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