Table 1 Effect of a dimetal ion binding ligand on the SmI2 promoted pinacol
coupling.a
Diastereosel
(±+meso)
Entry
1
Aldehyde
Ligand (equiv.)b
Yield (%)c
1 (0.5)
1 (1.0)
89
92
1+5.0
2
3
1+5.5
1+5.9d
Tetraglyme (1.0) 83
4
Tetraglyme (1.0) 29
10+1
5
6
1 (0.5)
1 (1.0)
81
38
10+1
13+1
7
8
9
Tetraglyme (1.0) 53
14+1e
11+1e
1.9+1
1 (0.5)
None
77
44
Scheme 2
10
11
12
Tetraglyme (1.0) 24
1 (0.5) 44
Tetraglyme (1.0) 45
10+1
10+1
3.0+1
In conclusion, we have shown that dimetal binding ligands
can have a substantial effect on the SmI2 induced pinacol
coupling of alkyl aldehydes compared to that of tetraglyme.
Efforts are underway to increase the utility of such ligands. The
introduction of either planar chirality in the connector or
asymmetric carbon centers in the tetraglyme units may also lead
to the development of an asymmetric version of the pinacol
coupling reaction with alkyl aldehydes.
13
14
1 (0.5)
None
68
79
2.2+1
8.5+1
15
16
a
Tetraglyme (1.0)
1 (0.5)
0
0
—
—
We thank the University of Aarhus and the Danish National
Science Foundation for generous financial support.
Conditions: complexed or uncomplexed SmI2 (0.1 M, 1.5 equiv.),
aldehyde (1 equiv.), THF, 20 °C. b Based on the number of equivalents of
c
SmI2 added. Based on isolated, chromatographically pure material.
d Taken from ref. 6. e Determined after catalytic hydrogenation (Pd/C, H2)
of the diastereomeric mixture of diols.
Notes and references
† Preliminary investigations of this complex by cyclic voltametry imply that
the metallocene center does not influence the oxidation potential of SmI2
(ref. 6 and 11).
selectivity is low suggesting that substitution at the a-position
of the alkyl aldehyde is necessary for obtaining the high ±+meso
ratio when using the polyethyleneglycol complexing agents.
Increasing the steric bulkiness, as exemplified with piv-
alaldehyde, surprisingly led to no pinacol coupling with either
tetraglyme or 1 (entries 15 and 16). Nevertheless, without these
ligands SmI2 promoted this dimerisation with an 8.5+1
selectivity (entry 14). The steric bulkiness of the aldehyde was
sufficient to induce a high diastereoselectivity. These results
again support the notion that uncomplexed SmI2 is not available
in solution when the one electron reducing agent is added to 0.5
equiv. of ligand 1.
The combined findings suggest the formation of a complex
between two samarium(ii) metal ions and 1 as illustrated in
Scheme 2. Coordination of the two aldehyde units to each of the
two oxophilic metallic centers may then take place. The slow
reactivity of alkyl aldehydes also suggests that there is only a
low concentration of a ketyl radical species in solution. The
close proximity of the carbon radical center to the other
aldehyde, which possesses a low-lying LUMO due to its
coordination to SmI2, could lead to a radical addition step with
concomitant reduction of the resulting alkoxyl radical by SmI2.6
A two electron reduction of the aldehyde to dianion 11, which
eventually couples to a second aldehyde, is a possible
alternative mechanism because ligand 1 improves significantly
the yields of these pinacol coupling reactions compared to that
of tetraglyme, and although little effect is seen on the
diastereoselectivities, these preliminary observations tend to
support the radical addition mechanism where the entropy
contribution is less important owing to the pseudo-intra-
molecular nature of this reaction.
1 For recent reviews on the application of SmI2 in organic synthesis: A.
Krief and A.-M. Laval, Chem. Rev., 1999, 99, 745; G. A. Molander and
C. R. Harris, Chem. Rev., 1996, 96, 307; G. A. Molander and C. R.
Harris, Tetrahedron, 1998, 54, 3321; T. Skrydstrup, Angew. Chem., Int.
Ed. Engl., 1997, 36, 345.
2 For several recent reviews on the pinacol coupling, see: T. Wirth,
Angew. Chem., Int. Ed. Engl., 1996, 35, 61; R. G. Dushin, in
Comprehensive Organometallic Chemistry II, ed. L.S. Hegedus,
Pergamon, Oxford, 1995, vol. 12, p. 1071; A. Fürstner, Angew. Chem.,
Int. Ed. Engl., 1993, 32, 164. See also: A. Gansäuer and D. Bauer, Eur.
J. Org. Chem., 1998, 2673; Y. Yamamoto, R. Hattori and K. Itoh, Chem.
Commun., 1999, 825 and references therein.
3 J. Souppe, L. Danon, J. L. Namy and H. B. Kagan, J. Organomet.
Chem., 1983, 250, 227; T. Honda and M. Katoh, Chem. Commun., 1997,
369; R. Nomura, T. Matsuno and T. Endo, J. Am. Chem. Soc., 1996, 118,
11666; N. Miyoshi, S. Takeuchi and Y. Ohgo, Chem. Lett., 1993,
2129.
4 One exception is the SmI2–HMPA induced pinacol coupling of
benzaldehyde: J.-S. Shiue, C.-C. Lin and J.-M. Fang, Tetrahedron Lett.,
1993, 34, 335.
5 N. Taniguchi, N. Kaneta and M. Uemura, Tetrahedron, 1998, 54,
12775.
6 H. L. Pedersen, T. B. Christensen, R. J. Enemærke, K. Daasbjerg and T.
Skrydstrup, Eur. J. Org. Chem., 1999, 565.
7 T. Imamoto, T. Hatajima, N. Takiyama, T. Takeyama, Y. Kamiya and
T. Yoshizawa, J. Chem. Soc., Perkin Trans. 1, 1991, 3127.
8 G. W. Gokel, J. C. Medina and C. Li, Synlett, 1991, 677.
9 A.-S. Carlström and T. Frejd, J. Org. Chem., 1990, 55, 4175.
10 A. Sonoda and I. Moritani, J. Organomet. Chem., 1971, 26, 133.
11 R. J. Enemærke, K. Daasbjerg and T. Skrydstrup, Chem. Commun.,
1999, 343.
Communication 9/04278D
2052
Chem. Commun., 1999, 2051–2052