Facile Peroxidation of Cyclohexane Catalysed by In Situ Generated Triazole-Functionalised Schiff Base Copper Complexes

Article abstract of DOI:10.1007/s10562-021-03732-3

A set of facile room temperature catalytic systems for the oxidation of cyclohexane C–H bonds was developed from in situ generated triazole-functionalised Schiff base copper complexes. The combination of a new triazolium-functionalised Schiff base, [(E)-3-methyl-1-propyl-4-(2-(((2-(pyridin-2-yl)ethyl)imino)methyl)phenyl)-1H-1,2,3-triazol-3-ium hexafluorophosphate(V), 2] with a range of bench-top Cu(I) and Cu(II) salts (Cu₂O, CuO, Cu(CH₃CN)₄PF₆, CuSO₄·5H₂O, Cu₂(OAc)₄·2H₂O, CuCl₂, Cu(NO₃)₂·3H₂O) as catalysts were screened under varying reaction conditions for the peroxidation of cyclohexane using hydrogen peroxide as a green source of oxygen. High conversions to oxidised products were obtained with up to 80% in 6?h for the 2/CuSO₄·5H₂O system at 1?mol% catalyst concentration under optimised reaction conditions. All the copper salts yielded the ketone–alcohol (K–A) oil containing varying ratios of cyclohexanol and cyclohexanone. The results also showed that at room temperature, the various in situ generated copper catalysts exclusively yielded only the K–A oil. Furthermore, by changing the reaction temperature to reflux in acetonitrile and depending on the starting substrate (cyclohexane, cyclohexanol or cyclohexanone), 23–100% of adipic acid was also obtained. The kinetics study for the peroxidation reaction reveals activation energy of 12.29 ± 2?kJ/mol following a copper initiated radical mechanism. Graphic Abstract: [Figure not available: see fulltext.]

Full text of DOI:10.1007/s10562-021-03732-3

Catalysis Letters
https://doi.org/10.1007/s10562-021-03732-3
Facile Peroxidation of Cyclohexane Catalysed by In Situ Generated
Triazole‑Functionalised Schif Base Copper Complexes
Nasir S. Lawal1 · Halliru Ibrahim1 · Muhammad D. Bala1
Received: 18 February 2021 / Accepted: 11 July 2021
© The Author(s), under exclusive licence to Springer Science+Business Media, LLC, part of Springer Nature 2021
Abstract
A set of facile room temperature catalytic systems for the oxidation of cyclohexane C–H bonds was developed from
in situ generated triazole-functionalised Schif base copper complexes. The combination of a new triazolium-function-
alised Schif base, [(E)-3-methyl-1-propyl-4-(2-(((2-(pyridin-2-yl)ethyl)imino)methyl)phenyl)-1H-1,2,3-triazol-3-ium
hexafuorophosphate(V), 2] with a range of bench-top Cu(I) and Cu(II) salts (Cu2O, CuO, Cu(CH3CN)4PF6, CuSO4·5H2O,
Cu2(OAc)4·2H2O, CuCl2, Cu(NO3)2·3H2O) as catalysts were screened under varying reaction conditions for the peroxidation
of cyclohexane using hydrogen peroxide as a green source of oxygen. High conversions to oxidised products were obtained
with up to 80% in 6 h for the 2/CuSO4·5H2O system at 1 mol% catalyst concentration under optimised reaction conditions.
All the copper salts yielded the ketone–alcohol (K–A) oil containing varying ratios of cyclohexanol and cyclohexanone.
The results also showed that at room temperature, the various in situ generated copper catalysts exclusively yielded only the
K–A oil. Furthermore, by changing the reaction temperature to refux in acetonitrile and depending on the starting substrate
(cyclohexane, cyclohexanol or cyclohexanone), 23–100% of adipic acid was also obtained. The kinetics study for the per-
oxidation reaction reveals activation energy of 12.29 2 kJ/mol following a copper initiated radical mechanism.
Graphic Abstract
Keywords Cyclohexane · Adipic acid · Peroxidation · Copper salt · Triazolium salt · Schif base · In situ catalysis
1 Introduction
Nylons, resins, plasticisers and lubricants are produced from
adipic acid (AA), which is amongst the most widely used
feedstock in the textile and allied chemicals industries. At
the industrial scale, AA is obtained from the nitric acid-
catalysed terminal oxidation of cyclohexane (Cy-H) via its
primary oxidation products, cyclohexanone (Cy=O, K) and
cyclohexanol (Cy-OH, A), the so-called (ketone-alcohol)
* Muhammad D. Bala
bala@ukzn.ac.za
1
School of Chemistry & Physics, University of KwaZulu-
Natal, Westville Campus, Private Bag X54001,
Durban 4000, South Africa
Vol.:(0123456789)
1 3

N. S. Lawal et al.
K–A oil [1, 2]. Processing the K–A oil to AA usually
requires harsh conditions (i.e. high temperatures, use of
strong oxidants, and stoichiometric additives), which often
lead to poor selectivity, low yields, and undesired environ-
mental and economic costs [3].
to be active in a wide variety of homogeneous catalytic
reactions such as enantioselective C–C coupling [22, 23],
α-alkylation of ketones with alcohols [24] and the oxida-
tion of alkyl and aryl alcohols [25]. Recently we reported a
set of in situ generated Cu(I)/L (L=1,2,3-triazol-5-ylidene)
catalysts that showed moderate conversions (up to 39%) for
the oxidation of Cy-H (Table 1, entry 13) [19] and have
previously developed a variety of azolium (imidazolium and
triazolium)- based transition metal complexes for application
in oxidation [26] and C-N coupling reactions [27].
The catalytic oxidation of Cy-H to the K–A oil using
green oxidants such as H2O2, tert-butyl hydroperoxide
and molecular oxygen continues to be a challenge. This is
because the majority of the methods for this process are
based on heterogeneous catalysts. Although in comparison
with homogeneous systems such as those reported herein,
heterogeneous catalysts ofer the advantage of catalyst reuse
and recyclability, however, they are more prone to unde-
sirable over-oxidation and the production of side products
[4–6]. Various homogeneous copper-based catalytic systems
have been developed over the years and used to catalyse
Cy-H peroxidation at low temperatures with low to moderate
yields [7–20]. Table 1 shows the development of homoge-
neous copper catalysed systems for the mild oxidation of
cyclohexane to its products. The low conversions reported
for many of the catalysts made their industrial application
difcult, and thus far, the most active homogeneous catalysts
were reported in 2016 by Garcia-Bosch and Siegler, who
reported 56% conversion of the substrate (Table 1, entry 12)
[18]. The compounds listed in Table 1 serve as indicators
toward the development of more active and mild condition
oxidation catalysts based on copper.
Since the beginning of the twentieth century, Schif bases
have been adopted as exceptional donor ligands to stabi-
lise reactive metal ions [28] for efcient transition metal
catalysed homogenous reactions [29, 30]. Consequently,
coupling a Schif base moiety to an azolium core yields
multifunctional polydentate donor ligands [31–33] that have
shown unique and efcient stereoselectivity in asymmetric
catalysis by transition metals [34]. Hence, in this report,
we present the application of a new series of in situ gener-
ated copper complexes that are stabilised by such a multi-
functional ligand framework (iminopyridyl-1,2,3-triazole).
The three potential coordinating donor arms of the ligand
will serve to stabilise the metal centres as catalysts for the
peroxidation of Cy-H to its oxygenate products and, more
importantly, to AA. The formation of the in situ complexes
reported in this work and their application as viable cata-
lysts for cyclohexane oxidation illustrates the potential of
bench-top reagents as alternatives to more expensive con-
ventional metal precursors in the preparation of homogene-
ous catalysts. Consequently, the aforded catalyst systems
described herein are cheap to operate, work under mild
conditions and environmentally benign [35]. This report
is an efort to circumvent the shortcomings of previously
When compared to isolated metal complexes as catalysts,
the in situ generated variants are easy to prepare, cost-efec-
tive and require minimal to no work-up procedures for their
application. In situ catalysis featuring Cu/imidazolium salts
was pioneered by Woodward in 2001 [21]. And since then,
various Cu/imidazolium and triazolium salts were reported
Table 1 Homogeneous copper
catalysts for the mild oxidation
of cyclohexane
Entry
Catalysta
Oxidant
Time
Temp
Total conver-
References
sion (K–A oil)
1
2
3
4
5
6
7
8
Cu powder/pyridine/Ac
Cu(OH)2
H2O2 or O2
O2
H2O2
O2
H2O2
t-BuOOH
H2O2
6
24
3
24
2
24
5
r.t
20
r.t
70
r.t
70
80
60
5
4
15
4
7
11
18
12
[7]
[8]
[9]
[10]
[11]
[12]
[13]
[14]
[CuI(bpy)2]/pyridine
[CuCl2)4(18-crown-6)2(H2O)]
[Cu(bdpg)Cl]Cl
[Cu(tma)2]
[Cu(salen)]
[Cu(MeCN)4][BF4]
t-BuOOH or
5
CH3COOH
9
[Cu4(tea)-(BOH)4(m–O)]2+
[CuII(β-diketiminate)(OAc)]
Cu(I)-trispyrazolylborate
CuI/(R,R)-bpbp
H2O2
H2O2
H2O2
H2O2
H2O2
6
3
1
6
r.t
r.t
r.t
r.t
60
39
5
25
56
39
[15]
[16]
[17]
[18]
[19]
10
11
12
13
CuI/1,2,3-triazolylidene
24
aOnly the most active copper catalyst system for each reference is quoted
1 3

Facile Peroxidation of Cyclohexane Catalysed by In Situ Generated Triazole-Functionalised…
reported homogeneous catalyst systems by combining the
well-documented catalytic properties of the Schif base and
triazolylidene ligands with the potential hemi-lability of a
pyridyl N-donor to further improve the capability of copper
for oxidation catalysis.
7.60 (1H, d, J=6.9 Hz, Ar), 7.70 (2H, t, J=7.7 Hz, Py), 7.71
(1H, d, J=15.3 Hz, Ar), 7.92 (1H, d, J=7.5 Hz, Py), 8.37
(1H, s, HC = N imine), 8.48 (1H, d, J = 4.4 Hz, Py), 8.95
(1H, s, NNCH triazolyl). 13C NMR (DMSO, 100.6 MHz):
δ 10.3, 22.3, 54.6, 59.9, 120.4, 121.4, 123.0, 129.1, 130.6,
131.1, 132.1, 135.1, 136.4 (NNCH triazolyl), 141.6, 148.9,
159.1, 160.1 (HC=N imine). IR (ATR, cm−1): 3051 (C–H,
sp2), 1643 (C=N), 1591 (C=C), 1166 (N=N), 864 (PF−),
766 (Ar C–H). HRMS (ESI+): m/z 334.2028 [(M – PF6)6]+,
calculated for [(M – PF6]+ 334.2032.
2 Experimental
2.1 General Information
All reagents were purchased from Sigma-Aldrich and used
as received. Solvents (acetonitrile, tetrahydrofuran, metha-
nol and diethyl ether) were purchased from Merck and puri-
fed using a commercially available MBraun MB-SP Series
solvent purifcation system equipped with activated alu-
mina columns. Unless otherwise stated, all syntheses were
performed under a dinitrogen atmosphere using standard
Schlenk techniques. We recently reported the full charac-
terization of the starting aldehyde 4-(2-formylphenyl)-3-
methyl-1-propyl-1H-1,2,3-triazol-3-ium iodide, 1 [36].
NMR spectra were recorded on a Bruker Avance 400 MHz
spectrometer operated at ambient temperature with δ val-
ues reported in ppm referenced to (CH3)4Si as the internal
2.3 General Procedure for the Catalytic Oxidation
Reaction
Except when mentioned, all the processes involved in the
oxidation reaction were carried out in an aerobic environ-
ment in a Schlenk tube ftted with an efcient refux con-
denser. Cyclopentanone was used as the internal standard,
and all GC quantitative analysis and experiments were
conducted with an Agilent Technology 6820 GC System
equipped with a fame ionisation detector (FID) and an Agi-
lent DB wax column with a length of 30 m, an inner diam-
eter of 0.25 mm and a thickness of 0.25 mm. Retention times
from the catalytic mixture were compared with commercial
standards of cyclohexane, cyclohexanol, cyclohexanone and
adipic acid. The general stepwise peroxidation of Cy-H by 2/
CuSO4 is described; other catalyst systems were tested using
the same procedure:
1
standard for both H and 13C NMR data. Infrared spectra
were recorded on a Perkin Elmer universal ATR Spectrum
100 FT-IR spectrometer. UV–Vis spectra were measured on
a Shimadzu UV–Vis–NIR Spectrophotometer UV – 3600.
Mass spectrometric data were recorded on Waters Micro-
mass LCT Premier TOF MS-ES+.
The catalyst was generated in situ from CuSO4 (19.9 mg,
40 µmols) and compound 2 (38.4 mg, 40 µmols), which were
dissolved in 1 cm3 of CH3CN and placed in a 20 cm3 Schlenk
tube ftted with a stirrer and an efcient refux condenser.
Then, 1.52 cm3 of H2O2 solution (30% in H2O, 20 mmol)
was added to the 2/CuSO4 mixture. Immediately after that
(about 10 s), then the appropriate substrate, e.g. for Cy-H
(0.44 cm3, 4 mmol), was added to the mixture with the total
reaction volume of 3 cm3. The reaction mixture was stirred
at room temperature for peroxidation to Cy-OH and Cy=O
or at refux for the production of AA. At the end of the reac-
tion (6 h), the internal standard cyclopentanone (0.1 cm3,
95 mg, 1 mmol) and 5 cm3 of diethyl ether were added to
the reaction mixture. Then, 2 cm3 of the crude product were
added to a 5 cm3 vial containing PPh3 (1.5 mmol) and placed
in an ice bath in order to convert the Cy-OOH to Cy-OH
(excess H2O2 is converted to H2O). After 15 min, an aliquot
of the PPh3 treated sample was taken using a Pasteur pipette
and fltered through a cotton wool plug. The fltrate (0.5 μL)
was then injected into the GC for analysis and quantifcation.
Products were identifed by comparison with analytically
genuine samples, and yields were quantifed from peak areas
(average of two runs within 5%). Treatment of the product
with PPh3 is not required when Cy-OH and Cy=O were used
as substrates for the production of AA [35].
2.2 Synthesis of the Functionalised Schif Base
Ligand Precursor Salt
(E)-3-methyl-1-propyl-4-(2-(((2-(pyridin-2-yl)
ethyl)imino)methyl)phenyl)-1H-1,2,3-triazol-3-ium
hexafuorophosphate(V), 2
A conventional Schif base synthetic procedure was used
[37]. A methanolic solution of a mixture of 1 (0.357 g,
1 mmol), and 2 mol equivalent of 2-(pyridin-2-yl)ethan-
amine (0.244 g, 2 mmol) was stirred and refuxed for 4 h in
a 20 cm3 Schlenk tube. To the reaction fask was then added
KPF6 (0.18 g, 1 mmol), and the mixture continuously stirred
at room temperature for 12 h. Removal of the solvent fol-
lowed by extraction with DCM (3×25 cm3) gave the crude
2 as the DCM soluble fraction. The DCM soluble fraction
was then concentrated under reduced pressure, washed with
diethyl ether (3×25 cm3) and dried to aford the pure 2, as
yellow viscous oil. Yield 0.38 g, 80%. 1H NMR (DMSO-d6,
400 MHz): δ 0.96 (3H, t, J=7.4 Hz, CH3 Pr), 1.98 (2H, m,
J=7.2 Hz, –CH2– Pr), 2.89 (2H, t, J=14.2 Hz, –CH2–), 3.82
(2H, t, J=6.9 Hz, –CH2–N=), 3.86 (3H, s, CH3), 4.61 (2H,
t, J = 13.9 Hz, –CH2–N = pr), 7.22 (2H, t, J= 4.1 Hz, Ar),
1 3

N. S. Lawal et al.
aryl (C=C) functionalities were observed respectively at
1643 and 1591 cm−1 in the IR data of compound 2. The
absence of the C=O stretching vibration band previously
observed at 1702 cm−1 in 1 confrms the formation of the
Schif base functionality in 2 [39]. The pattern of the mass
ionisation spectrum of 2 is similar to those reported for
related compounds [40]. It is characterised by the loss of
the counterion with the [(M – PF6)]+ fragment as the parent
molecular ion, the HRMS analysis of which gave an m/z
value that is within acceptable limits of the calculated value.
3 Results and Discussion
3.1 Synthesis
We recently reported on the synthesis and full characterisa-
tion of 1 [36]. Compound 2 was synthesised from 1 by the
standard Schif-base condensation of the triazolium alde-
hyde and 2-(pyridin-2-yl)ethan-1-amine. The iodide salt was
subjected to counterion metathesis with KPF6 to give 2, in
good yield as a yellow viscous oil. All the compounds were
isolated and fully characterised (Scheme 1).
3.3 In Situ Catalytic Peroxidation of Cy‑H
3.2 Characterisation
A study of the catalytic oxidation of Cy-H was initiated at
room temperature, initially for 6 h in the preliminary opti-
misation stage (Scheme 2). Various parameters that included
the substrate to oxidant ratio, solvent to oxidant ratio, the
concentration of the catalyst and time of the reaction were
studied. The precatalyst at this stage was a mixture of 2 and
Cu(CH3CN)4PF6, which in solution generated the catalyst
in situ. It is important to highlight that excess triphenylphos-
phine was added to each aliquot before injection to the GC in
order to maximise the yield of Cy-OH. This has been noted
to result from the reduction of the major oxidation product
(Cy-OOH, Schemes 2–3) [41].
Formation of salt 2 was confrmed by the disappearance of
the aldehyde singlet proton signal that was initially observed
in 1 at δ 9.20 ppm, and the corresponding appearance of
two distinct singlet signals at 8.9 ppm and 8.4 ppm in the
1H NMR spectrum assigned to the triazolium (C5-H) and
imine (HC=N) protons, respectively. The corresponding
13C NMR spectrum showed the imine (HC=N) carbon at
160 ppm and shifted upfeld compared to the signal of the
unreacted triazolium aldehyde that was observed at 192 ppm
in 1. Aromatic and aliphatic protons of compound 2 were
within the expected regions in both the H and 13C NMR
1
spectra [38, 39]. The 31P NMR spectrum of compound 2
gave a multiplet peak at -148 ppm, accounting for the suc-
cessful incorporation of PF6 as a counterion in 2 [27]. In
addition, FTIR and HRMS complemented and unambigu-
ously confrmed the formation of the hexafuorophosphate
salt. Distinct signals corresponding to the imine (C=N) and
Much of the reported literature on copper catalysed oxi-
dation catalysis (Table 1) has indicated that an appropriate
ligand framework that supports the metal centre in a required
state of oxidation is a prerequisite for an efcient catalytic
system [42]. Hence, we carried out optimisation studies to
investigate the roles of the Cu and precursor salt 2 on the
Cy-H peroxidation studies. Analysis of the data obtained
showed the importance of the Cu species as the active centre
because no conversion of the Cy-H substrate to its oxidised
products was observed in the absence of the metal. However,
when the metal salt was added without the ligand precursor
2, a low substrate conversion of 8% was recorded, which
is attributed to a very unstable oxidant-generated copper
hydroperoxyl complex activating the substrate [42].
N
N
O
a
N
N
N
N
N
N
PF6
I
Then we studied the infuence of varying the oxidant
concentration on the catalytic process. The results pre-
sented in Fig. 1 show a direct dependence of substrate con-
version on oxidant concentration with optimum efciency
at 5 mol equivalents of H2O2. In context, many catalytic
1
2
Scheme 1 Synthesis of 1,2,3-triazolium Schif base salt, 2. Reaction
conditions: a) i. 2-(pyridin-2-yl)ethan-1-amine, MeOH, 4 h; ii. KPF6,
MeCN, rt, 12 h
Scheme 2 Catalytic peroxida-
PPh
3
tion of Cy-H to the alcohol
OOH
(Cy-OH) and ketone (Cy=O)
products
CH CN, (NHC:Cu),
3
OH
+
O
oxidant, rt, 6h
Cy=O
Cy-OOH
Cy-OH
Cy-H
1 3

Facile Peroxidation of Cyclohexane Catalysed by In Situ Generated Triazole-Functionalised…
+ H2O2
.
CuI/II
CuII/III
OO
(i)
L
L
OH + HO
OOH
H
H
.
+
+ HO
+
+ O2
- H2O
(ii)
(iv) Cy-OOH
(iii)
Cy-H
OH
OOH
O=PPh3
O
+
PPh3
+
(v)
Cy-OH
O
O
OH
O
O
O
[O]
[O]
[O]
OH
O
[O]
OH
H2O
OH
HO
O
OH
(ix)
(vii)
(viii)
(x)
AA
(vi) Cy=O
Scheme 3 Proposed reaction mechanism for the copper catalysed oxidation of Cy-H to AA
1000
800
600
400
200
80
60
40
20
0
TON
80
60
40
20
Conversion
0
1
2
3
Catalyst (mol%)
0
4
8
12
Fig. 2 Catalyst efciency as a function of concentration
H O (molar equiv)
2 2
Fig. 1 Dependence of Cy-H conversion to oxygenates on oxidant
An exploration of previous reports on this subject has
shown that catalyst concentrations above 1 mol% were asso-
ciated with slower reactions, decreased conversions supple-
mented by low turnover values, all of which were attrib-
uted to the decomposition of available active sites at higher
concentrations of a catalyst [15, 18, 19, 43]. In summary,
a precatalyst concentration of 1 mol% was adopted for the
remainder of this study.
concentration
systems for the peroxidation of Cy-H have reported the
use of up to 10 mol equivalents of an oxidant [15, 17–19].
Results on the optimisation of catalyst concentration
(Fig. 2) also indicated a direct correlation with substrate
conversion. A high conversion of 76% was obtained at a
catalyst concentration of 3 mol%. However, the best cata-
lyst efciency as turnover number (TON) was obtained
at a catalyst concentration of 0.25 mol%, albeit at a low
substrate conversion of 23% (Fig. 2).
A study on the reaction time profle presented in Fig. 3
shows that fve molar equivalents of the oxidant and 1 mol%
of the in situ generated catalyst led to 91% conversion in
24 h. However, evaluation of the catalyst efficiency as
turnover frequency (TOF) indicates that the highest TOF
1 3

N. S. Lawal et al.
100
80
60
40
20
0
100
80
60
40
20
0
Cyclohexanol
Cyclohexanone
0
5
10
15
20
25
Time (h)
Fig. 4 Catalytic performance of a range of bench-top Cu(I) and
Fig. 3 Time profle for the peroxidative conversion of Cy-H
Cu(II) salts for the oxidation of Cy-H to the K-A oil
of 103 h−1 was achieved at 6 h; hence the remainder of the
study was conducted using the optimised conditions of
1 mol% catalyst and 5 mol% H2O2 at 6 h reaction time.
Metal catalysed peroxidation reactions have been reported
to proceed through the formation of radical species or Fen-
ton-type routes. Thus, we added a stoichiometric amount of
a radical scavenger (PH2NH) at the start of the reaction as an
inhibitor to gain some insight into the route involved in the
peroxidative catalysis [35, 44]. This led to only 4% conver-
sion of Cy-H to the oxygenate products, which is an indica-
tion that the radical scavenger has shut down the catalysis,
clearly suggesting the active participation of radical species
in the reaction [14].
varying combinations, with the ketones being dominant for
the majority of the salts that are Cu(II) centered. This gen-
eral trend agrees with other literature reports where Cy=O
was the major product of Cy-H peroxidation reaction [7–17].
It is interesting to note that both Cu(I) and Cu(II) oxides and
the other Cu(I) salts preferentially produced the alcohol in
higher quantities, indicating that these salts produced more
of the major product (CyOOH) and curtailed secondary oxi-
dation to the ketone. However, in practical terms, this will
minimally afect the further oxidation of the K-A oils from
these salts to the terminal (AA) product (see Scheme 2).
A revision of previously reported literature indicates that
ligand stabilised metal centres serve as good catalysts in
reactions that proceeded via Fenton-type mechanisms, with
the supporting ligand serving to mediate the extent of metal-
substrate interactions [44]. In these regards, we recently
reported a monodentate triazolylidene copper complex that
actively yielded 39% conversion of Cy-H to a mixed ketone-
alcohol product [19]. Thus, we are of the opinion that the
multidentate ligand framework presented in this study in
conjunction with copper will lead to even better catalytic
activity due to enhanced ligand/metal interaction and coop-
eration. This assertion is further supported by our recent
report involving Cu(I) compounds as catalysts [45]. Hence,
we proceeded to study the efciency of a range of bench-top
Cu(I) and Cu(II) salts combined with the triazolium Schif
base ligand precursor 2 to generate in situ catalysts with
the results presented in Fig. 4. Interestingly, all the metal
salts gave good to excellent overall conversions in the range
of 51–80%. The Cu(II) salts accounted for higher conver-
sions to the oxygenate products, with CuSO4 yielding the
best conversion of 80%. All the salts yielded the K-A oil in
3.4 Catalytic Peroxidation of Cy‑H to AA
Previously reported studies on Cu-containing catalyst sys-
tems geared toward the oxidation of Cy-H were limited to
optimising the conversion of the substrate to the primary
oxygenate (K-A) product (Table 1), which in its own right
is a very important feedstock for the industrial produc-
tion of nylons and related products [11, 42]. In this study
(Table 2), we present a viable and environmentally benign
system for the direct quantitative production of AA, a very
important raw material for many industrial chemical pro-
cesses. The process involves the terminal oxidation of Cy-H
to AA under mild reaction conditions, such that instead of
the room temperature reactions studied thus far, refuxing
neat Cy-H (70 °C) under the optimised reaction conditions
(catalyst, oxidant, time) did not only produce higher conver-
sions (up to 96%) but, more signifcantly, directly yielded
23% of AA. This is contrary to the room temperature studies
in which AA was not observed at any stage of the reaction.
Furthermore, using Cy-OH as the starting substrate under
refux conditions gave about 80% total conversion with a
1 3

Facile Peroxidation of Cyclohexane Catalysed by In Situ Generated Triazole-Functionalised…
Table 2 Comparative analysis of substrate conversion to AA
OH
O
CH CN, (2/Cu),
COOH
COOH
3
o
H O , 70 C, 6h
2
2
Cy-H; Cy-OH; Cy=O
AA
#
Substrate Catalyst
Oxidant
Conva
(%)
Selectivityb (%)
Cy-OH Cy=O KAe AA others
TONc TOFd References
1
2
3
4
5
6
7
8
9
Cy-H
Cy-OH
Cy=O
Cy-H
Cy-H
Cy-H
Cy-H
Cy-H
Cy-H
2/Cu
2/Cu
2/Cu
H2O2
H2O2
H2O2
96
80
21
73
13
15
12
48
–
–
–
–
–
–
–
5
29
45
–
trace
–
–
–
–
–
–
–
58
62
84
–
23
55
100
73
25
23
12
–
–
–
–
27
17
15
4
–
55
960
800
210
–
–
–
–
–
380
160
130
35
–
–
–
–
–
–
*
*
*
[46]
[47]
[46]
[46]
[35]
[44]
Mn(acac)2/NHPI O2
Co isooctanoate
p-ClTPPCo
[p-ClFeTPP]2O
NHPI
O2
O2
O2
HNO3/TFA 82f
–
5
Cluster I
H2O2
95
–
45
Bold value indicates the importance of chemoselectivity to adipic acid
NHPI N-hydroxyphthalimide; TFA trifuoroacetic acid; p-ClTPPCo meso-tetra(p-chlorophenyl)cobalt(II)porphyrin; [p-ClFeTPP]2O μ-oxo-
bis[meso-tetra(p-chlorophenyl)porphyrinatoiron(III)]; Cluster I {[Cu4OCl6(BnNH2)4]2[CuCl2(BnNH2)2]}
aConversion=ratio of product(s) conc. to the substrate conc.×100 (average of two runs within 5%)
bSelectivity=ratio of moles of product of interest to the total moles of all products×100
cTON=ratio of moles of product(s) obtained to the moles of the catalyst
dTOF=TON/h (h−1
)
eKA selectivity of cyclohexanone, cyclohexanol and cyclohexylhydroperoxide
fYields were determined by 1H NMR spectroscopy using DMF as an internal standard
*This work. Reaction conditions: 0.4 mmol substrate, 1 mol% CuSO4·5H2O, 1 mol% 2, 5 equiv. H2O2 (30% in H2O), total reaction volume (in
CH3CN)=3 cm3, 70 °C, 6 h
much-improved selectivity (55%) to AA. Even more impres- 12–73% selectivity to AA (Table 2, entries 4–7). Another
sive is the direct oxidation of Cy=O that exclusively yielded interesting homogeneous system (Table 2, entry 8) that
100% AA, albeit at a low 21% conversion. It is clear from involves the use of N-hydroxyphthalimide as an organo-
this set of results that the peroxidation of Cy-H or its main catalyst required a harsh oxidation environment (HNO3/tri-
products (K-A) at an elevated refux temperature has resulted fuoroacetic acid) to selectively yield 82% AA from Cy-H.
in the production of quantifable quantities of the termi- This catalytic system resembles the industrial conversion of
nal adipic acid product. The refux condition supplied the KA oil to AA, only that in this case, a metal catalyst is not
required energy input to the catalytic system for overcom- required, which is an advantage for end-uses where extreme
ing the energy barriers of Cy-H ring scission via C–C bond high purity of the product is required. A current study by
cleavage for the production of the AA product (Scheme 2).
Becker and co-workers, which is the most closely related
In order to contextualise the current study, Table 2 pre- to this study (Table 2, entry 9), in which H2O2 was used as
sents a comparative analysis on the activities of related an oxidant with a Cu cluster as the catalyst under very mild
homogeneous catalysts reported in the literature for the per- conditions produced what is to date the best mild reaction
oxidation of Cy-H to oxygenated products, including AA. It conversion of Cy-H at 95% with an impressive 45% selectiv-
is important to note that there is a paucity of reports for the ity to AA. In comparison, the 2/Cu in situ catalyst system
homogeneous mild condition direct production of AA from presented herein (Table 2, entry 1) has closely mirrored the
Cy-H (or its primary products, Cy-OH and Cy=O) [35, 44, efciency of the Cu cluster system. In fact, we report here
46, 47]. The use of metal-based precatalysts and oxygen as that when the starting substrate is Cy-OH, the selectivity to
the oxidant has dominated the reports in the literature with AA is improved to 55% (Table 2, entry 2) and up to 100%
1 3

N. S. Lawal et al.
(Table 2, entry 3) when Cy=O was used as the substrate.
Hence, we opine that the 2/Cu system is a very robust, sim-
pler and highly efcient system for the production of AA
from components of the KA oil based on a bench-top Cu
salt that is comparable to the best systems reported to date.
constants. Plots of the rates vs concentration of the precata-
lyst (Fig. 5B) gave a linear relationship consistent with a
frst-order dependence.
Similarly, the kinetic order with respect to the oxidant
was determined by varying the concentration of H2O2 only,
and the results presented in Fig. 6 show a linear increase
in the initial reaction rate with an increase in oxidant con-
centration, thereby signifying frst-order dependence as
well. However, initial rates determination with respect to
the substrate shows that a variation of the concentration of
Cy-H (Fig. 7) had no infuence, and the kinetic order of
the reaction was observed to remain constant, indicating a
zero-order dependence. The nature of the dependence of
reaction kinetics on experimental factors (catalyst, oxidant
and Cy-H concentrations) recorded herein seems standard
3.5 Kinetic Studies
Kinetic parameters were determined using the method of
initial rates, whereby in all the experiments, the rate of Cy-H
conversion to the oxygenate products was monitored by GC.
Firstly, the initial quantities of the oxidant, Cy-H, solvent
and temperature were kept constant, and the concentration
of the in situ generated precatalyst was varied (Fig. 5A).
The slope of the graph was used to determine the initial rate
A
B
1.00 mM
2.0
0.75 mM
0.50 mM
0.25 mM
0.04
0.02
0.00
1.5
1.0
0.5
0.3
0.6
0.9
0
10
20
30
[2/Cu] (mM)
Time (min)
Fig. 5 Infuence of catalyst [2/CuSO4·5H2O] concentration on Cy-H conversion (A); determination of initial rate thereof (B). Reaction condi-
tions: Room temperature; [Cy-H]=0.4 mmol; [H2O2]=2 mmol; total reaction volume [in acetonitrile]=3 cm3
5000 mM
A
B
3500 mM
2500 mM
1000 mM
1.4
0.04
0.7
0.0
0.02
0.00
0
10
20
30
0
1000
2000
3000
4000
5000
Time (min)
[H O ] (mM)
2 2
Fig. 6 Infuence of changes in oxidant [H2O2] concentration on Cy-H conversion (A); determination of initial rate thereof (B). Reaction condi-
tions: room temperature; [Cy-H]=0.4 mmol; [2/CuSO4.5H2O]=0.04 mmol; total reaction volume [in acetonitrile]=3 cm3
1 3

Facile Peroxidation of Cyclohexane Catalysed by In Situ Generated Triazole-Functionalised…
C
200 mM
400 mM
800 mM
A
2.4
1.6
0.8
0.04
0.02
0.00
0
10
20
30
300
600
900
Time (min)
[Cyclohexane] (mM)
Fig. 7 Infuence of variation in substrate concentration on Cy-H conversion (A); determination of initial rate thereof (B). Reaction conditions:
room temperature; [H2O2]=2 mmol; [2/CuSO4.5H2O]=0.04 mmol; total reaction volume [in acetonitrile]=3 cm3
for the substantial conversions of the substrates recorded
(see below) [50].
3.6 UV–Vis Spectral Study for the Determination
-5.6
-1
of Active Cu Species
E = 12.29 kJmol
a
A common observation that we noted whilst generating and
testing these series of catalytic systems is the vigorously
exothermic nature and visible colour changes that occurred
during the initial stages of each reaction run, usually from
colourless to yellow and fnally, light green (depending on
the source of copper) colour. These changes are attributed
to ligand coordination to Cu and the eventual generation of
a Cu(II) hydroperoxy complex, a key intermediate in the
oxidation reaction cycle. This is supported by a UV study
conducted on the ligand precursor salt 2, and the corre-
sponding in situ generated copper complexes (Fig. 9). The
UV spectrum of the ligand precursor salt (Fig. 9, black)
is featureless and only showed a shoulder band at 282 nm
assigned to the π–π* transition of the pyridyl and phenyl
rings [51, 52]. However, on the addition of the copper salt
(Fig. 9, red), a new shoulder tail appeared at 364 nm, which
we attribute to the binding of a ligand to the metal (via the
imine C=N–) and in agreement with reported literature [52,
53], it is assigned to the n-π* transition of Cu(I)–N bond.
Interestingly, the addition of 0.2 cm3 of the 30% H2O2 solu-
tion (1 mM) to the in situ generated complex solution led
to the emergence of a new band at 356 nm (Fig. 9, green)
assigned to the n-π* transition of copper hydroxide (Cu(II)-
OH) complex [52]. This new band only lasts for about 5 min
(Fig. 9; blue), and after 10 min (Fig. 9; cyan), it has reduced
to a shoulder tail which continued to decline in intensity
for 40 min (Fig. 9; brown) before merging with the 364 nm
shoulder tail of the starting copper complex. The addition
of a further 0.2 cm3 of fresh hydrogen peroxide led to an
-6.0
-6.4
0.0027
0.0030
0.0033
1/T
Fig. 8 Arrhenius plot for the determination of oxidation reaction acti-
vation energy (Ea)
and in agreement with recent reports on a series of copper(I)
catalysts bearing a variety of N-donor ligands that were also
utilised for the oxidation of Cy-H [13, 18].
The activation energy (Ea) was determined from an
Arrhenius plot (Fig. 8) as 12.29 2 kJ/mol, which is sub-
stantially lower than the average bond dissociation energy
of a standard C-H bond (416.11 kJ/mol) [48]. We attribute
the substantial lowering of the energy barrier required for the
activation of the cyclohexane C-H bond to the catalytic con-
ditions developed for the oxidation reaction, which points to
a catalyst moderation of the energy input needed for activa-
tion even though the reactions were conducted at mild room
temperature conditions or refux for AA [49]. We believe
that a metal–ligand cooperative efect is mainly responsible
1 3

N. S. Lawal et al.
Fig. 9 UV–Visible spectro-
scopic analysis of the peroxida-
tion of Cy-H catalysed by 2/
Cu(CH3CN)4PF6 using H2O2 as
oxidant
2
2/Cu
1.50
1.25
1.00
0.75
0.50
0.25
0.00
2/Cu +H O
2 2
2/Cu +H O + 5 min
2 2
2/Cu +H O + 10 min
2 2
2/Cu +H O + 20 min
2 2
2 2
2 2
2 2
2/Cu +H O + 30 min
2/Cu +H O + 40 min
2/Cu +H O + 50 min
2/Cu +H O + 50 min + H O
282 nm, π−π∗
356 nm Cu(II)-OH
2 2 2 2
364 nm, Cu(I)-N
250
300
350
400
450
500
550
600
650
700
Wavelength (nm)
increase in its intensity (Fig. 9; dark purple), a piece of clear
evidence for the formation of Cu(II)-OH complex, which is
the active species that generate the hydroxyl radical respon-
sible for activating the substrate (Scheme 3). The role of the
ligand precursor 2 is to stabilise this copper complex, which
was observed by UV for over 40 min after the initial addition
of the hydrogen peroxide. However, on the addition of Cy-H
to the reaction mixture, no change in the UV spectrum was
observed even after 6 h. Likewise, Garcia-Bosch and Sie-
gler also confrmed the generation of copper peroxo species
using UV–visible analysis [18].
from the oxidant to form the cyclohexylperoxyl radical (Cy-
OO., iii). The most crucial step is the reduction of Cy-OO.
.
to cyclohexyl hydroperoxide (CyOOH) and regenerating Cy
(iv). This is because the alkyl hydroperoxide species has
been identifed as the primary product in alkane peroxida-
tion reactions. In order to maximise product formation, the
standard practice is to reduce the CyOOH to cyclohexanol
(Cy-OH) using a suitable reductant, often PPh3 (v). This
procedure developed by Shulpin is well-recognised for the
oxidation of alkanes [54]. In addition, due to the higher reac-
tivity of Cy-OH relative to the substrate Cy-H, many per-
oxide catalysed reactions have reported the ketone (Cy=O,
vi) as the primary product (Table 1) which concurs with the
results presented in Fig. 4.
3.7 Proposed Reaction Mechanism
As noted earlier, kinetics studies indicated the dependence
of the reaction rate on the catalyst and oxidant concentra-
tions whilst the addition of a radical scavenger Ph2NH was
observed to shut down the oxidation reaction. Hence, a cop-
per initiated radical mechanism is proposed in line with our
report for related copper(I) complexes for the formation of
the K-A oil as the main product [19]. The proposed mecha-
nism (Scheme 3) involves a reversible one-electron redox
process of the in situ generated copper catalyst [Cu(I) or
Cu(II)]. The catalyst is frst oxidised to generate the active
Cu-OH species and a hydroxyl (HO.) radical (i). This radical
was consumed in the presence of Ph2NH, as noted above.
The HO. radical then activates the substrate (Cy-H) by
abstracting a proton leading to the formation of a cyclohexyl
In the second part of the reaction involving refuxing
catalytic conditions, Cy-OH and Cy=O are converted to the
terminal product (AA) following a series of sequential oxi-
dation and hydrolysis reactions [55–58]. The true nature of
the intermediary products (vii–ix) is still a subject of debate;
however, based on the fnal products observed by GC in this
study, a plausible route is presented in Scheme 3. The route
is an adaptation from a report by Usui and Sato, who also
noted their inability to observe any of the elusive intermedi-
ates (vii–ix) [57].
.
radical (Cy, ii), which further reacts with available oxygen
1 3

Facile Peroxidation of Cyclohexane Catalysed by In Situ Generated Triazole-Functionalised…
8. Andrzej S, Aimin QXL, Antoni L, Sawyer DT (1993) Copper(I)/
4 Conclusions
(t-BuO0H)-induced activation of dioxygen for the ketonization of
methylenic carbons. J Am Chem Soc 115(2):609
A simple strategy for the production of adipic acid from
cyclohexane and its immediate oxidation products
(cyclohexanol and cyclohexanone) was developed and pre-
sented. The key fnding of this study is the use of refuxing
conditions with bench-top copper salts and a new multi-
functional imino-triazolium ligand precursor salt to gener-
ate in situ catalysts for the oxidation reaction promoted by
a green oxidant, 30% H2O2 in water. When benchmarked
against previously reported homogeneous copper catalysts
for cycloalkane oxidation reactions, the current method is a
novelty for homogeneous systems for adipic acid production.
In addition, this method has the potential to be developed
into a viable route for the large-scale production of a very
desirable and large volume industrial feedstock that is a raw
material for a wide range of commercial applications, nota-
bly for the production of nylon polymers. A brief insight
into the kinetics and mechanism of the oxidation process
indicated a radical catalytic process that is catalysed by the
in situ generated copper catalysts.
9. Shun-Ichi M, Naruyoshi K, Yukiko H, Kumano T (2001) Copper
complexes for catalytic, aerobic oxidation of hydrocarbons. Pure
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Supplementary Information The online version contains supplemen-
tary material available at https://doi.org/10.1007/s10562-021-03732-3.
Acknowledgements We acknowledge fnancial support from the Uni-
versity of KwaZulu-Natal, ESKOM (TESP2019) and the NRF. NSL
thanks Ahmadu Bello University for a paid study fellowship.
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Declarations
Conflict of interest There are no conficts of interest to declare.
19. Mncube SG, Bala MD (2019) Homogeneous oxidation reac-
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