Phosphinooxazolines
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.[1]
Synthesis
[edit]The synthesis of phosphinooxazolines is modular. Methods exist for installing the phosphine ligand before the oxazoline and the reverse.[2] 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:
- When X = fluorine coupling involves anionic displacement with a alkali metal diphenylphosphide.[3]
- 2-Bromoaryl oxazolines can be converted into a Grignard reagent, which react with chlorodiphenylphosphine.[4] Alternatively, the aryl bromide can be coupled with diphenylphosphine via a copper iodide-catalysed reaction.[5]
- Aryl oxazolines undergo directed ortho lithiation, and the resulting 2-lithio derivative then can be treated with chlorodiphenylphosphine.[6]
Of these methods, the copper iodide catalysed reaction method is popular.[7]
Catalysis
[edit]Phosphinooxazoline complexes have been widely tested in homogeneous catalysis.[8][9][10]
Allylic substitutions
[edit]PHOX-based palladium complexes catalyse enantioselective allylic substitutions.
Substitutions include allylic alkylations (Tsuji-Trost reaction),[11] aminations,[12] and sulfonylations.[13]
Heck Reaction
[edit]
Palladium complexes containing chiral phosphinooxazolines are efficient catalysts for the Heck reaction.[14] [15] 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.[16]
Asymmetric Hydrogenation
[edit]In asymmetric hydrogenation iridium complexes of phosphinooxazolines catalyse 'classic' hydrogenation.[17] Related ruthenium and palladium catalysts effect transfer hydrogenation.[1] In addition to theoretical studies,[18] the structural[19] and kinetic properties[20]
See also
[edit]Other oxazoline based ligands
- (S)-iPr-PHOX - A specific PHOX ligand
- Bisoxazolines (BOX)
- Trisoxazolines (TRISOX)
Structurally related ligands
References
[edit]- ^ a b Braunstein, Pierre; Naud, Fre´de´ric; Rettig, Steven J. (2001). "A new class of anionic phosphinooxazoline ligands in palladium and ruthenium complexes: catalytic properties for the transfer hydrogenation of acetophenone". New Journal of Chemistry. 25 (1): 32–39. doi:10.1039/b004786o. ISSN 1144-0546.
- ^ Koch, Guido; Lloyd-Jones, Guy C.; Loiseleur, Olivier; Pfaltz, Andreas; Prétôt, Roger; Schaffner, Silvia; Schnider, Patrick; von Matt, Peter (2 September 2010). "Synthesis of chiral (phosphinoaryl)oxazolines, a versatile class of ligands for asymmetric catalysis". Recueil des Travaux Chimiques des Pays-Bas. 114 (4–5): 206–210. doi:10.1002/recl.19951140413.
- ^ Peer, Markus; de Jong, Johannes C.; Kiefer, Matthias; Langer, Thomas; Rieck, Heiko; Schell, Heico; Sennhenn, Peter; Sprinz, Jürgen; Steinhagen, Henning; Wiese, Burkhard; Helmchen, Günter (1996). "Preparation of Chiral Phosphorus, Sulfur and Selenium containing 2-Aryloxazolines". Tetrahedron. 52 (21): 7547–7583. doi:10.1016/0040-4020(96)00267-0. ISSN 0040-4020.
- ^ Sprinz, Jürgen; Helmchen, Günter (1993). "Phosphinoaryl- and phosphinoalkyloxazolines as new chiral ligands for enantioselective catalysis: Very high enantioselectivity in palladium catalyzed allylic substitutions". Tetrahedron Letters. 34 (11): 1769–1772. doi:10.1016/S0040-4039(00)60774-8.
- ^ Tani, Kousuke; Behenna, Douglas C.; McFadden, Ryan M.; Stoltz, Brian M. (1 June 2007). "A Facile and Modular Synthesis of Phosphinooxazoline Ligands" (PDF). Organic Letters. 9 (13): 2529–2531. doi:10.1021/ol070884s. PMID 17536810.
- ^ Zhang, Xumu; Liu, D.; Dai, Q. (27 June 2005). "A New Class of Readily Available and Conformationally Rigid Phosphino-Oxazoline Ligands for Asymmetric Catalysis". Tetrahedron. 61 (26): 6460–6471. doi:10.1016/j.tet.2005.03.111.
- ^ Krout, M. R.; Mohr, J. T.; Stoltz, B. M. (2009). "Preparation of (S)-tert-ButylPHOX". Organic Syntheses. 86: 181. doi:10.15227/orgsyn.086.0181. PMC 2805096. PMID 20072718.
- ^ Helmchen, Günter; Pfaltz, Andreas (June 2000). "PhosphinooxazolinesA New Class of Versatile, Modular P,N-Ligands for Asymmetric Catalysis". Accounts of Chemical Research. 33 (6): 336–345. doi:10.1021/ar9900865. PMID 10891051.
- ^ Yamagishi, Takamichi; Ohnuki, Masatoshi; Kiyooka, Takahiro; Masui, Dai; Sato, Kiyoshi; Yamaguchi, Motowo (1 October 2003). "Construction of P-stereogenic center by selective ligation of N–P–N type ligands and application to asymmetric allylic substitution reactions". Tetrahedron: Asymmetry. 14 (21): 3275–3279. doi:10.1016/j.tetasy.2003.09.004.
- ^ Armstrong, Paul B.; Dembicer, Elizabeth A.; DesBois, Andrew J.; Fitzgerald, Jay T.; Gehrmann, Janet K.; Nelson, Nathaniel C.; Noble, Amelia L.; Bunt, Richard C. (2012). "Investigation of the Electronic Origin of Asymmetric Induction in Palladium-Catalyzed Allylic Substitutions with Phosphinooxazoline (PHOX) Ligands by Hammett and Swain–Lupton Analysis of the 13C NMR Chemical Shifts of the (π-Allyl)palladium Intermediates". Organometallics. 31 (19): 6933–6946. doi:10.1021/om3007163.
- ^ 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.
- ^ von Matt, Peter; Loiseleur, Olivier; Koch, Guido; Pfaltz, Andreas; Lefeber, Claudia; Feucht, Thomas; Helmchen, Gunter (1994). "Enantioselective allylic amination with chiral (phosphino-oxazoline)pd catalysts". Tetrahedron: Asymmetry. 5 (4): 573–584. doi:10.1016/0957-4166(94)80021-9.
- ^ Eichelmann, Holger; Gais, Hans-Joachim (1995). "Palladium-catalyzed asymmetric allylic sulfonylation". Tetrahedron: Asymmetry. 6 (3): 643–646. doi:10.1016/0957-4166(95)00049-U.
- ^ Loiseleur, Olivier; Hayashi, Masahiko; Keenan, Martine; Schmees, Norbert; Pfaltz, Andreas (1999-03-15). "Enantioselective Heck reactions using chiral P,N-ligands". Journal of Organometallic Chemistry. 576 (1–2): 16–22. doi:10.1016/S0022-328X(98)01049-3.
- ^ Loiseleur, Olivier; Hayashi, Masahiko; Schmees, Norbert; Pfaltz, Andreas (1 November 1997). "Enantioselective Heck Reactions Catalyzed by Chiral Phosphinooxazoline-Palladium Complexes". Synthesis. 1997 (11): 1338–1345. doi:10.1055/s-1997-1341.
- ^ Ripa, Lena; Hallberg, Anders (1997). "Intramolecular Enantioselective Palladium-Catalyzed Heck Arylation of Cyclic Enamides". The Journal of Organic Chemistry. 62 (3): 595–602. doi:10.1021/jo961832b. PMID 11671454.
- ^ Roseblade, Stephen J.; Pfaltz, Andreas (December 2007). "Iridium-Catalyzed Asymmetric Hydrogenation of Olefins". Accounts of Chemical Research. 40 (12): 1402–1411. doi:10.1021/ar700113g. PMID 17672517.
- ^ Hopmann, Kathrin Helen; Bayer, Annette (2011). "On the Mechanism of Iridium-Catalyzed Asymmetric Hydrogenation of Imines and Alkenes: A Theoretical Study". Organometallics. 30 (9): 2483–2497. doi:10.1021/om1009507.
- ^ 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.
- ^ 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.