Inorg. Chem. 1994,33, 4999-5002
4999
Multidimensional lo9Ag,31P,and 'H HMQC and HSQC NMR Studies on a Model Homogeneous Catalyst. Reactions of a Chiral Ferrocenylphosphine with Ag(CF3S03) Francesca Lianza? Alceo Macchioni,' Paul Pregosin,*'$and Heinz Ruegger' Department of Chemical Pharmacy, University of Milan, 1-20131 Milan, Italy, and Department of Inorganic Chemistry, ETH-Zentrum, CH-8092 Zurich, Switzerland Received May 18, 1994@
The chiral ferrocene ligand Fe{ ~ - ( P P ~ ~ ) - ~ - ( ( R ) - C H ( M ~ ) N ( M ~ ) C H ~ C H ~ N M ~ ~ ) 1, C ~reacts H ~ }with {C~~P~Z}, Ag(CF3S03) to form a trinuclear complex Ag3( 1)2(CF3S03)3,3, in which there are two different silver coordination spheres. In one environment two silver cations complex to the 1-PPh2 P atom and the two N atoms of the side chain, from two ligands 1, whereas the third silver cation coordinates to two P atoms from two C5mPh2 moieties. Reaction of 3 with a 70-fold excess of the isonitrile CNCHzC02Me, 4, affords a mononuclear complex, 5, in which both P atoms of 1chelate to silver(1). The number of coordinated 31Pspins for 3 and 5 and the coordination of the chiral side chain in 3 were determined using HSQC and HMQC 1wAg-31P, lwAg-'H, and 31P-1H multidimensional methods. The relevance of these observations to the Au(1)-catalyzed aldol condensation of aldehydes with isonitriles is discussed.
Introduction The reaction of isocyanides with aldehydes to form dihydrooxazoles can be catalyzed by gold(1) salts,1,2as shown in eq 1. If the reaction is carried out in the presence of a chiral auxiliary, such as 1, the organic products show substantial
-
Au+ salt
R'CHO
+ CNCH,R~
phosphorus donors of 1, and at least one isonitrile molecule, although this structure has not been proven. After the isonitrile is deprotonated by a suitable base, its enolate form is attacked by the aldehyde to form product(s). For 1, the base is provided by the chiral side chain. Model studies using mercury(II) halide c ~ m p l e x e stogether ,~ with 1, led to the isolation and characterization of trinuclear complexes of type 2. These compounds contain two mercury
catalyst
11 _-
12 15
4
Fe P h z P 2 u t 5 ' 4 1
enantiomeric excesses. l s 2 Mechanistically, the reaction is presumed to proceed through an intermediate complex in which the coordination sphere is composed of the metal, the two 'University of Milan. ETH-Zentrum. Abstract published in Advance ACS Abstracts, October 1, 1994. (1) (a) Hayashi, T.; Sawamura, M.; Ito, Y. Tetrahedron 1992, 48, 1999. (b) Ito, Y.; Sawamura, M.; Shirakawa, E.; Hayashizaki, K.; Hayashi, T. Tetrahedron Lett. l988,29,235. (c) Sawamura,M.; Ito, Y.; Hayashi, T. Tetrahedron Lett. 1989, 30,2247. (d) Hayashi, T.; Sawamura, M.; Ito, Y. Tetrahedron Lett. 1988, 29, 6321. (e) Ito, Y.; Sawamura, M.; Sirkawa, E.; Hayashi-Zaki, K.; Hayashi, T. Tetrahedron Lett. 1988, 44, 5253. (f) Ito, Y.; Sawamura, M.; Kabayashi, M.; Hayashi, T. Tetrahedron Let?. 1988,29, 239. (g) Ito, Y.; Sawamura, M.; Hayashi, T. Tetrahedron Lett. 1987, 28, 6215. (h) Ito, Y.; Sawamura, M.; Hayashi, T. J. Chem. SOC., Chem. Commun. 1986,108, 1090. (i) Ito, Y.; Sawamura, M.; Hayashi, T. J. Am. Chem. Soc. 1986, 108, 6405.
*
@
0020- 166919411333-4999$04.50/0
atoms each with a coordinated tridentate and one mercury center with two monodentate tertiary phosphine donors. This coordination sphere was identified using lg9HgNMR together with 31P- 'H heteronuclear multiple-quantum correlation (HMQC) spectroscopy. It was concluded3 that the complex chemistry of 2 involved coordination of the nitrogen side chain. The mercury complexes did not show catalytic activity3 related to the chemistry of eq 1. Hayashi and co-workers4 have also reported an NMR study for a silver(1) complex whose behavior is thought to be related to the catalytic chemistry of gold(1). We offer here new HMQC lo9Ag and 31P multidimensional NMR results which help to clarify the behavior of 1 with Ag(CF3S03) in the absence of and in the presence of an excess of isonitrile. (2) (a) Togni, A,; Pastor, S. D. J. Org. Chem. 1990, 55, 1649. (b) Pastor, S. D.; Togni, A. Helv. Chim. Acta 1991, 74, 905. (c) Togni, A.; Pastor, S. D.; Rihs, G.J. Organomet. Chem. 1990, 381, C21. (d) Togni, A,; Pastor, S. D. Helv. Chim. Acta 1989, 72, 1038. (e) Togni, A.; Pastor, S. D. Tetrahedron Lert. 1989, 30, 1071. (f) Togni, A,; Hausel, R. Synlett 1990, 633. (3) Blumer, R. E.; Lianza, F.; Pregosin, P. S.; Ruegger, H.; Togni, A. Inorg. Chem. 1993, 32, 2663. (4) Sawamura, M.; Ito, Y.; Hayashi, T. Tetrahedron Lett. 1990, 31, 2723.
0 1994 American Chemical Society
5000 Inorganic Chemistry, Vol. 33, No. 22, 1994
Lianza et al.
Results and Discussion The reaction of 1 with silver(1) triflate, as shown in eq 2, gives an isolable yellow-brown complex in good yield. The
N-Me,
H-6
2:3 ratio of l:Ag(CF$03) was chosen on the basis of the results for the mercury chemistry (a 1:l ratio gives mostly 3, as determined by low-temperature NMR). Complex 3 gave a satisfactory microanalysis and a weak set of peaks in its FAB mass spectrum at mle = 1986.6 (
