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H. S. P. Rao et al. / Tetrahedron Letters 48 (2007) 4495–4497
Table 1. Reaction of 2-butene-1,4-diones 1a–c with the formic acid in
Ph
O
Ph
O
the presence of concd sulfuric acid (cat) under MW irradiation
i
Ph
Ph
Ph
Ph
Entry Ar
Dione Benzil Time Power Yielda
O
O
O
(min) (W)
(%)
6
3a
1
2
3
4
C6H5
C6H5
4-ClC6H4
4-CH3C6H4 1c
1a
3a
3a
3b
3c
2
2
2
3
200
180
180
180
96
86
88
76
E-1a
1b
Scheme 2. Reagents and conditions: (i) 85% HCOOH, concd H2SO4
(cat), MW, 0.5 min, 97%.
a Isolated yield after column chromatography, no other product was
isolated from the reactions.
lowed by cyclo-reversion in a metathesis manner to gen-
erate two benzil units.
enes, having electron withdrawing Cl (1b) and electron
donating Me (1c) groups on the aryl rings (Scheme 1
and Table 1). In both cases, only benzil derivatives 3b–
c were obtained in very good yields, and no traces of
furan derivatives 2b–c could be detected. Similarly, the
E-isomer (E-1a) of Z-1,2,3,4-tetraphenyl-2-butene-1,4-
dione 1a8 was also subjected to MW irradiation with
formic acid. In this case also, facile transformation to
benzil 3a occurred.
To test if molecular oxygen in the atmosphere or
dissolved oxygen in the medium was responsible for
this oxidative cleavage through [2+2] cycloaddition,
followed by [2+2] cyclo-reversion (metathesis), the
cleavage reaction was performed in an oxygen rich
polyethylene glycol (PEG-200) medium without formic
acid. There was no change in the starting materials even
after exposure to MW for 10 min. Thus, formic acid is
necessary for the conversion. Next, a reaction was per-
formed with 18O rich formic acid (48%). From this reac-
tion we isolated benzil, which had 25% incorporation of
Next, we sought to find out if the MW-mediated cleav-
age reaction was a mere reflection of thermal conditions.
Therefore, ene-dione 1a and formic acid were heated in
an oil bath. In this case the starting material 1a was
recovered unchanged even after refluxing for 2 h. There-
fore, we believe that MW superheating is a necessary
condition for this reaction. Since, formic acid failed to
cleave 1,2,4-triphenyl-2-butene-1,4-dione,9 it appears
that the steric hindrance encountered in 1,2,3,4-tetra-
aryl-2-butene-1,4-diones 1a–c is primarily responsible
for the change of the course of the reaction from furan
formation to oxidative cleavage.
18
the O label,11 which confirmed the role of formic acid
in the cleavage reaction. The MW-mediated transforma-
tion of 1a to benzil 3a took place in formic acid alone,
that is, without H2SO4, but in low yield (39%). Interest-
ingly, when we subjected the epoxide12 6 to MW irradi-
ation in formic acid, benzil 3a was formed in
quantitative yield within 30 s (Scheme 2).
In conclusion, we have described an interesting, unusual
oxidative cleavage of diaroylstilbenes to benzils with for-
mic acid under MW irradiation.
A possible mechanism for this transformation is given in
Figure 1. Since two-electron-withdrawing benzoyl
groups flank the olefin in 1, conjugate addition of a for-
mate anion could lead to an intermediate 4. This inter-
mediate could react with molecular oxygen to generate
5, which decomposes into two units of benzil 3.10 In
an alternative possible mechanism, atmospheric oxygen
could add to the double bond in a [2+2] fashion fol-
Acknowledgements
H.S.P.R. thanks the UGC-SAP, DST-FIST programs,
UGC and CSIR, India, for financial support. S.J. and
K.V. thank CSIR for a fellowship. We thank Professors
A. Srikrishna and G. Nagendrappa for the spectral data
and helpful discussions.
Supplementary data
H
O
Ar
Ar
HCOO-
O
Ar
Supplementary data associated with this article can be
Ar
Ar
Ar
Ar
Ar
O O
O
O
1
4
O2,
References and notes
H2O
1. Reutemann, W.; Kieczka, H. In Ullmann’s Encyclopedia of
Industrial Chemistry, 5th ed.; VCH: Weinheim, 1983; Vol.
A12, pp 13–33.
2. Gibson, H. W. Chem. Rev. 1969, 69, 673–692.
3. (a) Johnstone, R. A. W.; Wilby, A. H.; Entwistle, I. D.
Chem. Rev. 1985, 85, 129–179; (b) Brieger, G.; Nestrick, T.
J. Chem. Rev. 1974, 74, 567–580.
OH2
H
- HCOO-
- H2O
O
O
Ar
O
Ar
O
2 x
OH
Ar
Ar
Ar
O
Ar
O
O
3
5
4. Yur’Eu, V. M.; Pravednikov, A. N.; Medvedev, S. S. J.
Polym. Sci. 1961, 55, 353–359.
Figure 1.