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Neutral iridium catalysts with chiral phosphine-carboxy ligands for asymmetric hydrogenation of unsaturated carboxylic acids † † Electronic supplementary information (ESI) available: For CIF data of ( S )- 2d , experimental procedures, and characterization data. CCDC 1498903 . For ESI and crystallographic data in CIF or other electronic format see DOI: 10.1039/c6sc03764j Click here for additional data file. Click here for additional data file.

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ABSTRACT

We developed neutral iridium catalysts with chiral spiro phosphine-carboxy ligands (SpiroCAP) for asymmetric hydrogenation of unsaturated carboxylic acids. Different from the cationic Crabtree-type catalysts, the iridium catalysts with chiral spiro phosphine-carboxy ligands are neutral and do not require the use of a tetrakis[3,5-bis(trifluoromethyl)phenyl]borate (BArF–) counterion, which is necessary for stabilizing cationic Crabtree-type catalysts. Another advantage of the neutral iridium catalysts is that they have high stability and have a long lifetime in air. The new iridium catalysts with chiral spiro phosphine-carboxy ligands exhibit unprecedented high enantioselectivity (up to 99.4% ee) in the asymmetric hydrogenations of various unsaturated carboxylic acids, particularly for 3-alkyl-3-methylenepropionic acids, which are challenging substrates for other chiral catalysts.

No MeSH data available.


Synthesis of iridium complexes of chiral spiro phosphine-carboxy ligands. The crystal structure of (S)-2d (hydrogen atoms omitted for clarity). Selected bond lengths (Å) and angles (°) for (S)-2d: Ir–O 2.085(4), Ir–P 2.3516(17); O–Ir–P 90.30(15).
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sch1: Synthesis of iridium complexes of chiral spiro phosphine-carboxy ligands. The crystal structure of (S)-2d (hydrogen atoms omitted for clarity). Selected bond lengths (Å) and angles (°) for (S)-2d: Ir–O 2.085(4), Ir–P 2.3516(17); O–Ir–P 90.30(15).

Mentions: Recently, we developed iridium catalysts with chiral spiro phosphine–oxazoline ligands and chiral spiro aminophosphine ligands, which show high enantioselectivity for the asymmetric hydrogenation of unsaturated carboxylic acids.4 Mechanistic studies revealed that the carboxy group of the substrate anchors the molecule to the catalyst and triggers the hydrogenation reaction. On the basis of these results, we envisioned that iridium complexes with a chiral spiro phosphine-carboxy ligand might efficiently catalyze the asymmetric hydrogenation of unsaturated carboxylic acids. Carboxy groups can be introduced into chiral ligands to tune the electronic and steric properties of catalysts and thus offer new opportunities to achieve asymmetric hydrogenation of challenging substrates. Herein, we report the preparation of iridium complexes with chiral spiro phosphine-carboxy ligands (2, Scheme 1) and the use of the complexes for the hydrogenation of unsaturated carboxylic acids. These neutral iridium catalysts have several advantages: (1) they do not require the use of a tetrakis[3,5-bis(trifluoromethyl)phenyl]borate (BArF–, has a molecular weight of 863) counterion, which is necessary for stabilizing chiral cationic Crabtree-type catalysts (also called Pfaltz catalysts);5 (2) they have high stability and have a long lifetime in air; and (3) they exhibit unprecedented high enantioselectivity (up to 99.4% ee) in the asymmetric hydrogenation of various unsaturated carboxylic acids, particularly for 3-alkyl-3-methylenepropionic acids, which are challenging substrates.


Neutral iridium catalysts with chiral phosphine-carboxy ligands for asymmetric hydrogenation of unsaturated carboxylic acids † † Electronic supplementary information (ESI) available: For CIF data of ( S )- 2d , experimental procedures, and characterization data. CCDC 1498903 . For ESI and crystallographic data in CIF or other electronic format see DOI: 10.1039/c6sc03764j Click here for additional data file. Click here for additional data file.
Synthesis of iridium complexes of chiral spiro phosphine-carboxy ligands. The crystal structure of (S)-2d (hydrogen atoms omitted for clarity). Selected bond lengths (Å) and angles (°) for (S)-2d: Ir–O 2.085(4), Ir–P 2.3516(17); O–Ir–P 90.30(15).
© Copyright Policy - open-access
Related In: Results  -  Collection

License
Show All Figures
getmorefigures.php?uid=PMC5384563&req=5

sch1: Synthesis of iridium complexes of chiral spiro phosphine-carboxy ligands. The crystal structure of (S)-2d (hydrogen atoms omitted for clarity). Selected bond lengths (Å) and angles (°) for (S)-2d: Ir–O 2.085(4), Ir–P 2.3516(17); O–Ir–P 90.30(15).
Mentions: Recently, we developed iridium catalysts with chiral spiro phosphine–oxazoline ligands and chiral spiro aminophosphine ligands, which show high enantioselectivity for the asymmetric hydrogenation of unsaturated carboxylic acids.4 Mechanistic studies revealed that the carboxy group of the substrate anchors the molecule to the catalyst and triggers the hydrogenation reaction. On the basis of these results, we envisioned that iridium complexes with a chiral spiro phosphine-carboxy ligand might efficiently catalyze the asymmetric hydrogenation of unsaturated carboxylic acids. Carboxy groups can be introduced into chiral ligands to tune the electronic and steric properties of catalysts and thus offer new opportunities to achieve asymmetric hydrogenation of challenging substrates. Herein, we report the preparation of iridium complexes with chiral spiro phosphine-carboxy ligands (2, Scheme 1) and the use of the complexes for the hydrogenation of unsaturated carboxylic acids. These neutral iridium catalysts have several advantages: (1) they do not require the use of a tetrakis[3,5-bis(trifluoromethyl)phenyl]borate (BArF–, has a molecular weight of 863) counterion, which is necessary for stabilizing chiral cationic Crabtree-type catalysts (also called Pfaltz catalysts);5 (2) they have high stability and have a long lifetime in air; and (3) they exhibit unprecedented high enantioselectivity (up to 99.4% ee) in the asymmetric hydrogenation of various unsaturated carboxylic acids, particularly for 3-alkyl-3-methylenepropionic acids, which are challenging substrates.

View Article: PubMed Central - PubMed

ABSTRACT

We developed neutral iridium catalysts with chiral spiro phosphine-carboxy ligands (SpiroCAP) for asymmetric hydrogenation of unsaturated carboxylic acids. Different from the cationic Crabtree-type catalysts, the iridium catalysts with chiral spiro phosphine-carboxy ligands are neutral and do not require the use of a tetrakis[3,5-bis(trifluoromethyl)phenyl]borate (BArF–) counterion, which is necessary for stabilizing cationic Crabtree-type catalysts. Another advantage of the neutral iridium catalysts is that they have high stability and have a long lifetime in air. The new iridium catalysts with chiral spiro phosphine-carboxy ligands exhibit unprecedented high enantioselectivity (up to 99.4% ee) in the asymmetric hydrogenations of various unsaturated carboxylic acids, particularly for 3-alkyl-3-methylenepropionic acids, which are challenging substrates for other chiral catalysts.

No MeSH data available.