Effective Chiral Catalyst Research Articles

Chiral dirhodium catalysts derived from l-serine, l-threonine and l-cysteine: design, synthesis and application

A series of dirhodium complexes were synthesized and found to be effective chiral catalysts for nitrene and carbene transfer reactions.

ChemInform Abstract: Asymmetric Hydrogenation in the Core of Dendrimers

AbstractReview: 51 refs.

Asymmetric hydrogenation in the core of dendrimers.

The transition metal complexes containing chiral phosphorus ligands are the most widely and successfully used catalysts in asymmetric hydrogenation of unsaturated compounds. However, a major problem associated with these homogeneous catalytic systems is the separation and recycling of the often expensive and easily oxidized chiral catalysts. In addition, many hydrogenation reactions still lack efficient chiral catalysts, and the stereoselectivities in many hydrogenation reactions are substrate-dependent. Therefore, the development of highly effective and recyclable chiral phosphorus catalysts is highly desirable. Over the past few decades, a number of chiral catalysts have been successfully anchored onto different supports, such as cross-linked polymeric resins and inorganic materials. However, most of the classical supported chiral catalysts suffered from inferior catalytic properties to their homogeneous counterparts due to poor accessibility, random anchoring, and disturbed geometry of the active sites in the solid matrix. To overcome this drawback, dendrimers, which have well-defined and globular macromolecular architectures serve as a promising type of soluble catalyst support. The catalytic sites are generally located at the core or on the periphery of the dendrimer, and the resulting dendritic catalysts are designable. Incorporation of a chiral catalyst into a sterically demanding dendrimer will create a specific microenvironment around the catalytic site and thus influence the catalytic performance of the metal center, like an enzyme does. In this Account, we survey the development of core-functionalized chiral dendritic phosphorus ligands for asymmetric hydrogenation mainly by our research group. Several series of chiral dendritic phosphorus ligands, including diphosphines, monodentate phosphoramidites, and P,N-ligands, have been synthesized by attaching the corresponding chiral phosphorus units into the core or the focal point of Fréchet-type dendrons. Their transition metal (Ru, Rh, or Ir) complexes have been applied in the asymmetric hydrogenation of prochiral olefins and ketones, as well as some challenging imine-type substrates. All reactions were carried out in a homogeneous manner, and the structure-property relationships in some cases were established. The sterically demanding dendritic wedges were found to play important roles in catalytic properties, and better catalytic activities or enantioselectivities or both than those obtained from the corresponding monomeric catalysts were achieved in most cases. In addition, the dendritic catalysts could be readily recycled by means of solvent precipitation, water- or temperature-induced two-phase separation. Our study has thus demonstrated that dendrimer catalysis could combine the advantages of both classical heterogeneous and homogeneous catalysis.

ChemInform Abstract: Molecular Assembly of an Achiral Phosphine and a Chiral Primary Amine: A Highly Efficient Supramolecular Catalyst for the Enantioselective Michael Reaction of Aldehydes with Maleimides.

A combined catalyst system of a cinchonidine-derived primary amine and triphenylphosphine (CDNH 2 /PPh 3 ) exhibited high catalytic performance in the Michael reaction of aldehydes with maleimides, thereby affording the corresponding functionalized aldehydes in excellent yields (up to 99%) and enantioselectivities (>99% ee). More interestingly, the significance of the phosphine in enhancing the enantioselectivities in the chiral-primary-amine-catalyzed Michael reaction was revealed. Furthermore, we explored the origin of the reaction mechanism in the Michael addition promoted by the dual organocatalytic system. On the basis of experimental results and spectroscopic analysis, such as UV/Vis, fluorescence emission (FL), NMR, and circular dichroism (CD) spectroscopy, as well as ESI-MS, we found that the molecular assembly of phosphine and primary amine played a crucial role in this enantioselective reaction, in which a possible supramolecular complex was formed as an effective chiral catalyst through noncovalent molecular interactions of a cinchona alkaloid-derived primary amine with triphenylphosphine.

Molecular Assembly of an Achiral Phosphine and a Chiral Primary Amine: A Highly Efficient Supramolecular Catalyst for the EnantioselectiveMichael Reaction of Aldehydes with Maleimides

A combined catalyst system of a cinchonidine-derived primary amine and triphenylphosphine (CD-NH2 /PPh3 ) exhibited high catalytic performance in the Michael reaction of aldehydes with maleimides, thereby affording the corresponding functionalized aldehydes in excellent yields (up to 99 %) and enantioselectivities (>99 % ee). More interestingly, the significance of the phosphine in enhancing the enantioselectivities in the chiral-primary-amine-catalyzed Michael reaction was revealed. Furthermore, we explored the origin of the reaction mechanism in the Michael addition promoted by the dual organocatalytic system. On the basis of experimental results and spectroscopic analysis, such as UV/Vis, fluorescence emission (FL), NMR, and circular dichroism (CD) spectroscopy, as well as ESI-MS, we found that the molecular assembly of phosphine and primary amine played a crucial role in this enantioselective reaction, in which a possible supramolecular complex was formed as an effective chiral catalyst through noncovalent molecular interactions of a cinchona alkaloid-derived primary amine with triphenylphosphine.

D2-Symmetric Dirhodium Catalyst Derived from a 1,2,2-Triarylcyclopropanecarboxylate Ligand: Design, Synthesis and Application

Dirhodium tetrakis-(R)-(1-(4-bromophenyl)-2,2-diphenylcyclopropanecarboxylate) (Rh(2)(R-BTPCP)(4)) was found to be an effective chiral catalyst for enantioselective reactions of aryl- and styryldiazoacetates. Highly enantioselective cyclopropanations, tandem cyclopropanation/Cope rearrangements and a combined C-H functionalization/Cope rearrangement were achieved using Rh(2)(R-BTPCP)(4) as catalyst. The advantages of Rh(2)(R-BTPCP)(4) include its ease of synthesis, its tolerance to the size of the ester group in the styryldiazoacetates, and its compatibility with dichloromethane as solvent. Computational studies suggest that the catalyst adopts a D(2)-symmetric arrangement, but when the carbenoid binds to the catalyst, two of the p-bromophenyl groups on the ligands rotate outward to make room for the carbenoid and the approach of the substrate to the carbenoid.

The Catalytic Asymmetric Diels–Alder Reactions and Post‐cycloaddition Reductive Transpositions of 1‐Hydrazinodienes

The Catalytic Asymmetric Diels–Alder Reactions and Post‐cycloaddition Reductive Transpositions of 1‐Hydrazinodienes

Enantioselective Cyclopropenation of Alkynes

The cobalt(II) complex of new D2-symmetric chiral porphyrin 3,5-DiMes-ChenPhyrin, [Co(P2)], has been shown to be a highly effective chiral metalloradical catalyst for enantioselective cyclopropenation of alkynes with acceptor/acceptor-substituted diazo reagents such as α-cyanodiazoacetamides and α-cyanodiazoacetates. The [Co(P2)]-mediated metalloradical cyclopropenation is suitable to a wide range of terminal aromatic and related conjugated alkynes with varied steric and electronic properties, providing the corresponding tri-substituted cyclopropenes in high yields with excellent enantiocontrol of the all-carbon quaternary stereogenic centers. In addition to mild reaction conditions, the Co(II)-based metalloradical catalysis for cyclopropenation features a high degree of functional group tolerance.

Enantioselective Cyclopropenation of Alkynes with Acceptor/Acceptor-Substituted Diazo Reagents via Co(II)-Based Metalloradical Catalysis

The cobalt(II) complex of a new D(2)-symmetric chiral porphyrin 3,5-diMes-ChenPhyrin, [Co(P2)], has been shown to be a highly effective chiral metalloradical catalyst for enantioselective cyclopropenation of alkynes with acceptor/acceptor-substituted diazo reagents, such as α-cyanodiazoacetamides and α-cyanodiazoacetates. The [Co(P2)]-mediated metalloradical cyclopropenation is suitable to a wide range of terminal aromatic and related conjugated alkynes with varied steric and electronic properties, providing the corresponding trisubstituted cyclopropenes in high yields with excellent enantiocontrol of the all-carbon quaternary stereogenic centers. In addition to mild reaction conditions, the Co(II)-based metalloradical catalysis for cyclopropenation features a high degree of functional group tolerance.

A density functional theory study of the enantioselective reduction of prochiral ketones promoted by chiral spiroborate esters

Recently, chiral spiroborate ester [(R)-2-((1,3,2-dioxaborolan-2-yloxy)methyl)pyrrolidine] has been experimentally employed as an effective chiral catalyst in the borane-mediated asymmetric reduction of prochiral ketones to produce the corresponding secondary alcohols. In this article, we have theoretically investigated the mechanism of the reduction using density functional theory. The results reveal that this reaction is accomplished via four steps. Fully geometry optimized reactants, products, transition states, and intermediates were obtained at the B3LYP/6-31G (d, p) level. The analysis of these results reveals one pathway that is more energetically favorable, and its associated geometries correlate well with the final products of the reaction. The further calculations show that the solvent effect of THF has no great influence on enantioselectivity of this reduction. © 2009 Wiley Periodicals, Inc. Int J Quantum Chem, 2010

Diastereo‐ and Enantioselective Gold(I)‐Catalyzed Intermolecular Tandem Cyclization/[3+3]Cycloadditions of 2‐(1‐Alkynyl)‐2‐alken‐1‐ones with Nitrones

The mysterious cyclizations of gold: An asymmetric gold(I)-catalyzed formal [3+3] cycloaddition of 2-(1-alkynyl)-2-alken-1-ones with nitrones has been developed. Compound 1 was found to be an effective and reliable chiral catalyst, as well as the commonly used 2, for this gold-catalyzed reaction. Tf=trifluoromethanesulfonyl.

Preparation and characterization of the chirally modified rapidly quenched skeletal Ni catalyst for enantioselective hydrogenation of butanone to R-(−)-2-butanol

A rapidly quenched skeletal Ni (RQ Ni) prepared by the melt-spinning method followed by alkali leaching was used as a new category of Ni precursor to fabricate effective chiral Ni catalyst. A series of tartaric acid (TA)–NaBr–modified RQ Ni catalysts (MRQNi) were derived, and the textural and structural properties of the MRQNi catalysts were systematically investigated. It is found that the modification conditions influence profoundly the composition, texture, and Ni crystallite size of the MRQNi catalysts. In liquid phase hydrogenation of butanone to R-(−)-2-butanol, the optimized MRQNi catalyst exhibits much better activity and enantioselectivity than the similarly chirally modified Raney Ni catalyst. Based on the characterizations, the superior catalytic performance of the MRQNi catalyst is mainly attributed to the higher surface content of the sodium–nickel(II) tartrate complex than that on the chirally modified Raney Ni catalyst.

Enantioselective [2 + 2] Cycloaddition of Unactivated Alkenes with α-Acyloxyacroleins Catalyzed by Chiral Organoammonium Salts

We report the first example of an organocatalytic enantioselective [2 + 2] cycloaddition reaction of unactivated alkenes with α-acyloxyacroleins to give optically active 1-acyloxycyclobutanecarbaldehydes and subsequent ring expansion to give optically active 2-hydroxycyclopentanone derivatives. Organoammonium salt, H-L-Phe-L-Leu-N(CH2CH2)2-reduced triamine•2.6HNTf2, is a highly effective chiral catalyst for the above enantioselective [2 + 2] cycloaddition. Thus, we have developed a novel and useful formal enantioselective [2 + 3] cycloaddition through two steps.

Dirhodium Tetracarboxylates Derived from Adamantylglycine as Chiral Catalysts for Enantioselective C−H Aminations

The dirhodium tetracarboxylate, Rh(2)(S-TCPTAD)(4), derived from adamantylglycine, is an effective chiral catalyst for both inter- and intramolecular C-H aminations. [reaction: see text]

Dirhodium Tetracarboxylate Derived from Adamantylglycine as a Chiral Catalyst for Carbenoid Reactions

[reaction: see text] The dirhodium tetracarboxylate, Rh2(S-PTAD)4, derived from adamantylglycine, is a very effective chiral catalyst for carbenoid reactions. High asymmetric induction was obtained in Rh2(S-PTAD)4-catalyzed intramolecular C-H insertion (94% ee), intermolecular cyclopropanation (99% ee), and intermolecular C-H insertion (92% ee).

Enantioselective Addition of Phenylacetylene to Ketones Catalyzed by Titanium(IV) Complexes of N‐Sulfonylated β‐Amino Alcohols

AbstractThe easily prepared and recoverable chiral N‐sulfonylated β‐amino alcohol 2 in combination with Ti(OPr‐i)4was found to be an effective chiral catalyst for the enantioselective addition of alkynylzinc to ketones, which gave the useful products, i.e. chiral tertiary propargyl alcohols, with the ee up to 92%.

Catalysis of Enantioselective [2+1]-Cycloaddition Reactions of Ethyl Diazoacetate and Terminal Acetylenes Using Mixed-Ligand Complexes of the Series Rh2(RCO2)n (L*4-n). Stereochemical Heuristics for Ligand Exchange and Catalyst Synthesis

This paper describes the synthesis of mixed Rh(2)(II) complexes containing bridging acetate and R,R-diphenyl-N-triflylimidazolidinone (DPTI) ligands (1, 2, and 9-19), and their function as enantioselective catalysts for the conversion of ethyl diazoacetate and terminal acetylenes to chiral cyclopropenes. Of these catalysts, 1 and 10 functioned with the highest enantioselectivity, in accord with a mechanistic model in which one of the ligand bridges is broken in the intermediate Rh-carbene complex. The synthetic results allow conclusions with regard to kinetically and thermodynamically favored pathways for the synthesis of mixed acetate-DPTI complexes. A new C(2)-symmetric complex having only two anti-DTBTI bridges (23) is shown to be a highly effective chiral catalyst, as expected from the model.

Recent Advances in Catalytic Asymmetric Addition to Imines and Related C=N Systems

Addition reactions to imines are relatively less developed compared to those of carbonyl compounds due to low electrophilicity and ease of α-deprotonation. With the use of an appropriate activator, which can coordinate to the imine nitrogen atom, the electrophilicity of the imine can be enhanced and a range of nucleophilic addition reactions becomes possible. When the activator is chiral, it will also create chiral environment and will direct the approach of the nucleophile to one face of the imine over the other resulting in enantioselectivity. The potential of catalytic asymmetric addition to imines in organic synthesis is enormous, but is relatively underused. With the development of many highly effective chiral catalysts for imine additions, the situation will soon be changed. The present review will cover selected literature on catalytic asymmetric additions to aldimines, ketimines and related compounds including hydrazones, oximes and nitrones from 1999 to 2004. The reactions of interest include hydrogenation, alkylation, Mannich and related reactions such as aza-Baylis-Hillman and aza-Henry reaction, Strecker reaction, hydrophosphonylation, and 3-6 membered ring forming reactions from imines. Some applications of these methodologies in synthesis of nitrogen-containing biologically active compounds will also be presented. Keywords: nucleophilic addition, chiral auxiliary, tautomerization, hydrogenation, hydride transfer reactions, hydrosilylation, alkyllithium reagents, alkylzinc reagents

Highly Enantioselective Phenylacetylene Additions to Ketones Catalyzed by (S)‐BINOL—Ti Complex.

AbstractFor Abstract see ChemInform Abstract in Full Text.

Highly Enantioselective Phenylacetylene Additions to Ketones Catalyzed by (S)-BINOL−Ti Complex

The readily available and inexpensive (S)-BINOL ligand in combination with Ti(O(i)Pr)(4) is an effective chiral catalyst for the catalytic asymmetric addition of alkynylzinc to unactivated simple ketones. Good to excellent enantioselectivities were achieved. No previous case has been reported successfully using BINOL to catalyze the addition of phenylacetylene to unactivated ketones, and thus the utility of BINOL in asymmetric catalysis is expanded.