Synthesis and application of phosphinoferrocenylaminophosphine ligands for asymmetric catalysis

A new class of bidentate ligands utilizing a phosphine-aminophosphine structure has been prepared on a ferrocenylethyl backbone in a straightforward and scalable fashion from acetylferrocene. The unique property of the alpha-ferrocenyl carbonium ion that allows the replacement of a variety of "leaving groups" with retention of configuration greatly facilitates the synthesis, and a number of ligands have been prepared by varying the nitrogen and phosphorus substituents on the aminophosphine. These readily prepared phosphinoferrocenylaminophosphines, known as BoPhoz ligands, show surprising hydrolytic and air stability, with no degradation after 3 years open to the air. The rhodium complexes of these ligands show exceedingly high enantioselectivities (generally >95% ee) and activities often in excess of 50,000 catalyst turnovers per hour for the asymmetric hydrogenation of a wide variety of dehydro-alpha-amino acid and itaconic acid derivatives. They also show high activity and good to excellent enantioselectivity for the hydrogenation of a number of alpha-ketoesters.     

Synthesis of novel chiral phosphinocyrhetrenyloxazoline ligands and their application in asymmetric catalysis

Several novel planar chiral phosphinocyrhetrenyloxazolines have been synthesized, and their catalytic activities have been evaluated in a variety of asymmetric catalytic reactions. Preferable effects as compared to their ferrocenyl analogues have been observed in asymmetric allylic amination and asymmetric hydrosilylation, and up to 97% ee and 72% ee were reached, respectively. The Lewis basicity of the phosphorus on the ferrocene and the cyrhetrene, which contributes to their different behavior in catalysis, has been deduced by 31P NMR spectroscopy analysis, as indicated by 1J(77Se-31P) in the corresponding phosphine selenides.     

Synthesis of various camphor-based chiral pyridine derivatives

(+)-β-Hydroxymethylenecamphor 1 and enamines 2a-e were transformed into chiral camphor-based pyridine derivatives 3a-e via a tandem condensation reaction in good yields.     

Synthesis of phosphorylated sulfoximines and sulfinamides and their application as ligands in asymmetric metal catalysis

Starting from inexpensive materials and following simple protocols various N-phosphorylated sulfoximines and sulfinamides have been synthesized. The newly prepared compounds were then applied as chiral ligands in asymmetric transition metal catalysis. Phosphorus triamide-type ligands derived from (S)-glutamic acid were found to be the most efficient stereoselectors in enantioselective palladium-catalyzed allylic substitutions (up to 97% ee). On the other hand, diamidophosphite-type structures stemming from (S)-proline were the best ligands in rhodium-catalyzed hydrogenation reactions (up to 84% ee).     

Design and application of linked-BINOL chiral ligands in bifunctional asymmetric catalysis

The design and application of linked-BINOLs investigated in our group are reviewed. Linked-BINOLs are a kind of semi-crown ether, thus they are flexible and applicable to metals having various ionic radii (Ga3+, Li+, Zn2+, In3+, La3+, and Y3+). The flexible linker segment, containing a coordinative heteroatom, has a crucial role in the construction of a unique and effective chiral environment that is not accessible from BINOL itself. Applications of linked-BINOLs to an epoxide opening reaction, Michael reactions, a direct aldol reaction, direct Michael reactions of a hydroxyketone, and direct Mannich-type reactions of hydroxyketones and N-acylpyrrole are described.     

Pyroglutamate-derived hydroxyamide ligands: synthesis and application to asymmetric catalysis

The synthesis of a series of chiral hydroxy amide ligands is described. These ligands were used in a ruthenium-catalysed transfer hydrogenation reaction where one ligand gave the product in 72% ee. The ligands were also used in two titanium-catalysed reactions, an alkylation where ee’s of up to 74% were achieved and a phenyl acetylene addition where more modest selectivities were observed.     

Chiral phosphine-phosphoramidite ligands in asymmetric catalysis

Chiral phosphine-phosphoramidite ligands, featuring ready availability, easy modification, and stability towards air and moisture, have recently emerged as a new kind of robust ligands for asymmetric catalysis. They have been found to display wide utilities in various catalytic asymmetric reactions, giving excellent enantioselectivities in the Rh-, Ru-, and Ir-catalyzed asymmetric hydrogenation of C?C, C?O, and C?N double bonds; Rh-catalyzed asymmetric hydroformylation; Pd-catalyzed asymmetric hydrophosphorylation; Pd-catalyzed asymmetric allylic alkylation; Ag-catalyzed asymmetric [3 + 2] cycloaddition; and Cu-catalyzed conjugate addition and reduction. In this review, the progress on the development of chiral phosphine-phosphoramidite ligands in asymmetric catalysis has been summarized.     

手性胺膦配体在不对称催化中的应用

利用邻二苯基膦苯甲醛分别与多种手性二胺的缩合反应,设计合成了一系列新型手性四齿胺膦配体.这类多齿胺膦配体含有两个软的磷原子和两个硬的氮原子,具有丰富的配位化学性能和优秀的不对称诱导能力.本文综述了手性胺膦金属络合物催化剂在不对称转移氢化反应、氧化动力学拆分反应、烯烃的不对称环氧化反应和不对称环丙烷化反应、不对称D-A反应中的应用.     

Novel nitrogen ligands based on imidazole derivatives and their application in asymmetric catalysis

Recently prepared chiral amines have been used in the preparation of novel tridentate ligands based on an imidazole ring with an additional (hetero)ring. The synthesis was carried out by the reaction of chiral amines with suitable aldehydes (2-phenylimidazole-4-carbaldehyde, 2-hydroxybenzaldehyde or pyridine-2-carbaldehyde) under reductive conditions (H₂/Pd or NaBH₄). All ligands prepared showed strong hydrogen bonds in d₆-DMSO solution, which resulted in hindered imidazole tautomerism. The observed hindered tautomerism was studied by ¹H NMR spectroscopy. The structures of the prepared ligands were also confirmed by APCI mass spectroscopy. Both chiral amines and tridentate compounds have been applied as ligands in copper (II)-catalyzed nitroaldol reactions (Henry reaction). Various reaction conditions for the Henry reaction have been studied (influence of temperature, molar ratio, solvent or copper (II) precursors). The compounds prepared with the two imidazole rings showed fast reaction times and a reversal in enantioselectivity compared to other chiral amines.     

Synthesis, resolution and racemisation studies of new tridentate ligands for asymmetric catalysis

A method for the high-yielding preparation of two tridentate, isoquinoline-derived ligands, involving successive Suzuki cross-coupling reactions, is described. The first ligand could be resolved via molecular complexation with N-benzylcinchonidinium chloride, while the second was resolved by chromatographic separation of its epimeric camphorsulfonates. The barrier to rotation about the central biaryl axis was evaluated via racemisation studies, and the absolute configuration assigned by X-ray crystallography.     

Resolution of racemic 2-aminocyclohexanol derivatives and their application as ligands in asymmetric catalysis

A preparatively easy and efficient protocol for the resolution of racemic 2-aminocyclohexanol derivatives is described, delivering both enantiomers with >99% enantiomeric excess (ee) by sequential use of (R)- and (S)-mandelic acid. A simple aqueous workup procedure permits the isolation of the amino alcohols in analytically pure form and the almost quantitative recovery of mandelic acid. Debenzylation of enantiopure trans-2-(N-benzyl)amino-1-cyclohexanol by hydrogenation and subsequent derivatization give access to a broad variety of diversely substituted derivatives. Furthermore, the corresponding cis isomers are readily available. Applications of these optically active aminocyclohexanols in catalyzed asymmetric phenyl transfer reactions to benzaldehydes and transfer hydrogenations of aryl ketones lead to products with up to 96% ee.     

Aminophosphine–oxazoline and phosphoramidite–oxazoline ligands and their application in asymmetric catalysis

The synthesis of a novel series of aminophosphine–oxazoline and phosphoramidite–oxazoline is described. The efficacy of these aminophosphine–oxazoline ligands was investigated in the palladium catalysed asymmetric allylic alkylation of 1,3-diphenylprop-2-enyl acetate leading to a maximum of 38% ee at 64% conversion. Phosphoramidite–oxazoline ligands, however, gave ees of up to 87% at 71% conversion in the same reaction and also proved to be effective in the palladium catalysed asymmetric Suzuki coupling between 2-methylnaphthylboronic acid and 1-bromonaphthalene, leading to a maximum of 46% ee in 54% isolated yield at room temperature.     

Chiral olefins as steering ligands in asymmetric catalysis

Metal-catalyzed asymmetric processes offer one of the most straightforward ways to introduce stereogenic centers. Hence, the development of novel chiral ligands that can effectively induce asymmetry in reactions is crucial in modern organic synthesis. While many established chiral ligands bind to a metal through heteroatoms, structures that coordinate to metals through carbon atoms have received little attention so far. Here, we highlight the increasing number of such chiral chelating olefin ligands as well as their application in a variety of metal-catalyzed transformations.     

Novel sulfinyl imine ligands for asymmetric catalysis

[structure: see text] A novel class of P,N-sulfinyl imine ligands has been prepared that incorporates chirality solely at sulfur. The Pd complex of ligand 14 catalyzes the allylic alkylation reaction with high enantioselectivity (94%), and the first crystal structure of a Pd-bound sulfinyl imine provides insight into binding mode and origins of stereoselectivity.     

Synthesis of tetrahydroisoquinoline-diamine ligands and their application in asymmetric transfer hydrogenation

The use of the tetrahydroisoquinoline scaffold is well documented in biologically active compounds. However, reports of the utilisation of tetrahydroisoquinoline compounds in asymmetric catalysis are limited. The synthesis of novel diamine ligands possessing the tetrahydroisoquinoline (tetrahydroisoquinoline) backbone and evaluation of their activity in the asymmetric transfer hydrogenation of acetophenone are presented. The diamine ligands in conjunction with i-PrOH as the hydrogen source and [RhCl₂(Cp1)]₂ as the metal precursor proved to be the most effective of the tetrahydroisoquinoline derivatives for this catalytic system. Water was found to have a profound influence on the enantioselectivity of the reaction. Optimisation of the amount water, i-PrOH and catalytic loading content rendered the best result of 70% enantioselectivity for the (S)-1-phenylethanol isomer product.     

Application of novel enantiopure hydroxymethyl-substituted pyridine derivatives in asymmetric catalysis

The synthetic modification of enantiopure hydroxymethyl-substituted pyridine derivatives leading to novel chiral ligands is described. A set of these pyridine derivatives was examined as ligands in asymmetric transformations such as enantioselective alkylations or alkynylations of aldehydes, the asymmetric copper-catalyzed Henry reaction and the asymmetric allylation of benzaldehyde with allyl(trichloro)silane. This first screening revealed that several of the pyridine derivatives prepared are effective ligands affording high yields and good enantioselectivities. The asymmetric alkylation of aldehydes with diethylzinc provided yields of up to 93% with an enantiomeric excess of up to 88%. The asymmetric Henry reaction was also efficiently catalyzed by one of the prepared ligands affording (S)-2-nitro-1-phenylethanol in 68% yield and with 70% ee.     

Synthesis of chiral cyclohexane-backbone P,N-ligands derived from pyridine and their applications in asymmetric catalysis

P,N-ligands trans-4 and cis-5 with a cyclohexane backbone were easily synthesized. The key step was chiral resolution of (±)-trans-2-pyridylcyclohexanols with DBTA. Enantiopure trans isomer was subjected to Mitsunobu reaction and deprotection to give the corresponding cis isomer. These ligands have been successfully used in asymmetric hydrogenation of arylalkenes with up to 93% ee using 5 and 90% ee using 4 and asymmetric allylic alkylations with up to 95% ee using 5 and 93% ee using 4. Trans and cis P,N-ligands 4 and 5 all gave the product with the same configuration. It was suggested that the absolute configuration of the product was controlled by the configuration of the stereogenic pyridyl-bearing carbon of the ligands.     

Novel benzoferrocenyl chiral ligands: Synthesis and evaluation of their suitability for asymmetric catalysis

Four benzoferrocenyl phosphorus chiral ligands were conveniently prepared in good overall yields. These ligands were found to be stable in solid form and in solution. Two of the four ligands were resolved by chiral HPLC. Unlike a reported bis(phosphino-η⁵-indenyl)iron(II) complex, in which the indenyl ligands undergo ring flipping through an η¹-intermediate, these two ligands were found to be configurationally stable in solution and in solid state. The suitability of these ligands for enantioselective catalysis was assessed in studies on allylic alkylation reactions. When the two less sterically hindered ligands were used, excellent chemical yields were obtained, but the other two more sterically hindered ones gave lower yields. When the two enantiopure ligands were used, enantioselectivity of up to 51% ee was observed. These findings suggest that benzoferrocene derivatives may be used as chiral ligands for asymmetric catalysis.