinato and
dipicolinato complexes in the presence of H2O2 (90%) as terminal
oxidant [40]. Later, compounds of other metals, such as Co [23], Ni
[30], Cu [24], Sn [14], W [2], Re [17,38,41] and V/Mo [26], were also
shown to catalyse the BV oxidations. In particular, systems based
The recognized application of the versatile complex [(Me)ReO3]
(MTO) in oxidation catalysis [44–47], including the BV oxidation
of ketones [17,39,41], clearly demonstrates the ability of rhenium
centres to form highly active catalysts for the oxidation reactions
of olefins and other unsaturated substrates. However, the applica-
tion of other Re complexes, apart from MTO, as catalysts for the
BV oxidation is still very limited, although we have found that
certain rhenium complexes with oxo- or N-ligands (such as scor-
pionates, pyrazoles, benzoyl-diazenido, -hydrazido) are capable of
catalysing, in homogeneous systems, the oxidation and carboxyla-
tion of inert alkanes, under mild or moderate conditions [48–50].
Aiming at the extension of our oxidation studies to other sub-
strates and catalytic transformations and to a further development
of the catalytic applications of rhenium complexes, we embarked
in evaluating the catalytic potential of various Re complexes of our
interest, bearing the above types of ligands and with the metal
in a wide range of oxidation states, for the Baeyer–Villiger oxi-
dation of ketones. Hence, in the present work we report the BV
oxidation, by aqueous H2O2 in 1,2-dichloroethane, of cyclic and
linear ketones to the corresponding lactones and esters, catalysed
by a number of Re complexes bearing N-ligands, namely a scorpi-
onate [HC(pz)3, pz = pyrazolyl], pyrazole (Hpz), benzoyl-hydrazido
and -diazenido [␩2- or ␩1-N2C(O)Ph], acetonitrile and dinitrogen,
∗
Corresponding author. Tel.: +351 218419237; fax: +351 218464455.
E-mail address: pombeiro@ist.utl.pt (A.J.L. Pombeiro).
0926-860X/$ – see front matter © 2012 Elsevier B.V. All rights reserved.
http://dx.doi.org/10.1016/j.apcata.2012.07.007

28
E.C.B.A. Alegria et al. / Applied Catalysis A: General 443–444 (2012) 27–32
Scheme 2. BV oxidation of cyclic (reaction 1) and acyclic (reaction 2) ketones.
(Fluka) hydrogen peroxide (30%) (Fluka), Re2O7 10 (Aldrich),
cycloheptanone (Aldrich), diethyl ether (Riedel-de-Haën) and
dinitrogen gas (Air Liquid Portugal) were used as received from the
suppliers. The solvents were dried over appropriate drying agents
and degassed by standard methods.
2.2. Instrumentation
Gas chromatographic (GC) measurements were carried out
using a FISONS Instruments GC 8000 series gas chromatograph with
a FID detector and a capillary column (DB-WAX, column length:
30 m; internal diameter: 0.32 mm). The temperature of injection
was 240 ◦C. The initial temperature was maintained at 80 ◦C for
1 min, then raised 10 ◦C/min up to 140 ◦C (in the case of pinacolone
oxidation) or 180 ◦C, and held at this temperature for 1 min. Helium
was used as the carrier gas.
2.3. Typical procedures and product analysis
In typical conditions, Re catalysts were used as stock solu-
tions (for this, 10.0 mg of the compound 1–10 were dissolved in
2.00 mL of 1,2-dichloroethane, 1,2 DCE). The required amount of
this stock solution for the desired oxidant/catalyst molar ratio
(1000:1) was transferred to a second flask containing 3.00 mL
of 1,2-dichloroethane. 1.7 mmol of H2O2 as a 30% aqueous solu-
tion (102 ␮L) and 1.7 mmol of substrate were then added, and
the reaction solution was stirred for 6 h at the desired tempera-
ture (typically 70 ◦C) and normal pressure (dinitrogen atmosphere).
Then, 120 ␮L of cycloheptanone (internal standard) and 10.00 mL
of diethyl ether (to extract the substrate and the organic products
from the reaction mixture) were added. The obtained mixture was
stirred during 10 min and then a sample (1 ␮L) was taken from
the organic phase and analysed by GC using the internal standard
method. Blank tests indicate that no oxidation takes place in the
absence of the Re catalyst or the oxidant.
Scheme 1. Re catalyst precursors in this study for BV oxidation.
apart from the oxo-ligand (Scheme 1). Such ligands are expected
to promote coordinative unsaturation at the metal, in view of their
lability [N2, NCMe, Hpz, ␩3- or ␩2-HC(pz)3 towards lower dentic-
ity], and/or proton-transfer steps, in view of their basic character
[Hpz, HC(pz)3, N2C(O)Ph, oxo], features that are favourable [50–54]
to the occurrence of oxidation catalysis with H2O2.
2. Experimental
2.1. Materials and complexes
Complexes,
[ReCl2{N2C(O)Ph}(Hpz)2(PPh3)]
[ReCl2{␩2-N,O-C(O)Ph}(PPh3)2]
[55]
1,
[56]
2
(Hpz = pyrazole),
3. Results and discussion
[ReClF{N2C(O)Ph}(Hpz)2(PPh3)] [57] 3, [ReCl3{HC(pz)3}] [58]
4 (pz = pyrazolyl), [ReCl4{HC(pz)3}] [58] 5, [ReCl4(NCMe)2] [59] 6,
[ReCl3(NCMe)(PPh3)2] [60] 7 [ReCl(N2)(CO)2(PPh3)2] [61] 8 and
[ReOCl3(PPh3)2] [62,63] 9 (for their formulae see Scheme 1) were
prepared according to published procedures.
We found that the rhenium complexes 1–10 (Scheme 1) can
exhibit a remarkable catalytic activity (e.g. turnover numbers up
to ca. 2450 for complex 1, Table 2, entry 5) under relatively mild
conditions for the partial oxidation of different simple cyclic (2-
methylcyclohexanone, 2-methylcyclopentanone, cyclohexanone,
cyclopentanone, cyclobutanone) and acyclic (pinacolone) ketones
to the respective lactones and esters, in a single-pot process
(Scheme 2, Tables 1, 2, S1 and S2). The catalytic systems are based
on the above Re compounds, hydrogen peroxide (30% aqueous solu-
tion) as the oxidizing agent, and 1,2-dichloroethane (1,2 DCE) as the
solvent, under typical temperature of 70 ◦C and 6 h reaction time.
2-Methylcyclohexanone (Aldrich), 2-methylcyclopentanone
(Aldrich), cyclopentanone (Aldrich), cyclohexanone (Aldrich),
cyclobutanone (Aldrich), 3,3-dimethyl-2-butanone (pinacolone)
(Aldrich), delta-hexalactone (Aldrich), epsilon-caprolactone
(Aldrich), delta-valerolactone (Aldrich), beta-butyrolactone
(Aldrich), tert-butylacetate (Aldrich), 1,2-dichloroethane (Aldrich),
acetonitrile (Riedel-de-Haën), dichloromethane (Fluka), methanol

E.C.B.A. Alegria et al. / Applied Catalysis A: General 443–444 (2012) 27–32
29
Table 1
Baeyer–Villiger oxidation of several ketones catalysed by Re complexes 1–10 (selected data).a
Entry
Substrate
Catalyst
Yieldb
TONc
Conv.
Select.
Product
1
1
1
1
2
3
4
5
8
9
42
18
5
31
22
18
24
33
39
7
416
178
45
307
223
177
241
329
393
67
68
97
>99
77
79
74
65
81
67
60
61
18
4
39
28
24
37
41
58
11
2d
3e
4
5
6
7
8
9
10
10
11
12
13
14
15
16
17
18
19
1
2
3
4
5
6
7
9
10
27
22
10
21
19
37
13
26
10
270
223
102
209
192
365
131
263
104
42
28
40
37
37
77
58
79
26
67
80
26
57
52
47
22
33
40
20
21
22
23
24
25
26
1
2
3
4
5
17
16
7
18
5
172
158
74
180
53
57
24
47
44
46
33
55
30
69
16
41
12
43
5
9
10
14
3
138
27
27
2
23
231
63
37
28
29
3
9
11
31
109
306
58
36
41
85
30
31
32f
33
34
35
36
1
2
2
3
4
5
9
45
40
54
18
33
33
43
449
401
537
178
329
334
432
90
98
99
65
78
50
49
54
28
42
33
46
100
94
37
38
39
1
2
3
4
6
11
6
7
12
7
9
109
64
74
118
67
92
73
8
15
81
67
17
10
29
4
11
69
69
32
51
40
41
42g
43
9
10
2
18
a
Reaction conditions (unless stated otherwise): rhenium catalyst (1.7 ␮mol, used as a stock solution in 1,2-dichloroethane), 1.7 mmol of substrate, H2O2 (1.7 mmol, i.e.
1000:1 molar ratio of oxidant to Re catalyst), 1,2-dichloroethane (3.0 mL), 6 h, 70 ◦C, under dinitrogen; for more essays see Table S1 (supporting data). Percentage of yield,
TON determined by GC analysis.
b
Molar yield (%) based on the ketone substrate, i.e. moles of lactone (or ester) per 100 mol of ketone.
Turnover number (moles of product per mol of Re catalyst).
Oxidant/substrate = 2:1.
Oxidant/substrate = 4:1. No substrate was observed in the end.
c
d
e
f
30 h.
g
T = 50 ◦C.
All the tested Re complexes bearing N- or oxo-ligands exhibit
higher activities than the simple ReVII oxide Re2O7 10 (Table 1).
The ReV complexes, i.e. the oxo-chloro [ReOCl3(PPh3)2] 9 and
the benzoyl-hydrazido [ReCl2{␩2-N,O-C(O)Ph}(PPh3)2] 1, pro-
vide the most active catalysts under the conditions of this
study for the oxidation of 2-methylcyclohexanone, cyclopen-
tanone and cyclobutanone, whereas the ReIV acetonitrile complex
(typically 6 h) using a low concentration (<30%) of hydrogen per-
oxide solution and without the utilization of any additive.
In the presence of the Re complexes 1–9, the Baeyer–Villiger
oxidation of cyclic ketones to the respective lactones proceeds more
effectively than that of the acyclic pinacolone to tert-butylacetate,
in accord with the steric hindrance of the latter substrate.
As expected [41,45], for the simplest cyclic ketones the highest
reactivity was exhibited by the ring-strained cyclobutanone (con-
versions up to 99%, Table 1), that was easily converted into the
␥-butyrolactone, followed by cyclopentanone and cyclohexanone
(Table 1, entries 21, 22, 25, 27–29, 31, 33 and 36, for complexes
2, 3 and 9). The same tendency was reported in the case of the
[(CH3)ReO3]/H2O2 catalysed BV oxidation of cyclic ketones in ionic
liquids [21]. In this system, conversions (up to 98%) and yields (up
[ReCl4(NCMe)2]
6
and the ReIII tris(pirazolyl)methane com-
are the most active catalysts for
pound [ReCl3{HC(pz)3}]
4
2-methylcyclopentanone and cyclohexanone or pinacolone BV oxi-
dations, respectively.
The reactions are cleaner (water is formed as the only by-
product) than those promoted by peroxy acids [3], also giving rather
high yields (up to 45%, Table 1) in a relatively short reaction period

30
E.C.B.A. Alegria et al. / Applied Catalysis A: General 443–444 (2012) 27–32
Table 2
Effect of the oxidant-to-catalyst molar ratio in the BV oxidation of 2-
methylcyclohexanone and 2-methycyclopentanone catalysed by complex 2.a
Entry
Substrate
n(H2O2)/n(catalyst) × 103
Yield
TON
14
175
307
1167
2452
1
2
3
4
5
0.1
0.5
1
10
40
14
35
30
12
6
6
7
8
9
0.1
0.5
1
10
40
8
15
22
7
8
75
223
653
1730
10
4
a
Reaction conditions: rhenium catalyst (0.425–17 ␮mol, used as a stock solution
in 1,2-dichloroethane), 1.7 mmol of substrate, H2O2 (1.7 mmol), 1,2-dichloroethane
(3.0 mL), 6 h, 70 ◦C, under dinitrogen.
Fig. 1. Effect of H2O2 amount (H2O2/catalyst molar ratio) on TON in the BV
oxidation of 2-methylcyclohexanone and 2-methylcyclopentanone catalysed by
[ReCl2{N2C(O)Ph}(Hpz)2(PPh3)] 2. Reaction conditions are those of Table 2 entries.
to 98% and 70%) for cyclobutanone and cyclopentanone, respec-
tively [21], are higher those obtained in our Re catalytic systems,
but for the 6-membered ring ketones, such as cyclohexanone and
2-methylcyclohexanone, we have achieved comparable or even
better results.
Despite of high achieved conversions for some substrates (e.g. up
to 99% in case of cyclobutanone oxidation, Table 1), the selectivities
are moderate and depend on the type of the catalyst applied, e.g.
catalysts 2 and 9 are usually the most selective ones.
For the purpose of optimization of the reaction conditions, the
effects of various reaction parameters were examined. Hence, the
dependences of the yields of products and the TONs on the oxidant-
to-catalyst molar ratio, reaction time, temperature and type of
solvent were investigated for 1, 2 and 9 (Tables 2, S1 and S2).
3.1. Effect of the oxidant-to-catalyst molar ratio
We typically used one equivalent of H2O2 relatively to the
substrate. Hence, the yields and selectivities concerning H2O2 cor-
respond to those concerning the substrate. However, increasing
the amount of H2O2 relatively to the substrate (up to 4:1), for a
common amount of catalyst, leads to a decrease of product yield
(possibly due to overoxidation) concomitant with an increase of
the conversion and a lowering of selectivity (Table 1, entries 1–3).
In spite of structural similarities of the ReIII complexes 2 and 3,
the latter, i.e. the pyrazole-fluoro complex displays a considerably
lower activity for the oxidation of cyclic ketones than the former,
i.e. the analogous pyrazole-chloro compound (Table 1). Hence, the
presence of a fluoride ligand in 3, although particularly favourable
for other oxidation reactions [49,57], reduces its catalytic activity,
what eventually can be related to the stronger electron-donor char-
acter of the F− ligand to the metal in 3 compared with Cl− in 2 (the
former ligand exhibits lower values of the electrochemical Lever EL
and Pickett PL parameters, i.e. −0.42 V vs. NHE [64,65] and −1.3 V
[66], respectively for F−, and −0.24 V vs. NHE [64,65] and −1.19 V
[67] for Cl−) [68–70]. In accord, the fluoro-complex 3 is a weaker
Lewis acid and therefore would, by coordination, activate less the
carbonyl group of the ketones for a nucleophilic attack by H2O2.
In the most active species, [ReCl2{␩2-N,O-C(O)Ph}(PPh3)2]
The influence of the peroxide-to-catalyst molar ratio was
studied for the BV oxidation of 2-methylcyclohexanone and 2-
methylcyclopentanone in the presence of 2. Values of TON are given
in Table 2 for typical essays, and the dependences for both sub-
strates are shown in Fig. 1. The increase of the n(H2O2)/n(catalyst)
molar ratio, by decreasing the catalyst amount, results in higher
TONs, e.g. the TON for the oxidation of 2-methylcyclohexanone in
the presence of complex 2 enhances from 14 to 2452 upon chang-
ing that ratio from 100 to 40,000. However, the yield values decline
after reaching a maximum on account of the deceleration of reac-
tion rate upon lowering the catalyst concentration.
3.2. Effect of the reaction time and temperature
The effect of the reaction time was studied for complexes 1,
2 and 9 using 2-methylcyclohexanone, 2-methylcyclopentanone,
cyclohexanone, cyclopentanone and the acyclic pinacolone
(Tables S1 and S2). It was observed, for the cyclic substrates, that
the yield and TON sharply increase during the first 6 h, achieving
a maximum before droping slightly, after ca. 10 h (Fig. 2), conceiv-
ably due to product overoxidation or hydrolysis [75,76]. For the
cyclic 4-membered cyclobutanone, and in the presence of complex
2, there is an increase of both yield and conversion upon prolong-
ing the reaction time to 30 h (54% yield, 99% conversion). It is also
noted for this substrate that the good yield of ca. 20% is achieved
after 2 h, while for the larger ring ketones no significant reaction
is observed in the first 2–3 h, what is consistent with the known
higher reactivity of 4-membered ring ketones.
1
and [ReOCl3(PPh3)2] 9 the metal is in the 5+ oxidation
state. However, they are conceivably oxidized by H2O2 to
peroxo-complexes with the metal in a higher oxidation state.
In fact, the formation of the ␩2-peroxo complex [ReCl2(␩2-
O2){NNC(O)OMe}(PPh3)2] is reported [71] to occur in the reaction
of the ␩1-diazenido compound [ReCl2{NNC(O)OMe}(PPh3)2(dmf)]
(dmf = N,N-dimethylformamide) with O2. Moreover, oxo- and
peroxo-complexes of ReVII
,
such as [(CH3)ReO3(bipy)] [72],
The temperature is also an important factor and the use of 70 ◦C
appears to be the most adequate for the cyclic ketone oxidation.
In fact, e.g. we failed the attempt of performing the oxidation of
2-methylcyclohexanone in the presence of 9 at room temperature
(entry 10, Table S1) whereas the use of 50 ◦C resulted in the yield
reduction to half (from 39% at 70 ◦C to 17% at 50 ◦C, entries 12 and
11, respectively, Table S1). The heating effect above 70 ◦C is less pro-
nounced, corresponding to a slight yield decrease (e.g. from 39% at
70 ◦C to 35% at 100 ◦C, entries 12 and 13, respectively, Table S1)
ReO3[{(CH3)2N-C2H4}2NCH3,ReO4] [72], [(CH3)ReO(O2)2] [73,74]
and [(CH3)ReO(O2)2(H2O)] [72] are known catalysts for various
oxidation reactions of olefins and aromatics.
Similarly to an earlier proposed mechanistic pathway for MTO
catalysed BV oxidation [18,20,21], the reaction can proceed via the
formation of active peroxo-Re species derived from the starting
complexes in the presence of H2O2, which would undergo nucle-
ophic attack to the polar carbonyl group of the ketone substrate.

E.C.B.A. Alegria et al. / Applied Catalysis A: General 443–444 (2012) 27–32
31
The use of 1,2-dichloroethane as solvent leads to the highest
activity for all ketones, but compounds 1 and 9 (the most active
ones) can even operate in water as the only solvent.
The replacement of 1,2 DCE solvent by a mixed organic/water
medium deserves to be further explored by application of hydro-
soluble compounds, since it is particularly important for the
development of a green BV system.
500
400
300
200
100
0
Acknowledgements
This work has been partially supported by the Fundac¸ ão
para
a Ciência e a Tecnologia (FCT), Portugal, and projects
PTDC/QUI-QUI/102150/2008, PTDC-EQU-EQU-122025-2010 and
PEst-OE/QUI/UI0100/2011.
0
6
12
18
24
30
Time (h)
Appendix A. Supplementary data
Fig. 2. Effect of the reaction time on TON in the BV oxidation of 2-
methylcyclohexanone (—), 2-methylcyclopentanone (- - -) and cyclopentanone
(– –), catalysed by [ReOCl3(PPh3)2] 9 and [ReCl2{N2C(O)Ph}(Hpz)2(PPh3)] 2 (ꢀ).
Reaction conditions are those of Table S2 entries.
Supplementary data associated with this article can be
found, in the online version, at http://dx.doi.org/10.1016/j.apcata.
2012.07.007.
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are observed. On the other hand, its oxidation at room temper-
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