(1,5-Cyclooctadiene)(tricyclohexylphosphine)(pyridine)iridium(I) Hexafluorophosphate

[64536-78-3]  · C31H50F6IrNP2  · (1,5-Cyclooctadiene)(tricyclohexylphosphine)(pyridine)iridium(I) Hexafluorophosphate  · (MW 804.5)

(homogeneous catalyst for hydrogenation, especially of hindered and highly substituted alkenes;5,6 catalyst for directed hydrogenation;6 catalyst for alkene isomerization and hydroboration9)

Physical Data: mp 150 °C (dec).

Solubility: sol CH2Cl2, acetone, and THF; insol water.

Form Supplied in: yellow microcrystals; not commercially available.

Purification: recrystallization from CH2Cl2/Et2O.

Handling, Storage, and Precautions: air stable for days, but long term storage under N2 and in the dark recommended.

General Aspects.

[Ir(cod)(PCy3)(py)]PF6, a readily prepared1 complex, is one of the standard catalysts for hydrogenation and has also been used for the addition of Si-H and B-H bonds across C=C or C&tbond;C bonds. Room temperature should be employed in all stages of the preparation and use of the catalyst. Although the hexafluorophosphate salt has been used most often, the exact nature of the anion is not important as long as it is noncoordinating. The catalyst deactivates rapidly if exposed to H2 in the absence of substrate and deactivates slowly in any event; lower concentrations of catalyst (e.g. 2.5 mol %) are often most useful to give adequate catalytic rates without excessive deactivation. The catalyst also deactivates slowly in the presence of substrate, so the catalyst should be added to the substrate solution and hydrogen added promptly. It is not necessary that the solvents be specially dried or that the hydrogen be scrupulously air-free.

The cationic charge gives the catalyst special advantages over neutral catalysts such as Chlorotris(triphenylphosphine)rhodium(I): better stability to air and to oxidizing solvents, such as halocarbons, and good functional group tolerance. Its unusually high activity, even for hindered substrates, is in part a result of the noncoordinating solvent used, usually CH2Cl2, and the low ligand-to-metal (L:M) ratio. A special feature is the very high stereospecificity that can be seen when a functional group such as OH, CO2Me, or CONH2 binds to the catalyst and directs the attack to one face of the substrate. This has only been seen for catalysts with very low L:M ratios, which allow the functional group, the C=C bond, and H2 all to bind to the metal at the same time. The catalyst has so far proved most useful for alkene hydrogenation, isomerization, and hydroboration. Other applications have not yet been investigated. The closely related species (1,5-Cyclooctadiene)bis(methyldiphenylphosphine)iridium(I) Hexafluorophosphate and [Rh(cod)(Ph2P{CH2}4PPh2)]PF6 are sometimes useful for the same types of catalytic reactions, but it is not yet possible to predict which will be best in a given situation. Ideally, all three should be tried.

Isomerization of Allylic Alcohols and Ethers.

[Ir(cod)(PMePh2)2]PF6 in THF is activated by hydrogen at room temperature to give [IrH2(THF)2(PMePh2)2]PF6 which is an active isomerization catalyst, e.g. for the conversion of allylic alcohols to aldehydes or ketones; in eq 1 the reaction takes 30 min at room temperature.2

The conversion of allyl ethers to enol ethers3 can sometimes be accompanied by hydrolysis, especially on treatment with HgCl2-HgO.4 This constitutes a useful deprotection sequence for -OCH2CH=CH2 groups (eq 2).

Directed Hydrogenation of Alkenes.

[Ir(cod)(PCy3)(py)]PF6 is active for the reduction of hindered alkenes, even tri- and tetrasubstituted ones,5 while ketone, ester, nitro, amide, and aryl groups remain unaffected. The stereochemistry of the reduction product can be dominated by directing effects6 in suitable cases. This happens when neighboring carboxamide, hydroxyl, keto, methoxy, or ester groups (in approximate decreasing order of efficiency) bind to the catalyst and ensure addition of H2 from the face containing the directing group. Directivities of 0.92-1.0 are common, as shown in eq 3 where OH is the directing group. The catalyst seems to work best at low loadings, such as 2.5%. Conversions are high and isomerization is usually a minor pathway. In many cases Palladium on Carbon in EtOH gave the opposite isomer preferentially, although not exclusively.

Terpinen-4-ol is a particularly effective substrate, probably because of the favorable conformation of the OH and C=C groups shown in (1), which facilitates chelation of the Ir by simultaneous binding to both groups.

In the case of the methyl ketone shown, the catalyst shows both its directing effect and ability to reduce highly hindered alkenes (eq 4).

In indenones, the presence of a suitable hydroxyl group can even completely override the normal tendency to give a cis ring junction in the indanone product formed on hydrogenation (eq 5).6c

The directing effect of the catalyst has been useful in key steps of a number of total syntheses, such as of daunomycin7 and the pumilitoxins.8

Directed Hydroboration of Alkenes.

[Ir(cod)(PCy3)(py)]PF6 can be used at 4-5 mol % catalyst loading in the hydroboration of alkenes, a reaction which shows the same sort of directing effects as noted above for hydroboration.9

1. (a) Crabtree, R. H.; Morris, G. E. JOM 1977, 135, 395. (b) Evans, D. A.; Morrissey, M. M. JACS 1984, 106, 3866.
2. Baudry, D.; Ephritikhine, M.; Felkin, H. NJC 1978, 2, 355.
3. Baudry, D.; Ephritikhine, M.; Felkin, H. CC 1978, 694.
4. Ohvoort, J. J.; Van Boecke, C. A. A.; de Koenig, J. H.; van Boom, J. H. S 1981, 305.
5. (a) Crabtree, R. H.; Felkin, H.; Morris, G. E. JOM 1977, 141, 205. (b) Suggs, J. W.; Cox, S. D.; Crabtree, R. H.; Quirk, J. M. TL 1981, 22, 303.
6. (a) Crabtree, R. H.; Davis, M. W. OM 1983, 2, 681. (b) Crabtree, R. H.; Davis, M. W. JOC 1986, 51, 2655. (c) Evans, D. A.; Morrissey, M. M. TL 1984, 25, 4637. (d) Stork, G.; Kahne, D. E. JACS 1983, 105, 1072.
7. Mashado, A. S.; Olesker, A.; Castillon, S.; Lukacs, G. CC 1985, 330.
8. Schultz, A. G.; McCloskey, P. J.; Court, J. J. JACS 1987, 109, 6493.
9. (a) Evans, D. A.; Fu, G. C. JOC 1990, 55, 5678. (b) Evans, D. A.; Fu, G. C. JACS 1991, 113, 4042.

Robert H. Crabtree

Yale University, New Haven, CT, USA

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