Oxoiron(iv)-mediated Baeyer-Villiger oxidation of cyclohexanones generated by dioxygen with co-oxidation of aldehydes

Article abstract of DOI:10.1039/c5nj02093j

In this communication we describe detailed mechanistic studies on the [Fe^II(CH₃CN)(N4Py)(ClO₄)]ClO₄-catalyzed (N4Py = N,N-bis(2-pyridylmethyl)-N-bis(2-pyridyl)methyl-amine) Baeyer-Villiger oxidation of cyclohexanones by dioxygen with cooxidation of various aldehydes, including the kinetics of the formation and the reactivity of the trapped and spectroscopically characterized oxoiron(iv) intermediate.

Full text of DOI:10.1039/c5nj02093j

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COMMUNICATION
Oxoiron(IV)-mediated Baeyer-Villiger oxidation of
cyclohexanones generated by dioxygen with co-
oxidation of aldehydes.
Cite this: DOI: 10.1039/x0xx00000x
a
a
a
Dóra LakkꢀBogáth, Gábor Speier, and József Kaizer*
Received 00th January 2012,
Accepted 00th January 2012
DOI: 10.1039/x0xx00000x
www.rsc.org/
7
8
In this communication we describe a detailed mechanistic epoxidize alkenes, BaeyerꢀVilliger oxidation of ketones and
II
9
studies on the [Fe (CH CN)(N4Py)(ClO )]ClO -catalyzed oxidation of alcohols and of some hydrocarbons. These
3
4
4
(
N4Py
=
N
,
N-bis(2-pyridylmethyl)-N-bis(2-pyridyl)methyl- processes proceed via a free radical chain mechanism, and
amine) Baeyer-Villiger oxidation of cyclohexanones by without transition metal catalyst are often of low efficiency and
dioxygen with cooxidation of various aldehydes, including the selectivity. Metal complexes depending on their reduction
1
0
kinetics on the formation and the reactivity of the trapped power can accelerate or inhibit the above processes. There are
and spectroscopically characterized oxoiron(IV) some reports on the aerobic BaeyerꢀVilliger oxidation of
intermediate. ketones to esters or ꢀlactons catalysed by iron catalysts with
ε
aldehyde as reductant, but until now, no direct evidence for the
BaeyerꢀVilliger oxidation is one of the most important reactions
in organic chemistry because the lactones or esters are
important industrial intermediates in the production of
involvement of a highꢀvalent oxoiron species in the catalytic
1
1
process.
We have found that complex [Fe (CH CN)(N4Py)](ClO ) (
II
1
4 2
)
3
1
polymers, pharmaceuticals and herbicids. The most common
(
N4Py
amine) highly selectively and efficiently catalyse the Baeyerꢀ
Villiger (B.ꢀV.) oxidation of cyclohexanone derivatives to
=
N,Nꢀbis(2ꢀpyridylmethyl)ꢀNꢀbis(2ꢀpyridyl)methylꢀ
2
oxidants used for this reaction are peroxycarboxylic acids.
Over the past four decades transition metal complexes of a
variety of ligand systems like porphyrins, Schiff bases,
isoindolines, polypyridines etc. have been reported as active
εꢀ
caprolactones with molecular oxygen in the presence of various
aldehydes such as isobutyraldehyde and benzaldehyde (Fig. 1,
and Table S1, ESI†).
3
catalysts for the catalytic oxygen transfer reactions, and their
catalytic cycles often involve oxoiron(IV) intermediates as
4
oxidants. Within the past decade more than 60 oxoiron(IV)
4
f
5
complexes, including S=1 and S=2 ground states, have been
produced by reactions of precursor complexes with externally
prepared oxidizing compounds (in which oxygen is already
partly reduced) such as iodosylbenzene (PhIO),
chloroperoxybenzoic acid ( ꢀCPBA), and hydroperoxides
H O and RO H). However, only few examples exist for their
mꢀ
m
(
2
2
2
6
generation by activating dioxygen. Since the molecular oxygen
is a cheap, environmentally clean and readily oxidant, the use
of O as the primary oxidant is of fundamental importance.
2
Oxoiron(IV) species can be prepared in solvent mixtures of
6
a
6b
CH CN/alcohols or CH CN/ethers , furthermore in the
3
3
presence of an electron donor (dihydroxynicotinamide adenine
6
c
ꢀ
Fig. 1. Yields without and with catalyst for the 1-catalysed B.-V. oxidation of
cyclohexanones with benzaldehyde in MeCN at 60°C under oxygen atmosphere
dinucleotide (NADH) analogue or BPh ) and proton source
4
(
HClO ). Dioxygen activation using an aldehyde as a reductant
-5
-2
4
0 0
for 15 hours. [1] = 1.00 × 10 M, [cyclohexanone derivatives] = 1.00 × 10 M,
-
1
is a very simple and practical oxidative system that is able to [benzaldehyde] = 1.50 × 10 M.
0
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The reactions were carried out in MeCN at 298, 323, 333 and In order to prove that cyclohexanone has been oxygenated, we
1
8
16
3
43 K. The optimum conditions used for the cyclohexane carried out the reaction with a mixture of O and O 60 :
2 2
oxidation were: catalyst, substrate and aldehyde in a molar ratio 40%). Analysis of the GCꢀMS spectra of the products using
1
8
of 1 : 1000 : 15000, respectively at 333 K in acetonitrile
Oꢀlabeled compounds gave an answer to this question. After
solution under dioxygen. The consumption of the starting 24 h of stirring the reactants at 298 K, GCꢀMS analysis of the
1
8
16
ketones and formation of oxidized products were monitored by reaction mixture shows the presence of Oꢀ and Oꢀ
εꢀlactone
GC or GCꢀMS in the presence of inert internal standard in the appropriate ratio: m/z 116(M+, 7.7), 114(M+, 4.8). The
1
8
(
naphthalene) (Fig. S1ꢀS6, ESI†). The oxidation in the presence
Oꢀεꢀlactone derivative gave a molecular ion at m/z 116 (114 +
1
8
18
of isobutyraldehyde occurs with moderate conversion (6.2% 2), showing that one of the O atom of O are incorporated
2
(
5h) and 16.4% (15h) at 333 K). The highest conversions are into the cyclohexanone ring. The relative abundance of m/z 116
1
8
attained with benzaldehyde, 34% and 73% after 5 and 15 h, to that at m/z 114 (~62%) parallels the O enrichment used in
respectivel. In case of the Fe/(
oxidation occurs with much higher yields (59.1% (5h) and atom of O can be find in the forming benzoic acid: m/z
2
1
8
m
ꢀCPBA)ꢀcontaining system the the experiment (Fig. S2, and Table S1, ESI†). The second
O
1
8
2
1
9
00% (15h) at 333 K), the yields without catalyst are 16.8 and 124(M+, 64.6), 122(M+, 54.7). The smaller value of the
2.3% after 5 and 15h, respectively (Table S1, ESI†). It means relative abundance (54% compared to 62%) can be explained
that the conversion can be increased in both cases by the use of by water exchanged reaction. Spectral investigations on the
, compared to the classical B.ꢀV. reaction (Fig. S7 and S8, catalytic system above have confirmed the formation of
ESI†). Complex may have a dual effect, it can serve as an oxoiron(IV) species (Fig. 3, and Fig. S9, ESI†). The steadyꢀ
initiator and catalyst too. The relative reactivity of substrates is state concentration of [Fe =O(N4Py)]
in the following order: cyclohexanone (73%) 2Meꢀ be ~20% by comparing the intensity of the absorption band at
1
1
IV
2+
(2) was determined to
>
t
12a,b
cyclohexanone (62.5%) > 4Meꢀcyclohexanone (53%) > 4 Buꢀ 693 nm with the known
cyclohexanone (18%) > 3Meꢀcyclohexanone (15.5%, 3ꢀ and 5ꢀ
ε value of 2.
1
1d
methylꢀ
substituted benzaldehydes relative to that of benzaldehyde were
determined in the presence or in the absence of . Hammett
treatments of relative reactivities (krel = (log(Xf/Xi)/(log(Yf/Yi),
where Xi and Xf are the initial and final concentrations of para
εꢀcaprolactone (1:1) ). The reactivities of paraꢀ
1
ꢀ
substituted benzaldehydes, and Yi and Yf are the initial and final
concentration of benzaldehyde) of various substituents against
σ
0
gave
.998,
and Table S1ꢀ2, ESI†). The
ρ
values of ꢀ0.71 (
= 5) for , and no metal complex, respectively (Fig. 2,
values obtained with and no
r = 0.997, n = 5), and ꢀ0.68 (r =
n
1
ρ
1
catalyst were the same within experimental error, suggesting
that the formation of the PBA is the key step in both cases. It is
pertinent to mention that the 4ꢀhydroxyꢀ and 4ꢀ
methylaminobenzaldehyde are able to block both reactions,
probably via a hydrogen abstraction and electron transfer
processes.
IV
2+
Fig. 3. Visible spectral change for the formation of [Fe =O(N4Py)]
at 693 nm) derived from the B.-V. oxidation of cyclohexanone catalysed by
MeCN at 298 K. Inset: Time dependent formation of under the conditions
(
2
) species
(
1
in
2
-3
-2
described above. [
0 0
1] = 2.00 × 10 M, [benzaldehyde] = 8.00 × 10 M,
-
1
[
cyclohexanone] = 1.00 × 10 M.
0
To get insight into the formation of
2 generated by dioxygen
plus benzaldehydes, which serve as oxygen acceptors, detailed
kinetic studies were carried out at 293 K, in MeCN, without
cyclohexanones. The kinetic profile of the reaction is given in
Fig. 4. Addition of benzaldehyde at room temperature to
acetonitrile solution of the complex
changes in colour from red to green. Similar spectral behaviour
can be observed during the reaction of with ꢀCPBA (Fig.
S10, ESI†), where the formation of can be drawn by oxene
1 under air results in rapid
1
m
2
transfer between the precursor complex and the oxidant.
Influence of the structure and electronic properties of the
aldehyde was also studied on the formation reaction of
2.
Fig. 2. Hammett plot for the para-substituted benzaldehydes in the B.-V.
reactions without
reactions without
Isobutyraldehyde gave ~10% yield of (Fig. S11, ESI†). Upon
2
1
(□), and with
1 (■) in MeCN at 60 °C. Inset: yields in the
-
5
using paraꢀbenzaldehydes with electron donating groups the
yields were increased remarkably (Fig. S11, ESI†). Rate
constants (kobs) determined from these plots showed a good
1
(white), and with
1
(black).
0
[1] = 1.00 × 10 M,
-2
-1
= 1.50 × 10 M under O .
[
cyclohexanone]
0
= 1.00 × 10 M, [aldehyde]
0
2
2
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DOI: 10.1039/C5NJ020 J
linear correlation (
r = 0.99, n = 4) with the Hammett substituent enthalpies and the large negative activation entropies are typical
parameters ( = ꢀ1.61, Fig. S12, ESI†).
σ
of associative processes.
Fig. 5. . Visible spectral change for the decay of
2 (at 693 nm) derived from the
Fig. 4. Visible spectral change for the formation of
1
2 (at 693 nm, yield = ~45 %)
derived from the reaction of and benzaldehyde in MeCN under air at 298 K.
IV
reaction of
concentration (at 693 nm) derived from the reaction of Fe =O and cyclohexanone
), 2-methyl- ( ), 4-methyl-( ), 4-tert-butyl-( ), 3-methyl-cyclohexanone ( ) and
without substrate ( , t1/2 = 60 h) in MeCN at 298 K. [
2 with 2-methyl-cyclohexanone. Inset: The change of Fe =O
IV
-3
-2
0 0
[1] = 1.00 × 10 M, [benzaldehyde] = 8.00 × 10 M.
(
a
b
c
d
e
-
3
f
2]
0
= 2.00 × 10 M,
-1
Time analysis of these reactions showed an induction period [cyclohexanone] = 2.50 × 10 M.
0
(
lengths are dependent on the amount of dioxygen,
benzaldehyde, and complex concentration), and after that a
1
first order rate law was observed. The partial rate orders of the
reaction were investigated by plotting the initial concentrations
against the reaction rates, which gave straight lines, suggesting
an overall third order rate law of d[
with a thirdꢀorder rate constant k3 = 34±2 M s in MeCN at
98 K (Fig S13ꢀS18, Table S2 and S3, ESI†). The dependence
of k3 on temperature (T = 293 to 308 K) gave the activation
2]/dt = k3 [aldehyde][1][O2]
ꢀ
2 ꢀ1
2
≠
ꢀ1
≠
ꢀ1 ꢀ1
parameters
ꢀH = 44 kJ mol and ꢀS = ꢀ109 J mol K (Table
S3, Fig. S19, ESI†).
Finally, to get direct evidence for the involvement of the highꢀ
valent oxoiron(IV) species in the BaeyerꢀVilliger oxidation, the
reaction of
2 with various cyclohexanone derivatives was
Fig. 6. Dependence on the reaction rate values
concentrations for the decay of
), 2ꢀmethylꢀ (
) in MeCN at 298 K. [
(
k
obs
)
on the substrate
investigated. The oxoiron(IV) was generated by reaction of
with PhIO, and its reaction with cyclohexanones was cyclohexanone (
measured as a function of the concentration of added
cyclohexanone derivatives (Fig. 5 and 6). It was found that the
electrophilic oxoiron(IV) is able to oxidize the cyclohexanone
1
2
b
in the presence of cyclohexanone derivatives:
), 4ꢀmethylꢀ( ), 4ꢀtercꢀbuthylꢀ( ), and 3ꢀmethylꢀ
= 2.00 × 10 M.
1
2
a
e
c
d
ꢀ
3
cyclohexanone (
2]
0
In summary, a mechanism for the iron(II)ꢀcatalysed Baeyerꢀ
Villiger oxidation of cyclohexanones by O and aldehyde has
derivatives to the corresponding
εꢀcaprolactones, excluding it’s
2
been proposed (Scheme 1). It is well known that autoxidation
rates of aldehydes are very high with long propagation chains
nucleophile attack on the carbonyl group. Since no reaction has
been observed for acetophenone and benzophenone, this
indicates clearly that the conjugation of the carbonyl group
even at room temperature. In the absence of
the ꢀcaprolactones can be drawn from the reaction between the
forming peroxybenzoic acid and cyclohexanone via Criegee
adduct (Route in Scheme 1). We have found that complex
exhibited high catalytic performance for the oxidation of
cyclohexanone to ꢀcaprolactone comparing with that in blank
1 the formation of
1
1a
ε
decreases the reactivity of the ketone. The traces could be
fitted with a firstꢀorder kinetic law, with respect to , and the
2
A
1
calculated kobs values for different concentrations of the
appropriate substrate are reported in Table S4 (ESI†). The
relative reactivity of substrates is in the following order:
ε
experiment, indicating that iron is crucial for the aerobic
oxidation of cyclohexanones. It has a dual effects in the
catalytic cycle, firstly to aid in the generation of the
peroxycarboxylic acid (side effect) which is nessesary for
transforming the iron(II) complex into the active highꢀvalent
oxoiron(IV) species, and secondly to mediate the oxygen
transfer into the substrate via an electron transfer mechanism
cyclohexanone > 2Meꢀcyclohexanone > 4Meꢀcyclohexanone >
t
4
Buꢀcyclohexanone > 3Meꢀcyclohexanone, which is the same
trend, that was found for the catalytic system (Fig. 5, and Table
S4, ESI†). Kinetic experiments revealed 1stꢀorder dependence
on both the oxoiron(IV) and the substrate concentration with
k
2
ꢀ
1
ꢀ1
≠
ꢀ1
≠
ꢀ1
=
1.48 ± 0.04 M s , ꢀH = 41 kJ mol and ꢀS = ꢀ111 J mol
ꢀ
K at 298 K (Table S4, Fig. S20ꢀ21, ESI†). The low activation
(
main effect) between the ketone and the electrophilic
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oxoiron(IV) or iron(III)ꢀoxyl radical intermediate (Route
B
in
Scheme 1). Since the formation of the peroxybenzoic acid,
that is slightly accelerated by , and the formation of are fast,
Rios, E. Salazar, H. F. Olivo, Green Chem., 2007, , 459; (d) M. Y.
9
1
0b
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Michaud, J. K. Kochi, J. Am. Chem. Soc., 1986, 108, 2309; (c) T.
Katsuki, Coord. Chem. Rev., 1995, 140, 189; (d) R. H. Fish, R. H.
1
2
3
the rateꢀdetermining step is the reaction of the forming
oxoiron(IV) intermediate with the the appropriate carbonyl
compounds.
Fong, J. B. Vincent, G. Christou, J. Chem. Soc., Chem. Commun.
,
RCHO
1
988, 1504; (e) M. J. Gunter, P. Turner, Coord. Chem. Rev., 1991,
08, 115; (f) R. H. Holm, Chem. Rev., 1987, 87, 1401; (g) K. A.
FeIII O O
II
Fe + O
2
1
FeIII O OH
Jorgensen, Chem. Rev., 1989, 89, 431.
O
-OH
4
5
(a) L. Que, Jr., Acc. Chem. Res., 2007, 40, 493; (b) A. R. McDonald
and L. Que, Jr., Coord. Chem. Rev., 2013, 257, 414; (c) K. Ray, F. F.
Pfaff, B. Wang, W. Nam, J. Am. Chem. Soc., 2014, 136, 13942; (d)
M. Costas, M. P. Mehn, M. P. Jensen and L. Que, Jr., Chem. Rev.,
2004, 104, 939.; (e) E. Y. Tshuva and S. J. Lippard, Chem. Rev.,
2004, 104, 987; (f) W. Nam, Acc. Chem. Res., 2007, 40, 522.
(a) J. England, M. Martinho, E. R. Farquhar, J. R. Frisch, E. L.
Bominaar, E. Münck, L. Que, Jr., Angew. Chem. Int. Ed., 2009, 48,
R C
O2
O
O
Fe2+
RCO H
IV
FeIII_
O
R C O O
R C O OH
Fe =O
-
2
O
O
RCHO
3
622; (b) J. England, Y. Guo, K. M. Van Heuvelen, M. A.
A
B
R
R
Cranswick, G. T. Rohde, E. L. Bominaar, E. Münck, L. Que, Jr., J.
Am. Chem. Soc., 2011, 133, 11880; (c) D. C. Lacy, R. Gupta, K. L.
Stone, J. Greaves, J. W. Ziller, M. P. Hendrich, A. S. Borovik, J. Am.
Chem. Soc., 2010, 132, 12188.
FeIII
O
O
O
O
6
7
(a) S. O. Kim, C. V. Sastri, M. S. Seo, J. Kim, W. Nam, J. Am. Chem.
Soc., 2005, 127, 4178; (b) A. Thibon, J. England, M. Martinho, V. G.
Young, Jr., J. R. Frisch, R. Guillot, J.ꢀJ. Girerd, E. Munck, L. Que,
Jr., F. Banse, Angew. Chem., Int. Ed., 2008, 47, 7064; (c) S. Hong,
R
-
Fe2+
Scheme 1 Plausible mechanisms of B.-V. oxidation of cyclohexanones to
caprolactones by O with aldehydes in the presence of
R
ε-
2
1.
Y.ꢀM. Lee, W. Shin, S. Fukuzumi, W. Nam, J. Am. Chem. Soc.
,
2
009, 131, 13910.
In conclusion, a novel catalytic method for the BaeyerꢀVilliger
oxidation of cyclohexanone derivatives has been investigated
with nonꢀheme iron(II) complex as catalyst, aldehydes as
oxygen acceptors and dioxygen as oxidant. The experimental
results clearly indicated the formation of a highꢀvalent metalꢀ
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IV
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Ishii, J. Org. Chem., 1994, 59, 2915.
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Bravo, F. Fontana, F. Minisci, A. Serri, Chem. Commun., 1996, 1843;
Acknowledgments
The Hungarian National Research Fund (OTKA K108489),
(c) C. Einhorn, J. Einhorn, C. Marcadal, J.ꢀL. Pierre, Chem.
TÁMOPꢀ4.2.2.Aꢀ11/1/KONVꢀ2012ꢀ0071,
5/1/KONVꢀ2015ꢀ0004 and COST Action CM1003
Biological oxidation reactions –mechanism and design of new
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Commun., 1997, 447.
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“
1
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a
Department of Chemistry, University of Pannonia, Hꢀ8200 Veszprém,
Hungary. Fax: +36 88 624 469; Tel: +36 88 624 720; Eꢀmail:
kaizer@almos.vein.hu.
†
Electronic Supplementary Information (ESI) available: Electronic
Supplementary Information (ESI) available: Experiemental details of
catalyses, kinetic, and spectroscopic data. See DOI: 10.1039/c000000x/
1
2 (a) J. Kaizer, E. J. Klinker, N. Y. Oh, J.ꢀU. Rohde, W. J. Song, A.
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DOI: 10.1039/C5NJ02093J
Oxoiron(IV)-mediated Baeyer-Villiger oxidation of cyclohexanones generated by
dioxygen with co-oxidation of aldehydes.
Dóra Lakk-Bogáth, Gábor Speier, and József Kaizer*
Mechanistic studies on the Fe(II)-catalyzed Baeyer-Villiger oxidation is described, including
the formation and the reactivity of the trapped oxoiron(IV) intermediate.
O
O
O
R
R
O
II
Fe /O
FeII
2
IV
FeII
RCHO
R C O OH
Fe =O
-
RCO H
2
O
RCO H
R C O OH
2