1828
F. Löffler et al.
LETTER
using tert-butyl diazoacetates and ligands 1 the anti-prod-
ucts could be increased up to a ratio of 99:1. Remarkably,
however, the diastereomeric ratio is reversed when
ligands 2 are used and syn-products are the major products
for most cases, e.g., for indene (3g) a change from 99:1
(ligand 1b) to 24:76 (ligand 2a) has been observed. Syn-
selective cyclopropanations are very rare.3i
References and Notes
(1) Concave Reagents, 31. U. Lüning, W. Hacker, J. Prakt. Chem.
1999, 341, No. 7, 1999, 341, 662-667.
(2) a) P. Eilbracht, Methoden Org. Chem., (Houben-Weyl) 4th.
Ed. 1995, vol. E21c, 2650-2658. b) H. U. Reißig, ibid. 1995,
vol. E21c, 3179-3270. c) A. Pfaltz, Acc. Chem. Res. 1993, 26,
339-345. d) M. P. Doyle, M. N. Protopopova, Tetrahedron
1998, 54, 7919-7946. e) M. P. Doyle, M. A. McKervey, Chem.
Commun. 1997, 983-989. f) Z. Gross, N. Galili, L.
Simkhovich, Tetrahedron Lett. 1999, 40, 1571-1574. g) F.
Simal, D. Jan, A. Demonceau, A. F. Noels, Tetrahedron Lett.
1999, 40, 1653-1656.
(3) a) K. C. Brown, T. Kodadek, J. Am. Chem. Soc. 1992, 114,
8336-8338, and ref. cited. b) A. B. Charette, J. Lemay, Angew.
Chem. 1997, 109, 1163-1165; Int. Ed. Engl., 36, 1090-1092.
c) U. Leutenegger, G. Umbricht, C. Fahrni, P. von Matt,
A. Pfaltz, Tetrahedron 1992, 48, 2143-2156. d) C. Bolm,
Angew. Chem. 1991, 103, 556-558; Int. Ed. Engl. 30, 542-544.
e) V. K. Singh, A. DattaGupta, G. Sekar, Synthesis 1997, 137-
149. f) O. Tenne, S.-A. Taj, G. P. Anderson, J. Org. Chem.
1998, 63, 6007-6015. g) M. M.-C. Lo, G. C. Fu, J. Am. Chem.
Soc. 1998, 120, 10270-10271. h) L. Cai, H. Mahmoud,
Y. Han, Tetrahedron: Asymmetry 1999, 10, 411-427. i)
T. Uchida, R. Irie, T. Katsuki, Synlett 1999, 1163-1165.
(4) Copper Ligands: a) H. Fritschi, U. Leutenegger, A. Pfaltz,
Helv. Chim. Acta 1988, 71, 1553-1565. b) A. V. Bedekar, P.
G. Andersson, Tetrahedron Lett. 1996, 37, 4073-4076.
(5) M. Hagen, U. Lüning, Chem. Ber./Recueil 1997, 130, 231-
234.
The tendency of ligands 1 to preferably form anti-prod-
ucts and the tendency of ligands 2 to preferably form syn-
products become obvious for all alkenes 3 and even in the
side reaction forming the maleic and fumaric esters 10.
This second diastereoselective reaction can be seen as an
additional indicator for the diastereoselectivity generating
properties of the concave ligands 1 and 2.
Besides ethyl diazoacetate (4), also tert-butyl diazoacetate
(5) and methyl diazoacetate (6) have been investigated.
By using the larger tert-butyl ester 5 the diastereomeric
ratio for the cyclopropanes 8g could be increased up to
>99:1 with ligands 1 (see Table 2). However, with ligands
2 the preference to form syn-products syn-8 still remains
but lower than that with the ethyl ester 4.
A comparison of Tables 1 and 2 shows clearly that the size
of the ester 4 has an influence on the diastereoselectivity.
Larger esters increase the anti-selectivity with ligands 1
and decrease the syn-selectivity with ligands 2. Therefore,
also methyl diazoacetate (6) was used (see Table 3). By
using ligand 2a in combination with the methyl ester 6 the
anti/syn-selectivity for styrene (3a) was increased to
26:74, and for indene (3g) to 14:86. The smaller the ester
group, the larger the syn-selectivity for catalysis with
ligands 2. These differences in selectivity can be ex-
plained by the different geometry of the ligands.3a Details
shall be published later.7b
(6) U. Lüning, M. Müller, M. Gelbert, K. Peters, H. G. von
Schnering, M. Keller, Chem. Ber. 1994, 127, 2297-2306.
(7) a) H. Ross, U. Lüning, Tetrahedron Lett. 1997, 38, 4539-
4542. b) F. Löffler, diploma thesis, Universität Kiel, 1998.
(8) With cyclic alkenes 3c-g, exo- and endo-bicycloalkyl esters
7c-g, 8c-g and 9c-g are formed while the acyclic alkenes 3a-b
give trans- and cis-cyclopropanes. If discussed together the
prefix anti combines trans and exo, and syn combines cis and
endo.
(9) M. P. Doyle, Recl. Trav. Chim. Pays-Bas 1991, 110, 305-316.
(10) Under nitrogen or argon, ca. 10.0 mg (39.7 µmol) of copper(I)
triflate hemi-benzene complex (Aldrich) was placed in a vial
sealed with a teflon septum and was weight with an accuracy
of ± 0.1 mg. Then, 17.5 mmol of the alkene 3 and ca. 44 µmol
of the ligand 1 or 2 dissolved in 4 ml of 1,2-dichloroethane
(ligand/copper ratio: 1.1 - 1.2) was added. Dissolution was
supported by ultrasound. Then 2 mmol of the diazoacetate 4-
6 was carefully added via a syringe. Usually, nitrogen evolved
quickly. After 24 h, the copper salt was removed by filtration
through silica gel (5 x 3 cm, 150 ml of diethyl ether). After
concentration to ca. 5 ml, the flask was filled to 25 ml with
1,2-dichloroethane and n-hexadecane was added as GC-
standard (4.0 mg/ml).
GC-analysis: SE30/25 m, 80°C for 5 min, 10°C/min until
140°C, 1 min, 2°C/min until 160°C, 1 min, 20°C/min until
240°C, 2 min.
Acknowledgement
For characterization11 the same procedure was used but no
GC-standard was added. To determine selectivities only, the
amounts of starting materials could be reduced to ¼.
(11) M. Hagen, Ph. D. thesis, Kiel 1999.
This work was supported by the Deutsche Forschungsgemeinschaft
and the Fonds der Chemischen Industrie. M. Hagen thanks the Land
Schleswig-Holstein for a stipend.
Article Identifier:
1437-2096,E;1999,0,11,1826,1828,ftx,en;G20799ST.pdf
Synlett 1999, No. 11, 1826–1828 ISSN 0936-5214 © Thieme Stuttgart · New York