Catalytic and stoichiometric C[sbnd]H oxidation of benzylalcohols and hydrocarbons mediated by nonheme oxoiron(IV) complex with chiral tetrapyridyl ligand

Article abstract of DOI:10.1016/j.inoche.2019.04.008

Bioinspired chiral iron(II) complex [((R)-(?)-N4Py*)Fe^II(CH₃CN)]²⁺ (1) (N4Py* = N,N-bis(2-pyridylmethyl)-1,2-di(2-pyridyl)ethylamine) has been shown to efficiently catalyze the benzylic C[sbnd]H oxidation of ethylbenzene with tert-butyl hydroperoxide (TBHP), H₂O₂, and meta-chloroperoxybenzoic acid (mCPBA) resulting in enantiomerically enriched 1-phenylethanol up to 12.5% ee and the corresponding acetophenone, where the [Fe^IV(N4Py*)(O)]²⁺ (2) intermediate has been detected by UV/Vis spectrometry. The stoichiometric oxidation of benzyl alcohol and various hydrocarbon derivatives including the asymmetric hydroxylation of ethylbenzene with 2 has also been investigated. Detailed kinetic, and mechanistic studies (kinetic isotop effect (KIE) of 31 and 38, and Hammett correlation with ρ = ?0.32 and ?0.98 for PhCH₂OH and PhCH₃, respectively, and the linear correlation between the normalized bimolecular reaction rates and bond dissociation energies (BDE_CH)) lead to the conclusion that the rate-determining step in these reactions above involves hydrogen-atom transfer between the substrate and the Fe(IV)-oxo species. The stoichiometric 2-mediated hydroxylation of ethylbenzene affords 1-phenylethanol in up to 33% ee, suggesting clear evidence for the involvement of the oxoiron(IV) species in the enantioselective step. The moderate enantioselectivity may be explained by the epimerization of the long-lived substrate radical before the rebound step (non-rebound mechanism, where k_ep > k_reb). The kinetic resolution of the resulting chiral alcohol due to its metal-based overoxidation process into acetophenone in the catalytic metal-based ethylbenzene oxidation can be excluded.

Full text of DOI:10.1016/j.inoche.2019.04.008

Accepted Manuscript
Catalytic and stoichiometric C-H oxidation of benzylalcohols and
hydrocarbons mediated by nonheme oxoiron(IV) complex with
chiral tetrapyridyl ligand
Dóra Lakk-Bogáth, Balázs Kripli, Bashdar I. Meena, Gábor
Speier, József Kaizer
PII:
S1387-7003(19)30217-5
DOI:
Reference:
https://doi.org/10.1016/j.inoche.2019.04.008
INOCHE 7352
To appear in:
Inorganic Chemistry Communications
Received date:
Revised date:
Accepted date:
4 March 2019
1 April 2019
7 April 2019
Please cite this article as: D. Lakk-Bogáth, B. Kripli, B.I. Meena, et al., Catalytic and
stoichiometric C-H oxidation of benzylalcohols and hydrocarbons mediated by nonheme
oxoiron(IV) complex with chiral tetrapyridyl ligand, Inorganic Chemistry
Communications, https://doi.org/10.1016/j.inoche.2019.04.008
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ACCEPTED MANUSCRIPT
Catalytic and stoichiometric C-H oxidation of benzylalcohols and
hydrocarbons mediated by nonheme oxoiron(IV) complex with
chiral tetrapyridyl ligand
Dóra Lakk-Bogáth, Balázs Kripli, Bashdar I. Meena, Gábor Speier and József Kaizer*
aDepartment of Chemistry, Universtiy of Pannonia, 8201 Veszprém, Hungary
Abstract
Bioinspired chiral iron(II) complex [((R)-(-)-N4Py*)FeII(CH3CN)]2+ (1) (N4Py* = N,N-
bis(2-pyridylmethyl)-1,2-di(2-pyridyl)ethylamine) has been shown to efficiently catalyze the
benzylic C-H oxidation of ethylbenzene with tert-butyl hydroperoxide (TBHP), H2O2, and
meta-chloroperoxybenzoic acid (mCPBA) resulting in enantiomerically enriched 1-
phenylethanol up to 12.5% ee and the corresponding acetophenone, where the
[FeIV(N4Py*)(O)]2+ (2) intermediate has been detected by UV/Vis spectrometry. The
stoichiometric oxidation of benzyl alcohol and various hydrocarbon derivatives including the
asymmetric hydroxylation of ethylbenzene with 2 has also been investigated. Detailed kinetic,
and mechanistic studies (kinetic isotop effect (KIE) of 31 and 38, and Hammett correlation
with  = -0.32 and -0.98 for PhCH2OH and PhCH3, respectively, and the linear correlation
between the normalized bimolecular reaction rates and bond dissociation energies (BDECH))
lead to the conclusion that the rate-determining step in these reactions above involves
hydrogen-atom transfer between the substrate and the Fe(IV)-oxo species. The stoichiometric
2-mediated hydroxylation of ethylbenzene affords 1-phenylethanol in up to 33% ee,
suggesting clear evidence for the involvement of the oxoiron(IV) species in the
enantioselective step. The moderate enantioselectivity may be explained by the epimerization
of the long-lived substrate radical before the rebound step (non-rebound mechanism, where
kep>kreb). The kinetic resolution of the resulting chiral alcohol due to its metal-based
overoxidation process into acetophenone in the catalytic metal-based ethylbenzene oxidation
can be excluded.
Keywords: Biomimetic oxidation, C-H activation, Asymmetric hydroxylation, Chiral iron
complex, Kinetics.
*Corresponding author. Tel.: +36 88 62 4720; Fax: + 36 88 62 4469.
E-mail address: kaizer@almos.uni-pannon.hu.

ACCEPTED MANUSCRIPT
Biological oxidation reactions are amongst the most frequent reactions that occur in nature,
where they are catalyzed by metalloenzymes belonging to the oxidoreductases. The majority
of these metalloproteins exploit the oxidative power of dioxygen to catalyze a broad spectrum
of crucial metabolizing reactions, which have medical, pharmaceutical, agricultural or
commercial significance [1-7]. The synthesis of enantiopure compounds for the production of
drugs, agricultural or other specialty fine chemicals is one of the most active fields in organic
chemistry, but the asymmetric functionalization of sp3 C-H bonds, including the asymmetric
hydroxylation of alkanes still remains a challenge [8-11]. Nonheme iron enzymes [12-16],
and their corresponding biomimetic complexes, including the formation and reactivity of
high-valent oxoiron species, have been extensively studied [17- 21]. These systems are
responsible for a broad range of oxidative transformations. Fe(II)/α-ketoglutarate-dependent
taurine dioxygenase, that catalyzes the hydroxylation of taurine yielding sulfite and
aminoacetaldehyde, is one of the nonheme iron enzymes that is capable of carrying out
stereospecific and enantoselective C-H oxidation, where the formation of O2-derived high-
valent FeIVO was proposed as iron-based oxidant [15]. We have recently been successful in
obtaining well characterized synthetic analogues of high-valent non-heme iron species with chiral
ligands,
[FeIV(N4Py*)(O)]2+
(2)
(N4Py*
=
N,N-bis(2-pyridylmethyl)-1,2-di(2-
pyridyl)ethylamine), which serve as synthetic models for non-heme iron-dependent oxygenases
[22]. These chiral complexes have been studied in enantioselective oxidations such as
sulfoxidation [22], Baeyer-Villiger oxidation [23], and epoxidation [24]. Recently, Sun and co-
workers reported a chiral enantiopure L-prolin derived aminopyridine ligand, and the
corresponding oxoiron(IV) species could perform asymmetric C-H hydroxylation with 14%
ee [25]. We now want to describe herein the kinetics and mechanism of the oxidative C-H
activation processes of substituted benzyl alcohols, and various hydrocarbons mediated by
chiral nonheme oxoiron(IV) complex (Fig.1), compared to previously studied Fe(IV)O
intermediates with N4Py-type pentadentate ligands (N4Py = N,N’-bis(2-pyridylmethyl)-N-
bis(2-pyridyl)methylamine [26, 27]; N3PyBim = [N-(1-methyl-2-benzimidazolyl)methyl-N-
(2-pyridyl)methyl-N-(bis-2-pyridylmethyl)amine] [28]; N2Py2Bim = [N-bis(1-methyl-2-
benzimidazolyl)methyl-N-(bis-2-pyridylmethyl)amine]) [28], furthermore the enantioselective
hydroxylation of ethylbenzene under catalytic and stoichiometric conditions.

ACCEPTED MANUSCRIPT
Fig. 1. Oxoiron(IV) mediated C-H oxidation in this study.
We have shown earlier that Fe(II) complexes with chiral pentadentate aminopyridine
ligands can form relatively stable high valent oxoiron(IV) species with various oxidants such
as H2O2, tert-butyl hydroperoxide (TBHP), PhIO and meta-chloroperoxybenzoic acid
(mCPBA), which are possible candidates for stereoselective C-H oxidation reactions [22].
The catalytic activity including the enantioselective behaviour of enantiopure ligand-
containning [FeII(N4Py*)(CH3CN]2+ ((R)-(-)-1) was studied in the oxidation of ethylbenzene,
utilizing TBHP, H2O2 and mCPBA as co-oxidants. No oxidation products were obtained
when the reactions were carried out without any metal catalyst. Using an identical
concentration of precursor/substrate/co-oxidant (1/ethylbenzene/co-oxidant = 1/100/25),
Table 1 shows that there is an increase of the yield when the co-oxidants employed are
mCPBA, H2O2 and TBHP (from 4.6 to 21%), furthermore, moderate enantioselective
hydroxylation could be obtained in all cases. The reaction afforded a mixture of the 1-
phenylethanol and its overoxidized product, acetophenone. It can be seen that, with three
different co-oxidants mCPBA, H2O2 and TBHP, the enantiomeric excess values are similar
according to a probably same optically active intermediate. A decrease of the [1]/[TBHP]

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ratio decreases significantly the K/A ratio but increases slightly the ee from 7.5 to 14.4% (Fig.
2). Similar results have been observed for electron-rich iron porphyrin/H2O2 (yield 5%; 15 %
ee in MeOH/H2O) [29], and -oxo diferric/H2O2 systems (15 % ee) [30, 31]. The moderate
enantioselectivity may be explained by the parallel enantioselective metal-based and non-
selective Fenton-type radical processes [32], furthermore, by the epimerization of the long-
lived substrate radical before the rebound step (non-rebound mechanism, where kep>kreb) [33].
Complex 1 with mCPBA (50 equiv.) oxidizes the 1-phenylethanol in CH3CN at 0°C, and a
TON of 17.8 for acetophenone was observed with an overall yield of 35.6 % (based on
oxidant) and without any enantioselectivity. Based on this result the kinetic resolution of the
resulting chiral alcohol due to its metal-based overoxidation process (the selective
overoxidation of one of the isomer) into acetophenone in the catalytic metal-based
ethylbenzene oxidation can be excluded.
Fig. 2. Dependence of the K/A ratio and the enantiomeric excess (ee %) on the oxidant
concentrations for ethylbenzene oxidation in CH3CN at 0°C.

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Table 1
Catalytic oxidation of ethylbenzene carried out by (R)-(-)-1 with various co-oxidantsa.
[1] : [PhEt] : [Co-oxidant] TON (Ab)
TON (Kc)
Yield (%)d K/A ee (%)
1 . 100 :25 (mCPBA)
1 . 100 :25 (H2O2)
1 . 100 :25 (TBHP)
1 . 100 :50 (TBHP)
1 : 100 : 500 (TBHP)
0.58
1.40
0.63
0.84
1.2
0.57
5
0.98 13.4 (R)
2.08
12
20
21
24e
1.5
7.1
12.1 (R)
14.4 (R)
4.46
9.47
11.3 11.5 (R)
19.1 7.5 (R)
22.9
b
aReaction conditions: see Experimental section. 1-Phenylethanol. cAcetophenone. dBased on
oxidant. eBased on substrate.
Spectral investigations on the catalytic system above have confirmed the formation of
oxoiron(IV) species 2 (705 nm ( = 400 M-1 cm-1)) (Fig. 3A) [22], that undergoes a monotonic
decay, which is affected by the addition of 100 equivalent of ethylbenzene (Fig. 3B). These
results, including the observed enantioselectivity, may suggest that a high-valent oxoiron(IV)
species is able to control the stereochemistry of the reaction. However, nonselective
hydroxylation reactions involving free radicals such as HO•, tert-BuO• and dioxygen are
competing with the above process [34].
Fig. 3. The UV-Vis spectral change of 2 upon addition of TBHP to 1 in the presence of
ethylbenzene in CH3CN at room temperature (A), and time courses of the decay of 2 in the
absence and in the presence of ethylbenzene: [1]:[PhEt]:[TBHP] = 1:-:500 (a); 1:100:50 (b);
1:100:500 (c) in CH3CN at 25°C (B).

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Oxygenation, or oxygen atom transfer from an oxygen source to a metal center and
eventually to an organic substrate is a matter of great interest in organic synthesis and
transition metal catalysis. To get direct evidence for the involvement of the oxoiron(IV)
species in the C─H activation processes, the reactions of 2 with various substrates, such as
benzyl alcohol, 9,10-dihydroanthracene, fluorene, triphenylmethane, cumene, ethylbenzene,
toluene and cyclohexane were investigated (Table S1). The identities of the products
(anthracene, fluorenone, triphenylmethanol, benzaldehyde, ethylmethylketone (1-
phenylethanol), cyclohexanone (cyclohexanol), respectively) were confirmed by comparison
with authentic samples and GC-MS. The oxoiron(IV) complex was generated by in situ
reaction of 1 with 2 equivalent of PhIO (Fig. 4a), and its reaction with benzyl alcohol and
toluene was measured as a function of the concentration of added substrates (Fig. 4b). It was
found that the complex 2 is able to oxidize the benzyl alcohol (BA) to benzaldehyde (yields
~75 % based on 2), and the toluene (T) to benzyl alcohol and benzaldehyde (yields ~50 %) at
298 K, and the oxoiron(IV) species reverted back to the precursor complex 1 showing pseudo-
first-order decays (Fig. 4a). Kinetic experiments revealed 1st-order dependence on both the
oxoiron(IV) and the substrate concentration with k2 (BA) = 1.10 × 10-2 M-1 s-1, H≠ = 60 kJ
mol-1 and S≠ = -123 J mol-1 K for benzyl alcohol, and k2 (T) = 2.10 × 10-5 M-1 s-1, H≠ = 41
kJ mol-1 and S≠ = -84 J mol-1 K for toluene in MeCN at 298 K (Table 2). Reaction rates
obtained for the oxidation of benzyl alcohol and toluene are smaller by one order of
magnitude than those observed for oxoiron(IV) complex with N4Py ligand (k2 = 9.9 × 10-2 M-
1
s-1 [35], and 1.50×10-4 M-1 s-1 [24], respectively), and smaller by two and three order of
magnitude than those observed for FeIVO/toluene system based on modified N4Py type
ligands, N3PyBim (1.30×10-3 M-1 s-1), and N2Py2Bim (1.20×10-2 M-1 s-1), respectively [28].
The activation enthalpy of 60 kJ mol-1 is relatively higher than those obtained in the oxidation
of benzyl alcohol by [FeIV(N4Py)(O)]2+ (32 kJ mol-1) [35], and in the oxidation of
benzaldehyde by [FeIV(N4Py)(O)]2+ (32 kJ mol-1) [20]. The relatively large negative entropies
are typical of associative processes (Fig. 4g). The substrate C6H5CH2OH/C6D5CD2OH and
C6H5CH3/C6D5CD3 kinetic isotop effects (KIE) were also investigated. The involvement of
the benzylic C-H bond in the rate-determining step in both cases is indicated by the magnitude
of the kCH/kCD kinetic isotope effects of 31 for benzyl alcohol (Fig. 4c), and 38 for toluene
(Fig. 4d). These values are larger than „classical” KIE values (KIE ~ 7), and comparable to
those measured in the oxidation of benzyl alcohol, benzaldehyde and toluene by polypyridyl
oxoiron(IV) complexes (KIETPA = 58 and KIEN4Py = 48 for benzyl alcohol [35]; KIEN4Py
=

ACCEPTED MANUSCRIPT
26.5 and KIEN4Py* = 37 for benzaldehyde [20]; KIEN4Py = 20 for toluene [26]), indicating that
the oxoiron(IV) intermediate activates the benzyl alcohol, benzaldehyde and toluene in the
same manner by an H-atom abstraction from the benzylic position (HAT). Upon using para-
benzyl alcohols with electron donating groups the rate of the decay processes were increased
remarkably (Table 2, Fig. 4e). Relative rates determined from these plots showed a good
linear correlation (r = 0.99, n = 5) with the Hammett substituent parameters. The reaction
constant, , is negative ( = -0.32). It is 5-fold larger than those obtained for the
[FeIV(N4Py)(O)]2+/PhCH2OH ( = -0.07) and [FeIV(TPA)(O)]2+/PhCH2OH ( = -0.06)
systems, but comparable to that measured in the oxoiron(IV)porphyrin radical
cation)/PhCH2OH ( = -0.43) system [35], where -C-H hydrogen atom abstraction and
concomitant electron transfer was proposed. The relatively small  value suggests that the
metal-based oxidant is electrophilic, but there is only a small development of positive charge
on the substrate in the transition state. Competitive reactions were also performed with para-
substituted toluene derivatives, and the Hammett correlation (r = 0.98, n = 5,  = -0.98)
demonstrates that the rate constant for the oxidation of toluene by 2 is much more sensitive to
changes in the electronic properties of the toluene compared to the 2/PhCH2OH system
mentioned above. The magnitude of the  value in both cases is in a good agreement with a
hydrogen-atom-transfer (HAT) model [18]. A good linear correlation has been found between
logk2’ (Fig 4h, k2’ is the second-order rate constant divided by the number of equivalent C-H
bonds in the substrate) and C-H BDE for several hydrocarbons with a slope of -0.3, which is
comparable to those obtained in C-H bond oxidations by mononuclear nonheme oxoiron(IV)
species including rate-determining HAT process, following by rebound and/or non-rebound
mechanism (Scheme 1) [33].
Scheme 1. Schematic illustration of possible reaction pathways for oxoiron(IV)-mediated
oxidation of alkanes.

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Fig. 4. Reactions of [FeIV(N4Py*)(O)] (2) with benzyl alcohols and hydrocarbons in CH3CN
(Table S1). (a) UV-vis spectral change of 2 upon addition of 100 equiv. benzyl alcohol at 298
K. Inset shows time course of the decay of 2 monitored at 705 nm with toluene and benzyl
alcohol ([2]0 = 2 mM, [PhCH2OH]0= 0.2 M, [PhCH3]0 = 2.0 M). (b) Plot of kobs versus
[substrate] for reactions of 2 (2mM) with benzyl alcohol and toluene. (c) Plots of ln[2] versus

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time for reactions of 2 (2 mM) with 100 equiv. benzyl alcohol and d7-benzyl alcohol. (d) Plots
of ln[2] versus time for reactions of 2 (2 mM) with 1000 equivToluene and d8-toluene. (e)
Hammett plot of log krel against the p of para-substituted benzyl alcohols. (f) Hammett plot
of log krel against the p of para-substituted toluenes. (g) Eyring plots of logk/T versus 1/T for
benzyl alcohol and toluene ([2]0 = 2 mM, [PhCH2OH]0= 0.2 M, [PhCH3]0 = 2.0 M). (h) The
’
logk2 (second-order rate constant divided by the number of equivalent C-H bonds in the
substrate) versus BDEC-H plots.
Table 2
Kinetic parameters determined in the oxidation of benzyl alcohol and different alkane
substrates by oxoiron(IV) complexes in acetonitrile at 25°C.
H≠
(kJmol-1)
S≠
(Jmol-1K-1)
-184
G≠
(Jmol-1K-1)
complex
substrate
k2
(M-1s-1)
KIE
Refs.

N4Py
PhCH2OH
PhCH2OH
PhCH3
9.90×10-2
1.10×10-2
2.10×10-5
1.50×10-4
1.30×10-3
1.20×10-2
-0.07
-0.32
-0.98
48
32
82
[32]
N4Py*
31
38
20
14
11
60
-123
97
this work
this work
[26]
N4Py*
41
-84
66
N4Py
PhCH3
n.a.
n.a.
n.a.
n.a.
n.a.
n.a.
n.a.
N3PyBim
N2Py2Bim
PhCH3
n.a.
[28]
PhCH3
n.a.
[28]
Stoichiometric oxidation of ethylbenzene by [FeIV((R)-(-)-N4Py*)(O)] ((R)-(-)-2) in CH3CN
at 0°C gave 33% enantiomeric excess (ee) of 1-phenylethanol after 90 minutes, and 25% ee
after 180 minutes under Ar (Fig. S1). Since the oxoiron(IV) species was generated by PhIO,
the presence of hydroxyl and tert-butoxyl radicals and their non-selective reaction with the
substrate in the stoichiometric reaction can be excluded. Based on that, the moderate
enantioselectivity and the decrease on the ee value during the reaction can be explained by the
epimerization of the long-lived substrate radical (rotation process through C-C bond of the
radical species) before the rebound step (non-rebound mechanism, where kep>kreb) [33]. Much
lower value (14% ee) was observed in the oxidation of methyl 1-tetralone-2-carboxylate [25].

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Fig. 5. Time dependence on the K/A ratio and the enantiomeric excess (ee %) for the
stoichiometric oxidation of ethylbenzene with (R)-(-)-2 in CH3CN at 0°C
Efforts have been made to develop a highly efficient asymmetric catalyzed oxidation of
various alkanes by introducing the chiral moiety to ligands as well as their detailed
mechanistic aspects. In summary, we have reported one of the first example of
enantioselective C-H oxidation mediated by chiral oxoiron(IV) intermediate. Based on
detailed mechanistic studies on stoichiometric benzyl alcohol (KIE of 31, r = -0.32) and
hydrocarbon (KIE of 38, r = -0.98) oxidation that have been investigated with in situ
generated high-valent oxoiron(IV) complex, a plausible mechanisms have been proposed for
both systems, in which the oxidation of alcohols and hydrocarbons occurs in the same manner
by HAT in the rate-determining step. In this study we have also demonstrated that the
enantioselectivity depends on the nature of the following step (rebound versus non-rebound
mechanisms). Moderate yields and poor ee values can be expected for substrates with stronger
C-H bonds, where the radical dissociation pathway can become prominent. The oxidation of
ethylbenzene by the chiral oxoiron(IV) intermediate attains moderate enantioselectivities up
to 33% ee, which can be explained by the epimerization of the long-lived substrate radical
before the rebound step (non-rebound mechanism, where kep>kreb). Much lower ee values (up
to 14%) have been observed for the catalytic oxidation of ethylbenzene, which can be

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explained by the parallel enantioselective metal-based, oxoiron(IV)-mediated and non-
selective Fenton-type radical processes [34].
Supplementary material
Supplementary data for this article can be found, in the online version, at…
Acknowledgements
Financial support of the Hungarian National Research Fund (OTKA K108489), and
GINOP-2.3.2-15-2016-00049 are gratefully acknowledged.
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ACCEPTED MANUSCRIPT
Catalytic and stoichiometric C-H oxidation of benzylalcohols and
hydrocarbons mediated by nonheme oxoiron(IV) complex with chiral
tetrapyridyl ligand
Dóra Lakk-Bogáth, Balázs Kripli, Bashdar I. Meena, Gábor Speier and József Kaizer*
The reactivity of oxoiron(IV) complex, [FeIV(asN4Py)(O)]2+ with chiral pentadentate ligand,
has been investigated in the oxidation of various hydrocarbons and alcohols.
Graphical abstract
*Corresponding author. Tel.: +36 88 62 4720; Fax: + 36 88 62 4469.
E-mail address: kaizer@almos.uni-pannon.hu.

ACCEPTED MANUSCRIPT
Highlights
 Catalytic ethylbenzene oxidation with chiral Fe(II)-polypyridyl complex.
 Evidence for the formation of oxoiron(IV) intermediate (oxidant).
 Oxoiron(IV)-mediated C-H activation of benzyl alcohols and hydrocarbons.
 Reaction kinetics with UV-Vis spectroscopy.
 Oxoiron(IV)-mediated enantioselective oxidation.