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P. Saikia et al. / Tetrahedron Letters 43 (2002) 7525–7526
Table 1. YbI2/MgI2 mediated coupling of acyl cyanides into 1,2-diketones
Entry
R
Reaction time (h) YbI2
Yielda (%) of 2
Reaction time (h) MgI2
Yield (%) of 2
1a
1b
1c
1d
1e
1f
1g
1h
1i
C6H5
4-ClC6H4
4-IC6H4
5
6
7
7
6
6
5
6
8
80
75
78
75
82
75
78
70
70
6
7
8
8
7
8
6
8
9
75
76
75
76
78
75
75
70
60
PhCH2
4-MeC6H4
4-NO2C6H4
4-MeOC6H4
4-MeOOCC6H4
n-C8H11
a All the yields refer to isolated chromatographically pure compounds.
References
shown in Table 1. Aliphatic acyl cyanide 1i was also
coupled with YbI2 to directly form the 1,2-diketone,
however, the reaction completion time was about 9 h.
Further increase of reaction time gave no significant
improvement, rather decomposition of starting materi-
als occurred. When ytterbium iodide is replaced by
inexpensive magnesium iodide the coupling proceeded
effectively and the corresponding diketones were
obtained in almost comparable yields. In most cases the
reaction was over within 5–7 h with ytterbium iodide,
however, the reaction takes a little more time with
magnesium iodide. Enhancing further the reaction time
gave no improvement in yield but rather formation of
the corresponding benzoin occurred. Furthermore, the
reaction conditions are tolerant of the ester group
(entry 1h) and the ether group (entry 1g). Also, aro-
matic halides showed remarkable selectivity to give the
diketone without any dehalogenation (entries 1b and
1c) or formation of complex mixtures. Moreover, aro-
matic nitro compounds which are known to be reduced
by iron carbonyl11 coupled with ease with ytterbium
iodide to give the corresponding diketones in 75% yield
without any reduction of the nitro group.12 More
detailed studies aimed at expanding the scope of this
reaction and to understand its mechanism are in
progress.
1. Inamoto, T. In Comprehensive Organic Synthesis; Trost,
B. M.; Fleming, I., Eds.; Pergamon: Oxford, 1991; Vol. 4,
pp. 231–250 and pp. 251–282.
2. Molander, G. A. Chem. Rev. 1992, 92, 29–68.
3. McElvain, S. M. Org. React. 1948, 4, 256.
4. Using samarium diiodide to produce a-diketone and a-
ketol see: (a) Gotard, P.; Countgoal, R.; Kagan, H. B.;
Tetrahedron Lett. 1981, 22, 3959; (b) Souppe, J.; Namy,
J.-L.; Kagan, H. B. Tetrahedron Lett. 1984, 25, 2869;
using pyrophoric lead see: (c) Meszaros, I. Tetrahedron
Lett. 1967, 4951; using lithium wire and ultrasound see:
(d) Han, H. B.; Boudjouk, P. Tetrahedron Lett. 1981, 22,
2757.
5. Rabyohn, N. Org. React. 1949, 5, 331.
6. Adam, W.; Goesbeck, A.; Stach, E. Tetrahedron Lett.
1986, 27, 2839.
7. (a) Zibuck, R.; Seebach, D. Helv. Chim. Acta 1988, 71,
237. For using zerovalent complexes of nickel/enneacar-
bonyl diiron see: (b) Flood. T. C. Tetrahedron Lett. 1977,
3861; using beryllium metal powder in 40–50% yield see:
(c) Lapkin, I. I.; Anvarovs, G. Y.; Povarnitsyna, T. N. J.
Gen. Chem., USSR (Eng-trans) 1966, 36, 1945; yields are
difficult to reproduce according to (d) Seebach, D.
Angew. Chem., Int. Ed. 1969, 8, 639; footnote p. 641.
8. Olah, G. A.; An-harang, W. J. Org. Chem. 1991, 902.
9. Dams, R.; Malinowski, M.; West drop, I.; Geise, H. J. J.
Org. Chem. 1981, 46, 2406.
In conclusion, we have provided a novel and efficient
method for the coupling of keto cyanides into 1,2-dike-
tones employing ytterbium and magnesium iodide in
dry THF, which involves a simple work-up and will
make a useful and important addition to the existing
methodologies. This new procedure has the advantages
such as mild reaction conditions, lack of side products
and better yields than the classical methods and above
all the method is highly selective.
10. Girard, P.; Namy, J.-L.; Kagan, H. B. J. Am. Chem. Soc.
1980, 102, 2693.
11. Abbayes, H.; Alper, H. J. Am. Chem. Soc. 1977, 99, 98.
12. Several years ago we were investigating pentazocine syn-
thesis via a Grignard reaction and we observed the
dimerization of anisyl bromide to bianisyl with MgI2 in
quantitative yields. By analogy, we assume that the MgI2
triggers the coupling of acyl cyanides to 1,2-diketones via
a single electron transfer reaction.