Table 1 Conversion (%) at different reaction times for the oxidation in
chloroform of the phenylpropenoidic compounds, the β-O-4 and β-5
type dimeric compounds, shown in Schemes 1–3
1 : 1), yielding the phenyl-2,3-dihydrobenzofuran 7 (0.55 g,
50%); mp 82 ЊC (from methanol); 1H NMR(CDCl3): δH 7.63 (1
H, d, J = 15.0 Hz, 1 CH), 6.90–7.30 (7 H, m, 7 CH, aromatics),
6.31 (1 H, d, J = 15.0 Hz, 1 CH), 6.08 (1 H, d, J = 8.0 Hz, 1 CH),
5.50 (1 H, s, 1 OH), 4.33 (1 H, d, J = 8.0 Hz, 1 CH), 3.83 (3 H, s,
1 CH3), 3.80 (3 H, s, 1 CH3); MS (EI): m/z 354 (Mϩ) (100), 322
(98), 290 (60). Elemental analysis: C, 67.19; H, 5.12 (calculated
for C20H18O6); C, 67.68; H, 5.09 (experimental).
Conversion (%)
Compound
30 min
1 h
5 h
48 h
118
218
3
60
1
90
70
50 10
99
83
5
0.5
6
84
6
98
83
66
99
86
6
3
5
4
8
1
5
88
37
99
96
70
99
99
4
4
1
3
7
1
1
99
60
99
98
98
99
99
1
4
1
1
1
1
1
Apparatus and measurements
4
5
5
Melting points were determined with a Büchi MP 19 apparatus
and are uncorrected. Mass spectra were performed using the
direct injection system mode with positive electron impact on a
VG 7070 EQ instrument.
6
1
4
7
The paramagnetic species active in the oxidation mechanism
were identified using solutions prepared as follows: [Co(salen)]
(6 × 10Ϫ3 M) was added to a deaerated CHCl3 solution of the
substrate (6 × 10Ϫ2 M), then the solution was allowed to react
with oxygen (1 MPa pressure) at 298 K. After 20 min, the
solution was deaerated by repeated freeze–vacuum treatments,
then the spectrum was recorded at the indicated temperature
under an argon atmosphere, in order to avoid line broadening
due to the interaction of the paramagnetic species with
dioxygen. The spectra from 298 K to 10 K were recorded at 9.5
GHz microwave frequency, those from 200K to 10 K also at
190 GHz. The oxidation reactions were monitored by taking
aliquots of the reaction solution at the following reaction times
(expressed in minutes): 5, 10, 15, 20, 30, 45, 60, 120, 180, 210,
240, 270, 300, and by immediately cooling them in liquid nitro-
gen to slow down the reaction. The X-band EPR spectra were
recorded at 123 K.
IR spectra were recorded on a FTIR Perkin-Elmer 1725X
spectrophotometer. 1H and 13C NMR spectra were recorded by
a Bruker AC 300 or a Bruker AMX 300 instruments (in CDCl3
solutions). Chemical shift values (δ) are given as ppm relative to
the tetramethylsilane resonance and coupling constant values
(J) are measured in Hz. HPLC analyses were performed using a
combination of a Waters 600E pump, column (Kromasil C18,
250 × 4.6 mm, 5 µm) and HP 9153C photodiode array detector.
The column was eluted using a flow rate of 1.0 cm3 minϪ1 at
room temperature with the following gradient: 0–2 min,
CH3CN–H2O 60 : 40; 2–13 min, CH3CN–H2O 95 : 5; 13–15
min, CH3CN–H2O 60–40; 15–20 min, CH3CN–H2O 60 : 40.
The GLC-MS analyses were performed using a HP 5890
gas chromatograph, interfaced with a quadrupole detector
(HP 5970) operating in electron impact mode 70 eV. The gas
chromatograph was equipped with a Supelco SPB-5 (95%
dimethylpolisiloxan) capillary column (length 30 m, inner
diameter 0.25 mm, film thickness 0.25 µm). The column was
eluted at 333 K for 2 min, followed by a temperature gradient
from 333 K to 523 K at 283 K minϪ1. The carrier gas used was
The g values were measured by standardisation with diphenyl-
picrylhydrazyl (DPPH). The amount of paramagnetic species
were calculated by double integration of the resonance line
areas. All the experimental spectra were fitted by the 6/9/91
DOS version of the SIM14S simulation program.
helium at a flow rate of 40 cm3 sϪ1
.
The X-band CW EPR spectra were recorded on a Bruker
EMX spectrometer. Temperature control in the range 4–300 K
was achieved through an Oxford Instrument for X-band
spectroscopy. The CW high frequency (HF) EPR spectra were
recorded on a HF-EPR spectrometer that can operate at three
frequencies: 95, 190 and 285 GHz. Fundamental mono-
chromatic radiation in the range 94–96 GHz was delivered by a
Gunn effect diode (RPG Instruments, Meckenheim, Germany)
with an output power of 40 mW. The two upper harmonics
were generated by non-linear solid state devices which work as a
doubler and tripler. The frequency and phase of the source was
stabilized by means of a phase lock loop system. The magnetic
system was a superconductor magnet (Oxford Instruments
Ltd.) that can be continuously swept to a maximum field of 12
T with a constant relative homogeneity of 10 ppm. The magnet
system was equipped with a sweep coil that was used to sweep
the field through the EPR line, keeping the main coil constant at
a given field value. The maximum sweep range was 0.2 T. An
enhanced hot electrons bolometer (InSb) operating in liquid
helium (QMC instruments Ltd, Billingshurst, UK) was used as
the detector. The sample probe is an ultra-wide band probe
similar to those used in other laboratories.23 The system was not
equipped with a resonating structure. The sample was placed
in the volume of maximum homogeneity in an overmoded
cylindrical metallic wave-guide mounted axially in the static
magnetic field. The liquid sample was held in a 4 cm long Teflon
bucket of 9 mm internal diameter. A modulation coil was
mounted around the sample holder delivering an a.c. magnetic
field of 0.2 mT maximum amplitude. Temperature control in
the range 4–300 K was achieved through an Oxford Instrument
system based on the use of a static continuous flow cryostat
(CF1200, Oxford Instr.) and a PID temperature control
method. The temperature stability was 0.01 K at liquid helium
temperature.
Oxidation reactions
A CHCl3 solution (40 cm3) of substrate (6 × 10Ϫ2M) and
[Co(salen)] (6 × 10Ϫ3 M) was placed in a glass vessel (100 cm3)
and inserted into an autoclave (250 cm3). The autoclave was
charged with dioxygen (1 MPa) and left at 298 K for the time
specifically indicated in Tables 1–3. The solvent was then
evaporated under reduced pressure at room temperature and
the residue was resolved on a silica gel column with a gradient
of ethyl acetate–hexane (3 : 7 to 7 : 3) as eluents.
The identification of benzoic acid derivatives 9 and 11 and
benzaldehyde derivatives 10 and 12 (Scheme 1) has already been
reported.18 The quinones 13 and 14 (Scheme 2) were identified
by comparison with authentic samples.24 The conjugated
methylenic aldehyde 15 (Scheme 2), the open ring product 16
(Scheme 3) and the benzofuran structures 17 and 18 were
identified by mass spectrometry, H-NMR, 13C-NMR, IR and
1
UV spectroscopies.
Compound 15: MS (EI): m/z = 178 (Mϩ, 100%), 149 (38), 108
(60), 77 (50); 1H-NMR (CDCl3): δH 3.79 (3 H, s, 1 CH3), 5.05 (1
H, d, J = 2 Hz, 1 CH), 5.25 (1 H, d, J = 2 Hz, 1 CH), 6.85–7.25
(4 H, m, 4 CH, aromatics), 9.45 (1 H, s, 1 CHO); 13C-NMR
(CDCl3): δC 56.4 (CH3), 110.1 (CH2), 114.8 (CH), 118.3 (CH),
121.6 (CH), 123.7 (CH), 143.0 (C), 155.0 (C), 166.9 (C), 189.5
(CH); IR (Nujol): νmax/cmϪ1 1703 (C᎐C–CO), 1617 (C᎐C–Ar);
᎐
᎐
mp = 160 ЊC (from methanol).
Compound 16 (isolated after methylation): MS (EI): m/z =
472 (Mϩ, 15%), 457 (25), 442 (50), 428 (60); 1H-NMR (CDCl3):
δH 3.70 (3 H, s, 1 CH3), 3.75 (3 H, s, 1 CH3), 3.78 (3 H, s, 1 CH3),
3.85 (3 H, s, 1 CH3), 4.00 (3 H, s, 1 CH3), 4.06 (3 H, s, 1 CH3),
4.10 (3 H, s, 1 CH3), 6.80–7.20 (5 H, m, 5 CH), 6.31 (1 H, d, J =
16 Hz, 1 CH), 7.75 (1 H, d, J = 16Hz, 1 CH); 13C-NMR
J. Chem. Soc., Dalton Trans., 2002, 3007–3014
3009