A highly efficient non-heme manganese complex in oxygenation reactions

Article abstract of DOI:10.1039/b708976g

A non-heme manganese(ii) complex shows a high catalytic activity in the epoxidation of olefins by iodosyl benzene and in the oxidation of olefins, alcohols and alkanes by peracetic acid; a mechanism involving metal-based oxidants is proposed for the oxida

Full text of DOI:10.1039/b708976g

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A highly efficient non-heme manganese complex in oxygenation
reactions{
Kasi Nehru, Soo Jeong Kim, In Young Kim, Mi Sook Seo, Youngmee Kim, Sung-Jin Kim, Jinheung Kim and
Wonwoo Nam*
Received (in Cambridge, UK) 14th June 2007, Accepted 3rd September 2007
First published as an Advance Article on the web 20th September 2007
DOI: 10.1039/b708976g
CH3CN. Addition of diethyl ether into the reaction solution
afforded colorless crystals suitable for crystallographic analysis.
The crystal structure of 1 reveals a cis-a coordination mode
around the distorted octahedral metal center (Fig. 1), similar to the
structure of the iron analogue.9 The electrospray ionization mass
spectrum (ESI MS) of 1 exhibits a prominent ion peak at a mass-
to-charge ratio (m/z) of 546.2, whose mass and isotope distribution
pattern corresponds to [Mn(II)(BQEN)(CF3SO3)]+ (calculated m/z
of 546.1) (ESI,{ Fig. S1). The X-band EPR spectrum of 1 exhibits
an intense signal at g = 2.0 (ESI,{ Fig. S2), indicating a high-spin
(S = 5/2) MnII species.8c The high-spin state of 1 was further
confirmed by determining the magnetic moment of 5.4 mB at 25 uC
A non-heme manganese(II) complex shows a high catalytic
activity in the epoxidation of olefins by iodosyl benzene and in
the oxidation of olefins, alcohols and alkanes by peracetic acid;
a mechanism involving metal-based oxidants is proposed for
the oxidation reactions.
Metal-catalyzed oxygenations of organic substrates are of
significant importance in both synthetic chemistry and industrial
processes.1 Since metalloenzymes catalyze the oxygenation reac-
tions with high regio- and stereoselectivity under mild conditions,
biomimetic oxygenation reactions using their model compounds
have attracted much attention in the communities of bioinorganic
and oxidation chemistry.2 For example, it has been demonstrated
that synthetic iron complexes of heme and non-heme ligands are
capable of mimicking the chemistry of cytochrome P450 (CYP
450) and non-heme iron enzymes, respectively, and that the
oxygenation reactions by the model compounds proceed via a
mechanism involving metal-based oxidants (e.g., high-valent iron-
oxo intermediates).3 Similarly, manganese porphyrins have been
extensively investigated as chemical models of CYP 450 in various
oxygenation reactions.4 Manganese complexes bearing non-heme
ligands, such as salen- and tacn-derived ligands,5 have shown
promise as versatile catalysts in olefin epoxidation and alkane
hydroxylation.6,7 Very recently, Stack and co-workers8 reported a
highly efficient epoxidation reaction using peracetic acid as the
terminal oxidant, in which terminal olefins are epoxidized to the
corresponding epoxides in the presence of non-heme Mn(II)
catalysts (e.g., Mn(BPMEN)(CF3SO3)2 and Mn(BPMCN)-
(CF3SO3)2).5 We now report a mononuclear non-heme manganese
complex, Mn(BQEN)(CF3SO3)2 (1),5 that shows a high catalytic
activity and stereo- and regioselectivity in the oxidation of olefins,
alcohols and alkanes under mild conditions.
1
with the H NMR method of Evans.10
The catalytic activity of 1 was first investigated in olefin
epoxidation by iodosyl benzene (PhIO) and then compared to that
of Mn(BPMEN)(CF3SO3)2 (2) and Mn(BPMCN)(CF3SO3)2 (3)
under identical conditions (Table 1). In the epoxidation of
cyclohexene, cyclohexene oxide was the major product with the
formation of small amounts of allylic oxidation products such as
cyclohexenol and cyclohexenone (entry 1). The epoxidation of cis-
and trans-stilbene produced cis- and trans-stilbene oxide, respec-
tively, with the formation of trace amounts of isomerized stilbene
oxide products (entries 3 and 4), demonstrating that the olefin
epoxidation by 1 and PhIO is highly stereospecific. It is worth
noting that the formation of isomerized products was often
observed in non-heme manganese complex-catalyzed olefin
epoxidations.6,11 In the competitive epoxidation of cis- and
The manganese complex 1 was synthesized by reacting
equimolar amounts of Mn(CF3SO3)2 and the ligand BQEN9 in
Fig. 1 Molecular structure of Mn(BQEN)(CF3SO3)2 (1) showing 30%
probability thermal ellipsoids.{1 Hydrogen atoms have been omitted for
˚
clarity. The average Mn–N and Mn–O distances are 2.27 and 2.16 A,
Department of Chemistry, Division of Nano Sciences, and Center for
Biomimetic Systems, Ewha Womans University, Seoul 120-750, Korea.
E-mail: wwnam@ewha.ac.kr; Fax: 82 2 3277 4441; Tel: 82 2 3277 2392
{ Electronic supplementary information (ESI) available: Detailed experi-
mental procedures, Tables S1 and S2, and Fig. S1 and S2. See DOI:
10.1039/b708976g
respectively. The absolute configuration of the amine nitrogen atoms in
the stereoisomer is N2-R and N3-R. See ESI{ for the crystal data and
structure refinement of 1 (Table S1) and selected bond distances and
angles (Table S2). CCDC 650716. For crystallographic data in CIF or
other electronic format see DOI: 10.1039/b708976g
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Chem. Commun., 2007, 4623–4625 | 4623

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which cis-stilbene epoxidation is favored over trans-stilbene
epoxidationduetothesterichindrancecausedbythephenylgroups
of trans-stilbene.12 In contrast to the excellent catalytic activity of 1,
low epoxide yields and reduced selectivities were observed in olefin
epoxidations by 2and3(Table 1). To the best ofour knowledge, 1is
the most effective non-heme manganese catalyst that affords high
epoxide yields, small amounts of allylic oxidation products, and a
high stereospecificity in the epoxidation of olefins by PhIO.
However, only trace amounts of oxygenated products were
produced in the hydroxylation of cyclohexane by 1 and PhIO
(e.g., ,4% based on the PhIO added), indicating that 1 is not an
effective catalyst in alkane hydroxylation by PhIO (vide infra).
We then studied the oxidation of olefins, alcohols and alkanes
using peracetic acid as the terminal oxidant in the presence of
Mn(II) catalysts, inspired by the pioneering work of Stack and co-
workers who demonstrated that 3 is an excellent catalyst in the
epoxidation of terminal olefins by peracetic acid.8 The results in
Table 2 show that the product yields are very high and that the
catalytic activity of 1 is comparable to that of 3. In the epoxidation
of olefins, epoxides were the sole products detected (entries 1–4),
and excellent product yields were obtained even in the epoxidation
of terminal olefins.8 Alcohols were converted to the corresponding
ketone or aldehyde products (entries 5–7), and a kinetic isotope
effect (KIE) of 2.2 was determined in an intermolecular competitive
oxidation of benzyl alcohol and benzyl-d7 alcohol (entry 12).13
Table 1 Epoxidation of olefins by PhIO catalyzed by non-heme
manganese(II) complexesa
Yieldb,c (%)
Entry Substrate
Product(s)
1
2
3
1
Cyclohexene Cyclohexene oxide 82 (4) 45 (3) 45 (3)
8 (2)
8 (2)
Cyclohexenol
Cyclohexenone
3 (1) 10 (2)
3 (1) 11 (2)
2
3
Cyclooctene Cyclooctene oxide 95 (4) 35 (2) 26 (2)
cis-Stilbene cis-Stilbene oxide 33 (2) 16 (2) 12 (2)
trans-Stilbene oxide 3 (1) 7 (1) 13 (2)
Benzaldehyde 23 (3) 24 (3) 20 (3)
trans-Stilbened cis-Stilbene oxide
trans-Stilbene oxide 35 (2) 17 (2) 24 (2)
4
0
0
0
Benzaldehyde
23 (3) 20 (3) 27 (3)
5
cis-Stilbenee + cis-Stilbene oxide + 16 (2) + 4 (1) + 8 (1) +
trans-stilbene trans-stilbene oxide 40 (3) 24 (2) 22 (2)
a
Reaction conditions: Solid PhIO (100 equiv. per Mn) was added to a
reaction solution containing manganese catalyst (1 mM) and substrate
(0.5 M) in CH3CN (2 mL) at 25 uC. After 30 min stirring, the reaction
solution was filtered and directly analyzed by GC, GC-MS, or HPLC.
b
c
Product yields are based on the amount of PhIO added. The
numbers in parentheses represent standard deviation of three
experiments. Reaction was carried out with trans-stilbene (0.25 M)
a
d
and PhIO (50 equiv. per Mn) in acetone due to the low solubility of
the substrate. Equimolar amounts (0.25 M each) of cis- and trans-
stilbene and PhIO (50 equiv. per Mn) were used in acetone.
e
trans-stilbene,theratioofcis-totrans-stilbeneoxidewasdetermined
to be 0.4 (entry 5), indicating the preference of trans-olefin
epoxidation to cis-olefin epoxidation by the intermediate generated
in the reaction of 1and PhIO. This result is markedly different from
those frequently observed in competitive olefin epoxidations, in
ð1Þ
Table 2 Oxidation of olefins, alcohols, and alkanes by peracetic acid catalyzed by non-heme manganese complexesa
Yieldb,c (%)
Entry
Substrate
Product(s)
1
3
A. Olefin epoxidation
1
Cyclohexene
Cyclooctene
1-Hexene
Cyclohexene oxide
Cyclooctene oxide
1,2-Epoxyhexane
1,2-Epoxyoctane
95 (3)
95 (3)
96 (3)
95 (3)
95 (3)
95 (3)
98 (2)
98 (2)
2
3
4
1-Octene
B. Alcohol oxidation
5
6
7
Cyclohexanol
Cyclooctanol
Benzyl alcohol
Cyclohexanone
Cyclooctanone
Benzaldehyde
80 (5)
78 (5)
70 (5)
96 (3)
96 (3)
98 (2)
C. Alkane hydroxylation
8
9
Cyclohexane
cis-1,2-Dimethylcyclohexane
Cyclohexanol & cyclohexanone
cis-1,2-Dimethylcyclohexanol
2,3- and 3,4-Dimethylcyclohexanol and -one
trans-1,2-Dimethylcyclohexanol
2,3- and 3,4-Dimethylcyclohexanol and -one
Adamantan-1-ol
4 (1) & 41 (3)
58 (4)
20 (3)
18 (2)
30 (3)
3 (1) & 32 (3)
55 (4)
20 (3)
13 (2)
30 (3)
10
11
trans-1,2-Dimethylcyclohexane
Adamantaned
20 (2)
1 (1)
9 (1)
1 (1)
Adamantan-2-ol + adamantan-2-one
D. Competitive oxidation reactionse
12 Benzyl alcohol + benzyl-d7 alcohol
13
14
Benzaldehyde + benzaldehyde-d6
Cyclohexanol & cyclohexanone + cyclooctanone
Cyclohexanol + cyclohexanol-d12
Cyclohexanone + cyclohexanone-d10
cis-1,2-Dimethylcyclohexanol +
66 (3) + 30 (2)
0 + 90 (2)
22 (2) + 8 (1)
30 (2) + 4 (1)
52 (3) + 8 (1)
66 (3) + 32 (2)
0 + 98 (2)
26 (2) + 10 (1)
23 (2) + 5 (1)
46 (3) + 8 (1)
Cyclohexane + cyclooctanol
Cyclohexane + cyclohexane-d12
15
cis-1,2-Dimethylcyclohexanef +
trans-1,2-dimethylcyclohexane
trans-1,2-dimethylcyclohexanol
a
Reaction conditions: Peracetic acid (100 equiv per Mn, 32 wt% solution) was added to a reaction solution containing manganese catalyst
(1 mM) and substrate (0.5 M) over 20 min via a syringe method in CH3CN (2 mL) at 25 uC. After 10 min stirring, the reaction solution was
directly analyzed by GC and GC-MS. Product yields are based on the amount of CH3CO3H added. The numbers in parentheses represent a
b
c
d
e
standard deviation of three experiments. Low concentration of adamantane (0.25 M) was used due to the low solubility. Equimolar
amounts of competing substrates (0.25 M each) and oxidant (50 mM) were used unless otherwise noted. Other products, such as 2,3- and
f
3,4-dimethylcyclohexanol and -one, were formed (y30%).
4624 | Chem. Commun., 2007, 4623–4625
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In alkane hydroxylation, ketone was the dominant product
formed in the hydroxylation of cyclohexane (entry 8), and the
ketone formation was the result of the further oxidation of
alcohols at a fast rate (entry 13) [eqn. (1)]. By carrying out an
intermolecular competitive hydroxylation with cyclohexane and
cyclohexane-d12 (entry 14), a KIE value of 2.5 was determined for
the formation of alcohol. The alkane hydroxylation was found to
be highly stereospecific, in which the hydroxylation of cis-1,2-
dimethylcyclohexane afforded cis-1,2-dimethylcyclohexanol with
.99% retention and the hydroxylation of trans-1,2-dimethylcy-
clohexane yielded trans-1,2-dimethylcyclohexanol with no forma-
tion of its epimer (entries 9 and 10). In a competitive hydroxylation
of cis- and trans-1,2-dimethylcyclohexane, the ratio of cis- to trans-
1,2-dimethylcyclohexanol was y6.0 (entry 15), indicating that the
intermediate generated in the reactions of 1 and 3 with CH3CO3H
reacts faster with cis-alkane than trans-alkane. The alkane
hydroxylation was also highly regioselective, in which the
oxidation took place rigorously at the tertiary C–H bond in the
hydroxylation of adamantane (entry 12); the ratio of 3u/2u
oxygenated products was 60 after statistical corrections.§14 The
ESI MS and EPR spectra of 1, taken after the completion of the
oxidation reaction, were identical to those of the starting Mn
complex (ESI,{ Fig. S1 and S2), indicating that 1 is resistant
against the ligand destruction. However, the catalytic activity
decreases drastically upon the addition of further peracetic acid to
the reaction solution. A control reaction, carried out with
Mn(CF3SO3)2 instead of 1, revealed that only trace amounts of
oxygenated products were formed in the hydroxylation of
cyclohexane by CH3CO3H, implying that there is a significant
ligand effect in generating an active oxidant and/or tuning the
reactivity of the intermediate toward oxygenation reactions.
Finally, we carried out 18O-labeled experiments to understand
the source of oxygen found in oxygenated products.7b,15 When the
alkane hydroxylation was carried out in the presence of H218O, no
18O-incorporation from the labeled water into alcohol products
was observed. This result implies that the active species generated
in the reaction of 1 and CH3CO3H does not exchange with H218O
at a fast rate. In addition, no 18O-incorporation was observed in
oxygenated products when the alkane hydroxylation by 1 and
CH3CO3H was carried out under 18O2 atmosphere, demonstrating
that the oxygen in oxygenated products derived from the oxidant,
not from molecular oxygen.
oxygenating intermediates and the effect of non-heme ligands in
tuning the oxidizing power of the intermediates.
This research was supported by KOSEF/MOST through CRI
(to W. N.) and SRC/ERC (R11-2005-008-00000-0 to J. K.).
Notes and references
{ We make no claim to be reporting a chiral synthesis, and the Flack value
[0.20(3)] does not allow an unequivocal determination of absolute
structure of 1. What is shown in Fig. 1 has N2-R and N3-R
stereochemistry.
§ The amount of 1-adamantanol was divided by the sum of 2-adamantanol
and 2-adamantanone, and then multiplied by 3 to correct the number of
tertiary and secondary C–H bonds.
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In conclusion, all the results presented above strongly support
that the epoxidation of olefins by PhIO and the oxidation of
olefins, alcohols and alkanes by CH3CO3H does not occur via an
auto-oxidation reaction but via a mechanism involving metal-
based oxidants. Then, what are the oxygenating intermediates
involved in the PhIO and CH3CO3H reactions? Based on the
observations that the intermediates generated in the PhIO and
CH3CO3H reactions showed different reactivities in alkane
hydroxylation and different product distributions in competitive
oxygenations, we may propose that the intermediates involved in
the catalytic oxygenation reactions by PhIO and CH3CO3H are
different. However, all our efforts to characterize the reactive
species spectroscopically failed at this moment. It should be noted
that the nature of oxygenating intermediates in manganese
complex-catalyzed oxygenation reactions has been poorly under-
stood.16 Future studies will focus on elucidating the structure of
This journal is ß The Royal Society of Chemistry 2007
Chem. Commun., 2007, 4623–4625 | 4625