Hydrogenation Catalysts and Ligands

Chemfiles Volume 6 Article 1

Ir and Ru Diamine diphosphine Complexes

Sigma-Aldrich is proud to offer new catalysts for hydrogenation through our collaboration with Kanata Chemical Technologies.1 The Ir and Ru complexes highlighted herein are especially active in the hydrogenation of sterically congested and electronically deactivated ketones and imines and also exhibit extraordinarily high chemoselectivity in conjugated systems (Scheme 20).2 In particular, the Ru complexes have been found to catalyze the hydrogenation of several grams of various ketones in less than 12 h under 1 to 11 atm. of H2 at ambient temperature. The presence of an amine functionality is crucial for the facile H-atom transfer under the operating hydrogen pressures; therefore, this process is best thought of as a ligand-assisted outer-sphere hydrogenation.


Scheme 20.

The ruthenium catalysts 14 and 15 are air-stable and exhibit high activities in the hydrogenation of ketones under mild conditions. Importantly, these Ru(II) catalysts are selective enough to discriminate between C=O and C=C bonds under hydrogenation conditions, with the latter functional group remaining unreduced and available for additional functionalization (Scheme 21). The iridium(III) catalysts are also air-stable and are extremely active for the transfer hydrogenation of ketones under mild reaction conditions (Scheme 22). Hydrogenation reactions with the Ru and Ir catalysts have been performed on multigram scale utilizing bench top handling procedures.


Scheme 21.

Scheme 22.

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DuPhos and BPE Phospholane Ligands

Asymmetric hydrogenation reactions represent the ideal process for the commercial manufacture of single-enantiomer compounds, because of the ease by which these robust procedures can be scaled up and because of the low levels of byproducts generated in these asymmetric hydrogenations. The most effective hydrogenation systems rely on modifications of the electronic and steric properties of the ligands. Burk and co-workers succeeded in developing a highly-effective chiral phospholane class of ligands called DuPhos and BPE that contain 2,5-disubstituted groups allowing for systematic variation of the steric environment around the metal.3 Sigma-Aldrich is pleased to now offer Me-DuPhos and Me-BPE phospholane ligands in collaboration with Kanata Chemical Technologies that can be ligated to cationic Rh complexes to afford highly active catalysts for asymmetric hydrogenation (Scheme 23).4


Scheme 23. (665258) (665266) (665231) (665207)

The large-scale capacity of these robust catalysts is observed in the efficiency (substrate-to-catalyst (S/C) ratios up to 50,000) and the high activities (TOF > 5,000 h-1) in a myriad of enamide and ketone reductions. Under optimized conditions, (R,R)-Me-BPE-Rh reduced N-acetyl α-arylenamides in >95% ee to yield valuable α-1‑arylethylamines (Scheme 24).5 It should be noted that Me-DuPhos-Rh complexes were equally effective in asymmetric reductions of prochiral enamides. The general utility of these phospholane ligands is illustrated in the profound production of a vast array of chiral compounds (Scheme 25). Sigma-Aldrich is your dedicated source for a broad spectrum of building blocks that provide essential starting materials in the synthesis of complex organic molecules. Our growing portfolio of catalysis products, supplemented by the DuPhos/BPE family, strongly complements the existing Sigma-Aldrich chemical line and will accelerate your research success.


Scheme 24.

Scheme 25.

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Materials

     

References

  1. Ru and Ir catalysts featured above are sold in agreement with Kanata Chemical Technologies for the research market only.
  2. Abdur-Rashid, K. et al. Adv. Synth. Catal. 2005, 347, 571.
  3. Burk, M. J. Acc. Chem. Res. 2000, 33, 363.
  4. Sold in collaboration with Kanata Chemical Technologies Inc. for research purposes only. These phospholane compounds were made and sold under license from E.I. du Pont de Nemours and Company, which license does not include the right to use the Compounds in producing products for sale in the pharmaceutical field.
  5. Burk, M. J. et al. J. Am. Chem. Soc. 1996, 118, 5142.

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