Unprecedented Reactivities of Highly Reactive Manganese(III)-Iodosylarene Porphyrins in Oxidation Reactions

Article abstract of DOI:10.1021/jacs.0c10159

We report that Mn(III)-iodosylarene porphyrins, [MnIII(Porp)(sArIO)]+, are capable of activating the C-H bonds of hydrocarbons, including unactivated alkanes such as cyclohexane, with unprecedented reactivities, such as a low kinetic isotope effect, a saturation behavior of reaction rates, and no electronic effect of porphyrin ligands on the reactivities of [MnIII(Porp)(sArIO)]+. In oxygen atom transfer (OAT) reactions, the sulfoxidation of para-X-substituted thioanisoles by [MnIII(Porp)(sArIO)]+ affords a very unusual behavior in the Hammett plot with the saturation behavior of reaction rates and no electronic effect of porphyrin ligands on reactivities. The reactivities and mechanisms of [MnIII(Porp)(sArIO)]+ are then compared with those of the corresponding MnIV(Porp)(O) complex. The present study reports the first example of highly reactive Mn(III)-iodosylarene porphyrins with unprecedented reactivities in C-H bond activation and OAT reactions.

Full text of DOI:10.1021/jacs.0c10159

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Unprecedented Reactivities of Highly Reactive Manganese(III)−
Iodosylarene Porphyrins in Oxidation Reactions
Lina Zhang, Yong-Min Lee, Mian Guo, Shunichi Fukuzumi, and Wonwoo Nam*
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ABSTRACT: We report that Mn(III)-iodosylarene porphyrins, [MnIII(Porp)(sArIO)]+, are capable of activating the C−H bonds of
hydrocarbons, including unactivated alkanes such as cyclohexane, with unprecedented reactivities, such as a low kinetic isotope
effect, a saturation behavior of reaction rates, and no electronic effect of porphyrin ligands on the reactivities of
[MnIII(Porp)(sArIO)]+. In oxygen atom transfer (OAT) reactions, the sulfoxidation of para-X-substituted thioanisoles by
[MnIII(Porp)(sArIO)]+ affords a very unusual behavior in the Hammett plot with the saturation behavior of reaction rates and no
electronic effect of porphyrin ligands on reactivities. The reactivities and mechanisms of [MnIII(Porp)(sArIO)]+ are then compared
with those of the corresponding MnIV(Porp)(O) complex. The present study reports the first example of highly reactive Mn(III)−
iodosylarene porphyrins with unprecedented reactivities in C−H bond activation and OAT reactions.
eme and nonheme iron enzymes utilize various metal−
Scheme 1. Mn(III)−Iodosylarene Porphyrins and Their
Reactivities in C−H Bond Activation and Sulfoxidation
Reactions
H
oxygen intermediates, such as metal-oxo, -superoxo,
-peroxo, and -hydroperoxo species, in the oxidation of organic
compounds.1−4 To understand the geometric and electronic
structures and the reactivities of the intermediates, reactions of
model compounds with artificial oxidants have been
extensively investigated over the past several decades.1,2,4
Among the artificial oxidants, iodosylarenes (ArIO)5 have been
frequently used as terminal oxidants in generating metal-oxo
intermediates as well as in catalytic oxidation reactions.1,2,4
Indeed, a number of metal−oxo complexes have been
synthesized using the ArIO oxidant.1,2,4,6 In the catalytic
oxidation reactions, in addition to the metal-oxo intermediates,
metal−iodosylarene adducts (Mn+−ArIO), which are the
precursors of the metal−oxo species, have been proposed as
active oxidants that effect the oxidation reactions (i.e., one
oxidant versus multiple oxidants debate).7 Indeed, Mn+−ArIO
intermediates have been isolated and characterized spectro-
scopically and/or structurally.8−10 However, those Mn+−ArIO
complexes were not strong oxidants except a nonheme
iron(III)−iodosylarene complex;10c therefore, only a limited
number of oxidation reactions were investigated with the
isolated Mn+−ArIO complexes.9,10
Very recently, we reported the synthesis, characterization,
and reactivity of Mn(III)−iodosylarene porphyrins, such as
[MnIII(sArIO)(TDCPP)]+ (1a)11 (see Scheme 1 for struc-
ture); this intermediate showed high chemo- and stereo-
selectivity in olefin epoxidation reactions.8 In the present
study, we demonstrate that 1a is highly reactive in the C−H
bond activation of hydrocarbons and oxygen atom transfer
(OAT) reactions (Scheme 1); it is notable that the reactivity of
1a is comparable to that of the corresponding Mn(IV)−oxo
porphyrin complex, MnIV(O)(TDCPP) (2a), which is a highly
reactive oxidant that effects the C−H bond activation via an
oxygen non-rebound mechanism.12 Interestingly, the Mn-
(III)−iodosylarene adducts have shown unexpected reactivities
in the C−H bond activation and OAT reactions (see Scheme
1). The mechanisms of the oxidation reactions by
[MnIII(Porp)(ArIO)]+ are also discussed in this study.
The Mn(III)-iodosylarene porphyrins, such as 1a,
[MnIII(sPhIO)(TDFPP)]+ (1b),11 and [MnIII(sPhIO)-
(TPFPP)]+ (1c)11 (see Scheme 1 for structures), were
synthesized according to the published procedures (Support-
Received: September 23, 2020
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© XXXX American Chemical Society
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ing Information, Experimental Section).8 The reactivity of 1a
was then investigated in the C−H bond activation of
hydrocarbons. Upon addition of xanthene to 1a in CH2Cl2
at −60 °C, the peak at 462 nm due to 1a disappeared with the
increase of the peak at 477 nm due to [MnIII(TDCPP)]+
(Figure 1a). The first-order rate constants increased with
effect on the reactivities of Mn(III)−iodosylarene porphyrins.
This observation is of interest because the reactivities of iron-
oxo porphyrins are significantly affected by the electronic
nature of porphyrin ligands in oxidation reactions, suggesting
that the active intermediate may be different from the metal-
oxo porphyrin species (vide infra).14
More interestingly, when the C−H bond activation of
hydrocarbons, such as xanthene, DHA, CHD, indene, and
xanthene-d2, by 1a was performed with large amounts of
substrates, saturation plots were obtained (Figure 2a; Table S2
Figure 1. (a) UV−vis spectral changes showing the reaction of 1a
(0.10 mM, red line) and xanthene (15 mM) in CH2Cl2 at −60 °C.
Inset shows the plots of kobs against concentrations of xanthene (red
circles) and xanthene-d2 (blue squares) to determine k2 values with
1a. (b) Plots of log k2′ (k2′ = k2/number of equivalent target C−H
bonds) against the substrates C−H BDEs for 1a (red circles) and 2a
(blue circles).
Figure 2. Plots of kobs against concentrations of substrates [xanthene
(black circles), DHA (red circles), CHD (blue circles), and xanthene-
d2 (green circles)] in the reactions of (a) 1a and (b) 2a in CH2Cl2 at
−60 °C.
increasing xanthene concentration, giving a second-order rate
constant of 1.2 × 10−1 M−1 s−1 at −60 °C (Figure 1a, inset).
Similarly, a second-order rate constant of 8.4 × 10−2 M−1 s−1 at
−60 °C was determined with deuterated xanthene (xanthene-
d2) (Figure 1a, inset), giving a KIE value of 1.4. In contrast, a
KIE value of 11 was determined in the oxidation of xanthene
and xanthene-d2 by a Mn(IV)−oxo complex bearing the same
TDCPP ligand (2a) (Figure S1). Other Mn(III)−iodosylarene
porphyrins also afforded small KIE values in the oxidation of
xanthene and xanthene-d2, such as 2.0 for 1b and 1.4 for 1c
(Figure S2).
When the C−H bond activation by 1a and 2a was
investigated with other substrates, such as 9,10-dihydroan-
thracene (DHA, BDE = 77 kcal mol−1),13 1,4-cyclohexadiene
(CHD, BDE = 78 kcal mol−1),13 and indene (BDE = 79 kcal
mol−1)13 (Table S1 and Figures S3 and S4), good linear
correlations between the bond dissociation energies (BDEs) of
the substrates and the reaction rate constants were observed
(Figure 1b). Interestingly, the reactivity of 1a was slightly
greater than that of 2a (Figure 1b). In the reactions of DHA,
CHD, and indene with other Mn(III)−iodosylarene porphyr-
ins, such as 1b and 1c, good linear correlations between the
BDEs of the substrates and the reaction rate constants were
also observed with similar reactivities (Table S1 and Figures
S5−S7), indicating that there is no significant porphyrin ligand
and Figure S8). In contrast, a linear correlation between the
reaction rates and the substrate concentrations was observed in
the reactions of 2a with the substrates (Figure 2b); the
observation of the good linear correlation plot irrespective of
the concentration of substrates indicates that a hydrogen atom
(H atom) abstraction by 2a is the rate-determining step
(r.d.s.). In contrast, the saturation plots observed in the C−H
bond activation reactions by 1a indicate the presence of a
relatively fast equilibrium that precedes the r.d.s. H atom
abstraction (Scheme 2) (vide infra), as proposed in the C−H
bond activation by a Mn(IV)−(oxo)(hydroxo) complex by
Costas and co-workers; the latter complex also afforded a low
KIE of 2.0 in the C−H bond activation of xanthene.15
Other substrates with stronger C−H bonds, such as
ethylbenzene (BDE = 87 kcal mol−1),13 cyclooctane (BDE =
95.7 kcal mol−1),13 and cyclohexane (BDE = 99.5 kcal
mol−1),13 were also oxidized by 1a (Figure S9). By analyzing
products formed in the reactions of Mn(III)−iodosylarene
porphyrins and ethylbenzene in CH2Cl2 at −60 °C, we found
that 1-phenylethanol (78(4)%) was yielded as the oxygenated
product (Table S3). In addition, the product(s) formed in the
oxidation of cyclohexane by 1a was cyclohexanol (61(4)%).
These results are contrary to the previously reported oxidation
of hydrocarbons by Mn(IV)−oxo porphyrins, in which
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Scheme 2. Proposed Mechanism for the Oxidation of
Substrates by Mn(III)−Iodosylarene Porphyrins
halogenated products were yielded predominantly in halo-
genated solvents, such as CH2Cl2 (Table S3).12 Further, we
found out that oxygen atoms in the oxygenated products
formed in the oxidation of ethylbenzene and cyclohexane by
18O-labeled Mn(III)−iodosylarene porphyrins derived from
the Mn(III)−iodosylarene porphyrins (Figures S10 and S11).
Mn(III) porphyrins were the decay product of Mn(III)−
iodosylarene porphyrins (Figure S12).
Figure 3. (a) Hammett plots of log k2 (black circles, a low substrate
concentration) and log kox (red circles, a large substrate concen-
tration) against the σp values of para-X-substituted thioanisoles for
For the OAT reactions by Mn(III)−iodosylarene porphyr-
ins, we investigated the oxidation of para-X-substituted
thioanisoles and then compared their reactivities to that of a
Mn(IV)−oxo porphyrin complex. First, as observed in the C−
H bond activation reactions, saturation plots were obtained
when large amounts of substrates were used in the reactions of
1a, 1b, and 1c (Tables S4−S6 and Figures S13−S15),
indicating a relatively fast equilibrium that precedes the
oxygen transfer to the thioanisole substrates (Scheme 2)
(vide infra). In contrast, a linear correlation plot was obtained
in the oxidation of thioanisoles by 2a (Figure S16). Second,
Hammett plots of Mn(III)−iodosylarene porphyrins and
Mn(IV)−oxo porphyrin were compared in the oxidation of
para-X-substituted thioanisoles using small amounts of
substrates (i.e., with initial rates) (Figures S17−S19).
Interestingly, positive ρ values were obtained in the oxidation
of para-X-substituted thioanisoles by Mn(III)−iodosylarene
porphyrins when initial rates were plotted against the σp+ of the
substrates (Figure 3a, black line and Figure S20, black lines).
This observation is contrary to the known fact that
electrophilic oxidants exhibit a negative slope in Hammett
plot;16 we attribute the observed positive ρ value to the
existence of an equilibrium to form a precursor complex that
precedes the OAT to the thioanisole derivatives (Scheme 2)
(vide infra). It should be also noted that negative ρ values were
obtained in the Hammett plot when reaction rates determined
+
the sulfoxidation of thioanisole derivatives by 1a. (b) Hammett plots
+
of log k2 against the σp values of para-X-substituted thioanisoles for
the sulfoxidation of thioanisole derivatives by 2a.
Scheme 2, there is an equilibrium between the Mn(III)−
iodosylarene porphyrin and the substrate before the oxidation
reaction takes place. Accordingly, the saturation plot can be
fitted by eq 1,
kobs = koxKf[subs]/(1 + Kf[subs])
(1)
where Kf, which is k1/k−1, is the formation constant of
{[(Porp)MnIII(sArIO)]+·subs}‡ and kox is the rate constant of
the oxidation by {[(Porp)MnIII(sArIO)]+·subs}‡.15,17,18 Thus,
at high concentrations of substrate (i.e., Kf[subs] ≫ 1), kobs
becomes kox, and the existence of {[(Porp)MnIII(sArIO)]+·
subs}‡ can explain the independency of the substrate
concentration on the reaction rate, such as kobs = kox when
Kf[subs] ≫ 1.
With invoking the formation of {[(Porp)MnIII(sArIO)]+·
subs}‡ between Mn(III)−iodosylarene porphyrins and sub-
strates before the oxidation reaction takes place (Scheme 2),
we rationalize the observed positive ρ values in Hammett plots
as follows: the formation constant of {[(Porp)MnIII(sArIO)]+·
subs}‡, Kf, depends on the electronic interaction between
Mn(III)−iodosylarene porphyrins and substrates. That is, the
most electron-withdrawing substituent (e.g., para-CN-thioani-
sole) would afford the largest Kf, whereas the most electron-
donating substituent (e.g., para-MeO-thioanisole) would
afford the smallest Kf. Therefore, since the kox values depend
on the amount of {[(Porp)MnIII(sArIO)]+·subs}‡ generated,
the reaction of the intermediate species with para-CN-
thioanisole would occur faster than that with para-MeO-
thioanisole, which resulted in giving a positive ρ value in the
reactions performed with small amounts of substrates (Figure
3a, black line and Figure S20, black lines).
+
with high substrate concentrations were plotted against the σp
of the substrates (Figure 3a, red line and Figure S20, red lines).
In the case of Mn(IV)−oxo porphyrin, a negative ρ value was
obtained in the oxidation of thioanisole derivatives by 2a
irrespective of the concentrations of substrates (Figure 3b;
Figure S16). Product analysis revealed that [MnIII(TDCPP)]+
was formed as the decay product of 1a (Figure S21) and
methyl phenyl sulfoxide (>95%) as the organic product
(Figure S22). Finally, we observed no significant porphyrin
ligand effect in the oxidation of thioanisoles by Mn(III)−
iodosylarene porphyrins (Table S7 and Figures S13−S15), as
observed in the C−H bond activation reactions (vide supra).
Then, how do we interpret the saturation behavior of
reaction rates observed in the C−H bond activation and OAT
reactions by Mn(III)−iodosylarene porphyrins? As shown in
Finally, what is the nature of the active oxidant that oxidizes
substrates in the oxidation reactions by Mn(III)−iodosylarene
porphyrins? Recently, Wang and co-workers proposed two
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resonance valence-bond electronic structures from density
functional theory (DFT) calculations in an iron(III)−
iodosylarene complex-mediated sulfoxidation reaction.19 Sim-
ilarly, we propose here that in the cage, such as {[(Porp)-
MnIII(sArIO)]+·subs}‡ in Scheme 2, [(Porp)MnIII(ArIO)]+ can
oxidize substrate directly (Scheme 2, structure a). Alter-
natively, [(Porp)MnV(O)]+, which is formed via the O−I bond
cleavage of [(Porp)MnIII(ArIO)]+, oxidizes substrates
(Scheme 2, structure b). Since it has been known that
MnV−oxo complexes of porphyrin and nonporphyrin ligands
with a low-spin state (S = 0) are sluggish oxidants,20 we may
exclude a low-spin S = 0 [(Porp)MnV(O)]+ complex as an
active oxidant.21 Further, reactivities of the Mn(III)−
iodosylarene porphyrins obtained in this study, such as a low
KIE (e.g., ∼1.5) and no porphyrin ligand effect, are very
different from those of Mn(IV)−oxo porphyrins, such as a
large KIE (e.g., >10) and a significant porphyrin ligand effect
on reaction rates (see Scheme 3). Based on the results
AUTHOR INFORMATION
Corresponding Author
■
Wonwoo Nam − Department of Chemistry and Nano Science,
Ewha Womans University, Seoul 03760, Korea; School of
Chemistry and Chemical Engineering, University of Jinan,
Jinan 250022, China; orcid.org/0000-0001-8592-4867;
Email: wwnam@ewha.ac.kr
Authors
Lina Zhang − Department of Chemistry and Nano Science,
Ewha Womans University, Seoul 03760, Korea;
orcid.org/0000-0003-4539-2741
Yong-Min Lee − Department of Chemistry and Nano Science,
Ewha Womans University, Seoul 03760, Korea;
orcid.org/0000-0002-5553-1453
Mian Guo − Department of Chemistry and Nano Science,
Ewha Womans University, Seoul 03760, Korea;
orcid.org/0000-0001-7460-5852
Shunichi Fukuzumi − Department of Chemistry and Nano
Science, Ewha Womans University, Seoul 03760, Korea;
orcid.org/0000-0002-3559-4107
Scheme 3. Reactivity Comparison of Mn(III)−Iodosylarene
Porphyrin versus Mn(IV)−Oxo Porphyrin Intermediates
Complete contact information is available at:
https://pubs.acs.org/10.1021/jacs.0c10159
Notes
The authors declare no competing financial interest.
ACKNOWLEDGMENTS
■
This work was supported by the NRF of Korea through CRI
(NRF-2012R1A3A2048842).
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* Supporting Information
The Supporting Information is available free of charge at
https://pubs.acs.org/doi/10.1021/jacs.0c10159.
Experimental Section, Tables S1−S7 and Figures S1−
S22 (PDF)
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Journal of the American Chemical Society
pubs.acs.org/JACS
Communication
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moiety as a possible intermediate, since this Mn(V)−O species has
not been captured and/or synthesized yet.
F
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J. Am. Chem. Soc. XXXX, XXX, XXX−XXX