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Phosphinooxazolines

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Chiral phosphinooxazoline (PHOX) in its free and coordinated forms

Phosphinooxazolines (often abbreviated PHOX) are a class of chiral ligands used in asymmetric catalysis. Colorless solids, PHOX ligands feature a tertiary phosphine group, often diphenyl, and an oxazoline ligand in the ortho position. The oxazoline, which carries the stereogenic center, coordinates through nitrogen, the result being that PHOX ligands are P,N-chelating ligands. Most phosphine ligands used in asymmetric catalysis are diphosphines, so the PHOX ligands are distinctive. Some evidence exists that PHOX ligands are hemilabile.

Synthesis

The synthesis of phosphinooxazolines is modular. Methods exist for installing the phosphine ligand before the oxazoline and the reverse. Commonly a phenyloxazoline is combined with a source of diphenylphosphine. Methods for doing this depend on the nature of the substituent in the X position:

Of these methods, the copper iodide catalysed reaction method is popular.

Catalysis

Phosphinooxazoline complexes have been widely tested in homogeneous catalysis.

Allylic substitutions

PHOX-based palladium complexes catalyse enantioselective allylic substitutions.

Substitutions include allylic alkylations (Tsuji-Trost reaction), aminations, and sulfonylations.

Heck Reaction

Palladium complexes containing chiral phosphinooxazolines are efficient catalysts for the Heck reaction. Pd-PHOX catalysts have also been used for intramolecular Heck reactions and examples exist where they have been shown to be superior to more common ligands such as BINAP.

Asymmetric Hydrogenation

Further information: Asymmetric hydrogenation § P,N and P,O ligands

In asymmetric hydrogenation iridium complexes of phosphinooxazolines catalyse 'classic' hydrogenation. Related ruthenium and palladium catalysts effect transfer hydrogenation. In addition to theoretical studies, the structural and kinetic properties

See also

Other oxazoline based ligands

Structurally related ligands

References

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  11. Wiese, Burkhard; Helmchen, Günter (1998). "Chiral phosphinooxazolines with a bi- or tricyclic oxazoline moiety - applications in Pd-catalyzed allylic alkylations". Tetrahedron Letters. 39 (32): 5727–5730. doi:10.1016/S0040-4039(98)01173-3.
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  19. Smidt, Sebastian P.; Pfaltz, Andreas; Martínez-Viviente, Eloísa; Pregosin, Paul S.; Albinati, Alberto (2003). "X-ray and NOE Studies on Trinuclear Iridium Hydride Phosphino Oxazoline (PHOX) Complexes". Organometallics. 22 (5): 1000–1009. doi:10.1021/om020805a.
  20. Smidt, Sebastian P.; Zimmermann, Nicole; Studer, Martin; Pfaltz, Andreas (2004). "Enantioselective Hydrogenation of Alkenes with Iridium–PHOX Catalysts: A Kinetic Study of Anion Effects". Chemistry: A European Journal. 10 (19): 4685–4693. doi:10.1002/chem.200400284. PMID 15372652.
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