ORGANIC LETTERS
Formation of Enamides via Palladium(II)-Catalyzed Vinyl Transfer from Vinyl Ethers to Nitrogen Nucleophiles
2004 Vol. 6, No. 11 1845-1848
Jodie L. Brice, James E. Meerdink, and Shannon S. Stahl* UniVersity of WisconsinsMadison, 1101 UniVersity AVenue, Madison, Wisconsin 53706
[email protected] Received March 25, 2004
ABSTRACT
Palladium(II) complexes catalyze the formation of enamides via the formal cross-coupling reaction between nitrogen nucleophiles and vinyl ethers. These vinyl transfer reactions proceed in good yields with amide, carbamate, and sulfonamide nucleophiles, and the optimal catalyst is (DPP)Pd(OCOCF3)2 (DPP ) 4,7-diphenyl-1,10-phenanthroline).
Recent developments in palladium(0)-catalyzed carbonnitrogen bond formation have led to numerous useful synthetic methodologies, particularly for the cross-coupling of allyl,1 aryl,2 and vinyl3,4 halides and related electrophiles (1) (a) Trost, B. M.; Van Vranken, D. L. Chem. ReV. 1996, 96, 395422. (b) Johannsen, M.; Jørgensen, K. A. Chem. ReV. 1998, 98, 16891708. (c) Trost, B. M.; Crawley, M. L. Chem. ReV. 2003, 103, 29212943. (2) (a) Hartwig, J. F. Acc. Chem. Res. 1998, 31, 852-860. (b) Hartwig, J. F. Angew. Chem., Int. Ed. 1998, 37, 2046-2067. (c) Wolfe, J. P.; Wagaw, S.; Marcoux, J.-F.; Buchwald, S. L. Acc. Chem. Res. 1998, 31, 805-818. (d) Belfield, A. J.; Brown, G. R.; Foubister, A. J. Tetrahedron 1999, 55, 11399-11428. (e) Yang, B. H.; Buchwald, S. L. J. Organomet. Chem. 1999, 576, 125-146. (f) Muci, A. R.; Buchwald, S. L. Top. Curr. Chem. 2002, 219, 131-209. (3) (a) Barluenga, J.; Ferna´ndez, M. A.; Aznar, F.; Valde´s, C. Chem. Commun. 2002, 2362-2363. (b) Willis, M. C.; Brace, G. N. Tetrahedron Lett. 2002, 43, 9085-9088. (c) Wallace, D. J.; Klauber, D. J.; Chen, C.; Volante, R. P. Org. Lett. 2003, 5, 4749-4752. (d) Barluenga, J.; Ferna´ndez, M. A.; Aznar, F.; Valde´s, C. Chem.-Eur. J. 2004, 10, 494-507. (4) For related copper-catalyzed Cvinyl-N cross-coupling reactions, see: (a) Ogawa, T.; Kiji, T.; Hayami, K.; Suzuki, H. Chem. Lett. 1991, 14431446. (b) Shen, R.; Porco, J. A., Jr. Org. Lett. 2000, 2, 1333-1336. (c) Shen, R.; Lin, C. T.; Porco, J. A., Jr. J. Am. Chem. Soc. 2002, 124, 56505651. (d) Jiang, L.; Job, G. E.; Klapars, A.; Buchwald, S. L. Org. Lett. 2003, 5, 3667-3669. (e) Han, C.; Shen, R.; Su, S.; Porco, J. A., Jr. Org. Lett. 2004, 6, 27-30. 10.1021/ol0494360 CCC: $27.50 Published on Web 05/05/2004
© 2004 American Chemical Society
with amine and amide nucleophiles. These reactions are initiated by oxidative addition of a C-X bond (X ) halide, acetate, triflate) at a palladium(0) center. Palladium(II)catalyzed transformations have also been developed for C-N bond formation, and both oxidative and nonoxidative reactions are known.5 Examples include allylic substitution,6 rearrangement of allylic imidates,7 and hydroamination8 and oxidative amination8-10 of alkenes. This report describes a new method for carbon-nitrogen bond formation involving (5) For general references, see: Tsuji, J. Palladium Reagents and Catalysts; Wiley: New York, 1995. (6) Overman, L. E.; Remarchuk, T. P. J. Am. Chem. Soc. 2002, 124, 12-13. (7) (a) Hollis, T. K.; Overman, L. E. J. Organomet. Chem. 1999, 576, 290-299. (b) Overman, L. E.; Owen, C. E.; Pavan, M. M.; Richards, C. J. Org. Lett. 2003, 5, 1809-1812. (c) Anderson, C. E.; Overman, L. E. J. Am. Chem. Soc. 2003, 125, 12412-12413. (8) For leading references, see: (a) Mu¨ller, T. E.; Beller, M. Chem. ReV. 1998, 98, 675-703. (b) Zeni, G.; Larock, R. C. Chem. ReV. 2004, 104, 2285-2309. (c) Seligson, A. L.; Trogler, W. C. Organometallics 1993, 12, 744-751. (d) Kawatsura, M.; Hartwig, J. F. J. Am. Chem. Soc. 2000, 122, 9546-9547. (e) Utsunomiya, M.; Hartwig, J. F. J. Am. Chem. Soc. 2003, 125, 14286-14287. (f) Gaunt, M. J.; Spencer, J. B. Org. Lett. 2001, 3, 25-28. (g) Nobis, M.; Driessen-Ho¨lscher, B. Angew. Chem., Int. Ed. 2001, 40, 3983-3985.
palladium(II)-catalyzed vinyl transfer from vinyl ethers to nitrogen nucleophiles.11 During our recent studies on the palladium(II)-catalyzed oxidative amination of olefins (for example, eq 1),10b we observed that ethyl vinyl ether undergoes nonoxidative vinyl transfer to the nitrogen nucleophile (eq 2). This reaction represents a C-N cross-coupling reaction in which the vinyl ether is the electrophilic partner. However, palladium(0)catalyzed cross-coupling reactions are generally incompatible with an aerobic atmosphere, and vinyl ethers are not expected to undergo efficient oxidative addition to palladium(0). The limited precedent for this class of reactions, together with growing synthetic interest in enamides,12 prompted us to investigate this reactivity further.
A series of palladium(II) catalysts were explored to improve upon the reaction catalyzed by (PhCN)2PdCl2. The vinylation of 2-oxazolidinone was used to investigate the effects of both counterions and neutral donor ligands on palladium(II). The higher boiling point of butyl vinyl ether (BVE) relative to ethyl vinyl ether (eq 2) generally makes it a more convenient substrate in these reactions. BVE also served as the solvent in the reactions. Recent results by Schlaf and co-workers on vinyl transfer to alcohols13 prompted us to include complexes bearing phenanthroline ligands (Table 1), and the (4,7-diphenyl-1,10phenanthroline)palladium(II) trifluoroacetate complex, (DPP)Pd(OCOCF3)2, displayed the highest catalytic activity. Use of this catalyst resulted in a quantitative yield within 3 min (Table 1, entry 3). Nearly identical activity is obtained if this catalyst is prepared in situ. The (phen)Pd(OCOCF3)2 complex (entry 6) also displays high activity, but its low solubility can lead to lower yields under certain conditions. Mercuric salts are known to promote vinyl transfer to oxygen nucleophiles (alcohols and carboxylic acids);14 however, these complexes proved to be less effective (Table 1, entries 11-14). (9) (a) Hegedus, L. S. J. Mol. Catal. 1983, 19, 201-211. (b) van Benthem, R. A. T. M.; Hiemstra, H.; Longarela, G. R.; Speckamp, W. N. Tetrahedron Lett. 1994, 35, 9281-9284. (c) Ro¨nn, M.; Ba¨ckvall, J.-E.; Andersson, P. G. Tetrahedron Lett. 1995, 36, 7749-7752. (d) Larock, R. C.; Hightower, T. R.; Hasvold, L. A.; Peterson, K. P. J. Org. Chem. 1996, 61, 3584-3585. (e) Fix, S. R.; Brice, J. L.; Stahl, S. S. Angew. Chem., Int. Ed. 2002, 41, 164-166. (10) (a) Hosokawa, T.; Takano, M.; Kuroki, Y.; Murahashi, S.-I. Tetrahedron Lett. 1992, 33, 6643-6646. (b) Timokhin, V. I.; Anastasi, N. R.; Stahl, S. S. J. Am. Chem. Soc. 2003, 125, 12996-12997. (11) For a similar reaction using vinyl acetate, see: Bayer, E.; Geckeler, K. Angew. Chem., Int. Ed. Engl. 1979, 18, 533-534. (12) (a) Yet, L. Chem. ReV. 2003, 103, 4283-4306. (b) Xiong, H.; Hsung, R. P.; Shen, L.; Hahn, J. M. Tetrahedron Lett. 2002, 43, 4449-4453. (c) Wei, L.-L.; Xiong, H.; Hsung, R. P. Acc. Chem. Res. 2003, 36, 773-782. (13) (a) Handerson, S.; Schlaf, M. Org. Lett. 2002, 4, 407-409. (b) Bosch, M.; Schlaf, M. J. Org. Chem. 2003, 68, 5225-5227. 1846
Table 1. Catalyst Screening Resultsa
entry
catalyst
yieldb (%)
1 2 3 4 5 6 7 8 9 10 11 12 13 14
(PhCN)2PdCl2 Pd(OCOCF3)2 (DPP)Pd(OCOCF3)2c (DPP)Pd(OAc)2 (DPP)PdCl2 (phen)Pd(OCOCF3)2d (tmeda)Pd(OCOCF3)2e (PPh3)2Pd(OCOCF3)2 (diphos)Pd(OCOCF3)2f Na2PdCl4 Hg(OAc)2 HgSO4 HgCl2 Hg(OCOCF3)2
1 2 100 39
