T. Portada et al. / Tetrahedron Letters 46 (2005) 5957–5959
5959
azoacetate catalysed by Cu(I)–bisoxazoline–2 catalytic
complex. Experimental evidence shows that the nonlin-
ear effects are identical within experimental error for
both chiral diastereomeric products, cis- and trans-1.14
The occurrence of a (+)-NLE is explained by the forma-
tion of two catalytically inactive enantiomeric Cu(I)22
complexes and by one inactive heterochiral meso-
Cu(I)22 complex, the latter being responsible for the
occurence of the (+)-NLE.
References and notes
´
1. (a) Avalos, M.; Babiano, R.; Cintas, P.; Jimenez, J. L.;
Figure 1. Relationship of normalised eeꢀs of trans-1 (j) and cis-1 (d)
versus % ee of (R)-2 showing identical (+)-NLEꢀs for both diastero-
mers of 1.
Palacios, J. C. Tetrahedron: Asymmetry 1997, 8, 2997; (b)
Puchot, C.; Samuel, O.; Dunach, E.; Zhao, S.; Agami, C.;
˜
Kagan, H. B. J. Am. Chem. Soc. 1986, 108, 2353, and
referencies cited therein.
2. Kitamura, M.; Okada, S.; Suga, S.; Noyori, R. J. Am.
Chem. Soc. 1989, 111, 4028.
3. Pfaltz, A. Acta Chem. Scand. 1996, 50, 189.
4. Lowenthal, R. E.; Abiko, A.; Masamune, S. Tetrahedron
Lett. 1990, 61, 6005.
mers of a 1, higher eeꢀs were obtained than those calcu-
lated for the linear ee 1 versus ee (R)-2 relationship.
Hence, a clear positive NLE is observed (Fig. 1). The
cis-1/trans-1 diastereoselectivity in all cases was 30:70
and the chemical yields were in the range of 87–98%.
5. Lowenthal, R. E.; Masamune, S. Tetrahedron Lett. 1991,
32, 7373.
The observation of (+)-NLEs is in accordance with the
formation of the catalytically inactive meso-Cu(I)22
complex. Since the optical purity of the catalytic
Cu(I)(R)-2 complex influences the enantioselectivity
and not the diastereoselectivity of reaction, the magni-
tudes of the (+)-NLEs should be the same for both dia-
stereomers of 1. As shown in Figure 1, the normalised
eeꢀs of trans- and cis-1 versus optical purity of (R)-2
show identical curves within the experimental error.
This result is of significance demonstrating for the first
time that identical (+)-NLEs are obtained for two dias-
teromeric chiral products formed in a single reaction
step. Desimoni and co-workers13 performed Diels–Alder
and 1,3-cycloaddition reactions catalysed by zinc and
magnesium complexes of 2, respectively. Only in the
case of zinc complex the (+)-NLE was observed. The
effect was explained by the formation of a stable hetero-
chiral meso-Zn(II)22 complex; its X-ray structure is
analogous to that of the proposed meso-Cu(I)22 com-
plex shown in Scheme 2.
6. Evans, D. A.; Woerpel, K. A.; Hinman, M. M.; Faul, M.
M. J. Am. Chem. Soc. 1991, 113, 726.
7. Evans, D. A.; Woerpel, K. A.; Scott, M. J. Angew. Chem.,
Int. Ed. Engl. 1992, 31, 430.
8. Pique, C.; Fahndrich, B.; Pfaltz, A. Synlett 1995, 491.
9. Hoarau, O.; Ait-Hadou, H.; Castro, M.; Balavoine, G. G.
A. Tetrahedron: Asymmetry 1997, 8, 3755.
10. Pfaltz, A. Enantioselective Catalysis with Chiral Co- and
Cu-complexes. In Modern Synthetic Methods 1989;
Springer, 1989.
´
11. Hollosi, M.; Mayer, Z.; Vass, E.; Raza, Z.; Tomisˇic, V.;
ˇ
ˇ
´
´
Portada, T.; Piantanida, I.; Zinic, M.; Sunjic, V. Eur. J.
Inorg. Chem. 2002, 1738.
12. Tepfenhart, D.; Moisan, L.; Dalko, P. I.; Cossy, J.
Tetrahedron Lett. 2004, 45, 1781.
13. Crosignani, S.; Desimoni, G.; Faita, G.; Filippone, S.;
Mortoni, A.; Righetti, P. P.; Zema, M. Tetrahedron Lett.
1999, 40, 7007–7010.
14. To a 1,2-dichloroethane solution (1 mL) of styrene (0.26 g;
2.5 mmol), CuOTfÆ1/2PhMe (2.6 mg; 0.01 mmol) and 2
(6.7 mg; 0.02 mmol) of defined optical purity 0.5 mL of a
1 M ethyl diazoacetate solution in 1,2-dichloroethane was
slowly added under argon at room temperature over 4 h
using a syringe pump. Samples were taken and analysed
by chiral GC using a Chirasil Dex-CB column.
In conclusion, we report the first observation of a (+)-
NLE in the cyclopropanation of styrene with ethyl di-