Iridium Catalysts1


[7439-88-5]  · Ir  · Iridium  · (MW 192.22)

(moderately active hydrogenation catalysts useful for selective hydrogenation of a,b-unsaturated aldehydes to allylic alcohols;2 for stereoselective hydrogenation of C-C double bonds;3 for selective hydrogenation of aromatic nitro compounds4,5 and quinolines6)

Physical Data: mp 2454 °C; bp ca. 4800 °C; d (20 °C) 22.65 g cm-3.

Solubility: sol aqua regia after fusion with Na2O2 or a mixture of KOH and KNO3.

Form Supplied in: Ir/C (usually 5% Ir), Ir black, and IrO2 (a precursor of Ir black) are commercially available.

Preparative Methods: IrCl3.3H2O, IrCl4.xH2O, Na2IrCl6, (NH4)2IrCl6 are usual starting reagents for the preparation of supported and unsupported Ir catalysts. Unsupported Ir catalysts have been prepared by reducing IrO2 (an Adams' type) at 165 °C under a stream of hydrogen4 or by reducing Ir(OH)3 or Ir2O3.3H2O (precipitated by adding aq NaOH or LiOH to an aq solution of IrCl3.3H2O) in H2O at 80-90 °C under 8 MPa H2.7 Unsupported and supported Ir catalysts have also been prepared by the reaction of IrCl4.xH2O with NaBH4 in EtOH.8

Handling, Storage, and Precautions: Ir catalysts are rather stable and can be stored in the air without losing activity. Not attacked by any of the acids nor by aqua regia; only superficially oxidized on heating in air; oxidized at red heat to form IrO2; direct contact with explosive gas mixtures containing H2 or with alcoholic solvents, especially such as MeOH or EtOH, should be avoided.

Selective Hydrogenation of a,b-Unsaturated Aldehydes to Allylic Alcohols.

Allylic alcohols have been obtained in high yields from the hydrogenation of a,b-unsaturated aldehydes in EtOH with 5% Ir/C catalyst (eqs 1 and 2).2 With 2-propenal (acrolein), however, the yield of allyl alcohol was lower (60%).

A number of alternative methods for the enal reductions using heterogeneous catalysts have also been reported.9 Some of the effective catalyst systems are: Pt,10 Pt/C or Pt/CaCO3,9 modified by FeSO4-Zn(OAc)2; Pt-Ge-Nylon (3% Ge);11 prereduced Re2O7/pyridine;12 and Raney type Ag-Zn.13

Stereoselective Hydrogenation of C-C Double Bonds.

Ir catalysts are highly stereoselective in the hydrogenation of alkenic and aromatic compounds. Thus hydrogenation of 1,2-dimethylcyclohexene and o-xylene with Ir black in i-PrOH at 25 °C and atmospheric pressure of H2 gave cis-1,2-dimethylcyclohexane in 98.7 and 98.6% yields, respectively.14 Eq 3 compares the stereoselectivity of the six platinum metals in the hydrogenation of 1,2-dimethylcyclohexene and o-xylene (data in parentheses) in t-BuOH.3 Similarly, deuteration of D9,10-octalin with 5% Ir/C (25 °C, 2.5-2.7 MPa D2) gave cis-decalin in 97.8% selectivity.15 However, as has been observed with 1,2- and 1,6-dimethylcyclohexenes,16 the deuteration products obtained with Ir catalyst are extensively exchanged in spite of high stereoselectivity in the formation of the cis isomer. The high stereoselectivity of Ir catalysts is associated with their low alkene isomerization activity during hydrogenation.3 Excellent examples of stereoselective synthesis using Ir catalysts have been reported in the hydrogenation of the 16-methylene steroid (1) to the 16b-methyl derivative (2) (eq 4)17 and in the hydrogenation of the unsaturated ketone (3) to the saturated ketone (4) (eq 5),18 which was an important step in the total synthesis of (±)-9-isocyanopupukeanane. Both the hydrogenations gave mixtures of stereoisomers with other metal catalysts.

Selective Hydrogenation of Aromatic Nitro Compounds.

Hydrogenation of aromatic nitro compounds with Ir catalysts often slows down after the uptake of 2 mol of H2 and the corresponding hydroxylamines are obtained in good yields (eqs 6-8).4,5 Usually higher yields are obtained at neutral pH, at low temperatures, and with substrates having electron-withdrawing substituents.4,19

Hydrogenation of halonitrobenzenes with Ir catalysts is accompanied by only slight dehalogenation. For example, hydrogenation of 3,4-dichloronitrobenzene with 5% Ir/C in i-PrOH gave the corresponding dichloroaniline almost quantitatively.5 The rate of the dechlorination was only 1/250th of the rate of the hydrogenation. Other effective methods for suppressing dehalogenation have also been described using Pt/C and Ni catalysts in the presence of some inhibitors or a sulfided Pt/C catalyst.20

Selective Hydrogenation of Quinolines in the Heterocyclic Ring.

Both 2- and 4-methylquinolines are hydrogenated with iridium catalysts exclusively to the corresponding 1,2,3,4-tetrahydroquinolines without any additives (eqs 9 and 10).6 With Pd, Pt, Ru, Rh, and Ni catalysts the same selective hydrogenation can be achieved only in the presence of sulfur compounds or carbon monoxide.

1. (a) Livingstone, S. E. In Comprehensive Inorganic Chemistry; Bailar, Jr., J. C. et al., Eds.; Pergamon: Oxford, 1973; Vol. 3, pp 1163-1189, 1254-1274. (b) Rylander, P. N. Catalytic Hydrogenation in Organic Syntheses; Academic: New York, 1979. (c) Bond, G. C.; Webb, G.; Wells, P. B.; Winterbottom, J. M. J. Catal. 1962, 1, 74.
2. Bakhanova, E. N.; Astakhova, A. S.; Brikenshtein, Kh. A.; Dorokhov, V. G.; Savchenko, V. I.; Khidekel', M. L. IZV 1972, 9, 1993 (CA 1973, 78, 15 967).
3. Nishimura, S; Sakamoto, H.; Ozawa, T. CL 1973, 855.
4. Taya, K. CC 1966, 464.
5. Savchenko, V. I. et al. IZV 1978, 2329 (CA 1979, 90, 54 588).
6. Shaw, J. E.; Stapp, P. R. JHC 1987, 24, 1477.
7. (a) Takagi, Y.; Ishii, S.; Nishimura, S. BCJ 1970, 43, 917. (b) Nishimura, S.; Katagiri, M.; Watanabe, T; Uramoto, M. BCJ 1971, 44, 166.
8. (a) Brown, H. C.; Brown, C. A. JACS 1962, 84, 1494. (b) Brown, H. C.; Sivasankaran, K. JACS 1962, 84, 2828.
9. Rylander, P. N. Catalytic Hydrogenation in Organic Syntheses; Academic: New York, 1979; Chapter 5-II.
10. Tuley, W. F.; Adams, R. JACS 1925, 47, 3061.
11. Galvagno, S.; Poltarzewski, Z.; Donato, A.; Neri, G; Pietropaolo, R. CC 1986, 1729.
12. Pascoe, W. E.; Stenberg, J. F. In Catalysis in Organic Syntheses; Jones, W. H.; Ed.; Academic: New York, 1980; pp 1-9.
13. Nagase, Y.; Hattori, H.; Tanabe, K. CL 1983, 1615.
14. Nishimura, S.; Mochizuki, F.; Kobayakawa, S. BCJ 1970, 43, 1919.
15. Weitkamp, A. W. J. Catal. 1966, 6, 431.
16. Nishimura, S.; Tsuchimoto, M.; Ohkubo, K. CL 1984, 1625.
17. Gregory, G. I.; Hunt, J. S.; May, P. J.; Nice, F. A.; Phillipps, G. H. JCS(C) 1966, 2201.
18. Yamamoto, H.; Sham, H. L. JACS 1979, 101, 1609.
19. Makaryan, I. A.; Savchenko, V. I.; Brikenshtein, Kh. A. IZV 1983, 760 (CA 1983, 99, 21 744).
20. Strätz, A. M. In Catalysis of Organic Reactions; Kosak, J. E., Ed.; Dekker: New York, 1984; Chapter 17.

S. Nishimura

Tokyo University of Agriculture and Technology, Japan

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