Catalytic and stoichiometric oxoiron(IV) assisted oxidation of hydrocynnamaldehyde under air

Article abstract of DOI:10.1016/j.poly.2020.114505

Nonheme iron(II) complexes, [(N4Py)Fe^II(CH₃CN)](ClO₄)₂ (1) and [(N4Py*)Fe^II(CH₃CN)](ClO₄)₂ (2) with pentadentate tetrapyridyl ligands (N4Py = N,N′-bis(2-pyridylmethyl)-N-bis(2-pyridyl)methylamine, N4Py* = N,N-bis(2-pyridylmethyl)-1,2-di(2-pyridyl)ethylamine) have been shown to catalyze the oxidation of hydrocinnamaldehyde (HCA) with H₂O₂ under air resulting hydrocinnamic acid as the predominant product with phenylacetaldehyde, phenethyl alcohol and benzaldehyde side-products as a result of a free-radical chain process via the formation of reactive phenylpropionyl radical and its consecutive reaction with molecular oxygen. The stoichiometric oxidation of HCA with in situ generated high-valent oxoiron(IV) species under air was also investigated and based on the catalytic and stoichiometric results plausible mechanism including free radical process and high-valent intermediate (Fe^IV[dbnd]O) with rebound and non-rebound routes was proposed.

Full text of DOI:10.1016/j.poly.2020.114505

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Catalytic and stoichiometric oxoiron(IV) assisted oxidation of hydrocynnamaldehyde under air
Flóra Viktória Csendes, József Kaizer
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S0277-5387(20)30162-5
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https://doi.org/10.1016/j.poly.2020.114505
POLY 114505
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7 March 2020
Catalytic and stoichiometric oxoiron(IV) assisted oxidation of hydrocynnamaldehyde
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Catalytic and stoichiometric oxoiron(IV) assisted oxidation of
hydrocynnamaldehyde under air
Flóra Viktória Csendes and József Kaizer*
aDepartment of Chemistry, Universtiy of Pannonia, 8201 Veszprém, Hungary
Abstract
Nonheme
iron(II)
complexes,
[(N4Py)FeII(CH3CN)](ClO4)2
(1)
and
[(N4Py*)FeII(CH3CN)](ClO4)2 (2) with pentadentate tetrapyridyl ligands (N4Py = N,N’-bis(2-
pyridylmethyl)-N-bis(2-pyridyl)methylamine, N4Py* = N,N-bis(2-pyridylmethyl)-1,2-di(2-
pyridyl)ethylamine) have been shown to catalyze the oxidation of hydrocinnamaldehyde
(HCA) with H2O2 under air resulting hydrocinnamic acid as the predominant product with
phenylacetaldehyde, phenethyl alcohol and benzaldehyde side-products as a result of a free-
radical chain process via the formation of reactive phenylpropionyl radical and its consecutive
reaction with molecular oxygen. The stoichiometric oxidation of HCA with in situ generated
high-valent oxoiron(IV) species under air was also investigated and based on the catalytic and
stoichiometric results plausible mechanism including free radical process and high-valent
intermediate (FeIV=O) with rebound and non-rebound routes was proposed.
Keywords: Bioinspired oxidation, C-H activation, Oxidative cleavage, Nonheme oxoiron(IV)
complex, Kinetics.
1. Introduction
The oxidation and oxidative cleavage products of hydrocinnamaldehyde (HCA) such
as hydrocinnamic acid and phenylacetaldehyde are important organic chemicals for food,
pharmaceutical and chemical industries [1-3]. Hydrocinnamic acid is used in the preparation
of pharmaceuticals, including Rosagiline and other protease inhibitors used in the treatment of
Parkinson’s disease and HIV, respectively [4], while phenylacetaldehyde is responsible for
the antibiotic activity of maggot therapy [5]. As natural sources, hydrocinnamic acid is a
substrate of the enzyme oxidoreductases [EC 1.14.12.19] in the pathway of phenylalanine
metabolism [6-8], and phenylacetaldehyde can be also biosynthetically derived from the
phenylalanine [9]. The oxidation of aldehydes to give carboxylic acids has been reported
using stoichiometric reagents such as Cr(IV)-based Jones [10-11], the Ag(I)-based Tollen’s
*Corresponding author. Tel.: +36 88 62 4720; Fax: + 36 88 62 4469.
E-mail address: kaizer@almos.uni-pannon.hu.

[12], the Cu(II)-based Fehling’s [13], and permanganate reagents[14]. Carboxylic acids such
as hydrocinnamic acid can be also obtained by the heterogeneous catalytic aqueous-phase
oxidation of aldehydes using silver(I)/copper(II) oxide [15], and by homogeneous catalytic
oxidation of aldehyde using water-soluble N-heterocyclic carbene- and a silver(I)-catalyst
with oxygen in water [16-17]. Phenyl acetaldehyde can be obtained e.g. by Wacker oxidation
of styrene [18-20] and dehydrogenation of 2-phenylethanol with silver or gold catalysts [21].
Synthetic iron complexes as model catalysts of nonheme oxidase and oxygenase
enzymes, have been widely used for the selective oxidation (oxygenation) of hydrocarbons,
alcohols, alkenes, aldehydes and sulfides, where in general high-valent iron-oxo (FeIV=O or
FeV=O) intermediates were proposed as key metal-based oxidants [21-34]. In a series catalytic
studies, e.g. oxidative N-dealkylation of N,N-dimethylanilines [35], epoxidation of olefins
[36], and hydroxylation of ethylbenzene [37], mononuclear iron complex
[FeII(N4Py*)(CH3CN)](ClO4)2 with chiral pentadentate ligand, N4Py* (N4Py* = N,N-bis(2-
pyridylmethyl)-1,2-di(2-pyridyl)ethylamine) was shown to react with H2O2, generating a
highly reactive high-valent nonheme iron species [FeIV(N4Py*)(O)](ClO4)2, and non-selective
hydroxyl radicals [37]. The moderate (enantio)selectivities in these systems can be explained
by the non-selective radical processes including also the metal-based oxidation via radical,
dissociative, non-rebound (non-RM) mechanism. In the case of the rebound mechanism (RM)
the rate-determining step of the C-H bond activation is the hydrogen atom abstraction (HAA)
resulting in the formation of the alkyl (acyl) radical species in the cage [FeIIIOH--•CR2 or
•C(O)R)], which is followed by a rebound of the radical to the OH moiety of the FeIII
intermediate, forming alcohol (carbonic acid) product in a fast step [38]. However, in the case
of non-RM pathway the radical dissociates, and the leaving radical can undergo
rearrangement before the rebound step, or react with O2 at diffusion-controlled rates [38]. We
have recently reported the hydroxylation of aromatic aldehydes to carboxylic acids by
[FeIV(N4Py)(O)](ClO4)2 (N4Py = N,N’-bis(2-pyridylmethyl)-N-bis(2-pyridyl)methylamine)
and [FeIV(N4Py*)(O)](ClO4)2 complexes under argon, where the RM pathway including a
rate-controlling HAA step has been established based on detailed kinetic and mechanistic
results: high yield of products (FeII and PhCO2H), Bell-Evans-Polanyi plot (log k2 versus
BDEC-H, BDE = bond dissociation energy), and large kinetic isotope effect (KIE = 27, 37,
respectively) (Scheme 1) [25]. As a continuity of this study we have choosed and investigated
the stoichiometric and catalytic oxidation of arylalkyl aldehyde, hydrocinnamaldehyde (HCA)
by the aforementioned FeIV=O complexes and their precursor FeII complexes with H2O2 under

air to get more insight into the mechanism (RM versus non-RM pathway) of these curious
reactions.
Scheme 1. Previously proposed mechanism for the stoichiometric oxoiron(IV) mediated
oxidation of benzaldehyde under argon [25].
2. Experimental section
2.1. Materials and methods
The ligands N4Py and N4Py*, and their complexes [FeII(N4Py)(CH3CN)](ClO4)2 (1)
and [FeII(N4Py*)(CH3CN)](ClO4)2 (2) were prepared according to published procedures
[27,28,39,40]. UV/Vis spectra were recorded with an Agilent 8453 diode-array
spectrophotometer with quartz cells. Infrared spectra were recorded with an Avatar 330 FTIR
Thermo Nicolet instrument. UV/Vis spectra were recorded with an Agilent 8453 diode-array
spectrophotometer with quartz cells. Microanalyses were performed by the Microanalytical
Service of the University of Pannonia and Atlantic Microlab. The consumption of the starting
hydrocinnamaldehyde and formation of benzaldehyde, phenylacetaldehyde, phenethyl alcohol
and hydrocinnamic acid were monitored by GC and GC-MS (Shimadzu GCMS-QP2010-SE
equipped with a secondary electron multiplier detector with conversion dynode and a 30 m
HP-5MS column).
2.2. Description of the catalytic oxidation reactions

In a typical reaction, 0.5 cm3 of H2O2 (diluted from 35% solution) solution in CH3CN
was delivered by syringe pump in air to a stirred solution (1 mL) of catalyst 1, 2 or Fe(ClO4)2
salt, and hydrocinnamaldehyde (HCA) substrate inside a vial. The final concentrations were 3
mM catalyst (1, 2 or Fe(ClO4)2), 300 mM H2O2, and 300 mM HCA. After syringe pump
addition (10 min the solution was stirred for 5 minutes and a known amount of PhBr (150
mM) was added as an internal standard (Table 1). The iron complex was removed by passing
the reaction mixture through a neutral alumina column followed by elution with ethyl acetate
(0.15 cm3). The products (benzaldehyde, phenylacetaldehyde, phenethyl alcohol and
hydrocinnamic acid) were identified by GC/MS and confirmed by comparison with authentic
samples. GC-MS spectra for benzaldehyde m/z: 106 (93.60 %), 105 (88.80 %), 77 (100 %),
74 (9.60 %), 52 (14.00 %), 51 (49.60 %), 39 (7.20 %), for phenylacetaldehyde m/z: 120
(15.57 %), 108 (5.82 %), 92 (29.42 %), 91 (100 %), 79 (12.75 %), 77 (9.79 %), 65 (36.14 %),
63 (23.11 %), 51 (21.60 %), 39 (38.96 %), for phenylethyl alcohol m/z: 122 (20.36 %), 92
(51.15 %), 91 (100 %), 77 (10.60 %), 65 (34.80 %), 51 (19.84 %), 39 (28.95 %), for
hydrocinnamic acid m/z: 150 (38.48 %), 131 (7.60 %), 104 (43.61 %), 103 (45.66 %), 92
(24.85 %), 91 (100 %), 79 (22.39 %), 77 (57.67 %), 65 (43.65 %), 51 (48.94 %), 39 (42.45
%), for hydrocinnamic acid (Me-ester) m/z: 164 (5.47 %), 150 (26.25 %), 104 (71.12 %), 91
(100 %), 77 (60.03 %) 65 (19.32 %), 51 (41.94 %), 39 (33.08 %).
Table 1. Catalytic oxidation of hydrocinnamaldehyde (HCA) carried out by 1 and 2 with
H2O2 in CH3CN at 25◦C under air.
[1] : [HCA] : [H2O2] (mM)a Yield (%)b Yield (%)b
Yield (%)b
Yield (%)b
Yield (%)b
PhCHO
1.93
PhCH2CHO
5.09
PhCH2CH2OH PhCH2CH2CO2H
0.5 : 300 : 700
1.5 : 300 : 700
3 . 300 : 700
5.61
8.32
8.99
10.3
8.82
7.33
19.96
50.03
80.55
47.71
39.83
10.67
12.93
10.7
9.46
21.57
49.95
17.75
14.12
8.68
3 . 300 :300
8.96
3 : 300 : 200
8.56
8.33
[2] : [HCA] : [H2O2]
1 : 300 : 700
2.12
7.17
2.46
1.96
6.02
7.94
6.60
3.58
5.02
5.99
6.62
3.43
19.43
36.53
13.25
7.25
32.59
57.63
28.93
16.22
3 : 300 : 700
3 . 300 : 300
3 . 300 : 100
[FeII]c : [HCA] : [H2O2]
3 . 300 : 700
0.29
0.91
0.52
0.24
1.96

b
aReaction conditions: see Experimental section. Based on substrate, calculated from the
average of two independent measurements. cFeII = FeII(ClO4)2.
2.3. Stoichiometric oxidation of hydrocinnamaldehyde with [FeIV(N4Py or N4Py*)(O)]2+ (3
or 4) under air
All reactions were carried out under thermostated conditions at different temperature
in 1 cm quartz cuvettes with stirring under argon. The [FeII(N4Py)(CH3CN)](ClO4)2 or
[FeII(N4Py*)(CH3CN)](ClO4)2 complex (1.00 × 10-3 M) was dissolved in acetonitrile (1.5
cm3), then iodosobenzene (2.00 × 10-3 M) was added to the solution. The mixture was stirred
for 50 minutes, and a known amount of PhBr (1 mM) was added as an internal standard then
excess iodosobenzene was removed by filtration. The rate of substrate oxidation was followed
by monitoring the decrease in absorbance at 705 nm ( = 400 M-1 cm-1 for
[FeIV(N4Py*)(O)](ClO4)2 (4) [40]) or 695 nm ( = 450 M-1 cm-1 for [FeIV(N4Py)(O)](ClO4)2
(3) [27,28]) respectively, under pseudo-first order condition of excess HCA (20-200 equiv.).
The iron complex was removed by passing the reaction mixture through a neutral alumina
column followed by elution with ethyl acetate (0.15 cm3). The products (benzaldehyde,
phenylacetaldehyde, phenethyl alcohol and hydrocinnamic acid) were identified by GC/MS
and confirmed by comparison with authentic samples: Yields (based on FeIV) are
PhCH2CH2CO2H (~20%), PhCH2CH2CH2OH (~18%), PhCH2CHO (~19%) and PhCHO
(~5%).
Table 2. Kinetic data for the stoichiometric oxidation of hydrocinnamaldehyde with
[FeIV(N4Py)(O)](ClO4)2 (3) and [FeIV(N4Py*)(O)](ClO4)2 (4) complexes under air in CH3CN
(1.5 cm3).
No.
T
PhCH2CH2CHO
[3]
[4]
kobs
k2
(K)
298
298
298
298
298
298
293
288
283
(10-3M)
(10-3M)
(10-3M)
(10-2s-1)
(10-1M-1s-1)
3.09±0.19
2.68±0.13
2.28±0.05
2.61±0.11
2.60±0.09
2.57±0.10
2.13±0.07
1.71±0.06
1.32±0.04
1
2
3
4
5
6
7
8
9
20
1
1
1
1
1
1
1
1
1
0.618±0.038
1.070±0.051
1.370±0.03
2.090±0.088
3.120±0.108
5.145±0.200
1.707±0.060
1.367±0.054
1.063±0.026
40
60
80
120
200
80
80
80

10
11
12
13
14
15
16
17
18
19
20
21
278
298
298
298
298
298
298
298
293
288
283
278
80
20
1
0.899±0.051
0.182±0.009
0.328±0.014
0.494±0.023
0.613±0.025
0.819±0.023
0.914±0.053
1.310±0.092
0.369±0.016
0.259±0.014
0.178±0.008
0.127±
1.12±0.05
0.91±0.05
0.82±0.04
0.82±0.04
0.77±0.03
0.82±0.02
0.76±0.04
0.66±0.05
0.46±0.02
0.32±0.02
0.22±0.01
0.16±0.01
1
1
1
1
1
1
1
1
1
1
1
40
60
80
100
120
200
80
80
80
80
3. Results and discussion
3.1. Catalytic oxidation of hydrocinnamaldehyde (HCA)
The catalytic activity of [FeII(N4Py)(CH3CN]2+ (1) and [FeII(N4Py*)(CH3CN]2+ (2)
was investigated in the oxidation of HCA, utilizing H2O2 as the co-oxidant under air. The
reactions were carried out under standard catalytic conditions (1 : 100 : 100 ratio for the
catalyst : co-oxidant : substrate) in CH3CN under air at 25◦C. Co-oxidant was added by
syringe, and after a definite time the reaction mixture was analyzed by GC-MS (Table 1). It
took less than 15 min to have about 48 and 29 % yields (based on the substrate) for complex 1
and 2, respectively. A much lower yield was obtained for the Fe(ClO4)2 (~2 %) (Fig.1A and
Table 1). In case of catalyst 1 and 2, the oxidation reactions produced hydrocinnamic acid as
expected major product with an overall yield of 17.8 and 13.3 % in addition to three minor
products; benzaldehyde (11 and 2.5 %), phenylacetaldehyde (9 and 6.6 %) and phenethyl
alcohol (10 and 6.6 %), respectively. Similar results have been obtained during the aerobic
oxidation of phenyl acetaldehyde in the presence of cobalt and manganese acetate as catalysts
[41]. Significant increase in the yields of hydrocinnamic acid was observed when the amount
of catalyst (1, 2) and/or H2O2 was increased (50 % for 1 and 36.5 % for 2), whilst the yields of
the minor products didn’t changed remarkably (Fig. 1). Similarly to our previous results,
[FeII(N4Py)(CH3CN]2+ is a far better catalyst than [FeII(N4Py*)(CH3CN]2+ for the oxidation
of HCA, which may be explained by the different rigidity around the metal centres.

Fig. 1. (A) Comparison of the product formation in the iron-catalyzed (Fe(ClO4)2 vs. 1 and 2)
oxidation of hydrocinnamaldehyde with H2O2 as co-oxidants in CH3CN at 25°C under air.
Reaction conditions: [Fe, 1 or 2]0 = 3 mmol, [HCA]0 = 300 mM, [H2O2] = 300 mM. (B, C)
Effect of the amount of catalyst (1, 2); [HCA]0 = 300 mM, [H2O2] = 700 mM. (D, E) Effect of
the amount of H2O2; [1 or 2]0 = 3 mmol, [HCA]0 = 300 mM.
The oxidative cleavage of HCA to phenylacetaldehyde (benzaldehyde) and phenethyl
alcohol indicates an autooxidation process via Russel-type termination mechanism involving
reactions of the PhCH2CH2C(O)• and PhCH2CH2• radicals with dioxygen to form equimolar
amount of PhCH2CH2OH and PhCH2CHO (Scheme 2) [42]. The formation of
PhCH2CH2C(O)• radical may be explained either by the forming oxoiron(IV) intermediate,
and the appearance of hydroxyl radical during its formation. The oxidative cleavage of HCA
in the presence of oxoiron(IV) species could also occur via reaction between cage-escaped

PhCH2CH2C(O)• radical and dioxygen. It can be concluded that the moderate selectivity can
be explained by the parallel selective metal-based, and non-selective radical processes
including non-RM route with oxoiron(IV) species, resulting in hydrocinnamic acid and
oxidative cleaved products (consecutive products of the arylalkyl radicals formed by
decarbonylation of the phenylpropionil radical), respectively. It is worth to mention that in the
case of H2O2 the presence of dioxygen can not be excluded by the use of inert atmosphere due
to the metal-based disproportionation of H2O2.
Scheme 2. Proposed mechanism in the iron(II)-catalyzed oxidation of hydocinnamaldehyde
with H2O2 under air.

3.2. Stoichiometric oxidation of hydrocinnameldehyde (HCA) with 3 and 4 under air
In general free radical mechanism can be proposed for the oxidation of aldehydes with
molecular oxygen in the presence of transition metal complexes such as manganese and iron,
where the formation of acyl radical is the first step, which then reacts with dioxygen forming
acylperoxy radical, or easily decarbonylates into alkyl radical. Previously we have found clear
evidence
for
the
formation
of
high-valent
oxoiron(IV)
species
in
[FeII(N4Py*)(CH3CN]2+/H2O2-catalyzed systems. We have also found that the forming
oxoiron(IV) species is capable for the oxidation of benzaldehyde to benzoic acid under argon,
including rebound-type mechanism. In order to obtain more insight into the role of high-
valent oxoiron(IV) intermediate on the oxidation of arylalkyl aldehydes under air, we have
synthesized the oxoiron(IV) complex 3 and 4 by in situ reaction of 1 and 2 with PhIO, and
investigated the stoichiometric oxidation of HCA with it under air. In this way (by the use of
PhIO as an oxidant), the formation of hydroxyl radicals and its non-selective reactions can be
excluded, and the possible role of the oxoiron(IV) species can be directly investigated.
The oxoiron(IV) complexes 3 and 4, which are well-characterized iron-based oxidants
with an absorbance band in the visible region (max = 695 nm (450 M-1 cm-1) and max = 705
nm (400 M-1 cm-1), respectively) were generated by reaction of 1 and 2 with PhIO in MeCN,
and the rate of the decay of the absorption band at 695 nm and 705 nm was measured as a
function of the concentration of added HCA (Table 2 and Fig. 2). To generate 3, complex 1 (1
mM) was dissolved in MeCN and treated with 2 equiv of PhIO, which led to the maximum
generation of 3 (~40 min), as judged by UV-vis spectroscopy, followed by a fast
decomposition of the green species upon addition of HCA, which coincided with the
regeneration of 1 (max = 453 nm, 6250 M-1 cm-1) (Fig. 2A). The yield of 1 was ~25 % at the
end of the reaction of 3 with HCA based on an absorbance measurement using its extinction
coefficient under these conditions. Similar result has been obtained for complex 4. The
absorption bands at 680 nm and 690 nm (~200 M-1 cm-1) observed at the end of the reactions
can be assigned to the formation of [{FeIII(N4Py or N4Py*)2}(-O)]4+ complexes from the
elimination reaction of the forming [(N4Py or N4Py*)FeIII(OH)]2+ (~70 %) complexes. Both
complexes 3 and 4 were able to oxidize the hydrocinnamaldehyde under aerobic conditions, at
25˚C in CH3CN, and the GC/MS analysis of the products indicated the formation of
hydrocinnamic acid (~20 % based on 3 and 4), phenethyl alcohol (~18), phenylacetaldehyde
(~19) and benzaldehyde (~5%). These results including the low yield of Fe(II) and
hydrocinnamic acid (RM) beside the formation of [{FeIII(N4Py or N4Py*)2}(-O)]4+ and

oxidative cleaved products (non-RM) are consistent with a complex system including a
rebound and non-rebound mechanisms, where the dioxygen can interfere with the rebound
step and results in oxidative cleaved products by reacting with the cage-leaved acyl radical.
Kinetic experiments under pseudo-first-order conditions (excess of HCA) revealed 1st-order
dependence on both the oxoiron(IV) species and HCA with k2 = 2.60 × 10-1 M-1 s-1, ΔH# = 27
kJ mol-1 and ΔS# = -165 J mol-1 K-1 at 298 K (Fig. 3 and Table 3). Similarly, 4 was reacted
with HCA under the same conditions, and a slower rate with respect to complex 3 was
obtained. The rate constant (k2) for complex 4 was found to be k2 = 0.77 × 10-1 M-1 s-1 with
ΔH# = 51 kJ mol-1 and ΔS# = -106 J mol-1 K-1 at 298 K. The large negative activation
entropies are typical of associative processes. Reaction rates obtained for the oxidation of
HCA are 3-4 times smaller than those observed for benzaldehyde with oxoiron(IV) complexes
3 and 4 (k2 = 8.2 × 10-1 and 2.2 × 10-1 M-1 s-1, respectively [25]). Similar reaction rate values
but much higher yields for hydrocinnamic acid and the Fe(II) precursor complexes (~70-80
%) were observed under argon, excluding the involvement of dioxygen in the rate-
determining step.
Fig. 2. Reaction of 3 and 4 (derived from the reaction of 1 and 2 with PhIO) with HCA in
CH3CN at 25°C under air: (A) UV-Vis spectral changes of 3 species upon addition of 60
equiv of HCA ([3] = 1 mM, l = 1 cm). Inset shows time course of the decay of 3 monitored at
695 nm with HCA. (B) UV-Vis spectral changes of 4 species upon addition of 100 equiv of
HCA ([4] = 1 mM, l = 1 cm). Inset shows time course of the decay of 4 monitored at 705 nm
with HCA.

Fig. 3. Reaction of 3 and 4 (derived from the reaction of 1 and 2 with PhIO) with HCA in
CH3CN under air: (A) Plot of kobs versus [HCA]0 for reactions of 3 and 4 with HCA at 25°C
([3 and 4] = 1 mM). (B) Eyring plots of log(k2/T) versus 1/T for the oxidation reaction of 3
and 4 with HCA ([3 and 4] = 1 mM, [HCA]0 = 80 mM)
Table 3
Kinetic parameters determined in the oxidation of benzaldehyde and hydrocinnamaldehyde by
oxoiron(IV) complexes 3 and 4 in acetonitrile at 25°C.
H≠
(kJmol-1)
S≠
(Jmol-1K-1)
-195
G≠
(kJmol-1)
complex
substrate
k2
Refs.
(M-1s-1)
N4Py
PhCHO
8.2×10-1
2.2×10-1
2.6×10-1
7.7×10-2
21
79
[25]
N4Py*
N4Py
PhCHO
33
27
51
-168
82
77
80
[25]
PhCH2CH2CHO
PhCH2CH2CHO
-165
this work
this work
N4Py*
-106
4. Summary and conclusions
Efforts have been made to work out an iron(II)-catalyzed oxidation reaction for aryl
alkyl aldehyde, hydrocinnamaldehyde by the use of H2O2 under air, as well as their detailed
mechanistic aspects. It was found that the selectivity, the product composition (hydrocinnamic
acid versus oxidative cleaved products: phenylacetaldehyde, benzaldehyde and phenethyl
alcohol) in both the stoichiometric and the catalytic systems remarkably depends on the
presence of dioxygen. Based on experimental observations a plausible mechanism was
proposed, where the high-valent oxoiron(IV) species is proposed as key oxidant in the C-H
activation process via radical dissociation pathway resulting FeIIIOH and acyl radical species.
Their consecutive reactions can be explained by metal-based rebound mechanism resulting
hydrocinnamic acid and non rebound free radical mechanism between the forming acyl

radical and dioxygen resulting oxidative cleaved products. Based on these results we have got
a further evidence for the previously proposed non-rebound mechanism.
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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Tel: +36-88-624-720

Fax: +36-88-624-469
e-mail: kaizer@almos.vein.hu
March 03, 2020
Professor C. E. Housecroft
Regional Editor
Polyhedron
Dear Professor C. E. Housecroft,
Contribution of authors:
Flóra Viktória Csendes: Reaction kinetics, catalysis studies.
Prof. Dr. József Kaizer: Project leader, writer of the manuscript.
I verify that this submission is original, and that all co-authors are aware of the
submission and agree to its publication in the Journal.
Sincerely,
Professor József Kaizer
Department of Chemistry, University of Pannonia
8200 Veszprém
Wartha V. u. 1.
Hungary
Declaration of interests
X The authors declare that they have no known competing financial interests or personal
relationships that could have appeared to influence the work reported in this paper.
☐The authors declare the following financial interests/personal relationships which may be
considered as potential competing interests:

Catalytic and stoichiometric oxoiron(IV) assisted oxidation of
hydrocynnamaldehyde under air
Flóra Viktória Csendes and József Kaizer*
Stoichiometric and catalytic oxidation of arylalkyl aldehyde, hydrocinnamaldehyde was
investigated by FeIV=O and their precursor FeII complexes with H2O2 under air to get more
insight into the mechanism (RM versus non-RM pathway) of these reactions.
*Corresponding author. Tel.: +36 88 62 4720; Fax: + 36 88 62 4469.
E-mail address: kaizer@almos.uni-pannon.hu.