J. Am. Chem. Soc. 1999, 121, 2933-2934
Scheme 1
2933
High Enantioselection in the Rearrangement of
Allylic Imidates with Ferrocenyl Oxazoline Catalysts
Yariv Donde and Larry E. Overman*
Department of Chemistry, Rowland Hall
UniVersity of California, IrVine, California 92697-2025
ReceiVed September 14, 1998
Current investigations in these laboratories focus on the
development of asymmetric catalysts for the addition of external
(nonmetal bound) nucleophiles to prochiral alkenes. As an entry
point to this area, we have been investigating the Pd(II)-catalyzed
1
rearrangement of allylic imidates to allylic amides (eq 1).
Substantial evidence points to a cyclization-induced rearrangement
mechanism for this reaction in which a pivotal step is attack of
the imidate nitrogen onto a palladium-complexed alkene 2.2
Subsequent deoxypalladation of 3 yields the rearranged amide 4
and regenerates the Pd(II) catalyst.
The first asymmetric catalysts described for this synthetically
important rearrangement were cationic Pd(diamine) and Pd(bis-
oxazoline) complexes1 that, like cationic complexes containing
a
3
phosphine and oxazoline ligands, gave the rearranged amides in
moderate yield and enantioselectivity. The major competing
reaction was ionization of the allylic imidate, presumably
promoted by coordination of the imidate nitrogen to the cationic
palladium catalyst. Our most recent work has demonstrated the
7
prepared in a similar fashion. Complexes 7a-11a were stable,
highly crystalline solids which could be chromatographed on silica
gel or florisil. The sequence illustrated in Scheme 1 is sufficiently
efficient to conveniently provide these complexes in multigram
quantities. Moreover, the presence of crystalline intermediates
along this route allowed for easy diastereomer separation at
various stages. Alternate approaches to this family of catalysts
involving cyclopalladation or transmetalation were much less
successful, presumably because the Pd(II) salts employed in these
procedures promoted oxidative decomposition of the ferrocene
potential of asymmetric Pd(II) catalysts having anionic ferrocenyl
ligands.1b,c These neutral catalysts promoted the rearrangement
of allylic N-arylbenzimidates in high yield within hours at room
temperature, although enantioselectivity was still moderate. In this
paper, we report a new family of cyclopalladated catalysts bearing
ferrocenyl-oxazoline ligands which catalyze the rearrangement
of allylic imidates in high efficiency at convenient rates and with
the highest enantioselectivities (up to 96% ee) reported to date.
8
unit. The structures of complexes 7a and 9a were confirmed by
X-ray crystallography.9
These novel palladacycle catalysts were initially evaluated for
the rearrangement of (E)- and (Z)-2-hexenyl-N-(4-trifluorometh-
ylphenyl)benzimidates (12 f 13, eq 2). Although iodide-bridged
complex 7a was inactive, 5 mol % of the trifluoroacetate
complexes 7b-11b, generated in situ by reaction of the corre-
sponding iodide-bridged dimer with 2 equiv of Ag(OCOCF
3
),
The synthesis of the ferrocenyl oxazoline palladacycles relies
on the high-yielding oxidative addition of Pd(0) to the corre-
sponding iodide as the key step and is exemplified in the synthesis
promoted the rearrangement of 12 in CH Cl at room tempera-
2
2
ture.10 Several trends are evident in Table 1. All catalysts except
10b show higher enantioselectivity for rearrangement of the Z
stereoisomer of 12. With the silyl-containing catalysts 7b-9b,
benzamide 13 is produced in 90% ee or greater from (Z)-12.
Catalyst 9b gives the opposite enantiomer of 13 to that produced
by the “pseudo” enantiomeric catalysts 7b or 8b. For both imidate
stereoisomers, increasing the size of the oxazoline substituent from
tert-butyl to 3-methoxy-3-pentyl had no discernible effect on
enantioselectivity, although reaction rate was somewhat decreased.
4
of complex 7a (Scheme 1). Thus, ortholithiation of the known
enantioenriched oxazoline complex 5, a 30:1 mixture of diaste-
5
reoisomers, followed by reaction with diiodoethane provided
iodide 6 as a single stereoisomer in 70% yield after recrystalli-
zation. A deficiency of tert-butyllithium was employed to
6
minimize side products resulting from dilithiation of 5. Treatment
of 6 with Pd
2
(dba)
3
3
‚CHCl then provided the iodide-bridged dimer
7a in 86% yield. Ferrocenyl oxazoline complexes 8a-11a were
(
1) (a) Calter, M.; Hollis, T. K.; Overman, L. E.; Ziller, J.; Zipp, G. G. J.
(7) Experimental details for preparing these catalysts and their characteriza-
tion data are provided in the Supporting Information.
Org. Chem. 1997, 62, 1449. (b) Hollis, T. K.; Overman, L. E. Tetrahedron
Lett. 1997, 38, 8837. (c) Cohen, F.; Overman, L. E. Tetrahedron: Asymmetry
(8) For a review of palladacycle synthesis, see: Canty, A. J. In Compre-
hensiVe Organometallic Chemistry II; Abel, E. W., Stone, F. G. A., Wilkinson,
G., Eds.; Pergamon: New York, 1995; Vol. 9, pp 242-248.
1
998, 9, 3213.
(
2) Overman, L. E. Angew. Chem., Int. Ed. Eng. 1984, 23, 579.
(
3) Uozumi, Y.; Kato, K.; Hayashi, T. Tetrahedron: Asymmetry 1998, 9,
(9) The authors have deposited coordinates for these compounds with the
Cambridge Crystallographic Data Centre. The coordinates can be obtained,
on request, from the Director, Cambridge Crystallographic Data Centre, 12
Union Road, Cambridge, CB2 1EZ, U.K.
1
065.
(
4) For other examples of the use of oxidative addition for the synthesis of
palladacycles, see: (a) Denmark, S. E.; Stavenger, R. A.; Faucher, A.-M.;
Edwards, J. P. J. Org. Chem. 1997, 62, 3375. (b) Mateo, C.; Cardenas, D. J.;
Fernandez-Rivas, C.; Echavarren, A. M. Chem.sEur. J. 1996, 2, 1596.
(10) The trifluoroacetate complexes generated in this way were much less
stable than the iodide-bridged dimers and consequently were directly employed
1
(
5) Sammakia, T.; Latham, H. A.; Schaad, D. R. J. Org. Chem. 1995, 60,
without purification. H NMR data for 7b: (300 MHz, CD
2
Cl
2
) δ 5.14 (d, J
1
0.
) 1.9 Hz 1 H), 4.76 (dd, J ) 8.9, 2.9, 1 H), 4.60 (app t, J ) 8.9 Hz, 1 H),
4.36 (s, 5 H), 4.24 (br m, 1 H), 3.69 (dd, J ) 8.5, 2.9 Hz, 1 H), 0.95 (s, 9 H),
0.22 (s, 9 H).
(
6) These conditions are adapted from the following: Rebiere, F.; Samuel,
O.; Kagan, H. B. Tetrahedron Lett. 1990, 31, 3121.
1
0.1021/ja983263q CCC: $18.00 © 1999 American Chemical Society
Published on Web 03/11/1999