Nucleophilic reactivity of copper(II)-alkylperoxo complexes

Article abstract of DOI:10.1039/c7cc03965d

Copper(ii)-alkylperoxo adducts, [Cu(CHDAP)(OOR)]⁺ (CHDAP = N,N′-dicyclohexyl-2,11-diaza[3,3](2,6)pyridinophane; R = C(CH₃)₂Ph and ^tBu), were prepared and characterized using various physicochemical methods. These are the first synthetic Cu(ii)-alkylperoxo complexes that can perform aldehyde deformylation (i.e., nucleophilic reactivity) under the stoichiometric reaction conditions, which was confirmed by kinetic studies.

Full text of DOI:10.1039/c7cc03965d

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Received 00th January 20xx
,
Accepted 00th January 20xx
Nucleophilic Reactivity of Copper(II)-Alkylperoxo Complexes†
DOI: 10.1039/x0xx00000x
Bohee Kim, Donghyun Jeong and Jaeheung Cho*
www.rsc.org/
character and the kinetics of aldehyde deformylation by Cu(II)-
Copper(II)-akylperoxo adducts, [Cu(CHDAP)(OOR)]+ (CHDAP
N,N’-dicyclohexyl-2,11-diaza[3,3](2,6)pyridinophane;
=
=
alkylperoxo species at low temperature. In addition, the Cu(II)-
alkylperoxo complexes exhibit different reactivity at higher
temperature, where the Cu(II)-alkylperoxo complexes are
capable of catalyzing C-H bond activation (i.e., electrophilic
reactivity).
R
t
C(CH3)2Ph and Bu), were prepared and characterized by various
physicochemical methods. These are the first synthetic Cu(II)-
alkylperoxo complexes that can perform aldehyde deformylation
(i.e., nucleophilic reactivity) under the stoichiometric reaction
conditions, which was confirmed by kinetic studies.
Mononuclear transition metal alkylperoxo complexes are important
in diverse biological and catalytic oxidation processes.1 Direct
observation of the formation of Fe(III)-alkylperoxo complexes in
biological system has been reported for homoprotocatechuate 2,3-
dioxygenase and purple lipoxygenase.2 The biomimetic studies on
Fe(III)-alkylperoxo model complexes provide significant insights into
the catalytic mechanism of iron containing enzymes and iron
catalyzed synthetic oxidations.3
A number of Cu(II)-alkylperoxo species have been characterized
and some of these complexes served as models of lipid
peroxidation.4,5 Kitajima’s group reported the first crystal structure
Scheme 1 Synthetic procedures for Cu(II)-alkylperoxo complexes.
The copper(II) precursor complexes, [Cu(CHDAP)](ClO4)2(H2O)
(
1-(ClO4)2∙H2O) and [Cu(CHDAP)(NO3)2]
(1-(NO3)2) were
of
the
Cu(II)-cumylperoxo
complex
bearing
a
hydrotris(pyrazolyl)borate ligand.6 Detailed studies on the
spectroscopic characterization and reactivity of the Cu(II)-
alkylperoxo complexes have been reported by Solomon and co-
workers.7 Most of the Cu(II)-alkylperoxo species exhibited
electrophilic reactivity towards organic substrates, which was
induced by the homolytic Cu-O and O-O bond cleavage. Itoh et al.
also investigated intrinsic reactivities of Cu(II)-alkylperoxo
complexes to demonstrate that the Cu(II) binding alkylperoxo
prepared and characterized by UV-vis absorption spectroscopy
and electrospray ionization mass spectrometry (ESI-MS) (see
ESI, Experimental Section and Figure S1 and S2†). The crystal
structure of
1-(NO3)2 reveals a tetragonally distorted
octahedral geometry characteristic for d9 ions (ESI, Figure S3
and Table S1 and S2†). The X-band electron paramagnetic
resonance (EPR) spectrum of
1 reveals an axial signal with g┴ =
2.07 and g║ = 2.30 (A║ = 171 G) consistent with a tetragonally
complexes have been considered as precursors of reactive Cu(II)-O• elongated octahedral d9 Cu(II) center (ESI, Figure S4†).11
species.8
complex,
The
Cu(II)-cumylperoxo
[Cu(CHDAP)(OOC(CH3)2Ph)]+
equiv of cumene hydroperoxide (cumyl-OOH) to a reaction
solution containing in the presence of equiv of
(2), was prepared by adding 1
In
general,
metal-alkylperoxo
complexes
display
electrophilic reactivity through the O-O bond cleavage.9 To the
best of our knowledge, the nucleophilic reactivity of metal-
alkylperoxo species itself has been rarely explored. Only one
example of the nucleophilic oxidation activity of a Ni(II)-tert-
butylperoxo complex was reported recently.10 Herein, we
report the first example of Cu(II)-alkylperoxo complexes that
affect aldehyde deformylation with an oxidative nucleophilic
1
1
triethylamine (TEA) in CH3CN/CH2Cl2 (1:1) at –40 °C; the color
of the solution changed from green to orange-brown (Scheme
1). The UV-vis spectrum of
2 shows an intense band at 475 nm
(ε = 1000 M–1 cm–1) and a weak band at 758 nm (ε = 70 M–1
cm–1) (Figure 1a), which is similar to those of the previously
reported Cu(II)-alkylperoxo complexes.4c,8a The former band
has been assigned as the LMCT band of the Cu(II)-OOR species,
and the latter band has been assigned as the d-d transition
band of the Cu(II) ion (vide infra).8a Complex
2 was stable at –
† Electronic Supplementary Information (ESI) available: Synthesis and characterization
data and kinetic details. CCDC-1537802 for [Cu(CHDAP)(NO3)2] and 1540900 for
[Cu(CHDAP)(CH3CN)2](BPh4)2(Et2O)2 contain the supplementary crystallographic data
for this paper. For ESI and crystallographic data in CIF or other electronic format, see
DOI: 10.1039/b000000x/
40 °C, allowing us to use it for spectroscopic characterization
and reactivity studies. The cold spray ionization mass
spectrometry (CSI-MS) of
2
exhibits a prominent ion peak at a
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In addition, the bands at 558 and 546 can be assigned to C-C-C/C-C-
O deformation modes of the moiety. The EPR spectrum of a frozen
CH3CN solution of 2 measured at 114 K shows an axial signal with g
= 2.08 and g║ = 2.34 (A║ = 141 G), suggesting a tetragonal
┴
geometry for the Cu(II) ion (Figure 1c),11 which is similar to that of
1. Spin quantification finds that the EPR signal corresponds to
96(4)% of the total copper content in the sample (ESI, Experimental
Section†).
The Cu(II)-tert-butylperoxo complex, [Cu(CHDAP)(OOBut)]+ (3),
was obtained by
a method similar to that of 2 with the
corresponding tert-butylhydroperoxide (tBuOOH) (Scheme 1).
Complex 3 is characterized by UV–vis, CSI-MS, resonance Raman
and EPR techniques (ESI, Experimental Secꢀon and Figure S5†). The
similarity of these spectroscopic features to those of 2 leads us to
assign 3 as a Cu(II)-tert-butylperoxo complex.
Density functional theory (DFT) calculations were performed on
the Cu(II)-alkylperoxo complexes to determine the minimal energy
structures. For the credibility of the DFT calculations, structural
comparisons between the crystal structure of 1 and that calculated
by DFT were carried out. The root mean square deviation (RMSD)
was determined to be 0.21 Å (ESI, Figure S6†), affording to
reliability of the subsequent calculations in which crystal structures
are not available.13 The geometry of the Cu(II) ions in 2 and 3 are, as
expected, described as
a tetragonally distorted octahedron
coordinated by the oxygen atom of the end-on (η1) bound
corresponding alkylperoxide and the five nitrogen atoms from
CHDAP ligand and CH3CN (Figure 1b, left inset and ESI, Figure S5,
left inset†), in agreement with the EPR results (vide supra). The O-O
bond lengths of 2 and 3 are calculated to be 1.46 and 1.47 Å,
respectively (ESI, Table S3†), which are in line with that observed
for Kitajima’s Cu(II)-cumylperoxo complex (1.460 Å). It is also
comparable to previous observation for the homoprotocatechuate
2,3-dioxygenase (1.5 Å).2a Time-dependent DFT (TD-DFT)
calculations were performed to further analyze the electronic
structures where the bands at 475 nm for 2 and 485 nm for 3 were
assigned as alkylperoxo to Cu(II) charge transfer transitions (ESI,
Figure S7 and Table S4†).
Fig.
1
(a) UV-vis spectrum of [Cu(CHDAP)(OOC(CH3)2Ph)]+ (2) in
CH3CN/CH2Cl2 (1:1) at –40 °C. Inset shows the resonance Raman spectra of
2 prepared with cumyl-16O16OH (red line) and cumyl-18O18OH (blue line)
which were obtained upon excitation at 442 nm in CH3CN at –30 °C. (b)
CSI-MS of 2 in CH3CN/CH2Cl2 (1:1) at –40 °C. The right inset shows the
observed isotope distribution patterns for 2-16O (lower) and 2-18O (upper).
The left inset shows DFT-calculated structure of 2 (gray, C; blue, N; red, O;
green, Cu). Hydrogen atoms are omitted for clarity. (c) X-band EPR
spectrum (black line) and simulation (red line) of 2 in frozen CH3CN at 114
K. Spectroscopic settings: frequency = 9.155 GHz, microwave power =
0.998 mW, modulation frequency = 100 kHz, and modulation amplitude =
0.6 mT.
The Cu(II)-alkylperoxo complexes 2 and 3 decayed slowly,
obeying first-order kinetics at 25 °C (kobs = 7.2(1) × 10–3 s–1 for 2 and
2.1(1) × 10–2 s–1 for 3) to give the Cu(II) starting complex which was
confirmed by ESI-MS (ESI, Figure S8 and S9†). It has been well-
known that the products obtained from a cumylperoxo complex
depend on the O-O bond scission manner.1e,14 Acetophenone and
cumyl alcohol were obtained in 40(10)% and 49(10)% yields from
the thermal decomposition of 2 in CH3CN. Further, we have
mass-to-charge ratio (m/z) of 618.3 (Figure 1b), whose mass
and
isotope
distribution
pattern
correspond
to
{Cu(CHDAP)(OOC(CH3)2Ph)}+
(
2
-16O) (calcd m/z 618.3). When
the reaction was carried out with isotopically labeled cumyl- investigate the product analysis of the self-decomposition of 2 in
18O18OH,
{Cu(CHDAP)(18O18OC(CH3)2Ph)}+ (
CD3CN/CD2Cl2 (1:1), which shows that acetophenone and cumyl
a
mass
peak
corresponding
to
2
-18O) appeared at m/z 622.3
alcohol were obtained in 54(8)% and 21(10)% yield. These results
indicate that the homolytic cleavage of the O-O bond of
cumylperoxide occurs mainly during the self-decomposition of 2.4c
The reactivities of the Cu(II)-alkylperoxo complexes were
investigated in oxidative electrophilic and nucleophilic reactions at
low temperature. The electrophilic character of 2 and 3 was tested
in the oxidation of thioanisole and cyclohexadiene. Upon addition
of the substrates to 2 and 3 in a solvent mixture of CH3CN/CH2Cl2
(1:1) at –40 °C, 2 and 3 remained intact without showing UV-vis
absorption spectral changes. Product analysis of the reaction
solutions revealed that no oxidized products were formed in these
(calcd m/z 622.3) (Figure 1b, right inset). The four mass shifts
upon substitution of 16O with 18O demonstrate that
two oxygen atoms.
2 contains
The resonance Raman spectrum of 2 was collected using 442 nm
excitation in CH3CN at –30 °C. 2-16O exhibits isotope-sensitive bands
at 877, 835, 625, 558 and 546 cm–1, which shifted to 854, 803, 607,
554 and 539 cm–1 in samples of 2-18O (Figure 1a, inset). The isotope-
sensitive multiple bands of 2 are similar to those of previously
reported Cu(II)-cumylperoxo complexes, where the bands at 877
and 835 cm–1 can be assignable to mixed O-O/C-O/C-C vibrations,
and the band at 625 cm–1 to the Cu-O stretching vibration.4c,4d,8a,12
2 | J. Name., 2012, 00, 1-3
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Fig. 3 Hammett plot of lnkrel against σp+ of benzoyl chloride derivatives in
the reaction of 2 (0.25 mM) with para-substituted benzoyl chloride. The
krel values were calculated by dividing kobs of para-X-Ph-COCl (X = Me, tBu,
H, Cl, Br) by kobs of benzoyl chloride at –40 °C in CH3CN/CH2Cl2 (1:1).
examined using para-X-Ph-COCl (X = Me, tBu, H, Cl, Br) and
+
Hammett plot of the pseudo-first-order rate constant vs σp gave a
ρ value of 3.7(2) (Figure 3). Product analysis of the final reaction
mixture revealed the formation of para-substituted benzoic acids.
The positive ρ value is consistent with the process involving a
nucleophilic character.
Similarly, the kinetic data were determined in the reaction of 3
with 2-PPA and para-substituted benzoyl chlorides (ESI,
Experimental Secꢀon and Figure S11 and Figure S12†). Based on the
results of kinetic studies of 2 and 3, we propose that the aldehyde
oxidation by the Cu(II)-alkylperoxo complexes is initiated by the
nucleophilic attack of the carbonyl group of aldehydes by the
alkylperoxo group, and then the O-O bond cleavage on the
peroxyhemiacetal-like intermediate to afford products (Scheme 2).
It has been well documented that the Cu(II)-alkylperoxo
complexes are capable of performing the C-H bond activation
of hydrocarbons by the O-O bond homolytic cleavage.4c,8a We
Fig.
2
Reactions of [Cu(CHDAP)(OOC(CH3)2Ph)]+ (2) with 2-PPA in
CH3CN/CH2Cl2 (1:1). (a) UV-vis spectral changes of 2 (0.25 mM) upon
addition of 30 equiv of 2-PPA at –40 °C. Inset shows the time course of the
absorbance at 475 nm. (b) Plot of kobs against 2-PPA concentration to
determine a second-order rate constant for 2 at –40 °C. (c) Plot of second-
order rate constants against 1/T to determine activation parameters.
also observed that the thermal decomposition of
2 gave
ketone (40(10)%, i.e., O-O bond homolysis) and alcohol
(49(10)%, i.e., O-O bond heterolysis) products (vide supra). In
addition, the Cu(II)-alkylperoxo complexes reverted back to
the Cu(II) precursor
1
after the decomposition was completed.
Thus, we tested catalytic oxidative reaction of with excess
amount of alkylperoxides. The reaction of and 100 equiv of
reactions, indicating that 2 and 3 are not capable of conducting
electrophilic oxidative reactions under the reaction conditions.
In contrast, addition of 2-phenylpropionaldehyde (2-PPA) to 2 in
CH3CN/CH2Cl2 (1:1) at –40 °C afforded the characteristic absorption
band changes in which a first-order decay profile was observed
(Figure 2a). The pseudo-first-order rate constants increased
proportionally with an increase of the concentration of 2-PPA,
giving a second-order rate constant (k2) of 1.2(1) × 10–1 M–1 s–1
(Figure 2b). Product analysis of the reaction solution revealed that
acetophenone (95(3)%) was produced in the oxidation of 2-PPA.
After the reaction was completed, 2 was reverted back to 1
(96(3)%), which was confirmed by ESI-MS and EPR (ESI, Figure
S10†).15 The rates were dependent on the reaction temperature,
where a linear Eyring plot was observed in the range of 233 – 263 K
with activation parameters of ꢀH‡ = 10(1) kJ mol–1 and ꢀS‡ = −220(2)
J mol–1 K–1 (Figure 2c). The observed negative entropy value and
second-order kinetics suggest that a bimolecular mechanism is
performing in the oxidation of 2-PPA by 2.
1
1
cyclohexadiene (CHD) in the presence of 2 equiv of TEA in
CH3CN/CH2Cl2 (1:1) at 25 °C was examined by the addition of
100 equiv of cumene hydroperoxide. Table S5 (ESI†) shows 1 is
capable of turning over at least 10 times. In the absence of the
catalyst, a trace amount of benzene was detected. In addition,
under the single turnover conditions, the decay rate of the
characteristic absorption bands of
almost same as that of the self-decomposition of
analysis of the resulting solution reveals the formation of
benzene (61(5)%) and cumyl alcohol (64(7)%). These results
2
in the presence of CHD is
. Product
2
indicate that the direct oxidation of CHD by
2 does not
proceed in the reaction conditions.4c Further, the catalytic
oxidation in C-H bond activation was investigated with
xanthene and dihydroanthracene (ESI, Table S5†).
In conclusion, mononuclear Cu(II)-alkylperoxo complexes,
[Cu(CHDAP)(OOC(CH3)2Ph)]+ ) and [Cu(CHDAP)(OOBut)]+ (
(2 3),
The reaction of 2 with para-substituted benzoyl chlorides, which
have a series of electron-donating and -withdrawing substituents at
the para position of the phenyl group provides mechanistic
information into the nature of the alkylperoxo group in 2. The effect
of para substituents on the benzoyl chloride oxidation process was
were prepared and characterized by various physicochemical
methods such as UV-vis, CSI-MS, resonance Raman and EPR. In
particular, EPR data clearly exhibit tetragonal geometry of 2
and 3, which was in good agreement with the results of DFT
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Asano, T. Suzuki, S. Itoh, J. Am. Chem. Soc., 2015, 137, 10870;
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Scheme 2 Proposed mechanism of the oxidation of aldehydes by
Cu(II)-alkylperoxo complexes.
calculations. Both
under the stoichiometric conditions at low temperature, and
the nucleophilic character of and was determined by the
2 and 3 can perform aldehyde deformylation
6
7
8
N. Kitajima, T. Katayama, K. Fujisawa, Y. Iwata, Y. Moro-oka, J.
Am. Chem. Soc., 1993, 115, 7872.
2
3
positive Hammett ρ values. In the catalytic conditions at high
temperature, conversely, the Cu(II)-alkylperoxo species are
capable of catalyzing C-H bond activation through the O-O
bond cleavage.
P. Chen, K. Fujisawa, E. I. Solomon, J. Am. Chem. Soc., 2000,
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We gratefully acknowledge research support of this work
by the NRF (2017R1A2B4005441) and the Ministry of Science,
ICT and Future Planning (CGRC 2016M3D3A01913243) of
Korea. We thank Prof. Takashi Ogura and Prof. Takehiro Ohta
for assistance in acquiring resonance Raman spectra.
9
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15 When we use NaOH instead of the Cu(II) precursor and TEA,
only 4(1)% of acetophenone was obtained. In addition, without
the Cu(II) precursor complex, no oxidized products were
formed in these reactions.
4
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