Allylic C-H activations using Cu(II) 2-quinoxalinol salen and tert-butyl hydroperoxide

Article abstract of DOI:10.1021/jo300372q

Using a Cu(II) 2-quinoxalinol salen complex as the catalyst and tert-butyl hydroperoxide (TBHP) as the oxidant, allylic activations of olefin substrates can be converted to the corresponding enones or 1,4-enediones. Excellent yields can be achieved (up to 99%) within a very short reaction time and with great tolerance for additional functional groups. Possible mechanistic pathways have been characterized using Raman spectroscopy, cyclic voltammetry, and theoretical calculations.

Full text of DOI:10.1021/jo300372q

Article
pubs.acs.org/joc
Allylic C−H Activations Using Cu(II) 2-Quinoxalinol Salen and tert-
Butyl Hydroperoxide
Yuancheng Li, Tae Bum Lee, Tanyu Wang, Audrey V. Gamble, and Anne E. V. Gorden*
Department of Chemistry and Biochemistry, College of Science and Mathematics, Auburn University, Auburn, Alabama 36849-5319,
United States
*
S Supporting Information
ABSTRACT: Using a Cu(II) 2-quinoxalinol salen complex as
the catalyst and tert-butyl hydroperoxide (TBHP) as the
oxidant, allylic activations of olefin substrates can be converted
to the corresponding enones or 1,4-enediones. Excellent yields
can be achieved (up to 99%) within a very short reaction time
and with great tolerance for additional functional groups.
Possible mechanistic pathways have been characterized using
Raman spectroscopy, cyclic voltammetry, and theoretical
calculations.
INTRODUCTION
though copper is widely used in naturally occurring metal-
■
32
loenzymes to facilitate synthesis. For example, galactose
oxidase (GOase), a copper-containing metalloenzyme secreted
by the fungus Fusarium spp., is critical in the oxidation of
The regioselective activation of allylic C−H bonds is of wide
interest, and the subsequent oxidation of the to the enone or
enedione products of such reactions are common building
blocks in multistep organic syntheses. The direct trans-
formations of alkenes to the corresponding α,β-unsaturated
enones or 1,4-enediones are particularly important in the
3
3,34
1
,2
primary alcohols to aldehydes.
Previously, we have reported a new catalytic system that
consists of Cu(II) 2-quinoxalinol salen (salqu) complex 1 as
catalyst and TBHP as the oxidant for the oxidation of benzylic
3
−9
synthesis of natural products and synthetic drug precursors.
Although alternative organocatalytic allylic oxidations have
35
methylenes into carbonyl groups. Quantitative yields are
obtained while different functional groups remain intact. In
allylic oxidation using this system, multiple isomeric products
are possible. Here, we describe the application of Cu(II) salqu
complex 2 in allylic oxidation of olefins to address commonly
encountered problems as found in other similar catalytic
systems, specifically, long reaction time, low selectivity, harsh
1
0−12
been reported,
oxidations is remarkable.
hydroperoxide (TBHP) has been used in combination with
the high efficiency of metal-catalyzed allylic
13−28
An oxidant such as tert-butyl
1
3,14
various metal catalysts including chromium compounds,
2
8
29
sodium chlorite, copper iodide, dirhodium caprolacta-
1
9−23
30
26
mate,
ruthenium trichloride, bismuth salt, cobalt
14−28
2
4,25
16−18
15
conditions, low functional group tolerance, and high cost.
acetate,
palladium(II) salts,
manganese(III) acetate,
27
In addition, we wanted to better characterize the reactivity of
the copper complex and the nature of the catalytic mechanistic
pathway, in the hopes of subsequently gaining insight as to how
we might further exploit these catalyst systems.
and ferric chloride. While these previous investigations have
reported promising results, numerous limitations remain, e.g.,
harsh reaction conditions, difficult workup and/or purification
procedures, production of harmful waste, low functional group
tolerance, and high costs. The two major drawbacks of current
methods using TBHP as an oxidant are long reaction time and
low regioselectivity of the tert-butyl peroxy radical, which
results in the formation of numerous isomeric byproducts (e.g.,
RESULTS AND DISCUSSION
■
We began our investigations by examining the allylic oxidation
of 1-acetyl-1-cyclohexene 3 using Cu(II) salqu complex 2 as the
catalyst (eq 2). For ease of comparison with other catalytic
systems, we carried out the oxidation on a millimole scale using
2
2
1).
The first disadvantage of long reaction times has been
addressed by Doyle and co-workers to some extent using a
dirhodium caprolactamate catalytic system in which the
0
.5 mol % catalyst loading. For the optimization of TBHP
required, the first solvent used was acetonitrile. The reaction
was monitored by gas chromatography (GC). Within 1 h of the
initiation of the reaction, the yield of the corresponding 1,4-
enedione product (4) increased from 65% to 99% when the
TBHP ratio changed from 1 equiv to 3 equiv. The addition of
19
reaction is found to be completed in much less time. In
•
this system, t-BuOO still formed and acts as an oxidant, and
2
2
hence, regioselectivity remains a problem. Although the most
effective metallic catalysts to date are those that are capable of
carrying out one-electron redox processes, such as Fe(II)/
Fe(III), Cu(I)/Cu(II), Co(II)/Co(III),
been done using Cu as a catalyst in allylic oxidation, even
19,31
less research has
Received: February 21, 2012
Published: May 7, 2012
©
2012 American Chemical Society
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Article
1
8% yield was observed after 1 h of reaction time as compared
to 99% yield in regular distilled acetonitrile. This result
indicated that dioxygen played a crucial role in this conversion.
In most metal-catalyzed allylic oxidations using TBHP as the
oxidant, the addition of base would accelerate the reaction
because of the facile decomposition of the tert-butyl peroxy
1
7,19,39
ether intermediates under basic conditions (Scheme 1).
However, in this case, the addition of 50 mol % of K CO did
2
3
not bring any change to the reaction rate in our system. This
result suggests two possibilities: (1) the decomposition of tert-
butyl peroxy ether intermediate is not the rate-determining step
or (2) the tert-butyl peroxy ether intermediate was not formed.
Since the decomposition of tert-butyl peroxy ether is known to
be slow and can be greatly accelerated by the addition of
1
7,19,39
base,
the reaction time should decrease with the addition
of a base if tert-butyl peroxy ether is formed in our system.
Thus, it is reasonable to rule out the first scenario. A reaction
mechanism bypassing the formation of tert-butyl peroxy ether
represents a new and alternative pathway to those reported
more TBHP did not result in a further improvement in the
yield (Figure 1). The Cu(II) salqu system only requires 3 equiv
of TBHP, much less than other reported catalytic methods that
15−19
previously.
This mechanism might subsequently lead to
19,22
required 5−10 equiv.
Theoretically, only 1 equiv of TBHP
improved selectivity because it would limit the formation of
additional isomers as side products along with the formation of
is consumed in the allylic oxidation of olefins to enones;
however, excess TBHP is required because of the decom-
position of TBHP during reaction process. The reduced
requirement for TBHP in the Cu(II) salqu system is indicative
of the better activation of TBHP leading to a different
improved efficiency mechanism.
1
7,18,22
desired peroxy ether (eq 1).
Using 3 equiv of TBHP and 0.5 mol % of complex 2, the
effect of different solvents on the oxidation of 1-acetyl-1-
cyclohexene 3 for 1 h (Figure 1) was also tested. The reaction
rates were evaluated on the basis of the yields found after 1 h.
Reactions proceeded much more slowly in dichloromethane
and chloroform than in acetonitrile. This is consistent with our
Allylic oxidations of a variety of representative olefins were
examined using 0.5 mol % of complex 2 and 3 equiv of TBHP
in CH CN at 70 °C (Table 1). Most of the substrates can be
3
converted to the corresponding products with excellent yields
with a very short reaction time. The yields typically ranged
between 74 and 99%. The nature of the substrates meant that
the products were often quite volatile and were, thus, best
measured by gas chromatography. In the case of the synthesis
of 5,6-dihydro-[1,1′-biphenyl]-3(4H)-one (Table 1, entry 3),
the product was isolated and purified using flash column
chromatography using hexane/ethyl acetate (4:1) as eluent,
resulting in a still quite impressive isolated yield of 88%. In the
one outlier, the low yield resulting from the 1-nitro-1-
cyclohexene precursor (Table 1, entry 11, only 11%) could
be due to the strong binding between the nitro group and the
Cu(II) of complex 2 that would poison the catalyst. It could
36
previous observations. The reaction in DMF had a moderate
yield, while the yield of the reaction in DMSO was extremely
low. One noticeable feature that distinguishes acetonitrile from
other solvents is the oxygen solubility. Oxygen has a greater
solubility in acetonitrile (8.1 mM) than most other commonly
used organic solvents, such as DMF (4.5 mM) and DMSO (2.1
3
7
mM). The surprisingly low yields in DMSO could be also be
ascribed to the ligation of the solvent molecule by the catalyst
complex, which would prevent the catalyst from functioning
3
8
normally. To determine whether the oxygen concentration
has an influence on the reaction rate, a control experiment was
performed in CH CN degassed with argon. In this case, only
3
Figure 1. Effects of oxidant ratio and solvents on reaction yields.
4
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Scheme 1. Decomposition of tert-Buytl Peroxy Ether under Basic Conditions
Table 1. Cu(II) Salqu Catalyzed Allylic Oxidation of Olefins
copper species in the reaction. It has been documented that
TBHP can easily bind to tridentate mononuclear Cu(II) species
to form a Cu(II) tert-butyl peroxo complex whose crystal
structure showed a distorted tetrahedral binding geometry of
40
the Cu(II) center. By comparing the resonance Raman
18
18
spectra of LCu(II)OO-t-Bu and LCu(II) O O-t-Bu obtained
at 77 K with an excitation at 568.2 nm, five peaks at 471, 640,
7
54, 834, and 884 cm− were assigned to the O−O moiety,
1
−1
41
while the one at 754 cm was very small. Meanwhile, little
has been done to demonstrate the binding of TBHP to a
tetradentate mononuclear Cu(II) complex. In our study,
resonance Raman was also employed to provide evidence
about the binding between TBHP and Cu(II) salqu (Figure 2).
Figure 2. Resonance Raman spectroscopy of Cu(II) salqu with TBHP.
The peaks at 596, 749, 915, and 955 cm− are in a similar
1
pattern with reported resonance Raman data associated with
41
the O−O moiety from TBHP upon binding. All these bands
shifted from 70 to 120 cm− to greater wavenumbers compared
to the previously reported data, indicating a higher vibration
frequency of the O−O moiety, i.e., a higher energy for the O−
O bond.
1
For the Cu(II) tert-butyl peroxo intermediate, the two
possible energetically favorable reaction pathways are (1)
reductive cleavage of the Cu−O bond forming Cu(I) and tert-
butyl peroxyl radical or (2) the homolytic cleavage of the O−O
bond caused by direct H abstraction from the substrate
4
1
resulting in a Cu(III)−oxo and tert-butyl alcohol (Scheme 2).
Given the fact that the addition of 50 mol % of K CO to the
2
3
a
GC yields with 1,2-dichlorobenzene added as internal standard.
oxidation of 1-acetyl-1-cyclohexene did not bring about a
change to the reaction rate, indicating that the intermediacy of
b
Isolated yield obtained from flash column chromatography using
c
hexane/ethyl acetate (4:1) as eluent. Isolated yield of reaction in
gram-scale.
Scheme 2. Possible Reaction Pathways of LCu(II)OO-t-Bu
also be the result of the strong electron-withdrawing capability
of the nitro group that could destabilize the radical
intermediate. Also worth noting is that 3-methylcyclohexene
and 1-methylcyclohexene yielded the same oxidation product
(entries 4 and 5). This is further evidence supporting a radical
mechanism.
To further explore the nature of this catalytic cycle,
experiments were performed to characterize the role of the
4
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Article
tert-butyl peroxy ether did not form, the reductive cleavage of
the Cu−O bond was not likely to occur in our catalytic system.
Although it is challenging to prove the formation of Cu(III)−
oxo, it is notable that Cu(III) can be stabilized by a
Scheme 3. Proposed Catalytic Cycle for Allylic Oxidation
Catalyzed by Cu(II) Salqu
42,43
noninnocent ligand such as salen.
An experiment using cyclic voltammetry with Cu(II) salqu
demonstrated two quasi-reversible oxidtions (E°′ = +547, +802
+
mV vs Fc /Fc) (Figure 3) that could be ascribed to the
Figure 3. Cyclic voltamagram of Cu(II) salqu with supporting
electrolyte 0.1 M tetrabutyl ammonium tetrafluoroborate in 5 mL of
CH Cl and 1 mM ferrocene internal standard, reference Ag/AgCl,
yielding tert-butyl alcohol, allylic radical 6, and LCu(II)-O•/
LCu(III)O 7. As proven earlier, dioxygen played a crucial
role in our system. One possible way for dioxygen to participate
is to react with the allylic radical to form allylic peroxyl radical 8
that can be converted to enone product. The active copper
complex 7 can also react with the allylic radical 6 resulting in
LCu(I) 9 and allylic oxyl radical 10, and the latter can be
2
2
2
counter electrode Pt gauze (A = 0.77 cm ), and the working electrode
2
was a glassy carbon disk (d = 0.3 cm, A = 0.071 cm ).
42
sequential single electron oxidations of the phenolic moieties.
The pattern of quasi-reversible CV peaks of Cu(II) salqu
complex suggested the capability of stabilizing Cu(III) by
•
converted to enone as well. Both LCu(I) and LCu(II)-O /
LCu(III)O are EPR-silent species. The postulated formation
+
• +
2,43
enabling the equilibrium between [Cu(III)L] and [Cu(II)L ]
•
4
of LCu(I) and LCu(II)-O /LCu(III)O is in agreement with
as seen in previous reports with salen complexes.
our observation in EPR experiment. Copper at an oxidation
state of +1 favors a tetrahedral geometry, while the rigid salqu
ligand only provides a square planar environment. Hence, the
oxidation of LCu(I) 9 to LCu(II) will easily occur. This could
be another place where dioxygen was involved. In addition, the
mismatch of Cu(I) and binding geometry gives an explanation
of our earlier observation about Cu(II) salqu catalyst
Calculation at the B3LYP/6-311+G(d,p) level also showed
that salqu ligand could stabilize Cu(III) better than salen by
enhancing the spin density dispersion on the heterocyclic
quinoxalinol backbone; i.e., the spin density of triplet
+
[
Cu(III)L] is nore delocalized with salqu ligand than salen
ligand (Figure 4).
36
consumption in steroids oxidation.
Using this proposed catalytic cycle, the regioselectivity and
other observed experimental results can be better explained.
The allylic radical 6 was formed by hydrogen abstraction when
olefin reacted with LCu(II)OO-t-Bu 5 instead of the hydrogen
•
abstraction by t-BuOO . Because of the bulkiness of the
reactive Cu(II) peroxo core induced by the two tert-butyl
groups from the ligand, the hydrogen abstraction is preferred at
the sterically less hindered position. The regioselectivity is thus
enhanced. For 3-methyl-1-cyclohexene (Table 1, entry 5), the
radical formed at the 3-position is thermodynamically more
favored, while the radical formed at the 6-position is sterically
more favored. Since the methyl group provides little bulkiness,
the formation of the energetically favored intermediate
overrides the other. Once the radical is formed at the 3-
position, the enone product is energetically more favored than
the allylic alcohol that can be formed by the oxidation at that
position. In addition, the reaction at the 1-position of the
radical intermediate was sterically favored. Affected by these
factors, the oxidation of 3-methyl-1-cyclohexene does not have
as good a yield as other substrates. In the cyclic voltammetry
experiment, the oxidation potentials of the Cu(II) salqu
complex were 100 and 150 mV higher than those of the
Figure 4. Spin density of [LCu(III)]+ with 0.0004 au isosurface
density (L = salen and salqu).
44
In addition, a EPR signal loss during reaction was observed,
suggesting the transformation of a EPR-active species (LCu(II),
9
d ) to a EPR-silent species. This could be rationalized in terms
1
0
of the formation of a LCu(I) d species or a low-spin,
8
diamagnetic Cu(III) d species that equilibrates with a
ferromagnetically coupled Cu(II)−ligand−radical species as
43
reported previously with Cu(II) salen complex.
From this, it is reasonable to believe that TBHP will bind to
Cu(II) salqu complex (LCu(II)) at the beginning of the
reaction (Scheme 3). As mentioned before, the resulting Cu(II)
tert-butyl peroxo complex 5 is likely to undergo a homolytic
cleavage of the O−O bond upon reacting with substrate,
+
Cu(II) salen (+450, +650 mV vs Fc /Fc in CH Cl at the same
2
2
4
2
scan rate), indicating a more difficult formation of the
[Cu(II)L ] . No direct electron transfer from the salqu ligand
•
+
4
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The Journal of Organic Chemistry
Article
was required in our system on the basis of the proposed
catalytic cycle.
ESI: (m + H)/z calcd for C42
This is consistent with previous reports.
Representative Procedure for Allylic Oxidation. To a 50 mL
round-bottomed flask charged with a stirring bar were sequentially
added complex 2 (2.5 μmol), CH CN (10 mL), 1-phenyl-1-
cyclohexene (0.5 mmol), and t-BuOOH (1.5 mmol). After the
reaction was stirred at 70 °C for 1 h, solvent was removed under
reduced pressure. The residue was purified by flash column
chromatography with hexane/ethyl acetate (3:1, v/v) as eluent to
yield 76.1 mg yellowish oil as product (88%).
H
55
N
4
O
3
Cu 726.3570, found 726.3575.
3
6,37
CONCLUSIONS
■
3
In summary, we have reported the first Cu(II) complex
catalyzed, regioselective allylic oxidation of olefins to enones
and 1,4-enediones. Excellent yields (up to 99%) can be
achieved in a very short reaction time (1 h). Tolerance of a
variety of functional groups is exhibited. The behavior of the
Cu(II) salqu complex as catalyst can be characterized to explain
the improved regioselectivity, and a different mechanism of
using TBHP from current methods involving the binding of
TBHP to Cu(II), homolytic cleavage of O−O bond, and the
formation of Cu(III)−oxo is proposed. Further experiments in
our ongoing investigations will be used to elucidate the role of
dioxygen in this reaction and whether it is possible to use
dioxygen as oxident or to use this for additional catalytic
reactions.
Representative Procedure for Allylic Oxidation (Gram
Scale). To a 400 mL round-bottomed flask charged with a stirring
bar were sequentially added complex 2 (50 μmol), CH CN (200 mL),
3
1-phenyl-1-cyclohexene (10 mmol), and t-BuOOH (30 mmol). After
the reaction was stirred at 70 °C for 1 h, solvent was removed under
reduced pressure. The residue was purified by flash column
chromatography with hexane/ethyl acetate (3:1, v/v) as eluent to
yield 1.38 g yellowish oil as product (80%).
Procedure for Yield Determination by GC. To the reaction
flask was added 1,2-dichlorobenzene (0.5 mmol) after 1 h. A 150 μL
aliquot of reaction solution was then taken to collect GC data. The
yield was determined by the equation yield = (area of the peak for
EXPERIMENTAL SECTION
General Methods. All reagents were obtained commercially
without further purification. The tert-butyl hydroperoxide used is
product)/(area of the peak for internal standard).
■
1
2
-Cyclohexenone (Entry 1, Table 1). H NMR: δ 7.03 (m, 1 H),
1
3
6
.01 (m, 1 H), 2.43, (m, 2 H), 2.37 (m, 2 H), 2.03 (m, 2 H).
C
NMR: δ 199.6, 150.9, 129.8, 38.1, 25.7, 22.8. HRMS-EI: m/z calcd for
5
.0−6.0 M solution in decane purchased from a commercial supplier.
1
13
C H O 96.0575, found 96.0582.
H NMR and C NMR spectra were recorded on 400 and 100 MHz,
6
8
1
3
-Acetyl-2-cyclohexenone (Entry 2, Table 1). H NMR: δ 6.58
respectively, as solutions in CDCl ; chemical shift (δ) are reported in
3
(
bs, 1 H), 2.50 (m, 2 H), 2.48 (m, 2 H), 2.41 (s, 3 H), 2.00 (m, 2 H).
C NMR: δ 201.5, 200.1, 154.6, 132.5, 37.9, 26.2, 23.4, 21.9. HRMS-
ppm relative to Me Si. Chromatographic purifications were performed
4
13
using (60 Å, 70−230 mesh) silica gel. HRMS data were collected with
EI: m/z calcd for C H O 138.0681, found 138.0687.
electronspray ionization. All products have been previously described,
8
10
2
1
1
13
3-Phenyl-2-cyclohenenone (Entry 3, Table 1). H NMR: δ
.55−7.53 (m, 2 H), 7.42−7.41 (m, 3H), 6.43 (t, J = 1.2 Hz, 1 H),
and H, C NMR data are in accordance with literature data. Raman
spectrscopic data was collected using the 785 nm line (6 mW) from an
air-cooled argon ion laser as the excitation source. Raman spectra were
collected and analyzed using a Renishaw via Raman microscope
system. In the cyclic voltammetry experiments, the electrochemical
circuit was controlled using an Epsilon electrochemistry workstation.
The CW EPR spectrum was measured at X-band (9 GHz)
frequency on a spectrometer fitted with an ER-4119-HS high
sensitivity perpendicular-mode cavity. General EPR conditions were
as follows: microwave frequency, 9.385 GHz; field modulation
frequency, 100 kHz; field modulation amplitude, 0.6 mT. The Oxford
Instrument ESR 900 flow cryostat in combination with the ITC4
temperature controller was used for measurements in the 4−300 K
range using a helium flow. Measurements at 77 K were performed by
fitting the cavity with a liquid nitrogen finger Dewar.
7
2
1
3
.78 (m, 2 H), 2.49 (t, J = 6.0 Hz, 2 H), 2.18−2.13 (m, 2 H).
C
NMR: δ 200.0, 159.9, 139.0, 130.0, 128.8, 126.1, 125.5, 37.3, 28.1,
2.8. HRMS-EI: m/z calcd for C H O 172.0888, found 172.0881,
2
12 12
1
3-Methyl-2-cyclohexenone (Entries 4 and 5, Table 1). H
NMR: δ 5.88 (d, J = 1.5 Hz, 1 H), 2.32 (t, J = 6.3 Hz, 2 H), 2.31−2.26
13
(
1
m, 2 H), 2.02−1.98 (m, 2 H), 1.96 (s, 3 H). C NMR: δ 199.9,
62.9, 127.0, 37.4, 31.4, 24.9, 23.1. HRMS-EI: m/z calcd for C H O
7 10
1
10.0732, found 110.0728.
-Acetoxy-2-cyclohexenone (Entry 6, Table 1). H NMR: δ
.92 (s, 1 H), 2.54 (t, J = 6.8 Hz, 2 H), 2.42 (t, J = 6.4 Hz, 2 H), 2.23
1
3
5
(
s, 3 H), 2.09 (m, 2 H). 13C NMR: δ 200.0, 170.0, 167.4, 117.5, 36.6,
2
1
8.3, 21.2, 21.2. HRMS-EI: m/z calcd for C H O 154.0630, found
8 10 3
54.0626.
-Acetyl-2-cyclopentenone (Entry 7, Table 1). H NMR: δ
1
3
Procedure for Cyclic Voltammetry Experiment. Electro-
chemical measurements were carried out at room temperature using
a three-electrode setup in a home-built glass cell (20 mL total volume).
The supporting electrolyte was 0.1 M tetrabutyl ammonium
tetrafluoroborate in 5 mL of CH Cl with 1 mM ferrocene as internal
6
2
.67 (t, J = 2.0 Hz, 1 H), 2.83−2.80 (m, 2 H), 2.56−2.51 (m, 2 H),
.50 (s, 3 H). 13C NMR: δ 210.6, 197.3, 169.3, 137.0, 35.4, 27.8, 26.3.
HRMS-EI: m/z calcd for C H O 124.0524, found 124.0520.
7
8
2
4
-Cyclopentene-1,3-dione Monoethylene Ketal (Entry 8,
2
2
1
Table 1). H NMR: δ 7.20 (d, J = 6.0 Hz, 1 H), 6.19 (d, J = 6.0
standard, the reference electrode was homemade Ag/AgCl wire, and
Hz, 1 H), 4.03 (m, 4 H), 2.60 (s, 2 H). 13C NMR: δ 204.0, 156.3,
2
the counter electrode was Pt gauze (A = 0.77 cm ). The working
2
135.4, 111.6, 65.2, 45.2. HRMS-EI: m/z calcd for C H O 140.0473,
7 8 3
found 140.0465.
electrode was a glassy carbon disk (d = 0.3 cm, A = 0.071 cm ). Before
electrochemical measurement, the solution was purged with N for 15
2
1
3
-Cyano-2-cyclohexenone (Entry 9, Table 1). H NMR: δ 6.52
min. Cyclic voltammogram of 1 mM Cu(II) salqu was recorded in 5
mL of CH Cl described above between 0.0 and 1.7 V using a scan rate
(
2
s, 1 H), 2.57 (dt, J = 6.0 Hz, 2.0 Hz, 2 H), 2.54 (t, J = 6.2 Hz, 2 H).
.13 (m, 2 H). C NMR: δ 196.6, 138.6, 131.1, 117.0, 37.2, 27.6, 22.0.
2
2
13
of 100 mV/s.
HRMS-EI: m/z calcd for C H NO 121.0528, found 121.0525.
7
7
Procedure for Raman Spectroscopy. In a 20 mL vial equipped
with a stirring bar, Cu(II) salqu complex (5 μmol) was dissolved in
CH Cl (10 mL) followed by the addition of TBHP (20 μmol). After
1
3
-Nitro-2-cyclohexenone (Entry 10, Table 1). H NMR: δ 6.91
(
m, 1 H), 2.10 (m, 2 H), 2.51 (t, J = 6.4 Hz, 2 H), 2.19−2.15 (m, 2
H). C NMR: δ 198.0, 164.0, 125.8, 37.1, 24.4, 21.0. HRMS-EI: m/z
2
2
13
1
5 min of strring, several drops of the Cu(II) salqu solution was taken
calcd for C H NO 141.0426, found 141.0431.
6
7
3
and allowed to evaporate on a gold foil. The residue was excited at the
7
2
1
85 nm line (6 mW), and the Raman spectrum was collected.
Synthesis of Cu(II) Salqu Complex 2. The Cu(II) salqu complex
ASSOCIATED CONTENT
Supporting Information
H and C NMR spectra and theoretical calculation data. This
material is available free of charge via the Internet at http://
pubs.acs.org.
■
3
6,37
was synthesized following a published procedure.
Cu(II) Salqu Complex 2. IR (KBr): 3429 (br), 2955, 2909, 2868,
*
S
1
13
−1
661, 1556, 1524, 1495, 1462, 1423, 1385, 1202 cm . UV−vis
−1
−1
−1
−1
(
CHCl ): 250 (ε = 35 600 M cm ), 285 (ε = 27 140 M cm ),
30 (ε = 31 340 M cm ), 460 nm (ε = 39 280 M cm ). HRMS-
3
−1
−1
−1
−1
3
4
632
dx.doi.org/10.1021/jo300372q | J. Org. Chem. 2012, 77, 4628−4633

The Journal of Organic Chemistry
Article
(
(
(
34) Whittaker, J. W. Arch. Biochem. Biophys. 2005, 433, 227−239.
35) Wu, X.; Gorden, A. E. V. Eur. J. Org. Chem. 2009, 503.
36) Li, Y.; Wu, X.; Lee, T. B.; Isbell, E. K.; Parish, E. J.; Gorden, A.
AUTHOR INFORMATION
Corresponding Author
E-mail: gordeae@auburn.edu.
■
*
E. V. J. Org. Chem. 2010, 75, 1807.
Notes
(37) Achord, J. M.; Hussey, C. L. Anal. Chem. 1980, 52, 601.
(38) Rothenberg, G. Catalysis: Concepts and Green Applications;
Wiley-VCH: Weinheim, 2008.
The authors declare no competing financial interest.
(
8
(
39) Kornblum, N.; DeLaMare, H. E. J. Am. Chem. Soc. 1951, 73,
80.
40) Kitajima, N.; Katayama, T.; Fujisawa, K.; Iwata, Y.; Moro-oka, Y.
ACKNOWLEDGMENTS
■
We thank Dr. Christian Goldsmith for the use of GC and
constructive suggestions, Dr. Curtis Shannon for the use of
Raman spectroscopy, and Axline Sanghapi for the help with
Raman. We would also like to thank Dr. Eduardus Duin for
assistance with EPR. We gratefully acknowledge funding for
this project from Auburn University.
J. Am. Chem. Soc. 1993, 115, 7872.
(41) Chen, P.; Fujisawa, K.; Solomon, E. I. J. Am. Chem. Soc. 2000,
122, 10177.
(42) Pratt, R. C.; Stack, T. D. P. J. Am. Chem. Soc. 2003, 125, 8716.
(
43) Storr, T.; Verma, P.; Pratt, R. C.; Wasinger, E. C.; Shimazaki, Y.;
Stack, T. D. P. J. Am. Chem. Soc. 2008, 130, 15448.
44) In the oxidation reaction of 1-acetyl-1-cyclohexene, after 30 min,
mL of solution was taken and EPR spectrum was collected from it.
Only typical signal from organic radical was observed.
(
1
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dx.doi.org/10.1021/jo300372q | J. Org. Chem. 2012, 77, 4628−4633