Nonheme manganese(III) complexes for various olefin epoxidation: Synthesis, characterization and catalytic activity

Article abstract of DOI:10.1016/j.ica.2021.120306

Three mononuclear imine-based non-heme manganese(III) complexes with tetradentate ligands which have two deprotonated phenolate moieties, ([(X₂saloph)Mn(OAc)(H₂O)], 1a for X = Cl, 1b for X = H, and 1c for X = CH₃, saloph = N,N-o-phenylenebis(salicylidenaminato)), were synthesized and characterized by ¹H NMR, ¹³C NMR, ESI-Mass and elemental analysis. Mn^III complexes catalysed efficiently various olefin epoxidation reactions with meta-chloroperbenzoic acid (MCPBA) under the mild condition. Mn^III complexes 1a and 1c with the electron-withdrawing group -Cl and electron-donating group –CH₃ showed little substituent effect on the epoxidation reactions. Product analysis, Hammett study and competition experiments with cis- and trans-2-octene suggested that Mn^IV = O, Mn^V = O, and Mn^III-OOC(O)R species might be key oxidants in the epoxidation reaction under this catalytic system. In addition, the use of PPAA as a mechanistic probe demonstrated that Mn-acylperoxo intermediate (Mn^III-OOC(O)R) 2 generated from the reaction of peracid with manganese complexes underwent both the heterolysis and the homolysis to produce Mn^V = O (3) or Mn^IV = O species (4). Moreover, the Mn^III-OOC(O)R 2 species could react directly with the easy-to-oxidize substrate to give epoxide, whereas the species 2 might not be competent to the difficult-to-oxidize substrate for the epoxidation reaction.

Full text of DOI:10.1016/j.ica.2021.120306

Inorganica Chimica Acta 520 (2021) 120306
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Research paper
Nonheme manganese(III) complexes for various olefin epoxidation:
Synthesis, characterization and catalytic activity
Sojeong Lee, Soyoung Park, Myoung Mi Lee, Jiyoung Lee, Cheal Kim*
Department of Fine Chem, Seoul National Univ. of Sci. and Tech., Seoul 129-743, South Korea
A R T I C L E I N F O
A B S T R A C T
Keywords:
Three mononuclear imine-based non-heme manganese(III) complexes with tetradentate ligands which have two
deprotonated phenolate moieties, ([(X2saloph)Mn(OAc)(H2O)], 1a for X = Cl, 1b for X = H, and 1c for X = CH3,
saloph = N,N-o-phenylenebis(salicylidenaminato)), were synthesized and characterized by 1H NMR, 13C NMR,
ESI-Mass and elemental analysis. MnIII complexes catalysed efficiently various olefin epoxidation reactions with
meta-chloroperbenzoic acid (MCPBA) under the mild condition. MnIII complexes 1a and 1c with the electron-
withdrawing group -Cl and electron-donating group –CH3 showed little substituent effect on the epoxidation
reactions. Product analysis, Hammett study and competition experiments with cis- and trans-2-octene suggested
that MnIV = O, MnV = O, and MnIII-OOC(O)R species might be key oxidants in the epoxidation reaction under
this catalytic system. In addition, the use of PPAA as a mechanistic probe demonstrated that Mn-acylperoxo
intermediate (MnIII-OOC(O)R) 2 generated from the reaction of peracid with manganese complexes under-
went both the heterolysis and the homolysis to produce MnV = O (3) or MnIV = O species (4). Moreover, the
MnIII-OOC(O)R 2 species could react directly with the easy-to-oxidize substrate to give epoxide, whereas the
species 2 might not be competent to the difficult-to-oxidize substrate for the epoxidation reaction.
Non-heme
Saloph
Manganese complexes
Olefin epoxidation
Mn=O
1. Introduction
epoxidations [7–9]. Therefore, the number of salen metal complexes
have been prepared and studied for several types of applications in
Developing catalysts which have high catalytic efficiency in olefin
epoxidation reactions have been considered important because the
epoxidation reactions have been used in a large field of the chemical
industry [1]. The epoxidation products, epoxides, are well known as
intermediates in the organic synthesis to produce perfumes, drugs, and
so on [2]. Consequently, improving the efficiency of the epoxidation
reaction with faster, cheaper, and more selective catalysts is meaningful
and significant in organic synthesis and industry.
bioinorganic and medicinal inorganic chemistry and drug developments
[7–9]. We have previously reported the MnIII complexes containing
4-
[Re4Q4(CN)12
]
(Q = Te and Se) cluster which catalyzed rapidly
epoxidation of various olefins under mild conditions [9]. However, they
required a complicated and high-cost synthesis process. These results led
us to develop new manganese complexes with simple, inexpensive, and
efficient catalysis.
Herein, we demonstrate the synthesis, characterization, and cata-
lytically efficient epoxidation reactions of Mn(saloph) complexes 1a-1c.
The Mn complexes showed an efficient catalytic reactivity for diverse
olefin epoxidations with MCPBA as oxidant under mild conditions. MnIII
complexes 1a and 1c with the electron-deficient group –Cl and electron-
rich group –CH3 showed little electronic substituent effect on the
epoxidation reactions. The mechanistic studies like product analysis,
Hammett plots, the use of PPAA (peroxyphenylacetic acid) , and
competition experiments with cis- and trans-2-octene led us to propose
that MnIV = O, MnV = O, and MnIII-OOC(O)R species might be key in-
termediates in the epoxidation reaction under this catalytic system.
Cytochrome P-450, a family of heme enzymes, catalyzes diverse
oxygenation reactions, including olefin epoxidation under environ-
mentally favorable conditions [3]. Thus, modeling these enzymes has
been paid attention to developing new olefin epoxidation catalysts with
the first low-transition metals such as Fe, Mn, and Cu [3–5]. In partic-
ular, the Mn-containing complexes such as [Mn(salen)] and [Mn(porp)]
have been well known as efficient catalysts toward olefin epoxidation
(salen = N,N-bis(salicylidene)ethylenediamine, porp = porphyrin)
[6,7]. For example, the homogeneous and heterogeneous Mn(salen)
complexes showed an excellent catalytic reactivity in various olefin
* Corresponding author.
E-mail address: chealkim@snut.ac.kr (C. Kim).
https://doi.org/10.1016/j.ica.2021.120306
Received 9 December 2020; Received in revised form 19 February 2021; Accepted 19 February 2021
Available online 25 February 2021
0020-1693/© 2021 Elsevier B.V. All rights reserved.

S. Lee et al.
Inorganica Chimica Acta 520 (2021) 120306
Scheme 1. Synthesis of ligands (La, Lb, and Lc) and Mn complexes (1a, 1b, and 1c).
However, the MnIII-OOC(O)R species may not be competent to the
difficult-to-oxidize substrate for the epoxidation reaction.
[Lc + H+ + 2MeCN + H2O]+: 445.23, found 445.08 (Fig. S3 (c)). Ele-
ments analysis calcd (%) for C22H20N2O2: C, 76.71; H, 5.84; N, 8.13.
Found: C, 76.85; H, 5.93; N, 8.11.
2. Experimental section
2.2. Preparation of the manganese complexes 1a-1c
Epoxides, olefins, 2-cyclohexen-1-one, ethanol, 2-cyclohexen-1-ol,
acetone, methanol, o-phenylenediamine, salicylaldehyde, benzalde-
hyde, acetonitrile, dichloromethane, Mn(OAc)3⋅2H2O, MCPBA (65%),
and dodecane were provided from Aldrich. PPAA (peroxyphenylacetic
acid) was synthesized as reported in the literature [10a].
All complexes were prepared with the previously reported literature
method [6,9,11,14]. They were synthesized by stirring the ligands
(1x10-3 mol, La or Lb or Lc) and Mn(III)(OAc)3⋅2H2O (1x10-3 mol) in
MeOH (5 mL) at 20 ◦C (Scheme 1) [6,9,11,14]. Brown powder was
formed, filtered and washed with MeOH. For 1a, ESI-MS: m/z calcd for
[1a + H+]+: 515.00, found 515.09 (Fig. S4(a)). Elements analysis calcd
(%) for C22H19MnN2O5: C, 59.20; H, 4.29; N, 6.28. Found: C, 59.24; H,
3.90; N, 6.11. For 1b, ESI-MS: m/z calcd for [1b + H+ + 2MeCN]+:
529.13, found 528.93 (Fig. S4(b)). Elements analysis calcd (%) for
Product analyses for PPAA experiment and alkene epoxidation were
achieved with a Perkin Elmer gas chromatograph with a 30 m capillary
column (HP-FFAP or DB-5). Product analysis for cis-stilbene epoxidation
were achieved with an Agilent 1100 series High-performance liquid
chromatograph system (Agilent Technologies, USA) with a 5 µm, 4.6
mm X 150 mm Octadecyl column (Waters Corp., Milford, MA). 1H and
13C NMR spectra were gained on a Varian spectrometer. Elemental an-
alyses were performed on a vario MICRO Cube analyzer. A single-
quadrupole liquid chromatography detector (ACQUITY QDa) was used
to collect ESI-MS spectra. UV/Visible spectra were provided with a
Perkin Elmer spectrometer (Lambda 2S UV/Visible).
C
22H19MnN2O5: C, 51.29; H, 3.33; N, 5.44. Found: C, 51.83; H, 2.87; N,
5.46. For 1c, ESI-MS: m/z calcd for [1c + H+]+: 475.11, found 474.98
(Fig. S4(c)). Elements analysis calcd (%) for C22H17Cl2MnN2O5: C,
60.76; H, 4.89; N, 5.91. Found: C, 60.74; H, 4.45; N, 5.69.
2.3. Olefin epoxidations by the manganese complexes 1a-1c with MCPBA
MCPBA (1x10-4 mol) was delivered to a mixture of Mn complex
2.1. Preparation of X2saloph ligands (La for X = Cl, Lb for X = H, Lc for
X = CH3)
(1x10-6 mol), substrate (3.5x10-5 mol) and solvent (1 mL, CH2Cl2/
◦
CH3CN = 1:1), which was stirred for 10 min at 20 C. Each reaction
analyzed by GC was run at least three times. Dodecane was employed as
an internal standard. Cis-stilbene epoxidation was analyzed by HPLC
with ethyl benzoate as an internal standard.
Compound Lb was synthesized according to the literature methods
(Scheme 1). [11–13] Benzene-1,2-diamine (108.1 mg, 1x10-3 mol) and
2-hydroxybenzaldehyde (268.7 mg, 2.2x10-3 mol) were dissolved in
MeOH (5 mL). The solution was stirred for 20 h at 20 ◦C. Yellow powder
was formed, filtered and washed with MeOH. 1H NMR (DMSO‑d6): δ
12.93 (s, 2H), 8.94 (s, 2H) 7.68 (d, 2H) 7.45 (m, 6H), 6.97 (t, 4H)
(Fig. S1 (a)). 13C NMR (DMSO‑d6): δ 117.1 (2C), 119.5 (2C), 119.9 (2C),
120.2 (2C), 128.2 (2C), 132.9 (2C), 133.9 (2C), 142.7 (2C), 160.8 (2C),
164.5 (2C) (Fig. S1 (b)). ESI-MS: m/z calcd for [Lb + H+]+: 317.13,
found 317.07 (Fig. S1 (c)). Elements analysis calcd (%) for C20H16N2O2:
C, 75.92; H, 5.11; N, 8.87. Found: C, 75.90; H, 5.59; N, 8.75.
2.4. Competitive epoxidations of styrene and para-substituted styrenes for
Hammett plots
MCPBA (5x10-5 mol) was delivered to a mixture of styrene (2x10-5
mol) and para(X)-substituted styrene (2x10-5 mol, X = –Cl, –CN, and
–OCH3, –CH3), Mn complex (1x10-6 mol) and solvent (1 mL, CH2Cl2/
CH3CN = 1:1), which was stirred for 10 min at 20 ◦C. The amounts of the
styrenes before and after epoxidations were analyzed by GC. The rela-
tive reactivity was analyzed with the following equation: kx/ky = log(Xf/
Xi)/log(Yf/Yi) where Yf and Yi are the final and initial concentration of
styrene and Xf and Xi are the final and initial concentration of para-
substituted styrenes [15].
A similar procedure was applied for La and Lc. [12] For La, 1H NMR
(DMSO‑d6): δ 12.53 (s, 2H), 8.98 (s, 2H) 7.80 (s, 2H) 7.67 (d, 2H), 7.42
(t, 2H), 6.98 (m, 4H) (Fig. S2 (a)). 13C NMR (DMSO‑d6): δ 117.2 (2C),
119.7 (2C), 119.9 (2C), 121.7 (2C), 129.8 (2C), 132.9 (2C), 134.4 (2C),
142.8 (2C), 160.8 (2C), 165.6(2C) (Fig. S2 (b)). ESI-MS: m/z calcd for
[La + H+ + DMSO + MeOH]+: 495.09, found 495.53 (Fig. S2 (c)). El-
ements analysis calcd (%) for C20H14Cl2N2O2: C, 62.36; H, 3.66; N, 7.27.
Found: C, 62.19; H, 3.77; N, 7.33. For Lc, 1H NMR (DMSO‑d6): δ 13.08
(s, 2H), 8.93 (s, 2H) 7.65 (d, 2H) 7.40 (m, 2H), 7.30 (s, 2H), 6.98 (t, 4H),
2.30 (s, 6H) (Fig. S3 (a)). 13C NMR (DMSO‑d6): δ 19.6 (2C), 117.1 (2C),
119.4 (2C), 119.9 (2C), 120.8 (2C), 132.8 (2C), 133.6 (2C), 136.6 (2C),
140.1 (2C), 160.8 (2C), 163.5 (2C) (Fig. S3 (b)). ESI-MS: m/z calcd for
–
2.5. Determination of the O
O bond cleavage products from the
epoxidation reactions of substrates by PPAA with Mn complexes
PPAA (3.5x10-5 mol) was delivered to a mixture of substrate
(0–1.6x10-4 mol), Mn complex (1x10-6 mol), and solvent (1 mL, CH2Cl2/
◦
CH3CN = 1:1), which was stirred for 10 min at 20 C. Each reaction
2

S. Lee et al.
Inorganica Chimica Acta 520 (2021) 120306
Table 1
Olefin epoxidations by MCPBA with manganese catalysts 1a-1c in CH3CN/CH2Cl2 (1:1).[a, 14]
Entry
Substrate
Product
1a
1b
1c
Conversion [%][b]
Yield [%][b]
Conversion [%][b]
Yield [%][b]
Conversion [%][b]
Yield [%][b]
1
cyclopentene
epoxide
~100
84
trace
64
71
82
3
~100
97
trace
76
72
79
3
99
89
trace
70
73
74
3
2-cyclopentene-1-one
epoxide
2
3
4
cycloheptene
cyclooctene
cyclohexene
~100
99
99
96
95
93
98
95
epoxide
epoxide
98
2-cyclohexene-1-ol
2-cyclohexene-1-one
epoxide
2
3
2
5
6
7
1-hexene
1-octene
51
57
98
43
38
77
3
42
47
94
39
35
84
4
59
54
94
29
33
78
3
epoxide
cis-2-hexene
cis-oxide
trans-oxide
8
9
trans-2-hexene
cis-2-octene
trans-oxide
84.
98
62
85
5
77
94
66
83
4
77
94
61
84
4
cis-oxide
trans-oxide
10
11
trans-2-octene
trans-oxide
67
97
65
63
6
62
98
52
57
6
64
91
54
57
5
styrene
epoxide
benzaldehyde
phenylacetaldehyde
cis-stilbene oxide
trans-stilbene oxide
benzaldehyde
2-phenylacetophenone
trans-stilbene oxide
benzaldehyde
2-phenylacetophenone
20
48
13
13
8
18
50
13
15
8
16
44
12
12
8
12
13
cis-stilbene
91
89
92
86
90
84
trans-stilbene
44
11
4
47
11
5
40
9
4
[a] Reaction conditions: olefins (3.5x10-5 mol), catalyst (1x10-6 mol), MCPBA (1x10-4 mol), and solvent (1 mL, CH3CN/CH2Cl2 = 1:1). [b] Based on substrate.
analyzed by GC was run at least three times. Dodecane was employed as
an internal standard.
mixture of olefin and manganese complex in 1 mL of CH2Cl2/CH3CN
(1:1). The reaction was complete for 1 min (Fig. S5). We certified that
the direct oxidation of olefins by MCPBA was negligible under the
conditions. UV/Visible experiments displayed that the Mn complexes
1a-1c were sturdy enough during the catalytic epoxidations (Fig. S6).
In Table 1 are summarized the epoxidation reactions. The epoxides
were predominantly formed in the presence of 1b. Cyclic alkenes of
cyclopentene, cycloheptene and cyclooctene were epoxidized in good
yields (72% ꢀ 97%, entries 1–3) with high conversions (96% ꢀ 100%).
In the reaction containing cyclohexene (entry 4), cyclohexene oxide
(79%) was mainly produced together with trace amounts of by-
products, 2-cyclohexen-1-one (3%) and 2-cyclohexen-2-ol (3%). This
result indicated that the radical-type oxidations were rarely involved
in the epoxidation reactions [11,13]. The poor reactive terminal ole-
fins, 1-hexene and 1-octene [15,16], showed low yields to 1-hexene
2.6. Competitive epoxidations of cis-2-octene and trans-2-octene by
MCPBA with manganese complexes 1a-1c
MCPBA (4x10-5 mol) was delivered to a mixture of cis-2-octene (1-
8x10-5 mol) and trans-2-octene (1-8x10-5 mol), Mn complex (1x10-6
mol), and solvent (1 mL, CH2Cl2/CH3CN = 1:1), which was stirred for
10 min at 20 ◦C. Each reaction analyzed by GC was run at least three
times. Dodecane was employed as an internal standard.
2.7. UV/Visible spectrophotometric studies
UV/Visible spectrophotometric experiments were perform as fol-
lows. Solutions of 1a, 1b, and 1c (1.0× 10-5 mol) in CH2Cl2/CH3CN
(1:1, 3 mL) were transferred into 1 cm UV cell. Cyclohexene (3.5x10-5
mol) and MCPBA (1x10-4 mol) were then added into the UV cell,
whereupon the spectral variations were monitored within 10 sec and
further analyzed for 10 min.
3. Results and discussion
3.1. Synthesis and characterization of manganese complexes 1a-1c
Saloph ligand Lb was synthesized as reported in the literatures
(Scheme 1) [11,12]. La having the electron-withdrawing group –Cl and
Lc the electron-donating group –CH3 were also synthesized with a
similar method, and verified by 1H and 13C NMR, ESI-MS and elemental
analysis. Manganese complexes 1a-1c were prepared by the reaction of
ligands La-Lc with Mn(OAc)3, and characterized by ESI-MS and
elemental analysis.
3.2. Epoxidation reactions catalyzed by manganese complexes
Catalytic epoxidation of dievrse alkenes by using MCPBA as oxidant
Fig. 1. Hammett plot for selective reactivity of styrene to para-substituted
styrenes by catalyst 1a with MCPBA.
with 1a, 1b, and 1c as catalysts was performed. MCPBA was added to a
3

S. Lee et al.
Inorganica Chimica Acta 520 (2021) 120306
Scheme 2. Possible mechanism for the production of the multiple active oxidants.
and 1-octene epoxides (39% and 35%, entries 5 and 6). cis-2-Hexene
and cis-2-octene were epoxidized to cis-2-hexene oxide and cis-2-octene
oxide (84% and 83%; entries 7 and 9) as well as a small amount of
trans-2-hexene oxide and trans-2-octene oxide (4% and 4%). These
outcomes suggested that the high stereochemical retentions were
retained in this epoxidation reaction. trans-2-Hexene and trans-2-
octene produced only trans-2-hexene oxide and trans-2-octene oxide
(66% and 52%; entries 8 and 10). Styrene formed styrene oxide (57%;
entry 11) with small amount of benzaldehyde (6%) and certain
amounts of phenylacetaldehyde (18%). cis-Stilbene was epoxidized to
cis-stilbene oxide (50%; entry 12) and trans-stilbene oxide (13%)
together with some benzaldehyde (15%) and 2-phenylacetophenone
(8%). However, trans-stilbene was only epoxidized to a trans-stilbene
oxide (47%; entry 13) as a major product along with minor amounts of
2-phenylacetophenone (5%) and benzaldehyde (11%). Various prod-
ucts observed from these aromatic olefins indicated that either the
peroxyl radical or oxomanganese(IV) is partially involved as the
epoxidizing oxidant because they were proposed to carry out non-
stereospecific or radical-induced epoxidations [11b,17,18].
On the other hand, complexes 1a and 1c showed similar results to
1b, thus implying that the electron-deficient group –Cl and electron-rich
group –CH3 showed little effect on the epoxidation reactions.
3.3. Competitive epoxidations of styrene and para-substituted styrenes for
Hammett plots
To figure out the electronic character of the reactive species in the
epoxidation reaction, the substituent electronic effect with styrene and
four different para-substituted styrenes was studied. The Hammett plots
gave the ρ values of ꢀ 0.68 for 1a (Fig. 1), ꢀ 0.66 for 1b (Fig. S7(a)), and
ꢀ 0.80 for 1c (Fig. S7(b)). The
ρ values suggested that the reactive in-
termediates are electrophilic, and the reaction rate of the electron-rich
styrenes is faster than those of electron-deficient ones.
–
3.4. Product distribution of the O
O bond cleavage of PPAA with
catalysts 1a-1c
–
To demonstrate the O O bond activation process of complexes 1a-
Table 2
Yield of products generated from PPAA catalyzed by the manganese catalysts 1a-1c in the absence and the presence of cyclohexene in a mixture of CH2Cl2/CH3CN
(1:1).[a, 14]
Entry
Catalyst
Cyclohexene[mM]
Heterolysis[b]
Homolysis[b]
Hetero (5)/Homo (6 + 7 + 8)
Oxidation products
5
6
7
8
oxide
-ol
-one
1
1a
0
57
55
81
83
89
70
66
58
76
83
62
69
58
69
83
22
19
19
15
12
22
20
15
14
12
20
18
16
13
12
2
1
1
1
1
2
1
1
1
1
2
1
1
1
1
–
–
–
–
–
–
–
–
–
–
–
–
–
–
–
2.4 (70.4/29.6)
2.8 (73.3/26.7)
4.1 (80.2/19.8)
5.2 (83.8/16.2)
6.9 (87.3/12.7)
2.9 (74.5/25.5)
3.1 (75.9/24.1)
3.6 (78.4/21.6)
5.1 (83.5/16.5)
6.4 (86.5/13.5)
2.8 (73.8/26.2)
3.6 (78.4/21.6)
3.4 (77.3/22.7)
4.9 (83.1/16.9)
6.4 (86.5/13.5)
–
–
–
2
20
40
80
160
0
45[d]
33[c]
51[c]
77[c]
–
3[d]
2[c]
2[c]
2[c]
–
2 [d]
1 [c]
2[c]
2 [c]
–
3
4
5
6
1b
1c
7
20
40
80
160
0
44[d]
34[c]
46[c]
60[c]
–
3[d]
2[c]
2[c]
2[c]
–
2[d]
1 [c]
2[c]
2[c]
–
8
9
10
11
12
13
14
15
20
40
80
160
22[d]
32[c]
35[c]
52[c]
1[d]
2[c]
2[c]
2[c]
1[d]
1[c]
2[c]
2[c]
[a]Reaction conditions: substrate (0–0.16 mmol), catalyst (1x10-6 mol), PPAA (4x10-5 mol), and solvent (1 mL).
[b]
Based on PPAA.
[c]
Based on oxidant, PPAA; oxide, -ol, and -one indicate cyclohexene oxide, 2-cyclohexen-1-ol, and 2-cyclohexen-1-one, respectively.
Based on cyclohexene.
[d]
4

S. Lee et al.
Inorganica Chimica Acta 520 (2021) 120306
Table 3
Yield of products generated from PPAA catalyzed by the manganese catalysts 1a-1c in the absence and the presence of 1-octene in a mixture of CH2Cl2/CH3CN (1:1).[a,
14]
Entry
Catalyst
1-Octene[mM]
Heterolysis[b]
Homolysis[b]
Hetero (5)/Homo (6 + 7 + 8)
Oxidation product
1-octene oxide
5
6
7
8
1
1a
0
57
75
76
67
70
70
72
73
75
73
62
66
58
68
65
22
27
26
26
25
22
23
28
22
22
20
19
19
18
17
2
3
3
3
3
2
2
3
2
2
2
3
2
2
2
–
–
–
–
–
–
–
–
–
–
–
–
–
–
–
2.4 (70.4/29.6)
2.5 (71.4/28.6)
2.6 (72.4/27.6)
2.3 (69.8/30.2)
2.5 (71.4/28.6)
2.9 (74.5/25.5)
2.9 (74.2/25.8)
2.4 (70.2/29.8)
3.1 (75.8/24.2)
3.0 (75.3/24.7)
2.8 (73.8/26.2)
3.0 (75.0/25.0)
2.8 (73.4/26.6)
3.4 (77.3/22.7)
3.4 (77.4/22.6)
–
2
20
40
80
160
0
16[d]
11[c]
16[c]
21[c]
–
3
4
5
6
1b
1c
7
20
40
80
160
0
14[d]
10[c]
15[c]
20[c]
–
8
9
10
11
12
13
14
15
20
40
80
160
11[d]
7[c]
11[c]
14[c]
[a] Reaction conditions: substrate (0–0.16 mmol), catalyst (1x10-6 mol), PPAA (4x10-5 mol), and solvent (1 mL).
[b]
Based on PPAA.
[c]
[d]
Based on oxidant, PPAA.
Based on 1-octene.
1c, PPAA was employed as a mechanistic probe, because it could
Table 4
–
Competitive oxidations of cis-2-octene and trans-2-octene by the manganese
determine the O O bond cleavage mode from degradation products
catalysts 1a-1c with MCPBA in a mixture of CH2Cl2/CH3CN (1:1).[a, 14]
Entry
–
(Scheme 2) [6,19]. When the O O bond of the Mn-acylperoxo species 2
is cleaved heterolytically, it produces phenylacetic acid (PAA (5),
Catalyst
Substrate
Concentration
[mM]
Product yield
(%)
Ratio of
cis- to
trans-
–
pathway (a)). The homolytic cleavage of the O O bond produces an
cis-
trans-
acyloxyl radical, which decomposes to benzaldehyde (6), benzyl alcohol
(7), and toluene (8) with a rapid β-scission (pathway (c)). The direct
oxidation reaction of the Mn-acylperoxo species and substrate produces
oxide
oxide
18 [b]
oxide
1
1a
cis-/trans-
10 / 10
20 / 20
40 / 40
80 / 80
10 / 10
20 / 20
40 / 40
80 / 80
10 / 10
20 / 20
40 / 40
80 / 80
32 [b]
25 [b]
34 [c]
41 [c]
30 [b]
26 [b]
33 [c]
38 [c]
36 [b]
25 [b]
38 [c]
44 [c]
1.78
2-octene
(64.0/
36.0)
2.08
–
PAA and influences the O O bond cleavage pattern (pathway (b)).
A control experiment by 1a with PPAA was performed without
substrate (Table 2, entry 1). The heterolysis product, PAA, and the ho-
molysis products, benzyl alcohol, benzaldehyde, and toluene, were
2
12 [b]
16 [c]
18 [c]
15 [b]
10 [b]
12 [c]
13 [c]
17 [b]
11 [b]
14 [c]
15 [c]
(67.6/
32.4)
2.13
3
formed in a ratio of 2.4. This result indicated that the heterolysis
(68.0/
32.0)
2.28
III
–
(70.4%) and the homolysis (29.6%) of the O O bond of Mn -OOC(O)R
intermediate 2 occurred simultaneously to afford high-valent MnV = O
(3) and MnIV = O (4).
4
(69.5/
30.5)
2.00
To comprehend the concentration dependence on the ratio variation
5
1b
cis-/trans-
–
of heterolysis to homolysis of the O O bond, we increased the amount
2-octene
(66.7/
33.3)
2.60
of cyclohexene, known as easy-to-oxidize substrate, and monitored the
ratio variation of heterolysis to homolysis (entries 2–5) [5,11a,19]. If the
MnIII-OOC(O)R 2 directly contributes to the epoxidation reaction, the
ratio of heterolysis to homolysis would increase with the increase of the
substrate concentration [19c]. The ratio of the heterolysis to homolysis
changed from 2.4 to 6.9 with the increase of the cyclohexene concen-
tration (entries 1–5). Similar outcomes were obtained for 1b and 1c
(entries 6–15). These results suggested that the MnIII-OOC(O)R (2)
might also be a possible active oxidant in the epoxidation, as formerly
proposed for [Mn(bpc)]-, [Mn(salen)]-, and Re4 cluster-supported Mn
complex-catalyzed epoxidations (bpc = 4,5-dichloro-1,2-bis(2-pyridine-
2-carboxamido)benzene) [5,7,10].
6
(72.2/
27.8)
2.75
7
(73.3/
26.7)
2.92
8
(74.5/
25.5)
2.12
9
1c
cis-/trans-
2-octene
(67.9/
32.1)
2.27
10
11
12
(69.4/
31.6)
2.71
We changed the substrate from cyclohexene to 1-octene which is
known as difficult-to-oxidize alkene to examine the cleavage pattern of
–
the O O bond on the kind of substrate (Table 3) [19c]. The distribution
(73.1/
26.9)
2.93
of the heterolysis and the homolysis was virtually identical, irrespective
to the change of 1-octene concentration. These results led us to propose
that the MnIII-OOC(O)R would not be active in the epoxidation of
difficult-to-oxidize alkene, 1-octene.
(74.6/
25.4)
[a] Reaction conditions: substrate (1-8x10-5 mol), catalyst (1x10-6 mol), MCPBA
(3.5x10-5 mol), and solvent (1 mL).
[b] Based on substrate. [c] Based on MCPBA.
3.5. Competition experiments of cis- and trans-2-octene by catalysts 1a-
1c
To further understand the electronic property of the reactive
5

S. Lee et al.
Inorganica Chimica Acta 520 (2021) 120306
Scheme 3. Possible mechanism for the production of the active oxidants from the reaction of the manganese complexes with peracids.
oxidants involved in the olefin epoxidation, competition experiments
with cis- and trans-2-octene were conducted (Table 4). The ratio of cis- to
trans-2-octene oxides was increased as the concentration of substrates
increased (from 1.78 to 2.28 for complex 1a; entries 1–4). 1b and 1c
showed a little bigger change with the increasing amounts of substrates
(entries 5–12). These outcomes indicated that the MnIII-OOC(O)R 2
would prefer to react with cis- olefins over trans-olefins owing to the
steric hindrance between 2 and trans-type substrate. Thus, we could
demonstrate that the species 2 might be somewhat involved in the
epoxidation reactions with easy-to-oxidize olefin [20].
epoxidation reactions with easy-to-oxidize olefin. Taken all together,
MnIII-OOC(O)R, MnIV = O, and MnV = O species might be key in-
termediates in olefin epoxidation reaction under this catalytic system.
However, the species 2 might not be competent to the difficult-to-
oxidize substrate for the epoxidation reaction. Future studies will
concentrate on attempts to understand the physical properties of the
active oxidants.
CRediT authorship contribution statement
Sojeong Lee: Conceptualization, Investigation, Validation, Visuali-
zation, Writing-Origianl draft. Soyoung Park: Investigation, Validation.
Myoung Mi Lee: Investigation, Validation. Jiyoung Lee: Investigation,
Validation. Cheal Kim: Supervision, Writing-review & editing.
3.6. Mechanism study
The most reasonable mechanism for the formation of the active in-
termediates responsible for epoxidation reaction with Mn complexes is
proposed in Scheme 3. Peracid reacts with manganese complexes to
produce a Mn-acylperoxo intermediate (MnIII-OOC(O)R 2), which un-
dergoes both the heterolysis and the homolysis to afford MnV = O (3) or
MnIV = O (4) in aprotic solvent (CH3CN/CH2Cl2 = 1:1). The MnV = O
intermediate might carry out the olefin epoxidation with high stereo-
specificity, whereas the MnIV = O complex might cause radical-type
oxidation including the production of allylic oxidation by-products
and loss of stereospecificity [6,10,13]. In addition, the MnIII-OOC(O)R
2 species could react directly with the easy-to-oxidize substrate to give
epoxide, whereas the species 2 might not be competent to the difficult-
to-oxidize substrate for the epoxidation reaction [4a,10,20]. On the
other hand, to observe the proposed active intermediates like MnIII-OOC
(O)R, MnV = O and MnIV = O, the spectroscopic instruments such as low-
Declaration of Competing Interest
The authors declare that they have no known competing financial
interests or personal relationships that could have appeared to influence
the work reported in this paper.
Acknowledgments
We kindly acknowledge the National Research Foundation of Korea
(NRF-2018R1A2B6001686).
Appendix A. Supplementary data
◦
◦
temp UV–Vis spectroscopy (-40 C) and ESI mass spectrometry (0 C)
were applied. However, they were not observed under our test
conditions.
Supplementary data to this article can be found online at https://doi.
org/10.1016/j.ica.2021.120306.
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