Rhodium(III) Chloride

RhCl3

[10049-07-7]  · Cl3Rh  · Rhodium(III) Chloride  · (MW 209.26) (hydrate)

[20765-98-4]

(isomerizes alkenes and strained carbocycles;1 reduction precatalyst;5 oxidation catalyst7)

Physical Data: mp 450 °C.

Solubility: insol water; sol aq sodium hydroxide, aq potassium cyanide.

Form Supplied in: available as anhydrous solid and hydrated solid.

Analysis of Reagent Purity: elemental analysis.

Isomerizations.

Rhodium trichloride catalyzes the isomerization of alkenes, typically generating the thermodynamically more stable isomer. The exposure of an arylmethylenechroman-4-one to this reagent results in the efficient migration of the exocyclic alkene (eq 1).1a This example nicely illustrates the mild nature of the isomerization method as basic media induce degradation of the chroman-4-one, acidic media do not produce a reaction, and Raney Nickel affords a mixture of the desired alkene isomer and a reduced compound. Enones to which alkenes are attached are transformed into phenols upon reaction with rhodium trichloride (eq 2).2 Unsaturated imines are converted to anilines via an analogous process (eq 3).2 This reagent also allows the removal of an allyl protecting group from a wide variety of heteroatoms through an isomerization-hydrolysis pathway (eq 4).3

Furthermore, rhodium trichloride can isomerize strained carbocycles (eq 5).4 In comparison, IrIII, RuIII, and PtIV are effective in bringing about similar rearrangements; FeIII, NiII, and HgII are not active in the same fashion. Deuterium exchange studies indicate that this process most likely involves a metal hydride.

Reductions.

A mixture of Sodium Borohydride and rhodium trichloride forms a selective reagent for the reduction of aromatic systems (eq 6).5 This method does not affect carboxylic acids, esters, or amides; ketones are partially reduced; alkenes are competitively reduced. The fact that aromatic nuclei typically require severe conditions for reduction underscores the usefulness of this mild process.

Rhodium trichloride, in the presence of the phase transfer catalyst Aliquat 336, serves as a hydrogenation catalyst that preferentially reduces alkenes in the presence of aromatic nitro groups (eq 7).6 In general, the reduction of the nitro functionality is the most facile hydrogenation process.

Oxidation.

Phase transfer catalysis allows the oxidation of alkenes to proceed in the presence of rhodium trichloride (eq 8).7 Tetrabutylammonium hydrogen sulfate (THS) is of general use as a catalyst. Other quaternary ammonium salts will also serve this role. Mechanistically, it is thought that this reaction is analogous to the Wacker oxidation. This is in contrast to another report describing the use of rhodium trichloride as a homogeneous oxidation catalyst.8

Acetylene Hydration.

Rhodium trichloride in acidic media catalyzes the hydration of Acetylene (eq 9).9 The rate of this reaction is accelerated by the introduction of ligating additives such as Lithium Chloride. In comparison, Ruthenium(III) Chloride also catalyzes alkyne hydration but at one-third the rate of rhodium trichloride.

Hydrosilylation Catalyst.

The hydrosilylation of a,b-unsaturated esters to form dimethylketene trimethylsilyl acetals is catalyzed by rhodium trichloride (eq 10).10 The use of rhodium trichloride rather than Chlorotris(triphenylphosphine)rhodium(I), the typical rhodium catalyst employed, results in a faster reaction with improved yields and product purities.


1. (a) Andrieux, J.; Barton, D. H. R.; Patin, H. JCS(P1) 1977, 359. For examples of other enone isomerizations see: (b) Grieco, P. A.; Nishizawa, M.; Marinovic, N.; Ehmann, W. J. JACS 1976, 98, 7102. (c) Genet, J. P.; Ficini, J. TL 1979, 1499. (d) Harrod, J. F.; Chalk, A. J. JACS 1964, 86, 1776.
2. Grieco, P. A.; Marinovic, N. TL 1978, 2545.
3. Moreau, B.; Lavielle, S.; Marquet, A. TL 1977, 2591.
4. Wiberg, K. B.; Bishop, K. C., III TL 1973, 2727.
5. Nishiki, M.; Miyataka, H.; Niino, Y.; Mitsuo, N.; Satoh, T. TL 1982, 23, 193.
6. Amer, I.; Bravdo, T.; Blum, J.; Vollhardt, K. P. C. TL 1987, 28, 1321.
7. Januszkiewicz, K.; Alper, H. TL 1983, 24, 5163.
8. Mimoun, H.; Machirant, M. M. P.; Seree de Roch, I. JACS 1978, 100, 5437.
9. James, B. R.; Rempel, G. L. JACS 1969, 91, 863.
10. Revis, A.; Hilty, T. K. JOC 1990, 55, 2972.

Jeffrey A. McKinney

Zeneca Pharmaceuticals, Wilmington, DE, USA



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