Bis(oxazoline) Copper(II) Complexes
J. Am. Chem. Soc., Vol. 123, No. 19, 2001 4481
reactions are encouraging, more general catalysts are needed
to provide a greater freedom in choice of nucleophile and
electrophile.
Michael reaction of alkylidene malonates.18 Previous research
from this laboratory has documented the elements of substrate
architecture necessary for designing reactions that are both
stereoselective and catalytic. Specifically, unsaturated acyl
oxazolidinones are excellent substrates for Diels-Alder cy-
cloadditions catalyzed by C2-symmetric bis(oxazoline) Cu(II)
Lewis acids (eq 6). Significant experimental data suggest that
the imide substrate coordinates to 1a in a distorted square-planar
geometry placing the s-cis planar enamide proximate to the C2-
symmetric ligand substituent, thereby imparting R-Re π-facial
selectivity in the Diels-Alder addition. We anticipated that the
selectivity afforded by bis(oxazoline) Cu(II) complexes in the
Diels-Alder reaction would be applicable to Michael reactions
of enolsilanes and appropriately activated acyl oxazolidinones
(eq 7). The details of this investigation, including scope, product
elaboration, and mechanism, are reported herein.19
Copper(II) Complexes. Bis(oxazoline) Cu(II) complexes8,9
are practical10 catalysts for a variety of carbon-carbon bond
forming processes, such as Diels-Alder,11 hetero Diels-Alder,12
carbonyl-ene,13 aldol,14-16 enolsilane amination reactions,17 and
Results and Discussion
(7) (a) Shibasaki, M.; Sasai, H.; Arai, T. Angew. Chem., Int. Ed. Engl.
1997, 36, 1236-1256. (b) Kim, Y. S.; Matsunaga, S.; Das, J.; Sekine, A.;
Ohshima, T.; Shibasaki, M. J. Am. Chem. Soc. 2000, 122, 6505-6507.
(8) Evans, D. A.; Rovis, T.; Johnson, J. S. Pure Appl. Chem. 1999, 71,
1407-1415.
(9) Johnson, J. S.; Evans, D. A. Acc. Chem. Res. 2000, 33, 325-335.
(10) The copper chloride complex that serves as a bench-stable precursor
to 1a can be routinely prepared in batches of 30 g: Evans, D. A.; Peterson,
G. S.; Johnson, J. S.; Barnes, D. M.; Campos, K. R.; Woerpel, K. A. J.
Org. Chem. 1998, 63, 4541-4544.
Reaction Development. Preliminary observations suggested
that a reactive Michael acceptor as well as a strong Lewis acid
catalyst were necessary to promote the addition process.
Previous experiments from this laboratory indicated that fumaryl
and acryloyl imides were the most reactive dieneophiles in the
copper(II) catalyzed Diels-Alder reaction with cyclopentadiene
and therefore initial efforts were focused on these two substrates.
The Lewis acid selected was [Cu((S,S)-tBu-box)](SbF6)2 (1a)
based on earlier catalytic asymmetric Diels-Alder investigations
which demonstrated that this complex was exceptionally active
(eq 6).20 The additions of enolsilanes 4a-c to fumarylimide 3
revealed that there was no correlation between enantioselection
and reactivity of the nucleophile (Table 1). Silylketene acetals
are the most reactive nucleophilic participants, but are generally
nonselective in their additions (not shown). Conversely, thioester-
derived ketene acetals and enolsilanes derived from both
(11) (a) Evans, D. A.; Miller, S. J.; Lectka, T.; von Matt, P. J. Am. Chem.
Soc. 1993, 115, 6460-6461. (b) Evans, D. A.; Miller, S. J.; Lectka, T.;
von Matt, P. J. Am. Chem. Soc. 1999, 121, 7559-7573. (c) Evans, D. A.;
Barnes, D. M.; Johnson, J. S.; Lectka, T.; von Matt, P.; Miller, S. J.; Murry,
J. A.; Norcross, R. D.; Shaughnessy, E. A.; Campos, K. R. J. Am. Chem.
Soc. 1999, 121, 7582-7594.
(12) Evans, D. A.; Johnson, J. S.; Olhava, E. J. J. Am. Chem. Soc. 2000,
122, 1635-1649 and references therein.
(13) Evans, D. A.; Tregay, S. W.; Burgey, C. S.; Paras, N. A.; Vojkovsky,
T. J. Am. Chem. Soc. 2000, 122, 7936-7943.
(14) Evans, D. A.; Kozlowski, M. C.; Murry, J. A.; Burgey, C. S.;
Campos, K. R.; Connell, B. T.; Staples, R. J. J. Am. Chem. Soc. 1999, 121,
669-685.
(18) Evans, D. A.; Rovis, T.; Kozlowski, M. C. J. Am. Chem. Soc. 1999,
121, 1994-1995.
(19) A preliminary account of this work has been communicated: Evans,
D. A.; Willis, M. C.; Johnston, J. N. Org. Lett. 1999, 1, 865-868.
(20) Evans, D. A.; Murray, J. A.; von Matt, P.; Norcross, R. D.; Miller,
S. J. Angew. Chem., Int. Ed. Engl. 1995, 34, 798-800.
(15) Evans, D. A.; Murry, J. A.; Kozlowski, M. C. J. Am. Chem. Soc.
1996, 118, 5814-5815.
(16) Evans, D. A.; Kozlowski, M. C.; Burgey, C. S.; MacMillan, D. W.
C. J. Am. Chem. Soc. 1997, 119, 7893-7894.
(17) Evans, D. A.; Johnson, D. S. Org. Lett. 1999, 1, 595-598.