OH
O
species. Further study concerning aspects of the mechanism is
currently underway.
We would like to express our appreciation to Professor Kan
Kanamori, Toyama University, and Dr Yasushi Oda, Kyowa
Hakko Kogyo Co., for their useful suggestions.
1 mol% VOCl3, O2
MeCN, room temp.
OMe
OMe
R
R
O
O
R = hexyl 95%
R = Ph 85%
Scheme 2
Notes and references
† We retrieved all the transformations of a-hydroxycarbonyl into a-
dicarbonyl using REACCS (MDL Co., Ltd.), and found over 100 methods.
Representative reports are cited in ref. 5.
‡ Two other aerobic oxidations of a-hydroxycarbonyls to a-dicarbonyls
have been reported (ref. 6).
of benzoin with a catalytic amount of VOCl3 in MeCN in air
also gave benzil in good yield (entry 2). It took longer to
complete the reaction in the cases of the a-diketones having
aliphatic substituents (entries 5–11). Interestingly, alcohols
without a carbonyl moiety at their a-position did not react under
the same conditions. a-Hydroxy esters were also efficiently
oxidized to a-keto esters (Scheme 2).
§ A typical experimental procedure is as follows: a mixture containing the
a-hydroxycarbonyls (5.0 mmol), VOCl3 (4.7 ml, 0.05 mmol) and MeCN (50
ml) was stirred at room temperature (or refluxed) under an oxygen
atmosphere for 1.5–22.5 h. The mixture was quenched with saturated aq.
NaHCO3 and extracted with 20 ml 3 3 of EtOAc. The combined organic
extracts were washed with saturated aq. NH4Cl and brine, dried over
MgSO4 and evaporated to afford the a-dicarbonyls.
As mentioned in a previous paper,4 this reaction is not a
simple radical oxidation, because this reaction was not
influenced by a radical inhibitor.¶ A stoichiometric amount of
VOCl3 reacted with benzoin to afford benzil in the absence of
oxygen (Scheme 3). This result means that molecular oxygen
acts as a co-oxidant and reoxidizes the low valent vanadium that
is formed.
¶ 2,6-di(tert-butyl)-p-cresol did not interfere with the reaction of VOCl3
with benzoin in MeCN.
We observed that the color of the reaction mixture was dark
yellow at first, and then turned green in most cases. The ‘green’
of the reaction mixture strongly suggests the presence of VIV
1 K. P. C. Vollhardt and N. E. Schore, Organic Chemistry, 2nd edn.,
Freeeman, New York, 1994, pp. 924–929.
2 G. B. Gill, in Comprehensive Organic Synthesis, ed. G. Pattenden,
Pergamon, New York, 1991, vol. 3, pp. 821–838.
3 I. Flament and M. Stoll, Helv. Chim. Acta, 1967, 50, 1754; H. W.
Rothkopf, D. Wo¨hrle, R. Mu¨ller and G. Kossmehl, Chem. Ber., 1975,
108, 875; H. Wynberg and H. J. Kooreman, J. Am. Chem. Soc., 1965, 87,
1739; W. W. Paudler and J. M. Barton, J. Org. Chem., 1966, 31, 1720.
4 M. Kirihara, S. Takizawa and T. Momose, J. Chem. Soc., Perkin Trans.
1, 1998, 7.
OH
O
100 mol% VOCl3, N2
Ph
Ph
Ph
Ph
MeCN, room temp.
5 min
O
O
95%
5 J. S. Buck and S. S. Jenkins, J. Am. Chem. Soc., 1929, 51, 2163; A.
McKillop, B. P. Swann, M. E. Ford and E. C. Taylor, Tetrahedron Lett.,
1970, 5281; A. McKillop, B. P. Swann, M. E. Ford and E. C. Taylor,
J. Am. Chem. Soc., 1973, 95, 3641; P. Girara and H. B. Kagan,
Tetrahedron Lett., 1975, 4513; M. Besemann, A. Cornelis and P. Laszlo,
C. R. Acad. Sci. Ser. C, 1984, 299, 427; G.-S. Zhang, Q.-Z. Shi, M.-F.
Chen and K. Cai, Synth. Commun., 1997, 27, 9534; R. S. Varma, D.
Kumar and R. Dahiya, J. Chem. Res. (S), 1998, 324.
OH
O
R
R
R
R
O
O
high valent
vanadium
low valent
vanadium
6 M. P. L. Caton, G. Darbrough and T. Parker, Tetrahedron Lett., 1980, 21,
1685; M. Singh and R. A. Misra, Synthesis, 1989, 403.
O2
Scheme 3
Communication 9/03622I
1388
Chem. Commun., 1999, 1387–1388