Chiral Mn(III) salen catalyzed oxidation of hydrocarbons

Article abstract of DOI:10.1007/s11243-010-9359-9

Four chiral manganese(III)-salen complexes (1-4) were employed as catalysts in the oxidation of hydrocarbons at room temperature using pentafluoroiodosylbenzene as terminal oxidant. The reactions were carried out in acetonitrile and dichloromethane. Norbornene has been selectively oxidized to exo-epoxynorborane in 85% yield. At room temperature, oxygenation of cyclohexane up to 14% in acetonitrile medium has also been achieved. Springer Science+Business Media B.V. 2010.

Full text of DOI:10.1007/s11243-010-9359-9

Transition Met Chem (2010) 35:527–530
DOI 10.1007/s11243-010-9359-9
Chiral Mn(III) salen catalyzed oxidation of hydrocarbons
Achintesh Narayan Biswas
Sujit Kumar Kandar Arunava Agarwala
Debkumar Bandyopadhyay
•
Purak Das
•
•
•
•
Pinaki Bandyopadhyay
Received: 5 March 2010 / Accepted: 7 April 2010 / Published online: 22 April 2010
Ó Springer Science+Business Media B.V. 2010
Abstract Four chiral manganese(III)-salen complexes
1–4) were employed as catalysts in the oxidation of hydro-
Jacobsen and Katsuki independently developed several
(
manganese(III) salen complexes with Schiff bases having
chiral centers in the diammine bridge. These chiral man-
ganese(III) salen complexes find their best application as
catalysts in enantioselective epoxidation of unfunctional-
ized olefins with different terminal oxidants [12, 15–19]. In
contrast, chiral manganese(III) salen complexes catalyzed
oxidation of hydrocarbons, where prochirality is absent, is
a less explored area [14, 20, 21]. Herein, we wish to report
the catalytic properties of a group of chiral manganese(III)
complexes toward hydrocarbon oxidation by penta-
fluoroiodosylbenzene (C F IO). The steric and electronic
carbons at room temperature using pentafluoroiodosylbenzene
as terminal oxidant. The reactions were carried out in
acetonitrile and dichloromethane. Norbornene has been
selectively oxidized to exo-epoxynorborane in 85% yield.
At room temperature, oxygenation of cyclohexane up to
1
4% in acetonitrile medium has also been achieved.
Introduction
6
5
The oxidative functionalization of hydrocarbons under
mild conditions is an important area of contemporary
chemical research [1–3]. Transition metal complexes have
been extensively studied as catalysts for hydroxylation of
alkanes and epoxidation of alkenes [4–9]. The epoxidation
of alkenes offers a convenient and important synthetic
route for transformation into variety of compounds via
highly regio- and stereoselective ring opening reactions
properties of the catalysts have been tuned by the substi-
tution pattern at the aldehyde moiety and the chiral diamine
fragment, respectively.
Experimental
Instruments and reagents
[
10]. Manganese(III) and iron(III) complexes of porphy-
rins, phthalocyanines, chlorins, triazacylononanes and
Schiff bases have been used as alkene oxidation catalysts
IR spectra were recorded in KBr palate with Nicolet 460
prot e´ g e´ IR spectrophotometer. Reactions were monitored
with a Perkin-Elmer Autosystem XL Gas Chromatograph.
Mass spectra were recorded with a Jeol SX-102 (FAB).
Elemental analyses were carried out with a Perkin-Elmer
Series II, CHNS/O Analyzer.
[
9, 11–14]. Kochi et al. introduced a group of manga-
nese(III) complexes with Schiff bases of N O donor set,
2
2
commonly known as salen complexes, as catalysts for the
oxidation of alkenes [14]. Subsequently, the groups of
All chemicals used for the synthesis of the ligands and
metal complexes were reagent grade. From Aldrich, 3,5-
dinitrosalicylaldehyde, (1R, 2R)-(?)-1,2-diaminocyclo-
hexane, (1R,2R)-(?)-1,2-diphenylethylenediamine, 2-tert
butyl-4-methylphenol, Tin(IV) chloride and triethylamine
were obtained and used as received. Cyclohexene and the
solvents used for the catalytic experiments were distilled
under argon prior to use.
A. N. Biswas ꢀ P. Das ꢀ P. Bandyopadhyay (&)
Department of Chemistry, University of North Bengal,
Raja Rammohunpur, Siliguri 734013, India
e-mail: pbchem@rediffmail.com
S. K. Kandar ꢀ A. Agarwala ꢀ D. Bandyopadhyay
Department of Chemistry, Indian Institute of Technology,
New Delhi 110016, India
123

5
28
Transition Met Chem (2010) 35:527–530
Synthesis of the ligands and the catalysts
Results and discussion
Nitro-substituted and alkyl-substituted salen ligands and
the Mn(III) complexes were prepared by literature methods
Four tetradentate chiral salen ligands were synthesized
according to reported methods [17]. Manganese(III) com-
plexes (1–4) (Fig. 1) were prepared from Mn(OAc) . 4H O
[
17]. Catalysts 1 and 2 were characterized by mass, spectral
and analytical data that corresponded to the reported data
17]. For the other metallosalen catalysts, the character-
ization data are given below.
2
2
and the corresponding ligands in the presence of LiCl. The
spectral (MS and IR) and elemental analysis data of two
known complexes (1, 2) are in agreement with the reported
values [17]. The two complexes (3, 4) were characterized
on the basis of by MS, IR and elemental analysis data.
Initially, the efficacy of the catalysts (1–4) was inves-
tigated with norbornene under a standard set of conditions.
The rationale behind the choice of norbornene as substrate
is based on the wide applicability of norbornene and its
derivatives like epoxy norborane, diol etc., in polymer
synthesis, pharmaceutical intermediates and general
organic synthesis [22]. The probable products during the
oxidation of norbornene include 2,3-epoxynorborane, 2-
norbornanone and exo, endo-norborneols. Oxidations of
norbornene were carried out by a number of catalytic
systems [23–26] but the selectivity and yield of epoxide are
moderate in most of the cases [27]. The present oxidizing
system selectively oxidizes norbornene to exo-epox-
ynorborane at room temperature. Mn(III)-salen complexes
(1 & 2) have emerged as more efficient catalysts, affording
the exo-epoxide in 72–85% yield, than the catalysts, 3 & 4
(Table 1). The introduction of electron donating groups
(tert-butyl and methyl) in the 3 and 5 positions of the
aldehyde fragment of the Schiff base increases significantly
the yield of norbornene epoxidation but the presence of
electron withdrawing nitro groups decreases the yield. Our
[
?
3. MS(FAB): 591 (M ). Anal.: Calcd for C H N
O MnCl. H O: C, 39.4; H, 2.9; N, 13.8. Found: C, 39.8;
20 16 6
1
0
2
-
1
H, 3.1; N, 14.0. IR (KBr) m (C=N)/cm : 1643.
?
4
N O MnCl.: C, 48.80; H, 2.6; N, 12.2. Found: C, 48.9; H,
. MS(FAB): 689 (M ). Anal.: Calcd for C H
28 18
6
10
-
1
2
.8; N, 12.2. IR (KBr) m (C=N)/cm : 1635.
Catalytic oxidations
Catalytic reactions were carried out in small screw capped
vials fitted with PTFE septa. In a typical reaction, catalyst
and substrate were dissolved in 2 mL of argon saturated
solvent. The final concentrations of the catalyst and the
substrate were 100 lM and 100–200 mM, respectively.
The oxidation reaction was initiated by adding C F IO
6
5
(
final concentration 2 mM), and the contents were mag-
netically stirred for 1 h. The standard solution of dodecane
was added to this reaction mixture, and an aliquot was
injected into a capillary column (carbowax, 30 m) of a
preheated GC. The identification and the quantitation of the
products were done from the response factors of standard
product samples as usual (Internal standard: Dodecane,
2
mM).
Fig. 1 Manganese (III)-salen
catalysts used in the study
Ph
N
Ph
N
N
O
N
O
Mn
Cl
Mn
Cl
H C
3
O
O
CH3
H C
CH3
3
tBu
But
tBu
But
1
2
Ph
N
Ph
N
N
N
Mn
Cl
Mn
O2N
O
O
NO2
O N
O
O
NO2
2
Cl
NO2
O N
2
NO2
O N
2
3
4
1
23

Transition Met Chem (2010) 35:527–530
529
Table 1 Mn(III) salen catalyzed oxidation of Norbornene by C
at 25 °C
6
F
5
IO
oxygenation of cyclohexene does not show much depen-
dence on the substituents at the salicylidine moiety or on
reaction medium; the percentage conversion of cyclohex-
ene by C F IO remained almost same in both acetonitrile
Entry
Catalyst
Solvent
% Yield
of epoxide
a
6
5
and dichloromethane (Table 2). Another distinct feature of
these reactions is that epoxidation is favoured over allylic
oxidation (Table 2) although appreciable amount of allylic
oxidation is observed in case of catalyst 2 (entry 4,
Table 2). Thus, conversions obtained are modest but the
formation of cyclohexene oxide without serious competi-
tion from the production of cyclohexenol and cyclohexe-
none is noteworthy as it is known that cyclohexene is prone
to allylic oxidation. This result is in contrast with the chiral
iron(III)-salen catalyzed oxidation [30] of cyclohexene,
where allylic oxidation dominates over epoxidation with
much lower selectivity.
1
2
3
4
5
6
7
8
a
1
1
2
2
3
3
4
4
CH
CH
CH
CH
CH
CH
CH
CH
3
2
3
2
3
2
3
2
CN
Cl
CN
Cl
CN
Cl
CN
Cl
77
72
80
85
45
45
52
52
2
2
2
2
Yields are with respect to total oxidant concentration
results are in agreement with the view that oxidized species
are stabilized by the introduction of the electron donating
substituents in the aldehyde fragment and destabilized by
electron withdrawing groups in Mn(III)-salen complexes
The Mn(III) salen complexes were also effective in
epoxidation of cyclooctene and 1-octene, the results are
summarized in Tables 3 and 4, respectively. For cyclooc-
tene, alkyl-substituted Mn(III) salen complexes appear to
be better catalysts. The conversion of cyclooctene to cyclo-
octene oxide with catalysts 1 and 2 was observed up to
[
20, 28].
The effect of chiral diamine fragment on the catalytic
activity of the Mn(III)-salen complexes can be appreciated
by the fact that catalysts 2 & 4 with (1R, 2R)-(?)-diphenyl
moiety afford better conversions than those (1 & 3) with
60%, whereas the epoxidation with catalysts 3 and 4 was
much lower (43 and 48%, respectively) (Table 3). Here
also, no allylic oxidation was observed. No distinct solvent
effect was observed for the nitro-substituted Mn(III)-
complexes (3 and 4), although the epoxidation of cyclo-
octene with catalysts 1 & 2 was found to be facilitated in
acetonitrile medium than in dichloromethane. In metal-
catalyzed epoxidations, terminal alkenes are known to be
least reactive among the olefins [14], yet 1-octene is readily
oxidized by C F IO in the presence of chiral Mn(III)-salen
(
1R, 2R)-(?)-cyclohexyl moiety. The catalytic reactions
were carried out separately in acetonitrile and dichloro-
methane but no solvent effect was observed. This suggests
the involvement of similar type of intermediates in both the
solvents. It is reasonable to believe that the active species
is [O=Mn(V)(salen)Cl] that transfers the oxygen to the
alkenes by a stepwise radical mechanism with C F IO [29].
6
5
High selectivity and conversion in epoxidation of nor-
bornene by the catalysts (1–4) encouraged us to extend our
study on oxidation of other hydrocarbons. At room tem-
perature, Mn(III)-salen complexes (1–4) catalyze the oxy-
genation of cyclohexene to epoxide as major product with
C F IO (Table 2). Interestingly, Mn(III)-salen catalyzed
6
5
catalysts (Table 4). Here, the nitro-substituted catalysts (3
4) appear more efficient than alkyl substituted catalysts
&
(
1 & 2) unlike the epoxidation of cyclooctene. It is well
0
documented that the substitution of nitro groups at 5, 5
positions of the Mn(III)-salen complexes can modulate the
donor properties of the oxomanganese(V) intermediates by
6
5
Table 2 Mn(III) salen catalyzed oxidation of Cyclohexene by
IO at 25 °C
6 5
C F
Table 3 Metallosalen catalyzed oxidation of Cyclooctene by C
at 25 °C
6
F
5
IO
a
Entry Catalyst Solvent Conversion (%) Product (% yields)
a
1
-ol 1-one Epoxide
Entry
Catalyst
Solvent
% Yield of epoxide
1
2
3
4
5
6
7
8
a
1
1
2
2
3
3
4
4
CH
CH
CH
CH
CH
CH
CH
CH
3
2
3
2
3
2
3
2
CN 31
Cl 31
CN 65
Cl 62
CN 46
Cl 44
CN 43
Cl 42
7
7
3
3
21
21
39
27
38
35
37
34
1
2
3
4
5
6
7
8
a
1
1
2
2
3
3
4
4
CH
CH
CH
CH
CH
CH
CH
CH
3
2
3
2
3
2
3
2
CN
Cl
CN
Cl
CN
Cl
CN
Cl
55
39
59
43
42
43
44
48
2
2
16
20
6
10
15
2
2
2
2
7
2
2
5
1
2
7
1
2
Yields are with respect to total oxidant concentration
Yields are with respect to total oxidant concentration
123

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Transition Met Chem (2010) 35:527–530
Acknowledgments The financial support (SR/S1/IC-08/2007) from
DST, Government of India, is gratefully acknowledged.
Table 4 Mn(III) salen catalyzed oxidation of 1-octene by C
5 °C
6
F
5
IO at
2
a
Entry
Catalyst
Solvent
% Yield of epoxide
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1
2
3
4
5
6
7
8
a
1
1
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CH
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CH
CH
CH
CH
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2
3
2
3
2
CN
Cl
CN
Cl
CN
Cl
CN
Cl
10
12
12
10
34
32
38
43
2
1
2
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-octene and oxidizing saturated hydrocarbons like cyclo-
hexane. Further studies on detailed mechanism of the
reactions are currently in progress.
1
23