Recent advances in transition metal-catalyzed enantioselective hydrogenation of unprotected enamines

Transition metal-catalyzed enantioselective hydrogenation of enamines is undoubtedly a useful and environment-friendly method for the preparation of optically pure chiral amines and amine derivatives. Over the last few decades, the use of transition metal catalysts containing chiral phosphorus or phosphine-oxazoline ligands attracted much attention for the hydrogenation of unprotected enamines. A number of efficient chiral catalysts have been developed, and some of them have shown high potential for the application in the synthesis of optical chiral amines in both laboratory and industry. This tutorial review focuses on the contributions concerning the transition metal-catalyzed enantioselective hydrogenation of unprotected enamines for the synthesis of chiral amines and amine derivatives.     

Organocatalytic enantioselective transformations involving quinone derivatives as reaction partners

This digest summarizes the recent progress made in the organocatalytic asymmetric reactions involving 1,4-quinone derivatives. The roles of quinones as Michael donors, Michael acceptors, dienophiles, 1,3-dipolarophiles and cascade reaction partners have been extensively investigated to access complex structural frameworks in the presence of chiral amine catalysts, phosphoric acid catalysts, bifunctional catalysts (Cinchona alkaloids- and thiourea-based), carboxylic acid catalysts, boronic acid catalysts and cationic oxazaborolidinium ions.     

Enantioselective 1,4‐Addition Reaction of α,β‐Unsaturated Carboxylic Acids with Cycloalkanones Using Cooperative Chiral Amine–Boronic Acid Catalysts

An enantioselective 1,4‐addition of α,β‐unsaturated carboxylic acids with cycloalkanones has been developed by using chiral amine–boronic acid cooperative catalysts. In the presence of a chiral amine and boronic acid, cycloalkanones and carboxylic acids are activated as chiral enamines and mixed anhydrides, respectively. The corresponding 1,4‐adducts are obtained in high yield with high enantioselectivity. Furthermore, subsequent oxylactonization of the 1,4‐adducts gives spirolactones with high diastereoselectivity.     

Enantioselective 1,4-Addition Reaction of α,β-Unsaturated Carboxylic Acids with Cycloalkanones Using Cooperative Chiral Amine–Boronic Acid Catalysts

An enantioselective 1,4-addition of α,β-unsaturated carboxylic acids with cycloalkanones has been developed by using chiral amine–boronic acid cooperative catalysts. In the presence of a chiral amine and boronic acid, cycloalkanones and carboxylic acids are activated as chiral enamines and mixed anhydrides, respectively. The corresponding 1,4-adducts are obtained in high yield with high enantioselectivity. Furthermore, subsequent oxylactonization of the 1,4-adducts gives spirolactones with high diastereoselectivity.     

Bronsted acid catalysis of achiral enamine for regio- and enantioselective nitroso aldol synthesis

Two types of chiral Br?nsted acid catalysts have been shown to catalyze regio- and enantioselective nitroso aldol synthesis between nitrosobenzene and achiral enamine. The combination of Br?nsted acidity and amine moiety of enamine realizes complete regioselectivity with high enantioselectivity. After a survey of Br?nsted acid catalysts, 1-naphthyl glycolic acid is found to be optimal in the O-nitroso aldol pathway, while 1-naphthyl TADDOL is the best catalyst for the N-nitroso aldol pathway. This is based on our finding on the control of regioselectivity by changing the amine moiety of enamine and the choice of Br?nsted acidity.     

烯烃的有机催化不对称环氧化研究进展及催化剂设计

介绍了用氧杂氮杂环丙烷、手性亚胺盐、手性酮、手性胺为催化剂的烯烃有机催化不对称环氧化反应,对各种体系催化机理进行介绍,并就手性催化剂的设计提出了构想.     

Organocatalytic Synthesis of Chiral Halogenated Compounds

This account summarizes our recent efforts in the enantioselective organocatalytic synthesis of chiral halogenated compounds. The enantioselective α-halogenation of aldehydes, decarboxylative chlorination of β-keto acids, and enantioselective C−C bond formation at the trifluoromethylated prochiral carbon to yield the corresponding organohalides with chlorinated, fluorinated, or trifluoromethylated chiral stereogenic centers are discussed. We applied common organocatalysts, such as Jørgensen-Hayashi catalyst and cinchona alkaloid-derivatived catalyst, and developed novel chiral amine catalysts for these reactions. This account also discusses stereospecific derivatizations of the resulting chiral halogenated compounds via nucleophilic substitution. Thus, we synthesized many novel chiral compounds that have not been reported, even as racemates.     

手性伯胺催化剂用于顺式选择性的不对称Cross-Aldol反应

基于酸碱作用,将手性伯胺与质子酸原位结合制得有机小分子催化剂,并用于醛与醛的不对称Cross-aldol反应.与一般手性仲胺催化剂不同,该类伯胺催化剂得到的是顺式选择性构型的Cross-aldol产物,其产率达90%,顺反比和ee值分别为9:1和90%.相比而言,采用简单易得的(1S,2S)-(+)-环己二胺即可得到较高选择性的顺式产物.     

Organocatalytic conjugate addition of malononitrile to conformationally restricted dienones

Organocatalytic conjugate addition of malononitrile to conformationally restricted dienones has been studied. A series of chiral primary and tertiary amine catalysts were screened. A piperidine-based thiourea-tertiary amine was found to be the efficient catalyst. Chiral pyran derivatives were obtained in excellent yields and enantioselectivities via a cascade conjugate addition-intramolecular cyclization pathway. The reaction is remarkably different for the corresponding reaction of conformationally flexible dienones.     

Fluorinated chiral secondary amines as catalysts for epoxidation of olefins with oxone

We have synthesized a series of chiral cyclic secondary amines having different substitution patterns and have screened them as catalysts for the asymmetric epoxidation of olefins using Oxone. The highest enantiomeric excess (61%) occurred for the epoxidation of 1-phenylcyclohexene catalyzed by a secondary amine bearing a fluorine atom at the beta-position relative to the amino center. Our experimental results provide further support to the notion that the amine plays a dual role--as a phase transfer catalyst and an Oxone activator--in these epoxidation reactions. The slightly acidic reaction conditions we employed in this work obviate the need to preform ammonium salts, which are the actual catalysts that mediate the epoxidations.     

Three‐Component Asymmetric Catalytic Ugi Reaction—Concinnity from Diversity by Substrate‐Mediated Catalyst Assembly

The first chiral catalyst for the three‐component Ugi reaction was identified as a result of a screen of a large set of different BOROX catalysts. The BOROX catalysts were assembled in situ from a chiral biaryl ligand, an amine, water, BH₃?SMe₂ , and an alcohol or phenol. The catalyst screen included 13 different ligands, 12 amines, and 47 alcohols or phenols. The optimal catalyst system (LAP 8‐5‐47) provided α‐amino amides from an aldehyde, a secondary amine, and an isonitrile with excellent asymmetric induction. The catalytically active species is proposed to be an ion pair that consists of the chiral boroxinate anion and an iminium cation.     

Enantioselective conjugate addition of both aromatic ketones and acetone to nitroolefins catalyzed by chiral primary amines bearing multiple hydrogen-bonding donors

A new class of chiral primary amine catalysts bearing multiple hydrogen-bonding donors have been designed and synthesized. The newly developed bifunctional organocatalysts efficiently catalyzed not only enantioselective conjugate addition of aromatic ketones to nitroolefins in good yields (up to 87%) with excellent enantioselectivities (97→99% ee) but also enantioselective conjugate addition of acetone to nitroolefins in excellent yields (90-96%) with high enantioselectivities (up to 97% ee).     

新手性铜络合物的制备及在烯烃不对称环丙烷化中的应用

设计并合成了2个新的手性源S-(一)-α-取代-2-羟基-5-甲基苄胺,从该手性源合成了4个光活性铜络合物,考察了它们对苯乙烯、1,1-二苯乙烯和2,4-二甲基-2,5-已二烯的不对称环丙烷化反应的光学诱导活性.结果表明,由S-(一)-1-(2-羟基-5-甲基苯基)-2-甲基丙胺与2-羟基萘甲醛缩合而得到的Schiff碱为配体的铜络合物的诱导活性最佳.     

Asymmetric direct vinylogous carbon-carbon bond formation catalyzed by bifunctional organocatalysts

The bifunctional chiral thiourea-tertiary amine organocatalysts have been applied to a direct asymmetric vinylogous Michael addition of α,α-dicyanoolefins to nitroolefins with 2-10 mol % catalyst loadings. The electronic properties of the thiourea-based catalysts have significant influences on this reaction. Moderate to excellent enantioselectivities (57-95% ee) have been achieved with low to good isolated yields through fine tuning the structures of the bifunctional organocatalysts. Much better ees were obtained for some α,α-dicyanoolefinic substrates compared with that catalyzed by modified cinchona alkaloids.     

Chiral Lewis base promoted trichlorosilane reduction of ketimines. An enantioselective organocatalytic synthesis of chiral amines

The reduction of the carbon-nitrogen double bond is an important transformation. Here we report our studies on a family of chiral organic catalysts able to promote the stereoselective reduction of ketimines with trichlorosilane. The very cheap, metal-free catalysts were easily prepared in one step from commercially available products, namely a chiral aminoalcohol and picolinic acid derivatives. The catalyst structure was extensively modified in a study that allowed to identify an extremely active species, able to promote the reduction on a large variety of substrates with high efficiency (up to 95% ee). A three component methodology has been also developed, where the enantiomerically enriched amine was isolated after performing the reaction by mixing a ketone and an amine in the presence of trichlorosilane and the catalyst. Its synthetic potentiality was demonstrated by employing the present metal-free catalytic procedure in the preparation of (S)-metolachlor, a potent and widely used herbicide.     

Chiral amine-polyoxometalate hybrids as highly efficient and recoverable asymmetric enamine catalysts

Chiral amine and polyoxometalate (POM) acids were combined to give highly efficient, stereoselective, and recoverable enamine-based catalysts. Less than 1 mol % of chiral amine loading was sufficient for good catalytic activity, and the catalyst could be recovered and reused 6 times with unchanged enantioselectivity and slightly decreased activity.     

Combining multi-catalysis and multi-component systems for the development of one-pot asymmetric reactions: stereoselective synthesis of highly functionalized bicyclo[4.4.0]decane-1,6-diones

We have developed a direct amine/acid-catalyzed stereoselective hydrogenation of a variety of Wieland-Miescher (W-M) ketones, Hajos-Parrish (H-P) ketones and their analogs with organic hydrides (Hantzsch esters) as the hydrogen source. This astonishingly simple and biomimetic approach was used to construct highly functionalized chiral bicyclo[4.4.0]decane-1,6-diones in a diastereoselective fashion. This is an example of the development of a new technology by the combination of multiple catalysts and components in one pot to deliver highly functionalized chiral molecules.     

Design of chiral organocatalysts for practical asymmetric synthesis of amino acid derivatives

A series of structurally rigid, chiral quaternary ammonium salts and several chiral sec-amine catalysts derived from commercially available (R)- or (S)-binaphthol have been designed as new C(2)-symmetric chiral phase-transfer catalysts and chiral bifunctional amino-catalysts. These chiral organocatalysts have been successfully applied to the highly practical asymmetric synthesis of various amino acid derivatives.     

A Preparation of enantiomerically enriched axially chiral beta-diketimines: synthesis of (-)- and (+)-IAN amine

The preparation of an enantiomerically enriched beta-diketimine composed of isoquinoline and 2-aminonaphthalene (IAN amine) is described, thereby offering new opportunities in the synthesis of nonracemic beta-diketimine- and pyridine-based chiral catalysts.     

A class of α-amino acids-derived multifunctional amidophosphane precatalysts: application to the highly enantio- and diastereoselective silver(I)-catalyzed 1,3-dipolar cycloaddition reaction

A class of multifunctional amidophosphanes derived from chiral α-amino acids have been developed with two amide bonds, a tertiary amine and a phosphine. In combination with Ag(I) salts, these amidophosphanes have been demonstrated as highly efficient multifunctional catalysts in the asymmetric 1,3-dipolar cycloaddition of azomethine ylides as well as the three-component reaction of the α-iminoesters in situ generated. Under optimal conditions, highly functionalized endo-8 pyrrolidines were obtained with good to excellent yields (up to 99% yield) and enantioselectivities (up to 98% ee).