A Novel Practical Reaction of Diarylalkynes With Sulfur Trioxide: Oxidation to 1,2-Diketones

Article abstract of DOI:10.1055/s-2001-14558

A new reaction of arylalkynes with the SO3-dioxane complex gives the corresponding aromatic 1,2-diketones in good yield.

Full text of DOI:10.1055/s-2001-14558

SHORT PAPER
1001
A Novel Practical Reaction of Diarylalkynes With Sulfur Trioxide: Oxidation
to 1,2-Diketones
Victor O. Rogatchov,1 Victor D. Filimonov,*1 Mehman S. Yusubov2
a
Department of Organic Chemistry, Tomsk Polytechnic University, Tomsk, 634034, Russia
b
Department of Chemistry Siberian Medical University, Tomsk, 634050, Russia
Fax +7(3822)415235; E-mail: filim@org.chtd.tpu.edu.ru
Received 19 January 2001
To demonstrate the scope of the reaction, a series of acet-
Abstract: A new reaction of arylalkynes with the SO3-dioxane
ylenic substrates 1a–h were subjected to oxidation by the
SO3-dioxane complex as a novel available reagent (Ta-
ble).
complex gives the corresponding aromatic 1,2-diketones in good
yield.
Key words: alkynes, oxidations, ketones, sulfur trioxide
So tolan (1a) is easily oxidized to benzil (2a) at 20-
100 °C. Best yields of benzil (2a) in this reaction are
achieved in dioxane as a solvent (Table). When chloro-
form or acetic anhydride have been used as solvents, the
It is common knowledge that alkynes in acidic media, for
example, in sulfuric acid, undergo hydration giving rise to
the corresponding ketones.1 Reactions of alkynes with the
complex of SO3 with dimethyl sulfide provided products
of conjugated addition to triple bonds.2 Terminal acety-
lenes are converted into corresponding sulfoacetylenes in
the reaction with the SO3-dioxane complex.3 Recently, we
have disclosed that phenylacetylene in the reaction with
the mixture of H2SO4/HOAc or with the SO3-dioxane
complex gives rise to δ-sultones – 4,6-diphenyl-1,2-oxati-
in.4,5
results were unsatisfactory. In most cases, the optimal
temperature is 60 °C, though oxidation of tolan (1a) takes
place at room temperature over a longer period. Temper-
ature increase up to 100 °C resulted in strong acceleration
of the oxidation reaction of tolan (1a), but the yield of
benzil (2a) was reduced (Table, Scheme 1).
Sulfur trioxide in this reaction is reduced to SO2, the latter
being fixed by qualitative probe with iodine. The oxida-
tion reaction of alkyne 1a by the SO3-dioxane complex
has been found to be equally effective both in the presence
and in the absence of air (argon medium). This significant
fact demonstrates that sulfur trioxide is the true oxidant in
the above reaction. The best preparative results (Table)
were achieved when 4 equivalents of SO3 per one equiva-
lent of substrate 1a-c was used.
In our attempts to extend this new reaction to some diary-
lalkynes (internal acetylenes), we have discovered that in-
stead of formation of the sultone cycles, oxidation of triple
bonds of the acetylenes into 1,2-diketones occurs. Al-
though there are many reagents for oxidation of internal
alkynes into 1,2-diketones, SO3 has been unknown as an
oxidant for this transformation (see review6). However,
many of the known reagents for this purpose (e.g.,
KMnO4, SeO2, OsO4, Tl(NO3)3, etc.) are toxic, rather ex-
pensive or require special reaction conditions.6 It is appar-
ent that using SO3 as an oxidant of alkynes is cheaper,
safer and more convenient.
It is important to note that the oxidation reaction with sul-
fur trioxide is very sensitive to the electronic nature of the
acetylenic substrates and it probably has electrophilic
character. Thus, methyltolan (1b) is oxidized just as fast
as tolan (1a) with an almost quantitative yield. In contrast,
alkyne 1c containing the strong electron-withdrawing ni-
tro-group is oxidized much slower and only provides a
small yield of diketone 2c (Table).
1. SO3/C4H8O2
2. H2O
O
Ph
R
R
+
SO2
19–99%
Ph
O
1a–c
2a–c
Scheme 1 a R=H, b R=CH3, c R=NO2
The reaction of the SO3-dioxane complex with dialkynes
1d–h provides oxidation of one or both triple bonds de-
pending on the structures of the substrates (Scheme 2). In
the case of dialkynes 1d, e with the close location of two
Synthesis 2001, No. 7, 01 06 2001. Article Identifier:
1437-210X,E;2001,0,07,1001,1003,ftx,en;E00601SS.
© Georg Thieme Verlag Stuttgart · New York
ISSN 0039-7881

1002
V. O. Rogatchov et al.
SHORT PAPER
Table Oxidation of Internal Alkynes by SO3/C4H8O2
Substrate
Temp. (°C)
Ratio of SO3/sub-Reaction time Producta
Yieldb (%)
Mp (°C)
Lit. mp (°C)
strate
(min)
(mol/mol)
1a
1a
1a
1a
1b
1c
1d
20
60
60
100
60
60
60
4
2
4
4
4
4
2
150
120
15
2a
2a
2a
2a
2b
2c
57
48
99
76
92
19
94–95
94–95
94–95
94–95
29–30
140–141
94–9511
5
10
3112
120
100
141–1427
2d
3d
70
5
108–109
123–124
106–10710
124–12510
1e
60
2
100
2e
3e
37
16
174–175
192–193
-
193–19413
1f
60
60
60
8
8
8
30
30
30
3f
97
98
95
202–204
105–106
141–142
205–20613
106–10713
141–14214
1g
1h
3g
3h
a All compounds give spectra (NMR, IR, MS) in accord with their proposed structures.
b Yields refer to isolated, chromatographically pure products.
1. SO3/C4H8O2
2. H2O
O
O
+
Ar
Ar
O
O
1d–h
3d–h (5–98%)
O
O
Ar
+
2d, e (37, 70%)
d
Ar=
e
Ar=
f
Ar=
O
O
g
CH2
Ar=
h
Ar=
Scheme 2
triple bonds, we have observed that the SO3-dioxane com- acetylene bond intact), but also led to the earlier unknown
plex oxidizes predominantly only one triple bond giving acetylene diketone 2e from alkyne 1e. Two moles of SO3
acetylenic diketones 2d, e. This circumstance not only al- per one mole of substrate 1d, e was used.
lowed the successful oxidation of compound 1d to 1e (one
Synthesis 2001, No. 7, 1001–1003 ISSN 0039-7881 © Thieme Stuttgart · New York

SHORT PAPER
Reaction of Diarylalkynes With Sulfur Trioxide: Oxidation to 1,2-Diketones
1003
Mp: 174-175 °C.
IR (KBr): ν = 1605, 1678 (C=O) cm−1.
Minor quantities of tetraketones 3d and 3e were also
formed in the reaction of 1d, e to diketones 2d and 2e. In
the case of two- or tetra-fold increase of SO3 per 1 mol of
substrate 1d, e, the formation of diketones 2d, e dominates
over the formation of tetraketones 3d, e, the total yield of
the oxidation products being considerably reduced. All at-
tempts to oxidize the remaining triple bond of compound
2d by means of SO3-dioxane complex have failed, and tet-
raketone 3d was obtained in negligible yield.
1H NMR (500 MHz, CDCl3): δ = 7.35 (m, 3H, Ar-H), 7.54 (m, 5H,
Ar-H), 7.67 (m, 3H, Ar-H), 8.02 (d, 1H, J = 1.31 Hz, Ar-H), 8.03(m,
1H, Ar-H), 8.15 (m, 2H, Ar-H), 8.56 (dd, 1H, J = 0.52, 1.79 Hz, Ar-
H).
13C NMR (80 MHz, CDCl3): δ = 194.4 and 193.4 (C=O), 160.1,
156.3, 134.8, 132.9, 131.8, 131.4, 129.9, 129.5, 128.9, 128.5, 128.3,
128.2, 124.4, 124.3, 123.7, 123.5, 123.0, 118.9, 112.5, 112.1 (Ar-
C), 89.6, 88.7 (alkynes).
Most likely, the selective oxidation of only one triple bond
in the compounds 1d, e is the result of the inhibitory effect
on the part of the electron-withdrawing COCOPh group in
diketones 2d, e as it took place in the case of oxidation of
Anal. Calcd. for C28H16O3: C, 83.98; H, 4.02. Found: C, 84.01; H,
4.10.
nitrotolan (1c). However, this selective oxidation of only Acknowledgement
one triple bond in diacetylenes 1d, e by means of SO3-di-
We thank the Russian Foundation for Fundamental Researches
(grant N 00–03–32812) for the financial support of this work.
oxane complex is useful for organic synthesis. Only three
oxidizers capable to transform selectively diacetylenes
with isolated triple bonds into corresponding acetylenic
1,2-diketones–PhI(OCOCF3)2,7
I2/DMSO,8
PdCl2/
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Uncorrected melting points were recorded on a Kofler apparatus.
C
Oxidation of Alkynes 1a–h by SO3-dioxane Complex
A mixture of compounds 1a−h (1 mmol) and SO3-dioxane complex
(4 mmol) was stirred at 60°C for the times shown in the Table. The
reaction mixture was poured into H2O (50 mL) and extracted with
Et2O (50 mL). The solvent was removed in vacuo and the residue
was purified and separated by chromatography (silica gel; benzene-
hexane, 1:1).
4-Phenylglyoxaloyl-7-(phenylethynyl)dibenzofuran (2e)
Synthesis 2001, No. 7, 1001–1003 ISSN 0039-7881 © Thieme Stuttgart · New York