Highly Chemoselective and Enantioselective Catalytic Oxidation of Heteroaromatic Sulfides via High-Valent Manganese(IV)-Oxo Cation Radical Oxidizing Intermediates

Article abstract of DOI:10.1021/acscatal.7b00968

A manganese complex with a porphyrin-like ligand that catalyzes the highly chemoselective and enantioselective oxidation of heteroaromatic sulfides, including imidazole, benzimidazole, indole, pyridine, pyrimidine, pyrazine, sym-triazine, thiophene, thiaz

Full text of DOI:10.1021/acscatal.7b00968

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Article
Highly Chemoselective and Enantioselective Catalytic
Oxidation of Heteroaromatic Sulfides via High-Valent
Manganese (IV)-Oxo Cation Radical Oxidizing Intermediates
Wen Dai, Sensen Shang, Ying Lv, Guosong Li, Chunsen Li, and Shuang Gao
ACS Catal., Just Accepted Manuscript • Publication Date (Web): 13 Jun 2017
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Highly Chemoselective and Enantioselective Catalytic Oxida-
tion of Heteroaromatic Sulfides via High-Valent Manganese
(IV)-Oxo Cation Radical Oxidizing Intermediates
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⊥
Wen Dai,*,†,‡ Sensen Shang,†,‡ Ying Lv,†,‡ Guosong Li,†,‡ Chunsen Li,*,§, and Shuang Gao*,†,‡
†Dalian Institute of Chemical Physics, the Chinese Academy of Sciences, Dalian 116023, China
‡Dalian National Laboratory for Clean Energy, Dalian 116023, China
§State Key Laboratory of Structural Chemistry, Fujian Institute of Research on the Structure of Matter, Chinese Academy of
Sciences, Fujian 350002, China
⊥Fujian Provincial Key Laboratory of Theoretical and Computational Chemistry, Xiamen, Fujian 361005, China
ABSTRACT: A manganese complex with porphyrinꢀlike ligand that catalyzes the highly chemoselective and enantioselective oxiꢀ
dation of heteroaromatic sulfides, including imidazole, benzimidazꢀ
ole, indole, pyridine, pyrimidine, pyrazine , symꢀtriazine, thiophene,
thiazole, benzothiazole, benzoxazole with hydrogen peroxide is deꢀ
scribed, furnishing the corresponding sulfoxides in good to excellent
yields and enantioselectivities (up to 90% yield, and up to >99% ee )
within a short reaction time (0.5 h). The practical utility of the methꢀ
od has been demonstrated in gramꢀscale synthesis of chiral sulfoxide.
Mechanistic studies, performed with 18Oꢀlabeled water (H218O), hyꢀ
drogen peroxide (H218O2) and cumyl hydroperoxide reveal that a
highꢀvalent manganeseꢀoxo species is generated as the oxygen atom delivering agent via carboxylic acidꢀassisted heterolysis of Oꢀ
O bond. Density functional theory (DFT) calculations were also carried out to give further insight into the mechanism of mangaꢀ
neseꢀcatalyzed sulfoxidation. On the basis of the theoretical study, the coupled highꢀvalent manganese (IV)ꢀoxo cation radical speꢀ
cies which bears obvious similarities with that of reactive intermediates in the catalytic oxygenation reactions based on the cytoꢀ
chromes P450 and metalloporphyrin models, has been proposed as reactive oxidant in nonꢀheme manganese catalyst system.
KEYWORDS: asymmetric sulfoxidation, manganese, heteroaromatic sulfides, porphyrinꢀlike, cation radical
INTRODUCTION
strates and products. Second, the sulfurꢀ or/and nitrogenꢀ
containing heteroaromatic rings moieties are potentially oxiꢀ
dizable (Scheme 1). Therefore, extremely high chemoselecꢀ
tivity and stereorecognition of the catalysts are essential to
solve this challenging objective. To date, only limited examꢀ
ples of asymmetric
Chiral sulfoxides are widely used as intermediates, auxiliaries,
and ligands in modern organic chemistry and constitute imꢀ
portant biologically active compounds, including several marꢀ
keted pharmaceuticals.1 Among the various methods develꢀ
oped for construction of enantioenriched sulfoxides, asymmetꢀ
ric sulfoxidation undoubtedly serves as one of the most
straightforward and reliable methods. Since the initial breakꢀ
through accomplished in asymmetric sulfoxidation using titaꢀ
nium/tartrate catalysts together with alky hydroperoxides as
oxidant by the groups of Kagan and Modena in 1984,2 other
transitionꢀmetal catalysts,3 organocatalysts4 and chemoenꢀ
zymatic methods5 have been extensively developed, and high
enantioselectivity has already been achieved in oxidation of
simple substrates such as phenyl alkyl and alkyl alky sulfides.
However, asymmetric oxidation of heteroaromatic sulfides has
been slow to develop, despite the special importance of enanꢀ
tiomerically enriched heteroaromatic sulfoxides in the syntheꢀ
sis of chiral drugs and natural products.1f,1g We postulated that
the main issues were as follows: First, oxidation reactions
involving this substrate have been plagued by catalyst deactiꢀ
vation owing to the strong coordinating ability of the subꢀ
Scheme 1. Challenges in Asymmetric Oxidation of Het-
eroaromatic sulfides.
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Scheme 2. Representative Examples of Asymmetric Oxida-
tion of Heteroaromatic sulfides, and limitations of these
catalyst systems.
Therefore, we reasoned that the chiral manganese complexes
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with ligands L might be extended to enantioselective oxidation
of heteroaromatic sulfides. Herein, we report a highly efficient
enantioselective oxidation of a remarkable broad range of hetꢀ
eroaromatic sulfides by an in situ formed porphyrinꢀlike chiral
manganese complex with hydrogen peroxide, providing the
corresponding sulfoxides in good to excellent yields and enanꢀ
tioselectivities (up to 90% yield, and >99% ee), under mild
conditions and in short reaction time as well as application of
the method to the gramꢀscale synthesis of optically pure sulꢀ
foxide. Detailed mechanism studies evidence that a highꢀ
valent manganeseꢀoxo species is generated as the reactive
oxidizing intermediate via carboxylic acidꢀassisted OꢀO bond
heterolysis. Specifically, on the basis of DFT analysis, the
coupled highꢀvalent manganese (IV)ꢀoxo cation radical speꢀ
cies (L+•)MnIVO, which bear obvious similarities with that of
oxidizing intermediates in the catalytic oxygenation reactions
based on the cytochromes P450 and metalloporphyrin models,
has been proposed as the reactive oxidant in nonꢀheme manꢀ
ganese catalyst system (Scheme 3).8
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oxidation of heteroaromatic sulfides have been documented.
For instance, enzymatic methods offer a partial solution to this
problem, but there are some inherent disadvantages associated
with such transformation such as instability, limited availabilꢀ
ity, high cost of the catalysts and narrow substrate tolerance
(Scheme 2, eq 1).5b,5e Although the titanium/tartrate systems
have also been applied to the oxidation of heteroaromatic sulꢀ
fides, only sulfides bearing imidazole substituents can achieve
a high enantioselectivity and environmentally unfriendly alky
hydroperoxides are used as terminal oxidant (Scheme 2, eq
2).6 Given these limitations, development of a highly efficient
and practical method that enables enantioselective oxidation of
heteroaromatic sulfides is highly desirable and of great signifiꢀ
cance.
RESULTS AND DISCUSSION
Optimization of catalytic activity with regard to identity of
carboxylic acid and oxidants. At the outset, 2ꢀ
(methylsulfinyl)pyridine (1a) was chosen as a model substrate
to optimize the reaction conditions. In the presence of 1.0
mol% loading of chiral manganese complex generated in situ
from Mn(OTf)2 and L4, 50% mol carboxylic acid as additive
and 1.3 equiv 45% H2O2 as the oxidant, the asymmetric oxidaꢀ
o
tion of 1a was conducted in acetonitrile at –20 C (see Table
S1 in SI). Among these aliphatic carboxylic acids, the adaꢀ
mantane carboxylic acid (aca) provided the best result (entry
6, Table S1). It is noteworthy that the aca loading could be
reduced to 20 mol% without erosion of the yield and enantiꢀ
oselectivity (entry 1, Table 1). A deleterious effect in the yield
and enantioselectivity was observed when the aca loading was
further lowered to 10% mol (entry 2, Table 1). Then oxidants
tꢀBuOOH and CumylꢀOOH were investigated in the presence
of aca (entries 3 and 4, Table 1). Interestingly, the ee values
were virtually the same irrespective of the type of oxidants,
suggesting that a common oxidizing intermediate was generatꢀ
ed in the reactions of H2O2, tꢀBuOOH and CumylꢀOOH in the
presence of carboxylic acid.
Inspired by porphyrin ligand, we recently developed a new
type of tetradentate N4 ligands (L) bearing relatively long
conjugation and two NꢀH moieties exhibiting strong σꢀ
donation that fulfilled the structural requirements of the porꢀ
phyrin ligand in some way, which has a highly conjugated
planar with strong donor moiety.7 The porphyrinꢀlike ligands
L exhibit extraordinary chemoselectivity and enantioselectiviꢀ
ty in various manganeseꢀcatalyzed oxidation reactions, includꢀ
ing electronꢀrich olefins epoxidation and alkyl phenyl sulfides
oxidation.7a,7b
Scheme 3. Strategy for the development of Asymmetric
Oxidation of Heteroaromatic sulfides.
Table 1. Optimization of Catalytic Activity with Regard to
Identity of Oxidants.
entry
1
Oxidant
H2O2
RCO2H
aca
% yielda % eeb
95
90
81
79
2c
H2O2
aca
3
4
tꢀBuOOH
91
89
82
80
aca
aca
CumylꢀOOH
aIsolated yield. bDetermined by chiral HPLC analysis. caca
(10 mol%).
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Screening of the other reaction parameters. Due to the oxiꢀ even when methyl on sulfur atom was replaced by branched or
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dative kinetic resolution process, the amount of oxidant has
been previously shown to play a positive role in improving
enantioselectivity in metalꢀcatalyzed oxidation of sulfides.3e,7b
Because of that, further studies were conducted to investigate
the amount of oxidant. A critical improvement in enantioselecꢀ
tivity was attained by increasing the equivalents of hydrogen
peroxide, although some loss in yield occurred upon switching
from 1.3 equiv. to 1.5 equiv. and finally 1.8 equiv. of hydroꢀ
gen peroxide (Table 2, entries 1 and 2). The use of 1.8 equiv.
hydrogen peroxide was the most beneficial in terms of yield,
enantioselectivity and cost (entry 2). A simple inspection on
the reaction temperature revealed that decreasing the temperaꢀ
longer alkyl or sterically hindered benzyl (entries 2a–2j). Noꢀ
tably, the substrate containing various functional groups such
as halogen, hydroxyl, ester and trifluoromethyl were equally
well tolerated. High yield and excellent enantioselectivity
were obtained for oxidation of 2ꢀ(phenylthio)ethanꢀ1ꢀol and
no trace of aldehyde was observed which indicated that our
catalyst system possessed high chemoselectivity of oxidation
of sulfide over alcohol (entry 2e). Various pyrimidine derivaꢀ
tives were also suitable reaction partners, thus affording the
corresponding sulfoxides in high yields and excellent enantiꢀ
oselectivities regardless of the electronic properties (entries
2k–2n). The oxidation reactions of pyrazine derivatives were
also carried out. Good yields and excellent enantioselectivities
were obtained (entries 2o and 2p). Subjecting the 1,3,5ꢀ
triazine derivative to the optimized reaction conditions providꢀ
ed the sulfoxide product in good yield and excellent enantioseꢀ
lectivity (entry 2q). Substrates bearing thiazole, benzothiazole,
1,3,4ꢀthiadiazole or thiophene substituents were also compatiꢀ
ble with the current catalyst system. No observation of oxidaꢀ
tion of sulfurꢀcontaining heteroaromatic rings further indicated
that the present sulfoxidation is highly chemoselective (entries
2r–2v). Benzoxazole
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o
ture to –30 C can afford a better enantioselectivity (entry 3).
o
Further lowering the temperature to –40 C resulted in no imꢀ
provement of enantioselectivity (entry 4). Subsequently, an
enhancement in yield and enantioselectivity failed to achieve
by prolonging the reaction time to 1 h (entry 5). From the
evaluation of various ligands, ligand L3 bearing secꢀButyl–
substituted oxazolines was identified as the ligand being the
most favorable in view of yield and enantioselectivity (entries
6–9). It is worth mentioning that the catalyst loading could be
further reduced to 0.5 mol% without loss of either reactivity or
enantioselectivity (entry 10). Further lowering the catalyst
loading to 0.25 mol% resulted in a marked decrease in yield,
albeit with an almost identical enantioselectivity (entry 11).
Scheme 4. Substrate Scope of Asymmetric Oxidation of
Heteroaromatic Sulfidesa,b,c
Substrate scope of asymmetric oxidation of heteroaromatic
sulfides. With the optimized conditions in hand, we next to set
out to explore the substrate scope, and the results were sumꢀ
marized in Scheme 4. A variety of pyridine derivatives could
be efficiently oxidized within short time to generate the
corresponding chiral sulfoxides in high yields with excellent
enantioselectivities. Excellent enantioselectivity was preserved
Table 2. Screening of the Other Reaction Parameters.
entry
ligand
H2O2
% yielda
% eeb
(equiv.)
1c
2c
3
4d
5e
6
1.5
1.8
1.8
1.8
1.8
1.8
1.8
1.8
93
88
87
86
87
79
81
88
84
86
88
88
88
80
79
90
L4
L4
L4
L4
L4
L1
L2
L3
7
8
9
1.8
1.8
1.8
83
89
85
83
90
89
L5
L3
10f
11g
L3
b
c
aIsolated yield. Determined by chiral HPLC analysis. –
20 oC. –40 oC. e1.0 h. fMn(OTf)2 (0.5 mol%), L3 (0.5
d
aReaction conditions: substrate (0.4 mmol), Mn(OTf)2 (0.5
mmol%), L3 (0.5 mmol%), aca (20 mo%), 45% H2O2 (1.8
mol%). gMn(OTf)2 (0.25 mol %), L3 (0.25 mol%).
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o
b
c
equiv.), CH3CN (1.5 mL), –30 C, 0.5 h. Isolated yield. Deterꢀ
mined by chiral HPLC analysis.
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could also serve as a good substrate, offering the sulfoxide
with satisfactory yield and excellent yield (entry 2w). When
the sulfide with imidazole substituent, which is identified as an
ideal model substrate in titanium/tartrate systems, was subjectꢀ
ed to the oxidation, the desired sulfoxide product was obtained
in good yield and enantioselectivity (entry 2x). Finally, the
present sulfoxidation could also be successfully applied to
pyrazole derivative, giving the corresponding sulfoxide in
good yield and enantioselectivity (entry 2y).
envisioned that the electronic structure of this active intermeꢀ
diate in the current catalyst system might bear obvious similarꢀ
ities with that of compound I. On the basis of our speculation
and literature precedence, we proposed a possible reaction
pathway shown in Scheme 6. Likewise, some experiments and
DFT calculations have been carried out to verify this mechaꢀ
nism.
Isotopically labeled experiments were employed as mechaꢀ
nistic tools, providing insights into the mechanism of OꢀO
lysis (Scheme 7). Oxidation of 1a using H218O2 as oxidant
afforded the corresponding sulfoxide 2a 94% 18O labelled, and
virtually no 18O–incorporation was observed when
H216O2/H218O was used, demonstrating unambiguously that the
exclusive source of oxygen atoms incorporated into the sulfoxꢀ
ide did stem from the hydrogen peroxide oxidant and also that
the incorporation of oxygen from the water into the sulfoxide
did not occur. The isotopic study argues against the implicaꢀ
tion of a waterꢀassisted OꢀO lysis and instead points toward
that the reaction should proceed via the carboxylic acidꢀ
assisted pathway that leads to form a highꢀvalent manganeseꢀ
oxo species, which is consistent with Scheme 6.
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Gram-scale synthesis of chiral sulfoxides. To further evaluꢀ
ate the synthetic potential of the catalytic system, asymmetric
sulfoxidation of benzothiazole derivative 1p and pyrazine
derivative 1t was performed under the optimal conditions on a
gram scale, giving the desired product 2p and 2t in 82% yield
with 91% ee and 88% yield and 90% ee, respectively (Scheme
5).
Scheme 5. The Gram-Scale Synthesis of 2p and 2t.
Scheme 7. Isotopic Analysis Studies.
Mechanistic studies. The original mechanism for the ironꢀ
catalyzed carboxylic acidꢀassisted epoxidation of olefins with
H2O2 was proposed by Que, et al and the reaction involved a
highꢀvalent iron (V)ꢀoxo species as oxygenꢀtransfer reagent
which is formed via acidꢀassisted heterolytic cleavage of OꢀO
bond in a FeIII(OOH)(HOAc) precursor.9 Experimental specꢀ
troscopic evidence and computational support in favor of this
“carboxylicꢀacid assisted” mechanism have been also recently
provided.10 On the other hand, the porphyrinꢀinspired ligands
used in the current catalyst system bear relatively long conjuꢀ
gation and two NꢀH moieties that exhibit strong σꢀdonation
that fulfill the structural requirements of the porphyrin ligand
in some way. In cytochrome P450 and iron porphyrin models,
high valent porphyrin πꢀcation radicals ferrylꢀoxo, the soꢀ
called compound I (Por+•)FeIVO, have been widely accepted as
reactive intermediates in the catalytic oxygenation reactions.8
Therefore, we
H218O2 reagent is a 2% solution in H216O
The enantioselectivity was approximately the same virtualꢀ
ly approximately common stereoselectivity irrespective of
Scheme 8. (a) Heterolytic and Homolytic O-O Bond Cleav-
age Mechanisms in the Presence of Carboxylic Acid, Re-
spectively, (b) Reaction of 1a with Cumyl-OOH as Oxidant
and Analysis of the Decomposition of the Cumyl-OOH.
Scheme 6. Proposed Mechanism.
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O
O
R
(L) MnIV
oxidant employed constitutes strong evidence that a common
oxygen atom transfer agent is formed with both CumylꢀOOH
O
B
O-O bond
heterolysis
LMnIII
OOH
OCOR
H2O
and H2O2 oxidant in the presence of carboxylic acids (Table 1,
entries 3 and 4). To further probe the O−O bond cleavage
mechanism in the current catalyst system, the oxidation of 1a
with cumyl hydroperoxide was performed and products origiꢀ
nated from the decomposition of the cumyl hydroperoxide
were analyzed to distinguish homolytic vs heterolytic O−O
bond cleavage pathways (Scheme 8). In this reaction, cumyl
alcohol was yielded quantitatively in the presence of aca, indiꢀ
cating that a highꢀvalent manganeseꢀoxo species was generatꢀ
ed from MnIII alkylperoxo species precursor via a heterolytic
OꢀO bond cleavage.
H
or
A
O
O
R
(L) MnV
O
B'
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The theoretical study at DFT level was also performed to
investigate the nature of the active oxidant and its mechanism
of formation in the current catalyst system. Consistent with the
experimental results, DFT calculations on the formation of
coupled highꢀvalent (L+•)MnIVO species (B) rather than
(L)MnVO species (B') from its precursor (L)MnIII(OOH) comꢀ
plex (A), revealed that the conversion of A to B proceeds in a
concerted heterolytic manner. Figure 1 displays the results for
this conversion in its triplet and quintet spin states. In accord
with previous experimental and theoretical results,11 the
ground state of complex A is quintet state and the correspondꢀ
ing triplet state lies higher by 7.4 kcal/mol. During the O−O
Figure 1. Energy profiles (in kcal/mol) and geometries of key
species (in Å) for the conversion of (L)MnIII(OOH)ꢀ(CH3COOH)
(A) to (L+•)MnIV(O)ꢀ(CH3COO)ꢀH2O (B). Energies were calculatꢀ
ed at the level of UB3LYP/Def2ꢀTZVPP//LACVP**. Zeroꢀpoint
energies (ZPE) and solvation were taken into account.
3
bond cleavage, the calculated transition state TS1 is lower
5
than TS1 by 1.4 kcal/mol. As such, the triplet state crosses
below the quintet state and hence mediates the conversion.
The spin density on the leaving OH group in the MnꢀOꢀOH
3
moiety is only ꢀ0.15 (see Scheme S1 in SI) on TS1 and the
equatorial ligand is coupled to the quartet pair in either a ferꢀ
romagnetic or antiferromagnetic manner, to give a triplet and
quintet spin states. As such, the
transition state leads directly to complex 3B without additional
steps. Thus, the overall O−O bond cleavage is effectively conꢀ
certed heterolytic. The nascent manganeseꢀoxo complex B has
a triplet ground state and a lowꢀlying excitedꢀquintet state with
a small energy difference of 2.5 kcal/mol as calculated with
UB3LYP. Inspection of electronic configurations of complex
B (Figure 2) shows that three unpaired spinꢀup electrons occuꢀ
py the dꢀorbital block of manganese center and one single
electron in the π orbital of the
Figure 2 Spinꢀnatural orbitals along with their occupancies for (a) 3B and (b) 5B.
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the dedicated grant for new technology of methanol conversion
1
2
from Dalian Institute of Chemical and Physics, Chinese Academy
of Science is gratefully acknowledged.
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REFERENCES
oxidation number on manganese is IV and complex B is repreꢀ
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with the electronꢀtransferred (L)MnVO complex B' caused it to
collapse to (L+•)MnIVO. Thus, as cytochrome P450 enzymes
utilize coupled porphyrin πꢀcation radical ferrylꢀoxo, the soꢀ
called compound I (Por+•)FeIVO, to efficiently activate subꢀ
strate, in this work the nascent manganeseꢀoxo complex emꢀ
ploys an analogous (L+•)MnIVO oxidant to catalyze the reacꢀ
tions.
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CONCLUSION
In conclusion, the present work describes a highly efficient
and practical method that enables enantioselective oxidation of
a broad range of heteroaromatic sulfides with a manganese
complex bearing porphyrinꢀlike ligand. Reactions are perꢀ
formed in short reaction times and low catalyst loading, emꢀ
ploying aqueous hydrogen peroxide as terminal oxidant and
utilizing simple operations. These attributes make the current
catalyst system an ideal alternative to enzymatic methods and
the titanium/tartrate systems. The practical utility of the methꢀ
od has been demonstrated in gramꢀscale synthesis of chiral
sulfoxides. The involvement of high valent Mnꢀoxo species as
oxidizing intermediate, which was formed via carboxylic acid
assisted heterolysis cleavage of the O−O bond, was evidenced
experimentally. Particularly of note, based on the DFT analyꢀ
sis, the coupled highꢀvalent manganese (IV)ꢀoxo cation radical
species as the reactive oxidizing intermediates which bear
obvious similarities with that of reactive intermediates in the
catalytic oxygenation reactions based on the cytochromes
P450 and metalloporphyrin models have been proposed for the
first time in nonheme manganese catalyst system. So, the preꢀ
sent chiral manganese complex is an enzymeꢀlike catalyst,
leading to highly chemoselective and enantioselective catalytic
oxidation of heteroaromatic sulfides. The above mechanistic
study may provide some useful hints for chiral oxidation caꢀ
talysis design and extend the scope of asymmetric oxidation
catalyzed by the porphyrinꢀlike manganese complex.
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ASSOCIATED CONTENT
Supporting Information.
The Supporting Information is available free of charge on the
ACS Publications website at DOI:xxxxx.
Procedures and NMR, HRMS, and HPLC data
AUTHOR INFORMATION
Corresponding Author
(6) (a) Seenivasaperumal, M.; Federsel, H. J.; Szabó, K. J. Adv.
Synth. Catal. 2009, 351, 903ꢀ919. (b) Seenivasaperumal, M.;
Federsel, H.ꢀJ.; Ertan, A.; Szabó, K. J. Chem. Commun. 2007, 2187ꢀ
2189.
*daiwen@dicp.ac.cn.
*chunsen.li@fjirsm.ac.cn
*sgao@dicp.ac.cn.
(7) (a) Dai, W.; Li, J.; Li, G.; Yang, H.; Wang, L.; Gao, S. Org.
Lett. 2013, 15, 4138ꢀ4141. (b) Dai, W.; Li, J.; Chen, B.; Li, G.; Lv,
Y.; Wang, L.; Gao, S. Org. Lett. 2013, 15, 5658ꢀ5661. (c) Dai, W.; Li,
G.; Chen, B.; Wang, L.; Gao, S. Org. Lett. 2015, 17, 904ꢀ907. (d) Dai,
W.; Lv, Y.; Wang, L.; Shang, S.; Chen, B.; Li, G.; Gao, S. Chem.
Commun. 2015, 51, 11268ꢀ11271. (e) Dai, W.; Mi, Y.; Lv, Y.; Chen,
B.; Li, G.; Chen, G.; Gao, S. Adv. Synth. Catal. 2016, 358, 667ꢀ671.
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Green, M. T. Science 2010, 330, 933ꢀ937. (c) Shaik, S.; Lai, W.;
Chen, H.; Wang, Y. Acc. Chem. Res. 2010, 43, 1154ꢀ1165. (d) Costas,
Notes
The authors declare no competing financial interest.
ACKNOWLEDGMENT
This work was financially supported by the National Natural Sciꢀ
ence Foundation of China (21502187 and 21573237), China Postꢀ
doctoral Science Foundation Funded Project (2015M581364) and
Natural Science Foundation of Fujian Province (2015J01069) and
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