Chiral Spiro Research Articles

Chiral Iridium Spiro Aminophosphine Complexes: Asymmetric Hydrogenation of Simple Ketones, Structure, and Plausible Mechanism

The iridium complexes of chiral spiro aminophophine ligands, especially the ligand with 3,5-di-tert-butylphenyl groups on the P atom (1c) were demonstrated to be highly efficient catalysts for the asymmetric hydrogenation of alkyl aryl ketones. In the presence of KOtBu as a base and under mild reaction conditions, a series of chiral alcohols were synthesized in up to 97% ee with high turnover number (TON up to 10,000) and high turnover frequency (TOF up to 3.7×10(4) h(-1)). Investigation on the structures of the iridium complexes of ligands (R)-1a and 1c by X-ray analyses disclosed that the 3,5-di-tert-butyl groups on the P-phenyl rings of the ligand are the key factor for achieving high activity and enantioselectivity of the catalyst. Study of the catalysts generated from the Ir-(R)-1c complex and H(2) by means of ESI-MS and NMR spectroscopy indicated that the early formed iridium dihydride complex with one (R)-1c ligand was the active species, which was slowly transformed into an inactive iridium dihydride complex with two (R)-1c ligands. A plausible mechanism for the reaction was also suggested to explain the observations of the hydrogenation reactions.

Enantioselective Copper-Catalyzed Intramolecular O−H Insertion: An Efficient Approach to Chiral 2-Carboxy Cyclic Ethers

A copper-catalyzed asymmetric intramolecular O-H insertion of ω-hydroxy-α-diazoesters has been accomplished by using chiral spiro bisoxazoline ligands. This highly enantioselective intramolecular O-H insertion reaction provides an efficient approach to a variety of synthetically important chiral 2-carboxy cyclic ethers with different ring sizes as well as substitution patterns.

Asymmetric Hydrogenation of Unfunctionalized Enamines Catalyzed by Iridium Complexes of Chiral Spiro N,N‐Diarylphosphoramidites

AbstractChiral spiro N,N‐diarylphosphoramidites were synthesized. These new chiral spiro monophosphoramidites were efficient ligands for iridium‐catalyzed asymmetric hydrogenation of unfunctionalized enamines derived from simple alkyl aryl ketones, providing chiral tertiary amines in good enantioselectivities (up to 90% ee).

Nickel-catalyzed enantioselective hydrovinylation of silyl-protected allylic alcohols: An efficient access to homoallylic alcohols with a chiral quaternary center

Asymmetric hydrovinylation of silyl-protected allylic alcohols catalyzed by nickel complexes of chiral spiro phosphoramidite ligands was developed. A series of homoallylic alcohols with a chiral quaternary center were produced in high yields (up to 97%) and high enantioselectivities (up to 95% ee). The reaction provides an efficient method for preparing bifunctional compounds with a chiral quaternary carbon center.

Enantioselective Nickel-Catalyzed Reductive Coupling of Alkynes and Imines

The nickel-catalyzed reductive coupling of alkynes and imines with Et(2)Zn as a reductant by using electron-rich phosphine ligands has been developed, affording various allylic amines with high yields and excellent chemoselectivities. Chiral induction was also achieved in this reductive coupling reaction when a nickel catalyst containing a chiral spiro phosphine ligand was used.

Nickel-catalyzed enantioselective hydrovinylation of silyl-protected allylic alcohols: An efficient access to homoallylic alcohols with a chiral quaternary center

An asymmetric hydrovinylation of silyl-protected allylic alcohols catalyzed by nickel complexes of chiral spiro phosphoramidite ligands was developed. A series homoallylic alchols with a chiral quaternary center were produced in high yields (up to 97%) and high enantioselectivities (up to 95% ee). The reaction provided an efficient method for preparing bifunctional compounds with a chiral quaternary carbon center.

Palladium-catalyzed asymmetric umpolung allylation of imines with allylic alcohols

A palladium-catalyzed asymmetric umpolung allylation reaction of imines with allylic alcohols has been developed. In the presence of chiral spiro phosphoramidite ligand 4, the allylation was accomplished with high yields and good enantioselectivities. The use of highly stable and easily available allylic alcohols instead of allylic metal reagents facilitated the preparation of chiral homoallylic amines.

Highly Enantioselective Hydrogenation of α-Arylmethylene Cycloalkanones Catalyzed by Iridium Complexes of Chiral Spiro Aminophosphine Ligands

The highly efficient asymmetric hydrogenation of alpha-arylmethylene cycloalkanones catalyzed by Ir-complexes of chiral spiro aminophosphine ligands was developed, providing chiral exo-cyclic allylic alcohols at high yields with excellent enantioselectivities (up to 97% ee) and high turnover numbers (S/C up to 10,000). This new reaction provided an efficient method for the synthesis of the key intermediate of the active form of the anti-inflammatory loxoprofen.

Enantioselective Hydrogenation of α-Aryloxy and α-Alkoxy α,β-Unsaturated Carboxylic Acids Catalyzed by Chiral Spiro Iridium/Phosphino-Oxazoline Complexes

The iridium-catalyzed highly enantioselective hydrogenation of alpha-aryloxy and alpha-alkoxy-substituted alpha,beta-unsaturated carboxylic acids was developed. By using chiral spiro phosphino-oxazoline ligands, the hydrogenation proceeded smoothly to produce various alpha-aryloxy- and alpha-alkoxy-substituted carboxylic acids with extremely high enantioselectivities (ee up to 99.8%) and reactivities (TON up to 10,000) under mild conditions. The hydrogenation of alpha-benzyloxy-substituted alpha,beta-unsaturated acids provided an efficient alternative for the synthesis of chiral alpha-hydroxy acids after an easy deprotection. A mechanism involving a catalytic cycle between Ir(I) and Ir(III) was proposed on the basis of the coordination model of the unsaturated acids with the iridium metal center. The rationality of the catalytic cycle, with an olefin dihydride complex as the key intermediate, was supported by the deuterium-labeling studies. The X-ray diffraction analysis of the single crystal of catalyst revealed that the rigid and sterically hindered chiral environment created by the spiro phosphino-oxazoline ligands is the essential factor that permits the catalyst to obtain excellent chiral discrimination. A chiral induction model was suggested on the basis of the catalyst structure and the product configuration.

Enantioselective Intramolecular Oxidative Aminocarbonylation of Alkenylureas Catalyzed by Palladium−Spiro Bis(isoxazoline) Complexes

An enantioselective synthesis of tetrahydropyrrolo[1,2-c]pyrimidine-1,3-diones via a palladium-catalyzed intramolecular oxidative aminocarbonylation is described. The carbon-carbon double bond of alkenylurea substrates has been shown to react intramolecularly with a nitrogen nucleophile in the presence of a palladium catalyst under a carbon monoxide atmosphere. The use of a chiral spiro bis(isoxazoline) ligand (SPRIX) is essential to obtain the desired products in optically active forms. In comparison with the coordination ability of other known ligands, this peculiar character of SPRIX originates from two structural characteristics: low sigma-donor ability of the isoxazoline coordination site and rigidity of the spiro skeleton.

From allylic alcohols to chiral tertiary homoallylic alcohol: palladium-catalyzed asymmetric allylation of isatins

A palladium-catalyzed asymmetric allylation of isatins with allylic alcohols as an allyl donor was developed by using chiral spiro phosphoramidite ligands. A variety of chiral tertiary homoallylic alcohols 3-allyl-3-hydroxy-2-oxindoles were prepared directly from allylic alcohols in one step with excellent yields and moderate enantioselectivities. This represents the first catalytic asymmetric allylation of ketones with allylic alcohol as the allylating agent.

Development of Chiral Spiro Ligands for Metal-Catalyzed Asymmetric Reactions

Abstract This account focuses our works on the development of chiral spiro ligands bearing N-heterocycles as metal-coordinating units and their applications in the metal-catalyzed asymmetric reactions. The spiro bis(isoxazoline) ligands (SPRIXs), spiro bis(isoxazole) ligands, and spiro (isoxazole–isoxazoline) ligands were readily synthesized through intramolecular double nitrile oxide cycloaddition of the corresponding dioximes as a key step. An unprecedented activation of olefins was displayed by the PdII complexes of these chiral spiro ligands in the enantioselective oxidative cyclizations, for example; the asymmetric tandem cyclization of dialkenyl alcohol via oxy-palladation produced a bicyclic ether in excellent enantioselectivity and enantioselective version of the aminocarbonylation proceeded for the first time. Other hitherto known chiral ligands we examined failed to promote these reactions.

Copper-catalyzed enantioselective carbenoid insertion into S–H bonds

An asymmetric carbenoid insertion into S-H bonds catalyzed by copper-chiral spiro bisoxazoline complexes has been developed, in which a series of alpha-mercaptoesters were produced in high yields with moderate to good enantioselectivities (up to 85% ee); this result represents the best enantioselectivity in the catalytic asymmetric carbenoid S-H bond insertion reaction.

Highly Enantioselective Copper‐Catalyzed Ring Opening of Oxabicyclic Alkenes with Grignard Reagents

A highly efficient copper-catalyzed enantioselective ring opening of oxabicylic alkenes with Grignard reagents has been developed by using chiral spiro phosphine ligands. Excellent trans selectivities, good yields, and high enantioselectivities are obtained for a broad range of Grignard reagents under mild reaction conditions. The catalyst system shows an extraordinary activity and the TON of the reaction reaches 9000.

Copper‐Catalyzed Highly Enantioselective Carbenoid Insertion into SiH Bonds

Complete control: A highly efficient, copper-catalyzed, asymmetric carbenoid insertion into SiH bonds, using chiral spiro diimine (SIDIM) ligands (see scheme, Ar=2,6-Cl2C6H3, OTf=trifluoromethanesulfonate), produces a wide range of α-silylesters, in high yields with excellent enantiomeric excesses. This catalytic system offers unprecedented levels of enantiocontrol for the SiH bond insertion reaction.

Chiral Diphosphine and Monodentate Phosphorus Ligands on a Spiro Scaffold for Transition-Metal-Catalyzed Asymmetric Reactions

The preparation of chiral compounds in enantiomerically pure form is a challenging goal in modern organic synthesis. The use of chiral metal complex catalysis is a powerful, economically feasible tool for the preparation of optically active organic compounds on both laboratory and industrial scales. In particular, the metals coordinated by one or more chiral phosphorus ligands exhibit amazing enantioselectivity and reactivity. Many chiral phosphorus ligands have been synthesized and used in transition-metal-catalyzed asymmetric reactions in past decades. However, a large number of reactions still lack effective chiral ligands, and the enantioselectivities in many reactions are substrate-dependent. The development of effective chiral phosphorus ligands, especially ligands having novel chiral backbones, is still an important task in the area of asymmetric catalysis. Molecules containing a spirocyclic framework are ubiquitous in nature. The synthesis of molecules with this spiro structure can be traced back to 100 years ago. However, the use of this spirocyclic framework to construct chiral phosphorus ligands is a recent event. This Account outlines the design and synthesis of a new family of chiral spiro phosphorus ligands including spiro diphosphines and spiro monodentate phosphorus ligands with 1,1'-spirobiindane and 9,9'-spirobifluorene backbone and their applications in transition-metal-catalyzed asymmetric hydrogenation and carbon-carbon bond formation reactions. The chiral spiro diphosphine lgands SDP with a 1,1'-spirobiindane backbone and SFDP with a 9,9'-spirobifluorene backbone, and the spiro monophosphorus ligands including phosphoramidites, phosphites, phosphonites, and phospholane with a 1,1'-spirobiindane backbone were synthesized in good yields from enantiomerically pure 1,1'-spirobiindane-7,7'-diol and 9,9'-spirobifluoren-1,1'-diol. The ruthenium complexes of chiral spiro diphosphine ligands proved to be very effective catalysts for asymmetric hydrogenations of ketones, alpha-arylaldehydes and alpha,beta-unsaturated acids. The rhodium complexes of chiral spiro monophosphorus ligands are highly enantioselective for the asymmetric hydrogenations of alpha- and beta-dehydroamino acid derivatives, alpha-arylethenyl acetamides and non- N-acyl enamines. The spiro monophosphorus ligands were demonstrated to be highly efficient for the Rh-catalyzed asymmetric addition of arylboronic acids to aldehydes and N-tosylarylimines, Pd-catalyzed asymmetric allylation of aldehydes with allylic alcohols, Cu-catalyzed asymmetric ring opening reactions with Grignard reagents, and Ni-catalyzed asymmetric hydrovinylation of styrene derivatives with ethylene. The chiral spiro phosphorus ligands show high enantioselectivities for a wide range of transition-metal-catalyzed asymmetric reactions. In most of these transformations, the enantioselectivities of spiro phosphorus ligands are superior to those obtained by using the corresponding phosphorus ligands with other backbones. These results arise from the intriguing chiral inducement of spiro structures of the ligands.

Recent developments on chiral ionic liquids: Design, synthesis, and applications

The recent progress in chiral ionic liquids with respect to their syntheses and applications in enantioselective reactions and chiral recognition is described. In addition to the conventional chiral ionic liquids derived from chiral natural products, a library of novel chiral spiro compounds, including spiro bis(pyridinium) and spiro bis(imidazolium) salt, is also described.

Synthesis of novel spiro imidazolium salts as chiral ionic liquids

A library of 13 novel chiral spiro imidazolium salts has been synthesized. The effects of N-substituents and counteranions on the melting point of spiro bis(imidazolium) salts are studied in efforts toward the development of room temperature chiral ionic liquids.

Highly Enantioselective Insertion of Carbenoids into O−H Bonds of Phenols: An Efficient Approach to Chiral α-Aryloxycarboxylic Esters

A highly efficient copper-catalyzed asymmetric carbenoid insertion into O−H bonds of phenols has been realized by using chiral spiro bisoxazoline ligands, affording α-aryloxypropionates and the related propionic acids in high yields with excellent enantiomeric excesses.

Enantioselective Glyoxylate‐ene Reaction Using a Novel Spiro Bis(isoxazoline) Ligand in Copper Catalysis.

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