Synthesis, characterization and catalytic activity of a mononuclear nonheme copper(II)-iodosylbenzene adduct

Article abstract of DOI:10.1016/j.jinorgbio.2021.111524

Iodosylbenzene (PhIO) and its derivatives have attracted significant attention due to their various applications in organic synthesis and biomimetic studies. For example, PhIO has been extensively used for generating high-valent metal-oxo species that have been regarded as key intermediates in diverse oxidative reactions in biological system. However, recent studies have shown that metal-iodosylbenzene adduct, known as a precursor of metal-oxo species, plays an important role in transition metal-catalyzed oxidation reactions. During last few decades, extensive investigations have been conducted on the synthesis and reactivity studies of metal-iodosylbenzene adducts with early and middle transition metals including manganese, iron, cobalt. Nevertheless, metal-iodosylbenzene adducts with late transition metals such as nickel, copper and zinc, still remains elusive. Herein, we report a novel copper(II)-iodosylbenzene adduct bearing a linear ligand composed of two pyridine rings and an ethoxyethanol side-chain, [Cu(OIPh)(HN₃O₂)]²⁺ (1). The copper(II)-iodosylbenzene adduct was characterized by several spectroscopic methods including UV–vis spectroscopy, electrospray ionization mass spectrometer (ESI MS), and electron paramagnetic resonance (EPR) combined with theoretical calculations. Interestingly, 1 can carry out the catalytic sulfoxidation reaction. In sulfoxidation reaction with thioanisole under catalytic reaction condition, not only two-electron but also four-electron oxidized products such sulfoxide and sulfone were yielded, respectively. However, 1 was not an efficient oxidant towards C–H bond activation and epoxidation reactions due to the steric hindrance created by the intramolecular H-bonding interaction between HN₃O₂ ligand and iodosylbenzene moiety.

Full text of DOI:10.1016/j.jinorgbio.2021.111524

Journal of Inorganic Biochemistry 223 (2021) 111524
Contents lists available at ScienceDirect
Journal of Inorganic Biochemistry
journal homepage: www.elsevier.com/locate/jinorgbio
Synthesis, characterization and catalytic activity of a mononuclear
nonheme copper(II)-iodosylbenzene adduct
Hyeri Jeon, Hana Oh, Seungwoo Hong*
Department of Chemistry, Sookmyung Women’s University, 04310, Seoul, 03722, Republic of Korea
A R T I C L E I N F O
A B S T R A C T
Keywords:
Iodosylbenzene (PhIO) and its derivatives have attracted significant attention due to their various applications in
organic synthesis and biomimetic studies. For example, PhIO has been extensively used for generating high-
valent metal-oxo species that have been regarded as key intermediates in diverse oxidative reactions in bio-
logical system. However, recent studies have shown that metal-iodosylbenzene adduct, known as a precursor of
metal-oxo species, plays an important role in transition metal-catalyzed oxidation reactions. During last few
decades, extensive investigations have been conducted on the synthesis and reactivity studies of metal-
iodosylbenzene adducts with early and middle transition metals including manganese, iron, cobalt. Neverthe-
less, metal-iodosylbenzene adducts with late transition metals such as nickel, copper and zinc, still remains
elusive. Herein, we report a novel copper(II)-iodosylbenzene adduct bearing a linear ligand composed of two
pyridine rings and an ethoxyethanol side-chain, [Cu(OIPh)(HN3O2)]2+ (1). The copper(II)-iodosylbenzene
adduct was characterized by several spectroscopic methods including UV–vis spectroscopy, electrospray ioni-
zation mass spectrometer (ESI MS), and electron paramagnetic resonance (EPR) combined with theoretical
calculations. Interestingly, 1 can carry out the catalytic sulfoxidation reaction. In sulfoxidation reaction with
thioanisole under catalytic reaction condition, not only two-electron but also four-electron oxidized products
Reactive intermediate
Iodosylbenzene adduct
Intramolecular H-bonding
Electron transfer
Catalytic reaction Cu(II) complex
–
such sulfoxide and sulfone were yielded, respectively. However, 1 was not an efficient oxidant towards C
H
bond activation and epoxidation reactions due to the steric hindrance created by the intramolecular H-bonding
interaction between HN3O2 ligand and iodosylbenzene moiety.
1. Introduction
reagents have led to an increased interest in rapid development in the
syntheses and functionalization of heterocyclic compounds via facili-
Copper containing metalloenzymes conducts reductive activation of
dioxygen mediated fundamental processes, which govern a myriad of
biological functions such as cellular respiration and oxidative trans-
formation of biomolecules [1–10]. Tremendous efforts towards under-
standing how these oxidation and oxygenation processes occur have led
many researchers to pinpoint spectroscopic features of plausible cop-
per oxygen intermediates, disclose their reactivity and provide detailed
mechanistic information [11–16]. Among proposed copper oxygen in-
termediates, less is known about high-valent mononuclear copper-oxo
[CuO]+ species, namely Cu(III)-oxo or Cu(II)-oxyl radical complexes
due to their intrinsic instability; only theoretical and gas-phase in-
vestigations predict their existence and their particularly high reactivity,
thereby, great controversy and debate persisted over pros and cons of
their formation [17–22].
tating atom transfer reactions [23]. While iodosylbenzene (PhIO) has
been tremendously used as an oxygen atom source in transition metal-
catalyzed functionalization of hydrocarbons, great efforts have been
devoted to uncover the mechanistic insights into high-valent metal-oxo
species and their reactivity patterns by promoting “shunt pathway” with
aliquots of PhIO. Multiple examples of heme and nonheme metal-oxo
species, which were central in catalytic properties of various interfaces
have been synthesized and spectroscopically characterized [24–30].
Therefore, one of solid consensus approach of generating the metal-oxo,
[MO]2+, is by reacting various model complexes with PhIO.
More recently, accumulated evidences that come under recent
scrutiny supported that the iodosylbenzene adducts of metal complexes,
[M-OIPh], were successfully prepared, isolated and characterized spec-
troscopically despite that their generation was transient and unstable
[31]. For example, the structure and reactivity of an iodosylarene adduct
Remarkable oxidizing properties of hypervalent iodine(III) and (IV)
* Corresponding author.
E-mail address: hsw@sookmyung.ac.kr (S. Hong).
https://doi.org/10.1016/j.jinorgbio.2021.111524
Received 30 April 2021; Received in revised form 11 June 2021; Accepted 25 June 2021
Available online 29 June 2021
0162-0134/© 2021 Elsevier Inc. All rights reserved.

H. Jeon et al.
Journal of Inorganic Biochemistry 223 (2021) 111524
of heme and nonheme Mn(III), Mn(IV), and Fe(III) complexes have been
extensively investigated [32–38]. On the basis of this discovery,
multiple-oxidant mechanism, in which metal-iodosylarene adducts were
proposed as a precursor of high-valent metal-oxo intermediates, was
surged and some of them exhibited the reactivity superior to their cor-
responding metal-oxo intermediates. Moving from iron to late transition
metals such as cobalt, nickel, and copper, the formation of high-valent
metal-oxo species is theoretically inaccessible in tetragonal geometry
by virtue of the “oxo wall” coined by Gray and Ballhausen [22,39]. It
spectra were recorded at 80 K using an X-band Bruker EMX-plus spec-
trometer equipped with a dual mode cavity (ER 4116DM). The low
temperatures were achieved and controlled by an Oxford Instruments
ESR900 liquid He quartz cryostat with an Oxford Instruments ITC503
temperature and gas flow controller. The experimental parameters for
EPR measurements were as follows: microwave frequency = 9.646 GHz,
microwave power
=
1.0 mW, modulation amplitude
= 10 G,
gain 104, modulation frequency =
=
1
×
100 kHz, time
constant = 40.96 ms, and conversion time = 85.00 ms. Product analyses
were performed with an Agilent 1220 infinity II high performance liquid
chromatograph (HPLC), and Agilent Technologies 6890 N gas chro-
matograph (GC). Electrochemical measurements were performed on a
CHI630B electrochemical analyzer (CH Instruments, Inc.) in CH3CN
containing 0.10 M Bu4NPF6 (TBAPF6) as a supporting electrolyte at
293 K. A conventional three-electrode cell was used with a platinum
working electrode (surface area of 0.30 mm2), a platinum wire as a
counter electrode, and a Ag/Ag+ electrode as a reference electrode. The
platinum working electrode was routinely polished with BAS polishing
alumina suspension and rinsed with acetone and acetonitrile before use.
The measured potentials were recorded with respect to a Ag/Ag+
(0.010 M) reference electrode.
predicts eventual electron population in an M–O π* antibonding orbital
resulting in the decrease of the bond order below 2. Instead, the for-
mation of iodosylarene adduct of cobalt(II) and cobalt(III) complexes
were evidenced crystallographically [40,41]. However, the iodosylarene
adduct of copper complexes still remained rare to date. Herein, we
report the synthesis and spectroscopic characterization of a copper(II)
iodosylbenzene adduct, [Cu(OIPh)(HN3O2)]2+ (1; HN3O2 ¼ 2-(2-(bis
(pyridin-2-ylmethyl)amino)ethoxy)ethanol) (Scheme 1). DFT optimized
structures suggest that H-bonding interaction can be formed to the
proximal oxygen atom in iodosylbenzene moiety in order to afford an
additional thermal stability. We then investigate its redox and electron-
transfer properties and examine the kinetic behaviors in catalytic re-
–
actions such as C H bond activation, oxygen atom transfer and olefin
2.3. Generation and characterization of [Cu(OIPh)(HN3O2)]2+ (1)
epoxidation reactions. Although slow reactivity was confirmed, 1 shows
moderate catalytic oxygen atom transfer reactivity, however, sluggish
–
reactivity in the C H bond activation, and olefin epoxidation reactions.
The mononuclear nonheme copper(II) iodosylbenzene adduct, [Cu
(OIPh)(HN3O2)]2+ (1), was prepared by treating a CH3CN-solution of
[Cu(HN3O2)(CH3CN)(H2O)](ClO4)2 with 3.0 equiv. of PhIO in CH3CN at
293 K. The formation of 1 was confirmed by monitoring the band growth
at 345 nm from UVꢀ vis spectral changes of the reaction solution. The
electron paramagnetic resonance (EPR) spectra of 1 (1.0 mM) were
recorded at 4 K. 18O-labeled sample of 1, [Cu(18OIPh)(HN3O2)]2+
(1-18O), was prepared by using 18O-labeled PhIO, which was synthe-
sized by incubating PhIO in MeOH (5% H128O).
2. Experiments
2.1. Materials
All chemicals were obtained from Aldrich Chemical Co. and used
without further purification unless otherwise indicated. Solvents were
dried according to the reported procedures and distilled under Ar prior
to use [42]. H128O (95% 18O-enriched) was purchased from ICON Ser-
vices Inc. (Summit, NJ, USA). The mononuclear nonheme copper(II)
complex, [Cu(HN3O2)(CH3CN)(H2O)](ClO4)2, was prepared according
to the literature methods [43].
2.4. Kinetic studies and product analysis
All reactions were run in 1 cm UV quartz cuvette and followed by
monitoring UVꢀ vis spectral changes of reaction solutions. Rate con-
stants were determined under pseudo-first-order conditions (e.g., [sub-
strate]/[1] > 10) by fitting the changes in absorbance for the
disappearance of bands at 345 nm for 1, 620 nm for ferrocene, and
650 nm for dimethylferrocene, respectively, in the phosphorus oxidation
and electron transfer reactions. The catalytic reactions of 1 with thio-
anisole, cyclohexene, and styrene were conducted in CH3CN at 293 K.
The kinetic experiments were run at least in triplicate, and the data
reported represent the average of these reactions.
Caution! The peroxidized compound and perchlorate ion should be
treated as potential bombs. This can be done safely for relatively low levels of
peroxides and perchlorate ions (2 mM and 2.0 mL volume).
2.2. Instrumentation
UVꢀ vis spectra were recorded on a Hewlett-Packard Agilent 8453
UVꢀ visible spectrophotometer equipped with a T2/sport temperature
controlled cuvette holder. Electrospray ionization mass spectra (ESI MS)
were collected on a Thermo Finnigan (San Jose, CA, USA) LTQ XL ion
Organic products formed in the catalytic reactions of 1 with thio-
anisole, cyclohexene and styrene were analyzed by high performance
liquid chromatography (HPLC) and gas chromatography (GC). The
product yields were determined by comparing the peak areas of sample
trap instrument, by infusing samples directly into the source at 5.0 μL/
min using a syringe pump. The spray voltage was set at 4.7 kV and the
capillary temperature at 200 ◦C. electron paramagnetic resonance (EPR)
Scheme 1. Schematic representation showing synthesis, characterization and reactivity of [Cu(OIPh)(HN3O2)]2+
.
2

H. Jeon et al.
Journal of Inorganic Biochemistry 223 (2021) 111524
products against standard curves prepared with known authentic
samples.
2.5. Density functional theory calculations
Geometry optimizations of 1a, and 1b were directly modified from
the single-crystal structure of [CuII(HN3O2)(CH3CN)(H2O)](ClO4)2. In
1b, the intramolecular H-bond interactions between the H atom of the
HN3O2 ligand and the O atom of iodosylbenzene was included. All
structures were fully optimized by using the Gaussian 09 [44] program
with the B3LYP density functional at the LANL2DZ/6-31G** level
[45–58], whereas no imaginary frequencies were observed. In addition,
the solvent effect of the polarizable continuum model (PCM) with
acetonitrile (ε = 35.688) and temperature (293 K) was also applied [59].
Energy evaluation were done at B3LYP/LANL2DZ/6–311 + G** level
[45–48,60–69] on the structure obtained by B3LYP/LANL2DZ/6-31G**.
3. Results and discussion
3.1. Synthesis and characterization of iodosylbenzene adduct copper(II)
complex
The copper(II) complex coordinated with a HN3O2 ligand, [Cu
(HN3O2)(CH3CN)(H2O)](ClO4)2, was synthesized according to the
literature methods [43]. Addition of iodosylbenzene (PhIO) into a
CH3CN-solution containing [Cu(HN3O2)(CH3CN)(H2O)](ClO4)2 affor-
ded gradual solution colour change from blue to pale blue along with
appearance of absorption band at 345 nm (
ε
= 2200 cmꢀ 1 Mꢀ 1) and at
641 nm (
ε
= 100 cmꢀ 1
M
ꢀ 1) 293 K (Fig. 1a). According to the spec-
troscopic titration, the absorbance at 345 nm due to 1 was saturated
when 3.0 equiv. of PhIO was added (Fig. S1). The colour of resulting
solution of this new species, denoted as 1, was metastable with a half-life
time (t1/2) of ~30 min allowing us to spectroscopically characterize 1. It
is noteworthy that the stability of other metal-iodosylbenzene adducts in
a tetragonal geometry were markedly increased in a mixture solvent
system (e.g, acetone:trifluoroethanol) or in the presence of acids (e.g.,
perchloric acid) [36–38,41]. However, the addition of perchloric acid
resulted in the fast decay of 1. To verify the formation of iodosylbenzene
adduct of copper complex, electrospray ionization mass (ESI MS),
electron paramagnetic resonance (EPR), and infrared (IR) spectroscopic
measurements were conducted. The ESI MS spectrum of 1 exhibited
minor mass peaks at m/z of 668.5, whose mass and isotope distribution
patterns were fully consistent with [Cu(OIPh)(HN3O2)(ClO4)]+ (calcu-
lated m/z of 668.9) (Fig. 1b). When 1 was synthesized with 18O isotope
labeling PhIO (PhI18O), a two mass unit shift from m/z of 668.5 to 670.5
was observed, suggesting the incorporation of an oxygen atom within 1
(Fig. 1b, inset). The IR spectrum of 1 showed isotopically sensitive
infrared bands at 715 cmꢀ 1, which was shifted to 678 cmꢀ 1 upon the use
of PhI18O (Fig. 1c). The observed shift of Δ = 37 cmꢀ 1 was in a good
agreement with the calculated value of 36 cmꢀ 1 expected for a diatomic
Fig. 1. (a) UV–vis spectra obtained in the reaction of 1 (0.10 mM) with iodo-
sylbenzene (0.30 mM) in CH3CN at 293 K. Inset shows the visible spectrum of 1
(2.0 mM) in the region of 400–900 nm. (b) ESI MS spectra of 1 (0.10 mM) and
18O-labeled 1. The peaks at m/z of 449.0 and 668.5 correspond to [Cu(HN3O2)
(ClO4)]+ (calculated m/z of 449.1) and [Cu(OIPh)(HN3O2)(ClO4)]+ (calculated
m/z of 668.9), respectively. Insets show the observed isotope distribution pat-
terns of 1 and 18O-labeled 1. (c) IR spectra of solid 1 (black line) and 18O-
labeled 1 (red line). * and # are indicative of solvent and ligand peaks,
respectively. (For interpretation of the references to colour in this figure legend,
the reader is referred to the web version of this article.)
–
–
O
I oscillator dictated by Hooke’s law. Compared to the O I bond
stretching frequencies of other reported metal-iodosylbenzene adduct
that appeared in the range of 600–750 cmꢀ 1 [32–38,40,41], 1 showed
–
relatively strong O I bond vibrational energy due to low oxidation state
of copper. The EPR spectrum of 1, recorded in a frozen CH3CN solution
at 4 K, exhibited a rhombic signal at g = 2.28 (A = 120 G), 2.05 and 1.98
(Fig. S2). Such moderate hyperfine coupling constant value (A) of 1
implied that the geometry of 1 is slightly distorted square planar,
thereby, a distorted tetragonally elongated geometry of 1 due to Jahn-
Teller disortion was anticipated.
interaction, we first constructed two possible structures of 1 with and
without H-bonding from the known crystal structure of [Cu(HN3O2)
(CH3CN)(H2O)]2+, 1a and 1b, respectively and carried out their struc-
tural optimization [43]. DFT optimized structures of both 1a and 1b are
shown in Fig. 2. For 1a without intramolecular H-bonding, the opti-
mized structure of the ground state showed tetragonally elongated ge-
3.2. Density functional theory (DFT) calculations
To gain insight into structural information, DFT calculations was
performed on the iodosylbenzene adduct of copper(II) complex, 1. Given
that HN3O2 ligand was prone to form the intramolecular H-bonding
–
ometry around the Cu center with a Cu O distance of 1.942 Å (Fig. 2,
left). This bond distance was in line with other structurally characterized
3

H. Jeon et al.
Journal of Inorganic Biochemistry 223 (2021) 111524
Fig. 2. DFT optimized structures of 1 without (left) and with H-bonding
interaction (right) between HN3O2 ligand and iodosylbenzene moiety. Colors
for the structures: Cu, orange; N, blue; O, red; I, purple; C, pale gray; H, white.
Relative energy diagram (bottom) of 1a and 1b. (For interpretation of the
references to colour in this figure legend, the reader is referred to the web
version of this article.)
iodosylbenzene adduct of metal complexes [32–38,40,41]. In case of 1b
–
with intramolecular H-bonding, the Cu O distance was slightly elon-
–
gated to 1.958 Å due to the weakening of the Cu O bond in comparison
to the structure of 1a without H-bonding at oxygen atom (Fig. 2, right).
The relative energies of 1a and 1b were illustrated in Fig. 2. The relative
energies diagram showed that 1a was energetically unfavorable than 1b;
the energy of 1b is lower (ca. 4.96 kcal/mol) than that of 1a; this clearly
suggested that the H-bonding interaction between the HN3O2 ligand and
the proximal oxygen atom of the iodosylbenzene moiety provided
additional stability to 1. Moreover, the TD-DFT calculated spectrum
showed an absorption band at 343 nm (Fig. S3). This absorption band
was due to a collection of electronic transitions from the HN3O2 ligand
to Cu d orbitals. Therefore, we proposed that H-bonding at the oxygen
atom of PhIO is more plausible structure.
3.3. Electron transfer reactivity of 1
The reactivity of the copper(II)-iodosylbenzene adduct, 1, was first
investigated in the electron transfer (ET) at 293 K. To examine the redox
properties of 1, the cyclic voltammetry was conducted in CH3CN at
293 K. Electrochemical measurements of 1 were carried out in the
ꢀ 600 mV to +1500 mV range under aerobic conditions at different scan
rates (Fig. 3a). While [Cu(HN3O2)(CH3CN)(H2O)]2+ exhibited a nearly
reversible CuII/I couple with the redox potential (Ered) value of 0.08 V (vs
SCE) [70], the irreversible oxidation (Epa) and reduction (Epc) peak
potential values at 1.35 V (vs Ag/Ag+) and ꢀ 0.28 V (vs Ag/Ag+),
respectively, indicated the low thermodynamic stability of 1 during
redox processes. Typically, the E1/2 value of irreversible redox processes
was found in the middle of Epa and Epc values. To estimate the E1/2 value
of 1, the ET reaction between 1 and ferrocene (Fc) derivatives was
explored. Upon addition of ferrocene (Eox = 0.66 V vs Ag/Ag+) into a
CH3CN-solution of 1, the ET from Fc to 1 was monitored by an increase
Fig. 3. (a) Cyclic voltammogram of 1 (2.0 mM) in CH3CN containing TBAPF6
(0.10 M) with a glassy carbon working electrode at various scan rates at 293 K.
(b) UV–vis spectral changes observed in the ET reaction from Fc (0.50 mM) to 1
(0.10 mM) in CH3CN at 293 K. (c) Plot of pseudo-first-order rate constants (kobs
)
against the concentrations of Fc in CH3CN at 293 K.
Ag+).
3.4. Catalytic reactivity of 1
of the absorption band at 620 nm (
ε
= 400 cmꢀ 1 Mꢀ 1) due to the for-
mation of ferrocenium ion (Fig. 3b). The pseudo-first order rate constant
(kobs) increased proportionally with an increasing concentration of Fc.
The second-order rate constant (ket) value (e.g., 1.3 × 10ꢀ 2 Mꢀ 1 sꢀ 1) of
ET from Fc to 1 was determined from the plot of the kobs values against
concentration of Fc (Fig. 3c). In parallel, the ket value of ET from 1,1′-
dimethylferrocene (Eox = 0.55 V vs Ag/Ag+) to 1 was determined to be
6.6 × 10ꢀ 2 Mꢀ 1 sꢀ 1 under identical reaction conditions (Fig. S4).
However, 1 barely reacted with bromoferrocene (Eox = 0.83 V vs Ag/
Ag+) under identical reaction conditions, indicating that the E1/2 value
of 1 might be positioned between 0.66 V (vs Ag/Ag+) and 0.83 V (vs Ag/
With the metastable copper(II)-iodosylbenzene adduct, 1 in hand,
the catalytic reactivity studies of 1 with substrates including thioanisole,
cyclohexene, and styrene were carried out using PhIO as an oxidant. The
oxygen atom transfer reaction was performed by adding 1 (0.10 mM) to
a CH3CN solution of thioanisole (100 mM) containing PhIO (10 mM) at
293 K under inert atmosphere (Fig. 4). The reactivity of 1 was expected
to be low due to the H-bonding interaction between HN3O2 ligand and
PhIO moiety. The catalytic reaction was carried over several hours. At
the initial stage of the reaction, the methyl phenyl sulfoxide (i.e., two
electron oxidized product) was obtained as the sole product with a
4

H. Jeon et al.
Journal of Inorganic Biochemistry 223 (2021) 111524
Scheme 2. The outlook of reactivity studies for 1 in the electron transfer and
catalytic sulfoxidation reactions.
–
such low catalytic reactivity towards epoxidation and C H bond acti-
vation reactions would be attributed to the intramolecular H-bonding
interaction, which created additional steric hindrance for substrates to
be in close interaction with iodosylbenzene moiety.
Fig. 4. Plot of concentrations of methyl phenyl sulfoxide (black dot), methyl
phenyl sulfone (blue dot) and turnover number (red dot) obtained in the cat-
alytic oxygen atom transfer reaction of thioanisole (100 mM) by [CuII(HN3O2)
(CH3CN)(H2O)](ClO4)2 (0.10 mM) in the presence of PhIO (10 mM). Control
experiments show that only trace amount of products (gray dot) were detected.
(For interpretation of the references to colour in this figure legend, the reader is
referred to the web version of this article.)
4. Conclusion
In summary, we have newly synthesized and characterized an
iodosylbenzene adduct of a mononuclear copper(II) complex supported
by a picolylamine based ligand with ethoxyethanol chain that created
the intramolecular H-bonding interaction between HN3O2 ligand and
iodosylbenzene moiety. DFT calculation suggested that the copper(II)-
iodosylbenzene adduct having the intramolecular H-bonding interac-
tion was energetically more favorable. The electron transfer reactivity of
the copper(II)-iodosylbenzene adduct along with electrochemical mea-
surement suggested that its redox potential was in the range of 0.66 V
and 0.83 V (vs Ag/Ag+). The present system displayed catalytic reac-
tivity towards sulfoxidation reaction in the presence and absence of acid;
the addition of acid accelerated the catalytic sulfoxidation presumably
due to the enhancement of electron transfer rates. However, the epoxi-
turnover number (TON) below 5 (i.e., 5% conversion yield; the TON and
product yield were calculated on the basis of the concentration of PhIO).
Interestingly, when the catalytic reaction proceeded over a day, the
formation of methyl phenyl sulfone (i.e., four electron oxidized product)
was detected. This evidently illustrated further oxidation of methyl
phenyl sulfoxide. The maximum TON of sulfoxidation reached over 75
(i.e., 75% conversion yield). As the amount of products increased line-
arly and slowly over several hours, the initial reaction rate of the reac-
tion was determined from the slope of the concentration of products
against the reaction time. The formation rate of methyl phenyl sulfoxide
and methyl phenyl sulfone were calculated to be 1.3 × 10ꢀ 4 M hꢀ 1 and
2.4 × 10ꢀ 5 M hꢀ 1, respectively. Control experiment was also carried out
under identical reaction conditions in the absence of copper(II) complex
and the negligible amount (less than 0.1% conversion yield based on
PhIO) was found (Fig. 4).
–
dation of styrene and the C H bond activation of cyclohexene did not
meet our expectation; the intramolecular H-bonding interaction might
cause strong steric hindrance and inhibit the catalytic reactivity of the
copper(II)-iodosylbenzene adduct. Further studies will be focused on the
–
cleavage of the I O bond processes in order to provide possible insight
Recently, accumulated reports have demonstrated that mechanisti-
cally, there are two different reaction pathways for the sulfoxidation of
thioanisole such as the direct oxygen atom transfer and electron transfer
pathway; the latter pathway could be accelerated in the presence of
acids. To evaluate the effect of acid on the catalytic activity of 1, the
catalytic sulfoxidation reactions were carried out in the presence of
H2SO4. It is noteworthy that while the TONs of the catalytic reactions
were similar, the initial reaction rates were clearly increased;
4.0 × 10ꢀ 4 M hꢀ 1 in the presence of 10 equiv. of H2SO4 and
2.0 × 10ꢀ 4 M hꢀ 1 in the presence of 5.0 equiv. of H2SO4 (Fig. S5).
Additionally, other oxidants such as tert-butylhydroperoxide, and
hydrogen peroxide were used under identical conditions, however, no
formation of oxidized product was confirmed (Fig. S6). Thus, the reac-
tive oxidant in the sulfoxidation reaction was the copper(II)-
iodosylbenzene adduct (Scheme 2).
into the involvement of high-valent copper complexes.
Author contributions
S.H. conceived, designed the research and wrote the manuscript. H.J.
and H.O. synthesized, run and analyzed the experimental and compu-
tational data. S.H. and H.J. discussed the results and contributed to the
scientific interpretation.
Declaration of Competing Interest
The authors have no competing interests to declare.
Acknowledgments
On the other hand, the oxidation of cyclohexene and styrene were
–
carried out in order to examine the catalytic activity of 1 in the C
H
This research was supported by the National Research Foundation of
Korea (NRF), funded by the Korean government (NRF-
2020R1C1C1008886 to S.H.)
bond activation and olefin epoxidation reactions. For instance, the
oxidation of cyclohexene by 1 resulted in the formation of cyclohexenol,
cyclohexenone, and cyclohexene oxide (Fig. S7). However, control
experiment produced identical products with similar composition and
yield, which led us to conclude that the catalytic oxidation of cyclo-
hexene did not involve 1 but only PhIO, which was acting as sole
reactive oxidant [71]. In case of styrene oxidation, the low catalytic
activity of 1 did barely produce styrene oxide over the course of time
(Fig. S8). For both cases, the presence of acids did not accelerate the
reaction rates nor increase the product yields. This result was contra-
dictory to that observed in the case of iodosylbenzene adduct of man-
ganese, iron and cobalt complexes [36–38,41]. Plausible explanation of
Appendix A. Supplementary data
Supplementary data to this article can be found online at https://doi.
org/10.1016/j.jinorgbio.2021.111524.
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