Preparation, Characterization and Reactivity of a Bis-hypochlorite Adduct of a Chiral Manganese(IV) Salen Complex

Article abstract of DOI:10.1021/acs.inorgchem.7b02661

A bis-hypochlorite adduct of a manganese(IV) salen complex having a chiral (R,R)-cyclohexane-1,2-diamine linkage (2-^tBu) is successfully prepared and characterized by various spectroscopic methods. The reactions of 2-^tBu with various organic substrates show that 2-^tBu is capable of sulfoxidation, epoxidation, chlorination, and hydrogen abstraction reactions. However, the enantioselectivity of the epoxidation reactions by 2-^tBu is much lower than that reported for the catalytic reactions by Jacobsen's catalyst. The low enantioselectivity is consistent with a planar conformation of the salen ligand, which is suggested by circular dichroism spectroscopy. This study suggests that 2-^tBu is not a reactive intermediate of Jacobsen's enantioselective epoxidation catalysis.

Full text of DOI:10.1021/acs.inorgchem.7b02661

Communication
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Cite This: Inorg. Chem. XXXX, XXX, XXX−XXX
Preparation, Characterization and Reactivity of a Bis-hypochlorite
Adduct of a Chiral Manganese(IV) Salen Complex
Ikuko Araki, Kaoru Fukui, and Hiroshi Fujii*
Department of Chemistry, Graduate School of Humanities and Sciences, Nara Women’s University, Kitauoyanishi, Nara 630-8506,
Japan
S
* Supporting Information
manganese(IV) salen complexes bearing a (R,R)-cyclohexane-
1,2-diamine linkage as the chiral unit (Scheme 1).
ABSTRACT: A bis-hypochlorite adduct of a manganese-
(IV) salen complex having a chiral (R,R)-cyclohexane-1,2-
diamine linkage (2-tBu) is successfully prepared and
Scheme 1. Synthesis of Manganese(IV) Bis-hypochlorite
Complexes
characterized by various spectroscopic methods. The
reactions of 2-tBu with various organic substrates show
that 2-tBu is capable of sulfoxidation, epoxidation,
chlorination, and hydrogen abstraction reactions. How-
ever, the enantioselectivity of the epoxidation reactions by
2-tBu is much lower than that reported for the catalytic
reactions by Jacobsen’s catalyst. The low enantioselectivity
is consistent with a planar conformation of the salen
ligand, which is suggested by circular dichroism spectros-
copy. This study suggests that 2-tBu is not a reactive
intermediate of Jacobsen’s enantioselective epoxidation
catalysis.
We examined the preparation of the hypochlorite adduct of
the manganese(IV) salen complex from the reactions of
dichloromanganese(IV) salen complexes (1) with tetra-n-
butylammonium hypochlorite (TBA-OCl), which was prepared
from tetra-n-butylammonium chloride and sodium hypochlorite
ransition-metal-catalyzed oxygenation reactions have re-
Tceived much attention because of their demand in organic
synthetic strategies and their biological relevance with respect to
metalloenzymes.1−7 These catalytic oxygenation reactions are
initiated by the reactions of transition-metal catalysts with
terminal oxidants such as hydrogen peroxide, iodosylarene,
sodium hypochlorite, and so on. These reactions result in the
generation of terminal oxidant adducts of the transition-metal
catalysts, followed by conversion to high-valent metal oxo
complexes that work as true reactive oxidants in the catalytic
reactions. The structure and reactivity of the terminal oxidant
adduct complexes and the high-valent metal oxo complexes have
been studied in many groups.8−27 Although the terminal oxidant
adduct complexes are unstable compounds, there have been
many reports of terminal oxidant adducts of hydrogen
peroxide,8−13 alkylperoxides,14−16 and m-chloroperoxybenzoic
acid.17−20 In contrast to these successful reports, there have been
only a few examples of hypochlorite adducts of metal
complexes.28−30 Recently, we succeeded in the preparation and
characterization of bis-hypochlorite and hypochlorite imidazole
adducts of an iron(III) porphyrin complex for the first time.28
More recently, hypochlorite adducts of nonheme nickel
complexes were detected as precursors of high-valent nickel
oxo species from the reactions of nickel(II) nonheme complexes
with sodium hypochlorite.29,30 However, the binding structure
and reactivity of metal-bound hypochlorite remain unclear. To
address the structure and reactivity relationship of metal-bound
hypochlorite, we report here the preparation, characterization,
reactivity, and enantioselectivity of hypochlorite adducts of
in dichloromethane. Figure 1 shows the absorption spectral
change for titration of the dichloromanganese(IV) salen complex
having a tert-butyl group at the para position (1-tBu) with TBA-
OCl in dichloromethane at −20 °C. With the addition of TBA-
OCl, the absorption spectrum of 1-tBu changes to a new one
Figure 1. Absorption spectral change for the titration of 1-tBu with
TBA-OCl in dichloromethane at −20 °C: (blue line) 1-tBu; (red line)
2-tBu. Inset: EPR spectra of (a) 1-tBu (blue line) and (b) 2-tBu (red
line) at 4 K.
Received: October 16, 2017
© XXXX American Chemical Society
A
DOI: 10.1021/acs.inorgchem.7b02661
Inorg. Chem. XXXX, XXX, XXX−XXX

Inorganic Chemistry
Communication
(2-tBu) having absorption peaks at 350, 428, and 650 nm with
clear isosbestic points. The titration of TBA-OCl indicates that 2
equiv of TBA-OCl is required for the formation of 2-tBu (Figures
1 and S1), suggesting the formation of a bis-hypochlorite
complex. Similar absorption spectral changes are obtained from
the reactions of dichloromanganese(IV) salen complexes having
chloro and nitro groups at the para position (1-Cl or 1-NO2)
with TBA-OCl (Figure S2). The broad strong absorption peak
around 650 nm shows a blue shift as the electron-withdrawing
effect of the para substituent of the salen ligand increases (2-Cl,
630 nm; 2-NO2, 610 nm). These results indicate that this band is
assignable to a ligand-to-metal charge-transfer band. To
investigate the manganese oxidation state of 2-tBu, we measured
electron paramagnetic resonance (EPR) spectrum (Figure 1).
The EPR spectrum of 2-tBu in the perpendicular mode shows
EPR signals at g = 4.93, 3.03, and 1.82, which are totally different
adduct (Figure S7). All of these spectroscopic characterizations
indicate that 2-tBu is a manganese(IV) bis-hypochlorite complex.
Conformation of 2-tBu in dichloromethane is investigated by
means of circular dichroism (CD) spectroscopy. Previously, we
reported that the intensity of the CD peak near 280 nm is a
sensitive marker to estimate the conformation of the chiral
manganese(IV) salen complex; the intensity of the band
increases as the conformation of the salen ligand shifts from
the planar conformation to the stepped conformation.31,32 The
CD spectrum of 2-tBu showed weak CD bands at 326 and 275
nm (Figure S8), suggesting a planar conformation.
To investigate the reactivity of the manganese-bound
hypochorite, we examined the reactions of 2-tBu with various
organic substrates. When thioanisole was added to 2-tBu,
prepared from 1 and 2 equiv of TBA-OCl at −60 °C, the
absorption spectrum of 2-tBu quickly changed to that of 1-tBu
(Figure S9). Product analysis of the reaction showed the
formation of phenyl methyl sulfoxide in 93% yield based on
OCl−, showing that both of the manganese(IV)-bound
hypochlorites are capable of oxygen-atom-transfer reaction to
sulfide. We further examined the reactions of 2-tBu with less
reactive thioanisoles, p-chlorothioanisole and p-nitrothioanisole,
at −60 °C. 2-tBu oxidizes p-chlorothioanisole and p-nitro-
thioanisole to the corresponding sulfoxides in 60 and 50% yield
based on OCl− for 15 and 60 s at −60 °C, respectively, although
TBA-OCl hardly oxidizes them under the same conditions (5%
and 0%). Phenyl methyl sulfones, further oxidation products of
phenyl methyl sulfoxides, are not detected in the present
reactions. Moreover, we studied the reaction of 2-tBu with
thioanisole by EPR spectroscopy. The EPR spectrum of 2-tBu
completely changed to that of 1-tBu in the perpendicular mode,
but there were no significant EPR signals in the parallel mode
immediately after the addition of 100 equiv of thioanisole (Figure
S10). These results are consistent with the formation of 1-tBu
from the reactions of 1-tBu with thioanisoles.
from those of 1-tBu, g = 5.43, 2.06, and 1.44.31 These EPR g
3
values are typical of the S =
/ spin state, suggesting a
2
manganese(IV) oxidation state. The manganese(IV) oxidation
state is further confirmed by the EPR measurement in the parallel
mode (Figure S3), which does not show a significant EPR signal.
1
The H NMR spectrum of 2-tBu showed paramagnetic NMR
signals resulting from the phenolic protons of the salen ligand at
−19.5 and −37.7 ppm from tetramethylsilane at −20 °C (Figure
S4). These NMR shifts were close to those of manganese(IV)
salen complexes reported in previous papers.31−33 2-tBu is stable
for hours in dichloromethane under −20 °C but slowly
decomposes to manganese(III) complexes over −20 °C. 2-tBu
can be isolated as a solid by the addition of excess cold hexane at
−40 °C. The isolation of 2-tBu was confirmed by its
spectroscopic data (Figure S5). We tried to get a single crystal
of 2-tBu, but we failed to do so. To confirm the coordination of
hypochlorite to the manganese(IV) center, we measured an IR
spectrum of 2-tBu (Figures 2 and S6). We found two sensitive
The nature of the manganese-bound hypochlorite is further
studied by a Hammett plot analysis. The reaction processes of
2-tBu with thioanisoles could be followed by rapid mixing
stopped-flow experiments at −20 °C (Figures S11 and S12). The
time courses of the absorbance for these reactions are simulated
well with double-exponential functions, suggesting two succes-
sive sulfoxidation reactions (Scheme 2). Hammett plot analyses
Scheme 2. Possible Reaction Mechanism of 2-tBu
Figure 2. Difference spectrum (16O − 18O) of the IR spectra (KBr) of
16O- and 18O-labeled 2-tBu.
bands for the 18O isotope labeling of hypochlorite (18OCl−) at
473 and 725 cm−1; these bands shifted to 455 and 696 cm−1,
respectively, for the 18O-isotope-labeled 2-tBu. The observed 18O
isotope shifts (18 and 29 cm−1) are in good agreement with those
(17 and 29 cm−1) calculated for a triatomic O−Mn−O
asymmetric stretching band and a diatomic O−Cl stretching
band, respectively. We also performed ESI-MS analysis of 2-tBu
but could not obtain ion peaks corresponding to a hypochlorite
show similar ρ values for these two processes (ρ = −0.84 and
−0.92), indicating electrophilic character of the manganese(IV)
bound hypochlorite. The estimated ρ values are smaller than
those reported for the sulfoxidation reactions of various
manganese(III) salen complexes with sodium hypochlorite (ρ
= −1.55 to −2.38),34 a nonheme oxomanganese(IV) complex (ρ
= −4.4),35 and oxomanganese(V) corrolazines (ρ = −1.29 to
B
DOI: 10.1021/acs.inorgchem.7b02661
Inorg. Chem. XXXX, XXX, XXX−XXX

Inorganic Chemistry
Communication
−2.99)36 but close to that of oxomanganese(V) porphyrin (ρ =
−0.65).37
epoxidation reaction would result from thermal decomposition
of the monohypochlorite complex formed after the first
epoxidation reaction of 2-tBu with olefin because the
decomposition reaction would be faster than the second
epoxidation reaction at 25 °C. In fact, the epoxidation rate for
the most reactive p-methoxystyrene is much smaller than the
sulfoxidation rates and comparable to the decomposition rate of
2-tBu under concentrations of less than 0.1 M (Figures S12 and
S17).
We further examined the chlorination and hydrogen
abstraction reactions by 2-tBu. 2-tBu reacted with 1,3,5-
trimethoxybenzene and 1,4-cyclohexadiene to afford 1-chloro-
2,4,6-trimethoxybenzene and benzene in 20% yield at 0 °C and
40% yield at −20 °C based on OCl−, respectively. A detailed
mechanistic study of these reactions is currently under
investigation.
We further examined the reactions of 2-tBu with olefins to
investigate the epoxidation reactivity. The absorption spectral
change for the reaction of 2-tBu with p-methoxystyrene (1.0 M)
at 25 °C is different from that with thioanisole at −60 °C (Figure
S13). The absorption spectrum of the reaction product was not
identical with that of 1-tBu but a manganese(III) salen complex.
This is further confirmed by EPR spectroscopy (Figure S14). As
expected, the EPR signals resulting from 2-tBu in the
perpendicular mode become weak, and new EPR signals
resulting from a manganese(III) complex in the parallel mode
become strong after the addition of p-methoxystyrene for 2 h.
Product analysis indicated the formation of p-methoxystyrene
oxide in 48% yield based on OCl−, suggesting epoxidation
reactivity of 2-tBu. The reactions of 2-tBu with other styrene
derivatives under single-turnover conditions are summarized in
Table 1. While trans-β-methylstryrene afforded trans-β-methyl-
In summary, we have reported the preparation, character-
ization, and reactivity of a bis-hypochlorite adduct of a chiral
manganese(IV) salen complex. The manganese(IV)-bound
hypochlorite is capable of sulfoxidation, epoxidation, chlorina-
tion, and hydrogen abstraction, although the enantioselectivity is
low.
Table 1. Reactivity and Enantioselectivity of Epoxidation
Reactions by 2-tBu
a
b
olefin
yield (%) ee (%) ee of Jacobsen’s catalyst (%)
styrene
13
13
nd
25
2
56
nd
85
62
ASSOCIATED CONTENT
* Supporting Information
The Supporting Information is available free of charge on the
ACS Publications website at DOI: 10.1021/acs.inorg-
chem.7b02661.
■
p-methoxystyrene
cis-β-methylstyrene
48
S
1 (cis)
5 (trans)
36
c
trans-β-methylstyrene
3
20
a
Yields are based on TBA-OCl. Conditions: 25 °C for 3 h.
Figures S1−S17 and experimental details (PDF)
b
c
Conditions: 4 °C for 3 h. Reaction with iodosylmesitylene at 25
°C.32
AUTHOR INFORMATION
Corresponding Author
*E-mail: fujii@cc.nara-wu.ac.jp.
ORCID
■
stryene oxide, cis-β-methylstyrene produced a mixture of cis- and
trans-β-methylstyrene oxides. These results suggest that the
reactivity of 2-tBu to olefin is lower than that of a reactive species
in the Jacobsen’s reaction.38−40
Hiroshi Fujii: 0000-0003-4611-2983
We investigate the enantioselectivity of the epoxidation
reactions of 2-tBu in order to investigate its relationship to a
reactive intermediate of Jacobsen’s enantioselective epoxidation
reactions (Figures S15 and S16).38−40 The results of the
enantioselectivity are summarized in Table 1 by enantiomer
excess (ee). The ee values of the epoxides produced by 2-tBu
under single-turnover reactions are much lower than those by
Jacobsen’s catalyst under catalytic conditions.38,39 The low
enantioselectivity of 2-tBu is consistent with the planar
conformation of the salen ligand, as suggested by the present
CD spectroscopy. 2-tBu seems not to be a reactive intermediate
in Jacobsen’s enantioselective epoxidation reactions.
Notes
The authors declare no competing financial interest.
ACKNOWLEDGMENTS
■
This study was supported by grants from JSPS (Grants 25288032
and 17H03032), CREST, The Naito Foundation, and The
Uehara Memorial Foundation. We thank IMS for assistance with
EPR measurements.
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