trans-Carbonyl(chloro)bis(triphenylphosphine)iridium(I)1

[14871-41-1]  · C37H30ClIrOP2  · trans-Carbonyl(chloro)bis(triphenylphosphine)iridium(I)  · (MW 780.264)

(catalyst precursor for homogeneous hydrogenations,2,3 decarbonylation of acyl halides,10 and the hydrosilation12 and hydroboration13 of alkenes)

Alternate Name: Vaska's compound.

Physical Data: mp 327-8 °C (dec); 1H NMR d (C6D6) 6.6 (m);15 IR nCO = 1953 cm-1 (nujol).

Solubility: sol toluene and chloroform; insol diethyl ether and alcohols.

Form Supplied in: yellow crystals, widely available.

Preparative Method: in a round-bottomed flask a mixture containing 3.52 g (10 mmol) of IrCl3.3H2O, 13.1 g (50 mmol) of PPh3, 4 mL of aniline, and 150 mL of DMF is heated under nitrogen at reflux for 12 h. After filtering the resulting red-brown solution hot, 300 mL of warm methanol is added rapidly and the mixture stirred. Upon cooling in an ice bath, yellow crystals of trans-IrCl(CO)(PPh3)2 are collected on a filter and washed with 250 mL of methanol and then with 25 mL of diethyl ether (87-90%).

Handling, Storage, and Precautions: the compound is stable in air but readily takes up oxygen in solution.

Hydrogenation.

Although trans-IrCl(CO)(PPh3)2 was initially prepared by Angoletta2 it was correctly formulated by Vaska3 and henceforth has commonly been referred to as Vaska's compound. While a number of alkenes and activated alkynes can be hydrogenated in the presence of a catalytic amount of Vaska's compound, isomerization is frequently a faster pathway and during the course of the reaction a number of isomers can accumulate.4 The catalyst precursor can be activated by weak UV radiation. Indeed, under these conditions 1,3-cyclohexadiene can be hydrogenated selectively (98%) to cyclohexene (eq 1), a reaction that does not take place thermally.5 In a related study, butynediol is selectively hydrogenated during photolysis using Vaska's compound in toluene/trifluoroethanol (1:1).7

Hydrogen Transfer.

Selective hydrogen transfer from Formic Acid to a,b-enones can be accomplished with a catalytic amount of trans-IrCl(CO)(PPh3)2 (eq 2).8 The polystyrene-supported reagent is an even more efficient catalyst for this transfer hydrogenation (with 70% conversion).9

Decarbonylation.

Vaska's compound can also be used to catalyze the decarbonylation and dehydrohalogenation of aliphatic acid halides into mixtures of alkenes selectively and under mild conditions (yields ca. 75%).10 Aromatic acid halides are not affected, making it possible to eliminate selectively one carbonyl functionality from the dichloride of 4-(p-carboxyphenyl)butyric acid, p-ClOCC6H4(CH2)3COCl. The resulting p-allylbenzoyl chloride is unstable, however, leading to extensive polymerization. Vaska's compound also catalyzes the decarbonylation of formate esters to alcohols and carbon monoxide.11

H-X Addition.

Vaska's compound catalyzes the regioselective hydrosilation of alkynes and styrenes with a number of silane reagents at high temperatures.12 For example, addition of Triethylsilane to phenylacetylene using Vaska's compound in catalytic amounts affords the btrans adduct in 88% yield. Photolysis of trans-IrCl(CO)(PPh3)2 leads to a moderately active catalyst for the hydroboration of 1-octene with Catecholborane (eq 3).13 Unfortunately, no reaction is observed with styrenes under identical conditions. More active and selective rhodium catalysts for the hydroboration of alkenes are well established.14

Related Reagents.

Carbonyl(chloro)bis(triphenylphosphine)rhodium(I); Chlorotris(triphenylphosphine)rhodium(I).


1. Collman, J. P.; Sears, C. T., Jr.; Kubota, M. Inorg. Synth. 1990, 28, 92.
2. Angoletta, M. G 1959, 89, 2359.
3. Vaska, L.; DiLuzio, J. W. JACS 1961, 83, 2784.
4. Strohmeier, W.; Fleischmann, R. JOM 1972, 42, 163 (CA 1972, 77, 113 581y).
5. Strohmeier, W.; Fleischmann, R.; Rehder-Stirnweiss, W. JOM 1973, 47, C37 (CA 1973, 78, 140 873x).
6. Strohmeier, W.; Weigelt, L. JOM 1974, 82, 417 (CA 1975, 82, 49 815b).
7. Strohmeier, W.; Grünter, K. JOM 1975, 90, C48 (CA 1975, 83, 57 993u).
8. Blum, J.; Sasson, Y.; Iflah, S. TL 1972, 1015.
9. Azran, J.; Buchman, O.; Blum, J. TL 1981, 22, 1925.
10. Blum, J.; Kraus, S.; Pickholtz, Y. JOM 1971, 33, 227.
11. Zahalka, H. A.; Alper, H.; Sasson, Y. OM 1986, 5, 2497.
12. Kopylova, L. I.; Pukhnarevich, V. B.; Voronkov, M. G. ZOB 1991, 61, 2606; JGU 1991, 61, 2418.
13. Westcott, S. A.; Marder, T. B.; Baker, R. T.; Calabrese, J. C. CJC 1993, 71, 930.
14. Westcott, S. A.; Blom, H. P.; Marder, T. B.; Baker, R. T. JACS 1992, 114, 8863.
15. Williams, A. F.; Bhaduri, S.; Maddock, A. G. JCS(D) 1975, 1958.

Stephen A. Westcott

University of North Carolina, Chapel Hill, NC, USA



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