Crabtree's catalyst

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Crabtree's catalyst
Crabtree's catalyst
Crabtree's-catalyst-cation-3D-sticks.png
Identifiers
CAS number 64536-78-3
Properties
Molecular formula C31H50F6IrNP2
Molar mass 804.9026 g/mol
Except where noted otherwise, data are given for materials in their standard state (at 25 °C (77 °F), 100 kPa)
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Infobox references

Crabtree's catalyst is the name given to a complex of iridium with 1,5-cyclooctadiene, tris-cyclohexylphosphine, and pyridine. It is a homogeneous catalyst for hydrogenation reactions, developed by Robert H. Crabtree, a professor at Yale University. The iridium atom in the complex has a square planar molecular geometry, as expected for a d8 complex.[1][2] Crabtree’s catalyst is air stable and available commercially as orange crystals.[3]

Development of Catalyst[edit]

Crabtree, graduate student George Morris and John Derek Woollins discovered this catalyst in the 1970s while working on iridium analogues of Wilkinson's rhodium-based catalyst at the Institut de Chimie des Substances Naturelles at Gif-sur-Yvette, near Paris.

Previous hydrogenation catalysts included Wilkinson’s catalyst and a cationic rhodium(I) complex with two phosphine groups developed by Osborn and Schrock.[4] These catalysts accomplished hydrogenation through displacement; after hydrogen addition across the metal, a solvent or a phosphine group dissociated from the rhodium metal so the olefin to be hydrogenated could gain access to the active site.[1] This displacement occurs quickly for rhodium complexes but occurs barely at all for iridium complexes.[5] Because of this, research at the time focused on rhodium compounds instead of compounds involving transition metals of the third row, like iridium. Wilkinson, Osborn, and Schrock also only used coordinating solvents.

Ligand dissociation step in Wilkinson catalysis (displacement by olefin). L’ can either be solvent of PPh3.[6]

Crabtree noted that the ligand dissociation step was not present in heterogeneous catalysis, and so posited that this step was limiting in homogeneous systems.[1] Because of this, he decided to study catalysts with “irreversibly creat[ed] active sites in a noncoordinating solvent.” This led to the development of the Crabtree catalyst, and use of the solvent CH2CL2, which was found to be noncoordinating.

Reactivity[edit]

Crabtree’s catalyst has a much higher turnover frequency than competing reactions for the hydrogenations of mono-, di-, tri-, and tetra-substituted substrates. In fact, while Wilkinson’s catalyst and the Schrock-Osborn catalyst are unable to catalyze the hydrogenation of a tetrasubstituted olefin, Crabtree’s catalyst is able to at high turnover frequencies, as displayed in the table.[1][7]

Substrate Wilkinson's Catalyst Schrock-Osborn Catalyst Crabtree's Catalyst
1-hexene 650 4000 6400
cyclohexene 700 10 4500
1-methylcyclohexene 13 -- 3800
tetramethylethylene -- -- 4000

The catalyst is reactive at room temperature.[8] The reaction is robust without drying solvents or meticulously removing air from added hydrogen. The catalyst is tolerant of functional groups like CO2R but not good ligands like CH2OH or strong bases like NR2.[1] The catalyst is sensitive to proton-bearing impurities.[9]

The catalyst becomes deactivated with time (roughly ten minutes, depending on the catalyst) due to an irreversible deactivation process. The catalysts turn yellow upon deactivation. Deactivation occurs due to the formation of coordination complexes with other molecules of the catalyst. The catalyst, as shown in the figure, forms a dimeric cation with another molecule of the catalyst through 3-center, 2-electron bonds between the Iridium metal centers and hydrogen atoms.[10] This can be prevented by keeping catalyst molecules further apart in solution - for example, by adding dropwise quantities of a dilute catalyst solution into the reaction as it proceeds.

Complexing of Iridium in crabtree’s catalyst with hydrogens via 3-center 2-electron bonds. Other substituents on iridum metals are not shown here.

An intermediate of the hydrogenation reaction catalysed by crabtree’s catalyst was able to be isolated.[11] After exposure to hydrogen gas, the red solution of catalyst precursor, [Ir(cod)L2]PF6, in dichloromethane turns colorless; this solution is stable at -80C for several hours.[1] The isolated intermediate is cis-[IrH2(cod)L2]PF6. It appears that the rest of the reaction proceeds through [IrH2(olefin)2L2]+, which collapses to form an alkane without further steps.[1] In this way the catalyst resembles a heterogenous catalyst more than a homogeneous one.

cis-[IrH2(cod)L2]PF6

Besides hydrogenation, the catalyst has been used for isomerization and hydroboration.[8]

An example of isomerization with Crabtree's catalyst. The reaction proceeds 98% to completion in 30 minutes at room temperature.
An example of somerization with Crabtree's Catalyst. The reaction proceeds 98% to completion in 30 minutes at room temperature.

Crabtree's catalyst can also be used in isotope exchange reactions. More specifically, it catalyzes the direct exchange of a hydrogen atom with its isotopes deuterium and tritium, without the use of an intermediate.[12] It has been shown that isotope exchange with Crabtree’s catalyst is highly regioselective.[13][14]

Enantioselectivity[edit]

Hydrogenation by Crabtree’s catalyst is an Enantioselective synthesis.[15] Because Crabtree’s catalyst is coordinatively unsaturated, it is able to bind and be directed by ligating groups, particularly OH.[16]

The hydrogenation of a certain terpen-4-ol is illustrative of this enantioselectivity, and the differences in enantioselectivity of different catalysts. With palladium on carbon in ethanol the product distribution is 20:80 in favor of the cis isomer (2B in scheme 1). The polar side (with the hydroxyl group) interacts with the solvent leaving the apolar to the catalyst surface. This is due to slight haptophilicty, an effect in which a functional group binds to the surface of a heterogeneous catalyst and directs the reaction.[17][18] In cyclohexane as solvent the distribution changes to 53:47 because haptophilicty is no long present (there is no directing group on cyclohexane). The distribution changes completely in favor of the cis isomer 2A when Crabtree's catalyst is used in dichloromethane. This selectivity is both predictable and practically useful.[19] Carbonyl groups are also known to direct the hydrogenation by the Crabtree catalyst is highly regioselective.[20]

Crabtree catalyst in hydrogenation

The directing effect that causes the enantioselectivity of hydrogenation of terpen-4-ol with Crabtree’s catalyst is shown below.

Directing effect of an OH group on enantioselectivity of hydrogenation by Crabtree's catalyst. Hydrogen is added from the direction of the iridium atom, selecting for the enantiomer shown above. Additional ligands on catalyst not shown.
Demonstration of the directing effect of a carbonyl group on hydrogenation with Crabtree's catalyst. Additional ligands on catalyst not shown.

References[edit]

  1. ^ a b c d e f g Crabtree, R. H. (1979). "Iridium compounds in catalysis". Acc. Chem. Res 12 (9): 331–337. doi:10.1021/ar50141a005. 
  2. ^ Brown, J. M. (1987). "Directed Homogeneous Hydrogenation". Angew. Chem. Int. Ed. 26 (3): 190–203. doi:10.1002/anie.198701901. 
  3. ^ Ir(cod)py(PCy 3 )]PF 6 Crabtree's catalyst - Homogeneous. (n.d.). Retrieved December 1, 2014, from http://jmcct.com/products-services/product_p64.html
  4. ^ Schrock, R., & Osborn, J. (1976). Catalytic hydrogenation using cationic rhodium complexes. I. Evolution of the catalytic system and the hydrogenation of olefins. Journal of the American Chemical Society, 98(8), 2134-2143. doi:10.1021ja0042a020
  5. ^ Osborn, J., & Shapley, J. (1970). Rapid intramolecular rearrangements in pentacoordinate transition metal compounds. Rearrangement mechanism of some fluxional iridium(I) complexes. Journal of the American Chemical Society, 92(23), 6976-6978. doi:10.1021/ja00726a047
  6. ^ Osborn, J., Jardine, F., Young, J., & Wilkinson, G. (1966). The preparation and properties of tris(triphenylphosphine)halogenorhodium(I) and some reactions thereof including catalytic homogeneous hydrogenation of olefins and acetylenes and their derivatives. Journal of the Chemical Society A: Inorganic, Physical, Theoretical, 1711-1711. doi:10.1039/J19660001711
  7. ^ White, M. (2002, October 15). Hydrogenation. Retrieved December 1, 2014, from http://www.scs.illinois.edu/white/lectures/week5.pdf
  8. ^ a b Crabtree, R. H. 2001. (1,5-Cyclooctadiene)(tricyclohexylphosphine)(pyridine)iridium(I) Hexafluorophosphate. e-EROS Encyclopedia of Reagents for Organic Synthesis. doi:10.1002/047084289X.rc290m
  9. ^ Mingos, M., Brown, J., & Xu, Y. (2008). Crabtree’s catalyst revisited; Ligand effects on stability and durability.Chemical Communication, 199-201. doi:10.1039/b711979h
  10. ^ Crabtree, R., Felkin, H., & Morris, G. (1977). Cationic iridium diolefin complexes as alkene hydrogenation catalysts and the isolation of some related hydrido complexes. Journal of Organometallic Chemistry, 141, 205-215. doi:10.1016/S0022-328X(00)92273-3
  11. ^ Crabtree, R., Felkin, H., & Morris, G. (1976). Activation of molecular hydrogen by cationic iridium diene complexes. Journal of the Chemical Society, Chemical Communications, 716-717. doi:10.1039/C39760000716
  12. ^ Schou, S. (2009). The effect of adding Crabtree's catalyst to rhodium black in direct hydrogen isotope exchange reactions. Journal of Labelled Compounds and Radiopharmaceuticals, 52, 376-381. doi: 10.1002/jlcr.1612
  13. ^ Valsborg, J., Sorensen, L., & Foged, C. (2001). Organoiridium catalysed hydrogen isotope exchange of benzamide derivatives. Journal of Labelled Compounds and Radiopharmaceuticals, 44, 209-214. doi: 10.1002/jlcr.446
  14. ^ Hesk, D., Das, P., & Evans, B. (1995). Deuteration of acetanilides and other substituted aromatics using [Ir(COD)(Cy3P)(Py)]PF6 as catalyst. Journal of Labelled Compounds and Radiopharmaceuticals, 36(5), 497-502. doi: 10.1002/jlcr.2580360514
  15. ^ Crabtree, R.H. Davis, M. W. (1986). "Directing effects in homogeneous hydrogenation with [Ir(cod)(PCy3)(py)]PF6". J. Org. Chem. 51 (14): 2655–2661. doi:10.1021/jo00364a007. 
  16. ^ Crabtree, R., & Davis, M. (1983). Occurrence and origin of a pronounced directing effect of a hydroxyl group in hydrogenation with [Ir(cod)P(C6H11)3(py)]PF6. Organometallics, 2, 681-682. doi: 10.1021/om00077a019
  17. ^ Thompson, H., & Naipawer, R. (1973). Stereochemical control of reductions. III. Approach to group haptophilicities. Journal of the American Chemical Society, 95(19), 6379-6386. doi: 10.1021/ja00800a036
  18. ^ Rowlands, G. (2002, January 1). Hydrogenation. Retrieved December 1, 2014, from http://www.massey.ac.nz/~gjrowlan/oxid/hydr.pdf
  19. ^ Brown, J. (1987). Directed Homogeneous Hydrogenation[New Synthetic Methods(65)]. Angewandte Chemie International Edition in English, 26(3), 190-203. doi: 10.1002/anie.198701901
  20. ^ Schultz, A., & Mccloskey, P. (1985). Carboxamide and carbalkoxy group directed stereoselective iridium-catalyzed homogeneous olefin hydrogenations. The Journal of Organic Chemistry, 50(26), 5905-5907. 10.1021/jo00350a105