Crabtree's catalyst

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Crabtree's catalyst
Crabtree's catalyst
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Names
IUPAC names
(SP-4)tris(cyclohexyl)phosphane
[(1-2-η:5-6-η)-cycloocta-1,5-diene]
pyridineiridium(1+) hexafluoridophosphate(1−)
Identifiers
64536-78-3
PubChem 5702647
Properties
C31H50F6IrNP2
Molar mass 804.9026 g/mol
Melting point 150 °C (302 °F; 423 K) (decomposes)[1]
Except where otherwise noted, 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 an organoiridium compound with the formula C8H12IrP(C6H11)3C5H5N]PF6. It is a homogeneous catalyst for hydrogenation and hydrogen-transfer reactions, developed by Robert H. Crabtree. This air stable orange solid is available commercially.[2][3]

Structure and synthesis

The complex has a square planar molecular geometry, as expected for a d8 complex. It is prepared from cyclooctadiene iridium chloride dimer.[4]

Reactivity

Crabtree’s catalyst is effective for the hydrogenations of mono-, di-, tri-, and tetra-substituted substrates. Whereas Wilkinson’s catalyst and the Schrock-Osborn catalyst do not catalyze the hydrogenation of a tetrasubstituted olefin, Crabtree’s catalyst does so to at high turnover frequencies (table).[2][5]

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.[1] The reaction is robust without drying solvents or meticulous deoxygenation of the hydrogen. The catalyst is tolerant of weakly basic functional groups such as ester, but not alcohols (see below) or amines.[2] The catalyst is sensitive to proton-bearing impurities.[6]

The catalyst becomes irreversibly deactivated after about ten minutes at room temperature, signaled by appearance of yellow color. One deactivation process involves formation of hydride-bridged dimers.[7]

File:CrabtreeHydrogen.png
Crabtree's catalyst is thought to operate via an intermediate such as this: cis-[IrH2(cod)L2] (cationic charge not shown).

Other catalytic functions: isotope exchange and isomerization

Besides hydrogenation, the catalyst catalyzes the isomerization and hydroboration of alkenes.[1]

File:CrabtreeIsomerization1.png
An example of isomerization with Crabtree's catalyst. The reaction proceeds 98% to completion in 30 minutes at room temperature.


Crabtree's catalyst is 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.[8] It has been shown that isotope exchange with Crabtree’s catalyst is highly regioselective.[9][10]

Influence of directing functional groups

The hydrogenation of a terpen-4-ol demonstrates the ability of compounds with directing groups (the OH group) to undergo diastereoselective hydrogenation. With palladium on carbon in ethanol the product distribution is 20:80 favoring the cis isomer (2B in scheme 1). The polar side (with the hydroxyl group) interacts with the solvent. This is due to slight haptophilicity, an effect in which a functional group binds to the surface of a heterogeneous catalyst and directs the reaction.[11][12] In cyclohexane as solvent the distribution changes to 53:47 because haptophilicity 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.[13] Carbonyl groups are also known to direct the hydrogenation by the Crabtree catalyst to be highly regioselective.[14][15] [16]

Crabtree catalyst in hydrogenation

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

File:CrabtreeDirectingEffect.png
Directing effect of an OH group on diastereoselectivity of hydrogenation by Crabtree's catalyst. Hydrogen is added from the direction of the iridium atom, selecting for the reactivity shown above. Additional ligands on catalyst not shown.

History

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.[17] 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.[2] This displacement occurs quickly for rhodium complexes but occurs barely at all for iridium complexes.[18] 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.

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.[19]

Crabtree noted that the ligand dissociation step does not occur in heterogeneous catalysis, and so posited that this step was limiting in homogeneous systems.[2] They sought 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.

References

  1. ↑ 1.0 1.1 1.2 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
  2. ↑ 2.0 2.1 2.2 2.3 2.4 Lua error in package.lua at line 80: module 'strict' not found.
  3. ↑ Lua error in package.lua at line 80: module 'strict' not found.
  4. ↑ Crabtree, R. H., Morris, G. E. (1977). "Some diolefin complexes of Iridium(I) and a trans-Influence Series for the Complexes [IrCl(cod)L]", J. Organomet. Chem, 135(3), 395-403, doi:10.1016/S0022-328X(00)88091-2
  5. ↑ White, M. (2002, October 15). Hydrogenation. Retrieved December 1, 2014, from http://www.scs.illinois.edu/white/lectures/week5.pdf
  6. ↑ 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
  7. ↑ 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
  8. ↑ 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
  9. ↑ 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
  10. ↑ 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
  11. ↑ 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
  12. ↑ Rowlands, G. (2002, January 1). Hydrogenation. Retrieved December 1, 2014, from http://www.massey.ac.nz/~gjrowlan/oxid/hydr.pdf
  13. ↑ 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
  14. ↑ 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
  15. ↑ Lua error in package.lua at line 80: module 'strict' not found.
  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. ↑ Schrock, R.; Osborn, J. A. (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
  18. ↑ 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
  19. ↑ Journal of the Chemical Society A: Inorganic, Physical, Theoretical, 1711-1711. doi:10.1039/J19660001711
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