Pyridine N-oxide mediated oxidation of diarylalkynes with palladium on carbon

Article abstract of DOI:10.1002/ejoc.201001641

Pyridine N-oxide works as an effective oxidant of 1,2-diarylalkynes at 120 °C to form benzil derivatives under Pd/C-catalyzed solvent-free conditions, and Pd/C could be reused up to five times after simple filtration.

Full text of DOI:10.1002/ejoc.201001641

FULL PAPER
DOI: 10.1002/ejoc.201001641
Pyridine N-Oxide Mediated Oxidation of Diarylalkynes with Palladium on
Carbon
Yoshinari Sawama,[a] Masato Takubo,[a] Shigeki Mori,[a] Yasunari Monguchi,[a] and
Hironao Sajiki*[a]
Keywords: Nitrogen oxides / Alkynes / Oxidation / Palladium / Heterogeneous catalysis
Pyridine N-oxide works as an effective oxidant of 1,2-di-
arylalkynes at 120 °C to form benzil derivatives under Pd/C-
catalyzed solvent-free conditions, and Pd/C could be reused
up to five times after simple filtration.
benzil derivatives. This harmless and environmentally
friendly oxidation system should garner the attention of or-
ganic and process chemists.
Introduction
Pyridine N-oxide derivatives, inexpensive and readily
available reagents, are quite useful oxidants to incorporate
oxygen atoms into organic frameworks because of their ease
of handling and harmless reaction residue (only pyridine
derivatives) after the oxidation step.[1–11] Meanwhile, direct
construction methods for the preparation of the 1,2-di-
ketone skeleton, which is valuable for the synthesis of phar-
maceutical agents[12–14] and functional materials[15] as key
intermediates, from alkynes has been eagerly studied as a
Results and Discussion
Our initial attempt to replace DMSO, which is trans-
very straightforward process.[16,17] Although the transfor- formed into dimethyl sulfide having an unpleasant odor,
mation of disubstituted alkynes into 2-alkenylated pyridine with pyridine N-oxide readily led to the desirable benzil de-
N-oxides[18] 1 by nickel(0)-catalyzed C–H functionalization rivatives and established a novel oxidation method for di-
at the 2-position of pyridine N-oxides due to the potential arylalkynes by comparing previous reports using pyridine
directing effect of the N-oxide moiety is known,[19] the pyr- N-oxides.[8,9,11]
idine N-oxide promoted synthesis of diketones from the
In the presence of a catalytic amount of Pd/C and under
corresponding alkynes has not been reported.
atmospheric O2 at 120 °C, pyridine N-oxide (5 equiv.)
Most recently, we reported the efficient Pd/C-catalyzed smoothly oxidized diphenylacetylene (2a, 0.5 mmol) to give
synthesis of 1,2-diaryl-1,2-diketones (benzil derivatives) the desired benzil (3a) in excellent yield (Table 1, Entry 2).
starting from diarylalkynes with the combination of di- The oxidation also proceeded under an atmosphere of ar-
methyl sulfoxide (DMSO) and molecular oxygen (O2) as ox- gon (Ar) and in air without losing any reactivity (Table 1,
idants; the use of O2 gas and DMSO is not generally ac- Entries 3 and 4). Lower reactivities were observed when
cepted by process chemists because of the oxidative and ex- using 4-methoxy- or 4-nitro-substituted pyridine N-oxides
plosive nature of O2 and because of the malodorous nature (Table 1, Entries 5 and 6). On the other hand, the tert-alk-
of dimethyl sulfide. In this reaction, DMSO played the role ylamine N-oxides and tert-butyl hydroperoxide (TBHP)
of solvent and oxidant, and the zero-valent palladium [Pd0] were not effective for the oxidation of the diarylalkynes
of Pd/C was oxidized by O2 gas to divalent palladium [PdII] (Table 1, Entries 7–9).
during the catalytic cycle.[20]
We next examined the effect of other activated carbon
We now report the O2/DMSO-free, pyridine N-oxide me- supported transition metal catalysts. Au/C and Pt/C main-
diated oxidation of 1,2-diarylalkynes by using Pd/C to form tained moderate catalyst activities for the oxidation, but the
reactions were not complete within 24 h (Table 2, Entries 2
and 3). In contrast, Ni, Ir, Ru, and Rh/C gave no reaction
[a] Laboratory of Organic Chemistry, Gifu Pharmaceutical
(Table 2, Entries 4–7). These results indicate that 10% Pd/
University
C is the optimal catalyst for the oxidation.
1-25-4 Daigaku-nishi, Gifu 501-1196, Japan
Fax: +81-58-230-8109
Detailed optimization of the reaction conditions revealed
that the oxidation was strongly influenced by the use of
Pd/C and pyridine N-oxide and the reaction temperature
E-mail: sajiki@gifu-pu.ac.jp
Supporting information for this article is available on the
WWW under http://dx.doi.org/10.1002/ejoc.201001641.
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Y. Sawama, M. Takubo, S. Mori, Y. Monguchi, H. Sajiki
Table 3. Optimization of reaction conditions.
FULL PAPER
Table 1. Pyridine N-oxide mediated benzil (2a) synthesis from di-
phenylacetylene (3a) with Pd/C.[a]
Entry
x
y
T
[°C]
Ratio[a]
3a/2a
Entry Gas
Oxidant
Yield [%][b]
1
2
3
4
5
6
7
8
none
5
5
5
5
3
2
1
5
5
120
120
120
120
120
120
100
80
0:100
84:16
100:0
91:9
85:15
47:53
58:42
31:69
1
2
3
4
5
6
7
8
9
O2
O2
Ar
air
air
air
air
air
air
DMSO
pyridine N-oxide
pyridine N-oxide
92
92
88
94
52
34
10
10
10
10
10
10
pyridine N-oxide
4-methoxypyridine N-oxide
4-nitropyridine N-oxide
trimethylamine N-oxide
N-methylmorpholine N-oxide
TBHP
complex mixture
1
0
3
[a] Ratio was determined by H NMR spectroscopy.
[a] 0.5 mmol of 2a was used. [b] Isolated yield.
N-oxide. Following formation of vinyl PdII complex B via
formation of PdII–π complex A and subsequent nucleo-
philic addition by pyridine N-oxide, the pyridine N-oxide-
mediated oxidation of B gives keto alcohol intermediate C
with concomitant elimination of pyridine. The continuous
elimination of pyridine from C produces desired product 3a
and the PdII species.
Table 2. Catalyst efficiencies of various carbon-supported catalysts.
[a]
Entry
Catalyst
Ratio [%]
3a/2a
1
2
3
4
5
6
7
10% Pd/C
10% Au/C
10% Pt/C
10% Ni/C
10% Ir/C
10% Ru/C
10% Rh/C
100:0
80:20
37:63
no reaction
no reaction
no reaction
no reaction
1
[a] Ratio determined by H NMR spectroscopy.
(Table 3). Pd/C is essential for the progress of the reaction
(Table 3, Entry 1), and the use of 5 mol-% Pd/C led to an
incomplete reaction even after 24 h (Table 3, Entry 2 vs. 3).
On the other hand, the oxidation efficiency gradually
dropped when the amount of pyridine N-oxide used was
decreased (Table 3, Entries 4–6).[21] A reduction in the oxi-
dation efficiency with a decrease in the reaction temperature
was also observed (Table 3, compare Entries 3, 7, and 8).
The pyridine N-oxide mediated oxidation also identically
and effectively proceeded when using homogeneous PdII
reagents [PdCl2 and Pd(OAc)2] instead of Pd/C [Equa-
tion (1)], which suggests that the catalytic cycle is probably
mediated by a PdII species.
Figure 1. Proposed reaction mechanism.
Because the present reactions were carried out at 120 °C,
which is a higher temperature than the melting point of
pyridine N-oxide (65–66 °C), the pyridine N-oxide and pyr-
idine[22] generated during the reaction play efficient roles as
solvents. However, the reaction rate dependently decreases
over time (Figure 2, ᭡), indicating a time course for the
oxidation of 2a because of catalyst poisoning resulting from
the generated pyridine.[23] These results indicate that the use
of a somewhat excess amount of pyridine N-oxide should
be required to accelerate the oxidation.[24]
(1)
The proposed oxidation mechanism is illustrated in Fig-
We next investigated the reusability of 10% Pd/C with
ure 1. The Pd0 species of Pd/C, which regularly consists of the aim of improving the cost performance and environ-
two Pd species, Pd0 as a chief element and PdII as a minor mental compatibility of the present oxidation process. Al-
or trace element, is initially oxidized to PdII by pyridine though 10% Pd/C has retained its catalytic properties over
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Pyridine N-Oxide Mediated Oxidation of Diarylalkynes
Figure 3. Pd leaching test.
The optimized reaction conditions were adapted for di-
arylalkynes 2b–r[26,27] to construct benzil derivatives 3b–r
(Table 5). Substrates 2b–d possessing an electron-with-
drawing group (Cl, CN, or NO2 group) on the aromatic
ring were transformed into 3b–d with high yields (Table 5,
Entries 1–3). Electron-donating Me- or MeO-substituted
substrates 2e–i underwent efficient oxidation to afford cor-
responding benzils 3e–i with high to excellent yields regard-
less of the steric or electronic effects at the substitution site
(ortho,[28] meta, or para position; Table 5, Entries 4–8). Di-
phenylacetylenes 2j and 2k bearing a hydroxy and formyl
group, respectively, also underwent the oxidation to afford
desired benzils 3j and 3k without forming any oxidized side
products (Table 5, Entries 9 and 10). The oxidation of di-
arylalkynes 2l–o bearing substituents on both aromatic
rings also successfully gave corresponding benzil derivatives
3l–o (Table 5, Entries 11–14). Furthermore, diarylalkynes
2p–r possessing a naphthyl, 2-pyridinyl, or 2-thienyl ring
within the molecule tolerated the oxidation conditions with-
out any significant deactivation of Pd/C to give 3p–r in
moderate to high yields (Table 5, Entries 15–17).
Figure 2. Time course of the oxidation of 2a: (᭡) first use of new
Pd/C, (♦) second use of recovered Pd/C.
three reaction cycles, its oxidation activity gradually de-
creased with repeated use due to catalyst poisoning re-
sulting from the residual amount of pyridine on the reco-
vered Pd/C (Table 4, left column). This problem could be
improved by washing the recovered Pd/C with 1 mm aque-
ous HCl after each oxidation to remove trace amounts of
pyridine on the Pd metal, that is, the acid-washed Pd/C
maintained a high catalytic activity, and the catalyst was
used until at least the fifth run (Table 4, right column).[25]
Furthermore, the reaction time course observed for the sec-
ond use of the recovered Pd/C catalyst also indicated no
loss of activity [Figure 2, dashed line (♦)].
Table 4. Reuse tests of Pd/C.[a]
Reuse
number
Yield [%]
Pd/C washed with HCl[c]
Pd/C[b]
1st
2nd
3rd
4th
5th
94
96
96
96
96
94
95
96
Conclusions
89 (8)[d]
84 (12)[d]
We have accomplished the novel Pd/C-catalyzed and pyr-
idine N-oxide-promoted oxidation of diarylalkynes as a
synthetic method to prepare benzil derivatives.[29] Pyridine
N-oxide is inexpensive and nontoxic and may play triple
roles: solvent, oxygen source for the transformation of al-
kynes into diketones, and oxidant of Pd0 to PdII. Moreover,
a high level of catalytic activity of the Pd species derived
from Pd/C was maintained over five runs by simple acid
wash after recovery. The presented reaction using Pd/C as
a reservoir of the Pd species could provide an easy-to-han-
dle, safe, and practical method for oxidation.
[a] Using 0.5 mmol of 2a under the conditions of 5 equiv. of pyr-
idine N-oxide and 10 mol-% of 10% Pd/C at 120 °C in air for 24 h.
[b] The recovered Pd/C was rinsed with CH2Cl2. [c] The recovered
Pd/C was washed with CH2Cl2, followed by 1 mm aqueous HCl.
See the Experimental Section for details. [d] The yield of the reco-
vered starting material is shown in parentheses.
The Pd leaching from Pd/C to the reaction mixture was
next examined. After the Pd/C-catalyzed oxidation of 2a
using pyridine N-oxide at 120 °C for 24 h, the catalyst was
removed by filtration, the filter was washed with CH2Cl2,
and the filtrate was concentrated in vacuo. The residue was
heated in the absence of Pd/C at 120 °C for another 24 h
after further addition of 2a and pyridine N-oxide to give
desired 3a in 90% yield and recovered 2a in 5% yield over
two steps, respectively (Figure 3). These results, including
ICP-AES analysis (see Experimental Section), indicated
that the leached Pd could be the actual catalyst of the oxi-
dation reaction and that Pd/C likely acts only as a reservoir
of the leached and soluble active Pd species.
Experimental Section
General: 10% Pd/C, Au/C, Pt/C, Ni/C, Ir/C, Ru/C, and Rh/C were
supplied by the N. E. Chemcat Corporation (Tokyo, Japan). Flash
column chromatography was performed using Silica Gel 60 N
1
(Kanto Chemical Co., Inc., 63-210 μm spherical, neutral). H and
13C NMR spectra were recorded with a JEOL EX 400, AL 400, or
ECA 500 spectrometer (1H NMR; 400 MHz for 3b, 3d, 3e, 3f, 3i,
3j, 3k, 3n, 3o, and 3r or 500 MHz for 3a, 3c, 3g, 3h, 3l, 3m, 3p, and
3q; 13C NMR; 100 MHz for 3f and 3n). Chemical shifts are intern-
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Y. Sawama, M. Takubo, S. Mori, Y. Monguchi, H. Sajiki
FULL PAPER
Table 5. Scope of substrates.[a]
[a] Reactions were carried out under the conditions indicated in Table 3, Entry 3.
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Pyridine N-Oxide Mediated Oxidation of Diarylalkynes
ally referenced (δ =7.26 ppm for CDCl3 for 1H NMR and 77.0 ppm
for CDCl3 for 13C NMR). EI mass spectra were taken with a JEOL
JMS-SX102A instrument.
in the aqueous layer and 120 ppm of Pd leaching in the CH2Cl2
layer were observed by ICP-AES.
General Procedure for Synthesis of Benzils: A mixture of diarylal-
kyne (500 μmol), 10% Pd/C (53.2 mg, 50.0 μmol), and pyridine N-
oxide (238 mg, 2.50 mmol) in a 15-mL test tube was stirred at
120 °C with an air balloon by using a Chemistation personal or-
ganic synthesizer (EYELA, Tokyo). After 24 h, the mixture was
diluted with CH2Cl2 (20 mL) and water (15 mL) and then passed
through a membrane filter (Millipore, Millex-LH, 0.45 μm). The
aqueous as separated and the CH2Cl2 layer were washed success-
ively with water (2ϫ15 mL) and brine (20 mL), dried with anhy-
drous Na2SO4, filtered, and concentrated in vacuo. Pure products
of 3a, 3e, 3m, 3n, 3q, and 3r were obtained by simple extraction.
The yields of 3b, 3c, 3d, 3f, 3g, 3h, 3i, 3j, 3k, 3l, 3o, and 3p were
determined after silica gel column chromatography (n-hexane/
EtOAc), because these products contained a slight amount of side
products.
Typical Procedure for the Synthesis of Benzil (3a) from 1,2-Diphen-
ylacetylene (2a): After the reaction mixture was passed through a
filter paper, crude Pd/C including pyridine was washed with
CH2Cl2 (25 mL). Subsequently, the residual pyridine on Pd/C was
washed with 1 mm aqueous HCl (2.5 mL). A mixture of 1,2-diphen-
ylacetylene (2a; 89.1 mg, 500 μmol), 10% Pd/C (53.2 mg,
50.0 μmol), and pyridine N-oxide (238 mg, 2.50 mmol) in a 15-mL
test tube was stirred at 120 °C with an air balloon by using a Chem-
istation personal organic synthesizer (EYELA, Tokyo). After 24 h,
the mixture was diluted with CH2Cl2 (20 mL) and water (15 mL)
and then passed through a membrane filter (Millipore, Millex-LH,
0.45 μm) to remove Pd/C. The aqueous layer was separated, and
the CH2Cl2 layer was washed successively with water (2ϫ15 mL)
and brine (20 mL), dried with anhydrous Na2SO4, filtered, and
concentrated in vacuo. Pure benzil (3a) could be obtained by simple
extraction (94% yield, Figure 4).
Benzil (1,2-Diphenylethane-1,2-dione, 3a): 1H NMR (500 MHz,
CDCl3): δ = 7.49 (dd, J = 6.8, 7.3 Hz, 4 H), 7.64 (t, J = 6.8 Hz, 2
H), 7.89 (t, J = 7.3 Hz, 4 H) ppm. 13C NMR (100 MHz, CDCl3):
δ = 128.9, 129.8, 132.9, 134.8, 194.5 ppm. The NMR spectrum of
3a was identical to that reported.[16t]
1-(4-Chlorophenyl)-2-phenylethane-1,2-dione (3b): 1H NMR
(500 MHz, CDCl3): δ = 7.45–7.51 (m, 4 H), 7.64 (m, J = 7.4 Hz, 1
H), 7.89–7.96 (m, 4 H) ppm. 13C NMR (100 MHz, CDCl3): δ =
129.0, 129.4, 129.9, 131.1, 131.3, 132.7, 135.0, 141.5, 193.0,
193.8 ppm. The NMR spectrum of 3b was identical to that report-
ed.[16t]
1
Figure 4. H NMR spectrum of 3a after the simple extraction.
1-(4-Cyanophenyl)-2-phenylethane-1,2-dione (3c): 1H NMR
(500 MHz, CDCl3): δ = 7.55 (t, J = 8.0 Hz, 2 H), 7.69 (t, J =
8.0 Hz, 1 H), 7.80 (d, J = 8.6 Hz, 2 H), 7.96 (d, J = 8.6 Hz, 2 H),
8.08 (d, J = 8.0 Hz, 2 H) ppm. 13C NMR (100 MHz, CDCl3): δ =
117.4, 117.8, 129.1, 129.9, 130.1, 132.3, 132.6, 135.2, 135.7, 192.2,
192.8 ppm. The NMR spectrum of 3c was identical to that re-
ported.[20]
Typical Procedure for Reuse of Pd/C: After the reaction mixture
was passed through a filter paper, crude Pd/C including pyridine
was washed with CH2Cl2 (25 mL). Subsequently, the residual pyr-
idine on Pd/C was washed with 1 mm aqueous HCl (5ϫ2.5 mL)
and MeOH (5ϫ2.5 mL), and the resulting Pd/C was dried in
vacuo.
1-(4-Nitrophenyl)-2-phenylethane-1,2-dione
(3d):
1H
NMR
General Procedure of Pd Leaching Test: After stirring a mixture of
diphenylacetylene (2a; 89.1 mg, 500 μmol), pyridine N-oxide
(238 mg, 2.50 mmol), and 10% Pd/C (53.2 mg, 50.0 μmol) for 24 h
at 120 °C, the reaction mixture was diluted with CH2Cl2 (20 mL)
and then passed through a membrane filter (Millipore, Millex-LH,
0.45 μm). The filtrate was concentrated in vacuo. Diphenylacety-
lene (2a; 89.1 mg, 500 μmol) and pyridine N-oxide (238 mg,
2.50 mmol) were added again to the residue, and the reaction mix-
ture was stirred at 120 °C in air by using a Chemistation personal
organic synthesizer (EYELA, Tokyo). After 24 h, the mixture was
diluted with CH2Cl2 (40 mL) and washed successively with water
(3ϫ20 mL) and brine (20 mL). The CH2Cl2 layer was dried with
anhydrous Na2SO4, filtered, and concentrated in vacuo to give a
mixture of 5% of 2a (8.1 mg, 0.05 mmol) and 90% of 3a (190.4 mg,
(500 MHz, CDCl3): δ = 7.55 (dd, J = 7.2, 7.7 Hz, 2 H), 7.71 (t, J
= 7.2 Hz, 1 H), 7.99 (d, J = 7.7 Hz, 2 H), 8.17 (d, J = 8.8 Hz, 2
H), 8.34 (d, J = 8.8 Hz, 2 H) ppm. 13C NMR (100 MHz, CDCl3):
δ = 124.1, 129.2, 130.0, 130.9, 132.4, 135.4, 137.3, 151.1, 192.0,
192.8 ppm. The NMR spectrum of 3d was identical to that report-
ed.[16t]
1-Phenyl-2-p-tolylethane-1,2-dione (3e): 1H NMR (400 MHz,
CDCl3): δ = 2.41 (s, 3 H), 7.29 (d, J = 8.3 Hz, 2 H), 7.48 (dd, J =
7.2, 7.3 Hz, 2 H), 7.63 (t, J = 7.3 Hz, 1 H), 7.86 (d, J = 8.3 Hz, 2
H), 7.96 (d, J = 7.2 Hz, 2 H) ppm. 13C NMR (100 Hz, CDCl3): δ
= 21.8, 128.9, 129.6, 129.7, 129.9, 130.4, 133.0, 134.7, 146.1, 194.2,
194.7 ppm. The NMR spectrum of 3e was identical to that report-
ed.[16t]
1
0.9 mmol). The yields were determined by H NMR spectroscopy.
1-Phenyl-2-m-tolylethane-1,2-dione (3f): 1H NMR (400 MHz,
CDCl3): δ = 2.40 (s, 3 H), 7.38 (dd, J = 7.2, 7.7 Hz, 1 H), 7.45–
7.52 (m, 3 H), 7.65 (t, J = 7.8 Hz, 1 H), 7.75–7.78 (m, 2 H), 7.97
(d, J = 7.2 Hz, 2 H) ppm. 13C NMR (100 MHz, CDCl3): δ = 21.2,
127.2, 128.9, 128.9, 129.8, 130.1, 132.9, 133.0, 134.8, 135.7, 138.9,
194.7, 194.8 ppm. MS (EI): m/z = 224 (20) [M]+, 51 (10), 65 (10),
91 (30), 105 (40), 119 (100), 224 (20). HRMS (EI): calcd. for
C15H12O2 [M]+ 224.0837; found 224.0841.
ICP-AES of Pd Leaching: After stirring a mixture of diphenylace-
tylene (2a; 891 mg, 5.0 mmol), pyridine N-oxide (2.38 g,
25.0 mmol), and 10% Pd/C (532 mg, 500 μmol) for 24 h at 120 °C,
the mixture was diluted with CH2Cl2 (30 mL) and then passed
through a glass filter with Celite. Furthermore, the filtrate was
passed through a membrane filter (Millipore, Millex-LH, 0.45 μm),
a second membrane filter (Millipore, Millex-LH, 0.20 μm), and
then concentrated in vacuo. After the filtrate was dissolved in
CH2Cl2 (30 mL) and extracted with water (30 mL), the aqueous 1-(4-Methoxyphenyl)-2-phenylethane-1,2-dione (3g): 1H NMR
layer and the CH2Cl2 layer were diluted with water or CH2Cl2,
respectively, in a 50-mL volumetric flask. Ͻ10 ppm of Pd-leaching
(400 MHz, CDCl3): δ = 3.88 (s, 3 H), 6.97 (d, J = 8.8 Hz, 2 H),
7.50 (dd, J = 7.9, 8.0 Hz, 2 H), 7.64 (t, J = 8.0 Hz, 1 H), 7.94–7.98
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Y. Sawama, M. Takubo, S. Mori, Y. Monguchi, H. Sajiki
(m, 4 H) ppm. 13C NMR (100 MHz, CDCl3): δ = 55.6, 114.3, (d, J = 8.0 Hz, 1 H), 9.33 (d, J = 8.6 Hz, 1 H) ppm. 13C NMR
FULL PAPER
126.1, 128.9, 129.9, 132.3, 133.2, 134.7, 165.0, 193.1, 194.7 ppm.
(125 MHz, CDCl3): δ = 124.4, 125.9, 127.1, 128.6, 128.8, 129.0,
129.4, 129.9, 130.9, 133.3, 134.1, 134.7, 135.1, 135.9, 194.6,
197.1 ppm. The NMR spectrum of 3p was identical to that report-
ed.[16t]
The NMR spectrum of 3g was identical to that reported.[16t]
1-(3-Methoxyphenyl)-2-phenylethane-1,2-dione (3h): 1H NMR
(400 MHz, CDCl3): δ = 3.86 (s, 3 H), 7.21 (dd, J = 1.6, 8.2 Hz, 1
H), 7.40 (t, J = 8.2 Hz, 1 H), 7.47–7.55 (m, 4 H), 7.66 (t, J =
7.2 Hz, 1 H), 7.97 (d, J = 7.7 Hz, 2 H) ppm. 13C NMR (100 MHz,
CDCl3): δ = 55.4, 112.8, 121.8, 123.1, 129.0, 129.8, 130.0, 132.9,
134.2, 134.8, 160.0, 194.4, 194.4 ppm. The NMR spectrum of 3h
was identical to that reported.[20]
1
1-Phenyl-2-(2-pyridinyl)ethane-1,2-dione (3q): H NMR (500 MHz,
CDCl3): δ = 7.48–7.53 (m, 3 H), 7.64 (t, J = 7.5 Hz, 1 H), 7.93–
7.95 (m, 3 H), 8.20 (d, J = 6.9 Hz, 1 H), 8.66 (d, J = 4.0 Hz, 1
H) ppm. 13C NMR (CDCl3): δ = 123.1, 128.1, 128.9, 129.5, 133.1,
134.6, 137.2, 149.8, 151.7, 195.1, 196.1 ppm. The NMR spectrum
of 3q was identical to that reported.[31]
1-(2-Methoxyphenyl)-2-phenylethane-1,2-dione (3i): 1H NMR
(400 MHz, CDCl3): δ = 3.56 (s, 3 H), 6.93 (d, J = 8.0 Hz, 1 H),
7.13 (t, J = 8.0 Hz, 1 H), 7.49 (dd, J = 1.2, 8.1 Hz, 2 H), 7.58–7.63
(m, 2 H), 7.92 (dd, J = 8.0, 8.1 Hz, 2 H), 8.03 (dd, J = 1.2, 8.1 Hz,
1 H) ppm. 13C NMR (100 MHz, CDCl3): δ = 55.7, 112.4, 121.6,
123.9, 128.7, 129.3, 130.6, 133.0, 133.7, 136.4, 160.4, 193.4,
194.6 ppm. The NMR spectrum of 3i was identical to that report-
ed.[16s]
1-Phenyl-2-(2-thienyl)ethane-1,2-dione (3r): 1H NMR (400 MHz,
CDCl3): δ = 7.18 (dd, J = 3.8, 4.8 Hz, 1 H), 7.51 (dd, J = 7.3,
7.7 Hz, 2 H), 7.66 (d, J = 7.7 Hz, 1 H); 7.80–7.85 (m, 2 H), 8.04
(d, J = 7.3 Hz, 2 H) ppm. 13C NMR (CDCl3): δ = 128.8, 128.9,
130.2, 132.5, 134.8, 136.7, 136.9, 139.8, 185.6, 192.0 ppm. The
NMR spectrum of 3o was identical to that reported.[32]
Supporting Information (see footnote on the first page of this arti-
1-(4-Hydroxyphenyl)-2-phenylethane-1,2-dione (3j): 1H NMR
(400 MHz, CDCl3): δ = 6.51 (br. s, 1 H), 6.90 (d, J = 8.4 Hz, 2 H),
7.50 (dd, J = 7.3, 7.7 Hz, 2 H), 7.65 (t, J = 7.3 Hz, 1 H), 7.87 (d,
J = 8.4 Hz, 2 H), 7.96 (d, J = 7.7 Hz, 2 H) ppm. 13C NMR
(100 MHz, CDCl3): δ = 116.1, 125.9, 129.0, 130.0, 132.8, 133.0,
135.0, 162.0, 193.4, 195.2 ppm. The NMR spectrum of 3j was iden-
tical to that reported.[16s]
1
cle): Copies of the H NMR spectra of the benzil products.
Acknowledgments
We thank the N. E. Chemcat Corporation for the kind gift of the
catalysts.
1-(4-Formylphenyl)-2-phenylethane-1,2-dione (3k): 1H NMR
(400 MHz, CDCl3): δ = 7.53 (dd, J = 7.7, 8.2 Hz, 2 H), 7.70 (t, J
= 7.7 Hz, 1 H), 7.97–8.03 (m, 4 H), 8.14 (d, J = 8.7 Hz, 2 H), 10.1
(s, 1 H) ppm. 13C NMR (100 MHz, CDCl3): δ = 129.0, 129.9,
129.9, 130.3, 132.4, 135.2, 136.8, 139.9, 191.3, 193.4, 193.5 ppm.
The NMR spectrum of 3k was identical to that reported.[16u]
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(400 MHz, CDCl3): δ = 3.88 (s, 6 H), 6.97 (d, J = 9.0 Hz, 4 H),
7.96 (d, J = 9.0 Hz, 4 H) ppm. 13C NMR (100 MHz, CDCl3): δ =
55.6, 114.2, 126.2, 132.3, 164.8, 193.4 ppm. The NMR spectrum of
3l was identical to that reported.[20]
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NMR (400 MHz, CDCl3): δ = 3.91 (s, 3 H), 7.00 (d, J = 8.9 Hz, 2
H), 7.97 (d, J = 8.9 Hz, 2 H), 8.16 (d, J = 9.0 Hz, 2 H), 8.34 (d, J
= 9.0 Hz, 2 H) ppm. 13C NMR (100 MHz, CDCl3): δ = 55.7, 114.6,
124.0, 125.4, 130.9, 132.6, 137.6, 151.1, 165.5, 191.3, 192.3 ppm.
[9] L. Ye, W. He, L. Zhang, J. Am. Chem. Soc. 2010, 132, 8550–
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1
The H NMR spectrum of 3m was identical to that reported.[30]
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1-(4-Methoxyphenyl)-2-(3-nitrophenyl)ethane-1,2-dione (3n): 1H
NMR (400 MHz, CDCl3): δ = 3.87 (s, 3 H), 6.97 (d, J = 9.0 Hz, 2
H), 7.70 (dd, J = 7.8, 8.2 Hz, 1 H), 7.93 (d, J = 9.0 Hz, 2 H), 8.27
(d, J = 7.8 Hz, 1 H), 8.45 (d, J = 8.2 Hz, 1 H), 8.76 (s, 1 H) ppm.
13C NMR (100 MHz, CDCl3): δ = 55.6, 114.5, 124.4, 125.2, 128.5,
130.2, 132.5, 134.4, 135.2, 148.4, 165.4, 191.0, 191.6 ppm. MS (EI):
m/z = 285 (5) [M]+, 44 (15), 50 (10), 64 (5), 77 (15), 92 (10), 107
(5), 135 (100), 285 (5). HRMS (EI): calcd. for C15H11NO5 [M]+
285.0637; found 285.0631.
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1-(4-Acetylphenyl)-2-(4-methoxyphenyl)ethane-1,2-dione (3o): 1H
NMR (400 MHz, CDCl3): δ = 2.65 (s, 3 H), 3.90 (s, 3 H), 6.99 (d,
J = 9.0 Hz, 2 H), 7.95 (d, J = 9.0 Hz, 2 H), 8.06 (m, 4 H) ppm. 13
C
NMR (CDCl3): δ = 26.9, 55.7, 114.5, 125.8, 128.6, 130.1, 132.4,
136.2, 141.2, 165.2, 192.2, 193.8, 197.2 ppm. The NMR spectrum
of 3o was identical to that reported.[20]
1
1-(1-Naphthyl)-2-phenylethane-1,2-dione (3p): H NMR (500 MHz,
CDCl3): δ = 7.46–7.53 (m, 3 H), 7.61–7.67 (m, 2 H), 7.73–7.77 (m,
1 H), 7.92 (t, J = 8.6 Hz, 2 H), 8.04 (dd, J = 1.5, 8.6 Hz, 2 H), 8.11
3366
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Eur. J. Org. Chem. 2011, 3361–3367

Pyridine N-Oxide Mediated Oxidation of Diarylalkynes
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Received: December 7, 2010
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Published Online: May 6, 2011
Eur. J. Org. Chem. 2011, 3361–3367
© 2011 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
www.eurjoc.org
3367