(Bicyclo[2.2.1]hepta-2,5-diene)[(2S,3S)-bis(diphenylphosphino)butane]rhodium Perchlorate1

(ClO4-)

[65012-74-0]  · C35H36ClO4P2Rh  · (Bicyclo[2.2.1]hepta-2,5-diene)[(2S,3S)-bis(diphenylphosphino)butane]rhodium Perchlorate  · (MW 720.97) (BF4-)

[79790-89-9]  · C35H36BF4P2Rh  · (Bicyclo[2.2.1]hepta-2,5-diene)[(2S,3S)-bis(diphenylphosphino)butane]rhodium Tetrafluoroborate  · (MW 708.33) (PF6-)

[99340-29-1]  · C35H36F6P3Rh  · (Bicyclo[2.2.1]hepta-2,5-diene)[(2S,3S)-bis(diphenylphosphino)butane]rhodium Hexafluorophosphate  · (MW 766.49)

(catalyst precursor used in the asymmetric hydrogenation of amino acid precursors, as well as in transfer hydrogenation and hydrosilation reactions2a,b)

Alternate Name: (bicyclo[2.2.1]hepta-2,5-diene)(chiraphos)rhodium perchlorate; (norbornadiene)(chiraphos)rhodium perchlorate.

Physical Data: 31P{1H} NMR (CD3OD) d 58.4 ppm, JRh-P = 154 Hz.

Solubility: sol CH2Cl2, MeOH, THF/MeOH, and H2O/MeOH mixtures; sparingly sol THF; insol Et2O.

Form Supplied in: cationic rhodium-chiraphos complexes are usually prepared in situ or from the procedure given below. In reactions employing the various rhodium-chiraphos complexes, the diene ligand is frequently hydrogenated prior to catalysis to afford the active solvated catalyst precursors [RhS2(chiraphos)]+.

Preparative Method: Bis(bicyclo[2.2.1]hepta-2,5-diene)rhodium Perchlorate (0.290 g) and (S,S)-chiraphos1,3 (0.308 g) (see 2,3-Bis(diphenylphosphino)butane) are dissolved in methylene chloride (5 mL) and THF (5 mL) under nitrogen. Hexane (6 mL) is then added and, after the mixture is allowed to stand at 25 °C for 1 h and then for 2 h at 5 °C, orange-red crystals of [Rh(nbd)(S,S-chiraphos)]ClO4 are collected (0.43 g).

Handling, Storage, and Precautions: cationic rhodium-chiraphos complexes are air-sensitive and should be handled and stored under an inert atmosphere.

Hydrogenation.

Rhodium(I) complexes containing chiral phosphine ligands are extremely useful in the preparation of enantiometrically pure amino acids.2,4 Asymmetric hydrogenation of a-(N-acylamino)acrylic acids proceeds in high chemical yields (95-100%) with essentially complete optical purity (eq 1). For example, DOPA, alanine, and tyrosine are obtained from asymmetric hydrogenations catalyzed by rhodium(I)-chiraphos cations with optical yields of 83, 91, and 92% respectively.2,4,5 Catalyzed hydrogenations of the analogous aliphatic compounds are somewhat less enantioselective.6 Indeed, hydrogenation of the (E) isomers gives products with even lower optical yields (eq 2).

The active herbicidal agent L-phosphinothricin7 is obtained with enantiomeric excesses up to 91% from the asymmetric hydrogenation of a-acylamidoacrylate precursors (eq 3). While yields of the primary hydrogenation products are quantitative, enantiomeric excesses increase slightly in reactions run in a H2O/MeOH mixture. Adverse effects are observed in reactions carried out in THF/MeOH.

Reactions with 1,1,1-trifluoro-2-(acetyloxy)-2-propene provide the first example of a vinyl acetate with a fully saturated substituent geminal to the heteroatom to be hydrogenated asymmetrically with high efficiency (eq 4).8 Rhodium-chiraphos cations also hydrogenate ketone9 and epoxide10 functionalities, albeit with low optical yields, and are, therefore, not synthetically useful. While this rhodium system seems somewhat limited to the preparation of amino acids, other rhodium11 and ruthenium12 catalyst precursors are currently available which show enhanced activity and selectivity for a much broader group of hydrogenation substrates.

Transfer Hydrogenation.

Transfer hydrogenation invokes the use of alcohols as a source of hydrogen for the reduction of organic functionalities. While rhodium-chiraphos cations catalyze the asymmetric transfer hydrogenation of acetophenone with high conversion (76%), optical yields are low (8.3%).13 Slightly higher enantiomeric excesses are obtained for the asymmetric reduction of ethyl phenyl ketone to give the corresponding alcohol (eq 5).14

Miscellaneous.

Intramolecular hydrosilation of internally substituted alkenes proceeds rapidly (t = 6 min) in the presence of rhodium-chiraphos cations to give cyclic siloxanes in high yields (eq 6).15 Basic workup proceeds with retention of configuration converting allylic alcohol derivatives to chiral 1,3-diols. Higher optical yields are obtained for analogous aryl alkenes. Hydroboration of vinylarenes with catecholborane employing cationic rhodium-chiraphos complexes affords secondary alcohols, upon oxidative workup, in high yields but with low optical purity. Much higher enantiomeric excesses are obtained in reactions using analogous rhodium-BINAP catalyst precursors.16


1. Fryzuk, M. D.; Bosnich, B. JACS 1977, 99, 6262.
2. (a) Ojima, I.; Clos, N.; Bastos, C. T 1989, 45, 6901. (b) Morrison, J. D., Asymmetric Synthesis; Academic: New York, 1983; Vol. 5. (c) Köttner, J.; Greber, G. CB 1980, 113, 2323.
3. Alcock, N. W.; Brown, J. M.; Maddox, P. J. CC 1986, 1532. Reaction between resolved iridium enamide complexes and racemic chiraphos mixture is highly enantioselective and permits in situ resolution for use in asymmetric catalysis.
4. Nugent, W. A.; RajanBabu, T. V.; Burk, M. J. Science 1993, 259, 479.
5. Scott, J. W.; Keith, D. D.; Nix, Jr., G.; Parrish, D. R.; Remington, S.; Roth, G. P.; Townsend, J. M.; Valentine, Jr., D.; Yang, R. JOC 1981, 46, 5086.
6. Weissermel, K.; Kleiner, H. J.; Finke, M.; Felcht, U. H. AG(E) 1981, 20, 223.
7. Zeiss, H-J. JOC 1991, 56, 1783.
8. Koenig, K. E.; Bachman, G. L.; Vineyard, B. D. JOC 1980, 45, 2362.
9. Genet, J. P.; Pinel, C.; Mallart, S.; Juge, S.; Thorimbert, S.; Laffitte, J. A. TA 1991, 2, 555.
10. Chan, A. S. C.; Landis, C. R. J. Mol. Catal. 1989, 49, 165.
11. Burk, M. J.; Feaster, J. E.; Nugent, W. A.; Harlow, R. L. JACS 1993, 115, 10125.
12. Noyori, R. CHEMTECH 1992, 22, 360.
13. Spogliarich, R.; Kaspar, J.; Graziani, M.; Morandini, F.; Piccolo, O. J. Catal. 1985, 94, 292.
14. Spogliarich, R.; Kaspar, J.; Graziani, M.; Morandini, F. JOM 1986, 306, 407.
15. Bergens, S. H.; Noheda, P.; Whelan, J.; Bosnich, B. JACS 1992, 114, 2121, 2129.
16. Hayashi, T.; Matsumoto, Y.; Ito, Y. TA 1991, 2, 601.

Stephen A. Westcott

University of North Carolina, Chapel Hill, NC, USA



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