Tris(1,2-diphenylethylenediamine)cobalt(III) Complexes: Chiral Hydrogen Bond Donor Catalysts for Enantioselective α-Aminations of 1,3-Dicarbonyl Compounds

Cinchona Organocatalyzed Enantioselective Amination for Quaternized Serines as Tertiary Amides

Herein, we describe a Cinchona-aminocatalyzed enantioselective α-hydrazination of an α-formyl amide for the production of protected quaternized serines as tertiary amides with ee's of generally >98% and ≤99% yields. The proposed TS model supported by density functional theory calculations involves a quinuclidinium ion Brønsted acid-assisted delivery of DtBAD, which occurs from the Re face of an H-bonded enaminone when using a 9S-cinchonamine catalyst, resulting in a hydrazide with the R-configuration as determined by X-ray analysis.

Chiral π-Cu(II) Catalysts for the Enantioselective α-Amination of N-Acyl-3,5-dimethylpyrazoles

We report the highly enantioselective α-amination of N-acyl-3,5-dimethylpyrazoles with dialkyl azodicarboxylates, catalyzed by in situ generated π-Cu(II) complexes that consist of Cu(OTf)₂ and N-(5H-dibenzo[a,d][7]annulen-5-yl)-l-alanine-derived amides, to give the corresponding products as d-α-amino acid derivatives (up to >99% yield and 99% ee). The site-selectivity and enantioselectivity can be satisfactorily explained by the coordination of dialkyl azodicarboxylate with π-Cu(II) complex. The synthetic potential of this one-pot transformation to the α-amino ester is also described.

Family of Well-Defined Chiral-at-Cobalt(III) Complexes as Metal-Templated Hydrogen-Bond-Donor Catalysts: Effect of Chirality at the Metal Center on the Stereochemical Outcome of the Reaction

A family of well-defined Λ- and Δ-diastereomeric octahedral cationic chiral-at-cobalt complexes were obtained by a simple two-step reaction of (R,R)-1,2-diaminocyclohexane, (R,R)-1,2-diphenylethylenediamine, or (S)-2-(aminomethyl)pyrrolidine and substituted salicylaldehydes with a cobalt(III) salt. It was observed for the first time that the use of an excess of cobalt(III) salt provides both the enantiopure Λ and Δ forms of the corresponding cobalt(III) complexes 1 and 2 in a ratio of diastereomers ranging from 1:1.6 to >20:1 (Λ/Δ) and in 31-95% combined yields. The obtained complexes were robust, air- and bench-stable, soluble in most of organic solvents, and insoluble in water. Through variation of the substituents in the phenyl ring of the salicylaldehyde moiety, it was shown that both steric and electronic effects of substituents have a significant impact on the formation of Λ and Δ isomers. Next, the efficacies of the enantiopure metal-templated complexes 1-3 were investigated in three benchmark asymmetric reactions in order to compare their catalytic activity. The chiral cobalt(III) complexes 1-3 were tested as enantioselective hydrogen-bond-donor catalysts in such important reactions as the Michael addition of the O'Donnell substrate to methyl acrylate, epoxidation of chalcone, and trimethylsilylcyanation of benzaldehyde. It was clearly demonstrated that the chirality at the cobalt center has an impact on the stereochemical outcome of the reactions. In particular, the Λ(R,R)-1 and Δ(R,R)-1 complexes acted as "pseudoenantiomeric" catalysts in the epoxidation and trimethylsilylcyanoation reactions, providing both enantiomers of the products with up to 57% enantiomeric excess.

Synthesis and a Catalytic Study of Diastereomeric Cationic Chiral-at-Cobalt Complexes Based on (R,R)-1,2-Diphenylethylenediamine

Here we report the first synthesis of two diastereomeric cationic octahedral Co(III) complexes based on commercially available (R,R)-1,2-diphenylethylenediamine and salicylaldehyde. Both diastereoisomers with opposite chiralities at the metal center (Λ and Δ configurations) were prepared. The new Co(III) complexes possessed both acidic hydrogen-bond donating (HBD) NH moieties and nucleophilic counteranions and operate as bifunctional chiral catalysts for the challenging kinetic resolution of terminal and disubstituted epoxides by the reaction with CO₂ under mild conditions. The highest selectivity factor (s) of 2.8 for the trans-chalcone epoxide was achieved at low catalyst loading (2 mol %) in chlorobenzene, which is the best achieved result currently for this type of substrate.

Enantioselective "organocatalysis in disguise" by the ligand sphere of chiral metal-templated complexes

Asymmetric catalysis holds a prominent position among the important developments in chemistry during the 20th century. This was acknowledged by the 2001 Nobel Prize in chemistry awarded to Knowles, Noyori, and Sharpless for their development of chiral metal catalysts for organic transformations. The key feature of the catalysts was the crucial role of the chiral ligand and the nature of the metal ions, which promoted the catalytic conversions of the substrates via direct coordination. Subsequently the development of asymmetric organic catalysis opened new avenues to the synthesis of enantiopure compounds, avoiding any use of metal ions. Recently, an alternative approach to asymmetric catalysis emerged that relied on the catalytic functions of the ligands themselves boosted by coordination to metal ions. In other words, in these hybrid chiral catalysts the substrates are activated not by the metal ions but by the ligands. The activation and enantioselective control occurred via well-orchestrated and custom-tailored non-covalent interactions of the substrates with the ligand sphere of chiral metal complexes. In these metal-templated catalysts, the metal served either as a template (a purely structural role), or it constituted the exclusive source of chirality (metal-centred chirality due to the spatial arrangement of achiral or chiral bi-/tridentate ligands around an octahedral metal centre), and/or it increased the Brønsted acidity of the ligands. Although the field is still in its infancy, it represents an inspiring combination of both metal and organic catalysis and holds major unexplored potential to push the frontiers of asymmetric catalysis. Here we present an overview of this emerging field discussing the principles, applications and perspectives on the catalytic use of chiral metal complexes that operate as "organocatalysts in disguise". It has been demonstrated that these chiral metal complexes are efficient and provide high stereoselective control in asymmetric hydrogen bonding catalysis, phase-transfer catalysis, Brønsted acid/base catalysis, enamine catalysis, nucleophilic catalysis, and photocatalysis as well as bifunctional catalysis. Also, many of the catalysts have been identified as highly effective catalysts at remarkably low catalyst loadings. These hybrid systems offer many opportunities in the synthesis of chiral compounds and represent promising alternatives to metal-based and organocatalytic asymmetric transformations.

Expanding the Family of Octahedral Chiral-at-Metal Cobalt(III) Catalysts by Introducing Tertiary Amine Moiety into the Ligand

Chiral metal-templated complexes are attractive catalysts for organic synthetic transformations. Herein, we introduce a novel chiral cobalt(III)-templated complex based on chiral trans-3,4-diamino-1-benzylpyrrolidine and 3,5-di-tert-butyl-salicylaldehyde which features both hydrogen bond donor and Brønsted base functionalities. The obtained complexes were fully characterized by 1H, 13C NMR, IR-, UV-vis, CD-spectroscopy and by a single X-ray diffraction analysis. It was shown that chlorine anion is connected with amino groups of the complex via a hydrogen bonding. DFT calculations of charges and molecular electrostatic potential of the cobalt(III) complex showed that the basicity of the complex is certainly diminished as compared with the routine tertiary amines but the acidity of the conjugated acid of the complex should be increased. Thus, the catalytic potential of the complex may be much greater as a chiral acid than a chiral base. We believe that this work opens a new way in chiral bifunctional catalyst design.

Regio- and enantioselective amination of acyclic branched α-alkynyl ketones: asymmetric construction of N-containing quaternary stereocenters

Direct regio- and enantioselective amination of acyclic α-branched ketones enabled by the α-alkynyl group.

An octahedral cobalt(iii) complex based on cheap 1,2-phenylenediamine as a bifunctional metal-templated hydrogen bond donor catalyst for fixation of CO2 with epoxides under ambient conditions

An octahedral cobalt(iii) complex based on cheap 1,2-phenylenediamine operates as an efficient bifunctional hydrogen bond donor catalyst in cycloaddition of epoxides with CO₂ under ambient conditions and solvent- and co-catalyst-free conditions.

Solvent free enantioselective catalysis with chiral cobalt(iii) Werner complexes via ball milling

The title technique is applied for the first time to cobalt(iii) hydrogen bond donor catalysts, a large family of recently developed salts that enable numerous enantioselective organic reactions and feature an earth abundant metal.

Enantioselective amination of 4-alkylisoquinoline-1,3(2H,4H)-dione derivatives

A mild and efficient enantioselective amination of 4-alkylisoquinoline-1,3(2H,4H)-dione derivatives was established, which is compatible with a broad range of substrates and delivers the final products in excellent yields (up to 99%) and ee values (up to 99%) with low catalyst loading (down to 1 mol%). The synthetic potential of this methodology was also demonstrated in the gram scale level.

Launching Werner Complexes into the Modern Era of Catalytic Enantioselective Organic Synthesis

Reactions catalyzed by transition metal complexes almost always entail binding of one or more reactants to the metal center, and nearly every corner of the "chiral pool" has been picked over in efforts to develop enantioselective catalysts. As reported by Alfred Werner in 1911-1912, salts of the formally D₃-symmetric [Co(en)₃]³⁺ trication (en = ethylenediamine) were among the first chiral inorganic compounds to be resolved into enantiomers. These air- and water-stable complexes are substitution-inert, so for 100 years they languished without application in organic synthesis. We then showed that when they are rendered soluble in organic media by lipophilic anions such as fluorinated tetraarylborates BAr_f^-, they become potent catalysts for a variety of carbon-carbon and carbon-heteroatom bond forming reactions.These involve substrate activation by hydrogen bonding to the coordinated NH₂ units (pK_a ca. 15), a "second coordination sphere" mechanism. Only modest enantioselectivities are obtained with [Co(en)₃]³⁺ 3BAr_f^- or related chromium, rhodium, iridium, and platinum salts. However, high enantioselectivities are achieved when the three en ligands are replaced by the 1,2-diphenyl analogues (S,S)- or (R,R)-H₂NCHPhCHPhNH₂. Here only one BAr_f^- anion is required to solubilize the trication, so a number of mixed-salt catalysts (2X^-BAr_f^-) have been evaluated. Alternatively, a dimethylamino group can be tethered to the backbone of one en ligand, providing bifunctional catalysts that obviate any need for an external base. Interestingly, the counteranions modulate the enantioselectivities somewhat. However, catalysts with chiral anions do not significantly outperform benchmark catalysts with achiral anions. Cagelike chiral hexaaminecobalt(III) complexes known as sepulchrates and sarcophagines, which feature secondary NH donor atoms, can also serve as catalysts, but the enantioselectivities are very low.In a spinoff application, certain salts are found to be superb "chiral solvating agents", leading to distinct sets of NMR signals for enantiomers of chiral analytes with Lewis basic functional groups. Loadings of 10-25 mol % generally suffice, providing the best way of assaying the enantiomeric purities of a host of compounds. Also, mixtures of several chiral compounds can be simultaneously analyzed. It is not surprising that complexes that perform well in chiral recognition phenomena also excel as enantioselective catalysts.In this Account, the stereochemical properties of the preceding complexes are treated, as well as arcana generally known only to specialists in the field. These include the use of charcoal for equilibrating configurations of the cobalt stereocenter and Sephadex for separating enantiomers and diastereomers. Other types of metal-containing hydrogen-bond-donor catalysts are briefly surveyed (noncoordinating NH units can also be effective), including several developed by other groups. However, the mechanisms of enantioselection in all of these transformations remain obscure. The optimum diastereomer and anion set varies from reaction to reaction, suggesting a "phenotypic plasticity" that allows adaption to a variety of processes.

Rendering classical hydrophilic enantiopure Werner salts [M(en)3]n+nX- lipophilic (M/n = Cr/3, Co/3, Rh/3, Ir/3, Pt/4); new chiral hydrogen bond donor catalysts and enantioselectivities as a function of metal and charge

Known hydrophilic halide salts of the title compounds are converted to new lipophilic BArf- (B(3,5-C6H3(CF3)2)4-) salts. These are isolated as hydrates (Λ- or Δ-[M(en)3]n+nBArf-·zH2O; z = 17-9) and characterized by NMR (acetone-d6) and microanalyses. Thermal stabilities are probed by capillary thermolyses and TGA and DSC measurements (onset of dehydration 71-151 °C). In the presence of tertiary amines, they are effective catalysts for enantioselective Michael type carbon-carbon or carbon-nitrogen bond forming additions of 1,3-dicarbonyl compounds (acceptors: trans-β-nitrostyrene, di-tert-butylazodicarboxylate, 2-cyclopenten-1-one; average ee = 33%, 52%, 17%). Effects of the metal and charge upon enantioselectivities are analyzed. A number of properties appear to correlate to the NH Brønsted acidity order ([Pt(en)3]4+ > [Cr(en)3]3+ > [Co(en)3]3+ > [Rh(en)3]3+ > [Ir(en)3]3+).

Enantioselective α-Amination of Acyclic 1,3-Dicarbonyls Catalyzed by N-Heterocyclic Carbene

Herein, we describe a method for the catalytic enantioselective α-amination of α-substituted acyclic 1,3-ketoamides and 1,3-amidoesters that affords the products possessing N-substituted quaternary stereocenters with a chiral N-heterocyclic carbene (NHC). The reaction is based on the utilization of an intrinsic Brønsted base characteristic of NHC that enables the catalytic formation of a chiral ion pair comprising the enolate and the azolium ion. A series of challenging open-chain α-substituted 1,3-dicarbonyls are aminated via this method with ee's of ≤99%.

Asymmetric catalytic construction of fully substituted carbon stereocenters using acyclic α-branched β-ketocarbonyls: the “Methyl Rule” widely exists

This review illustrates the recent advances in catalytic asymmetric α-functionalization of acyclic β-ketocarbonyls. A thorough survey of all these reactions indicates the existance of a general principle which is called the “Methyl Rule”.

The robust, readily available cobalt(iii) trication [Co(NH2CHPhCHPhNH2)3]3+ is a progenitor of broadly applicable chirality and prochirality sensing agents

When NMR spectra of chiral racemic organic molecules containing a Lewis basic functional group are recorded in the presence of air and water stable salts of the cobalt(iii) trication [Co((S,S)-NH2CHPhCHPhNH2)3]3+ (23+), separate signals are usually observed for the enantiomers (28 diverse examples, >12 functional groups). Several chiral molecules can be simultaneously analyzed, and enantiotopic groups in prochiral molecules differentiated (16 examples). Particularly effective are the mixed bis(halide)/tetraarylborate salts Λ-23+ 2X-BArf- (X = Cl, I; BArf = B(3,5-C6H3(CF3)2)4), which are applied in CD2Cl2 or CDCl3 at 1-100 mol% (avg 34 and 14 mol%). Job plots establish 1 : 1 binding for Λ-23+ 2Cl-BArf- and 1-phenylethyl acetate (4) or 1-phenylethanol (10), and ca. 1 : 2 binding with DMSO (CD2Cl2). Selected binding constants are determined, which range from 7.60-2.73 M-1 for the enantiomers of 10 to 28.1-22.6 M-1 for the enantiomers of 4. The NH moieties of the C2 faces of the trication are believed to hydrogen bond to the Lewis basic functional groups, as seen in the crystal structure of a hexakis(DMSO) solvate of Λ-23+ 3I-. These salts rank with the most broadly applicable chirality sensing agents discovered to date.

Enantioselective amination of nitroolefins under base-free and water-rich conditions using chiral bifunctional phase-transfer catalysts

A highly enantioselective amination of nitroolefins was realized with l-tert-leucine derived squaramide-scaffold bifunctional phase-transfer catalysts under base-free and water-rich conditions.

Readily Available Chiral Benzimidazoles-Derived Guanidines as Organocatalysts in the Asymmetric α-Amination of 1,3-Dicarbonyl Compounds

The synthesis and the evaluation as organocatalysts of new chiral guanidines derived from benzimidazoles in the enantioselective α-amination of 1,3-dicarbonyl compounds using di-t-butylazodicarboxylate as aminating agent is herein disclosed. The catalysts are readily synthesized through the reaction of 2-chlorobezimidazole and a chiral amine in moderate-to-good yields. Among all of them, those derived from (R)-1-phenylethan-1-amine (1) and (S)-1-(2-naphthyl)ethan-1-amine (3) turned out to be the most efficient for such asymmetric transformation, rendering good-to-high yields and moderate-to-good enantioselectivities for the amination products.

Stereogenic-Only-at-Metal Asymmetric Catalysts

Chirality is an essential feature of asymmetric catalysts. This review summarizes asymmetric catalysts that derive their chirality exclusively from stereogenic metal centers. Reported chiral-at-metal catalysts can be divided into two classes, namely, inert metal complexes, in which the metal fulfills a purely structural role, so catalysis is mediated entirely through the ligand sphere, and reactive metal complexes. The latter are particularly appealing because structural simplicity (only achiral ligands) is combined with the prospect of particularly effective asymmetric induction (direct contact of the substrate with the chiral metal center). Challenges and solutions for the design of such reactive stereogenic-only-at-metal asymmetric catalysts are discussed.

Enantioselective conjugate addition of hydroxylamines to α,β-unsaturated 2-acyl imidazoles catalyzed by a chiral-at-metal Rh(iii) complex

A highly efficient chiral-at-metal Rh(iii) complex catalyzed asymmetric aza-Michael addition was developed, affordingN-protected β-amino acid derivatives with excellent enantioselectivity.

Octahedral Werner complexes with substituted ethylenediamine ligands: a stereochemical primer for a historic series of compounds now emerging as a modern family of catalysts

As reported by Alfred Werner in 1911-1912, salts of the formally D₃ symmetric [Co(en)₃]³⁺ (en = ethylenediamine) trication were among the first chiral inorganic compounds to be resolved into enantiomers, the absolute configurations of which are denoted Λ (left handed helix) or Δ (right handed helix). After a >100 year dormant period during which few useful reactions of these substitution inert complexes were described, carbon substituted derivatives have recently been found to be potent catalysts for enantioselective organic synthesis. This review systematically outlines the fascinating range of stereoisomers that can arise, such as conformers associated with the five membered chelate rings (λ/δ), alignment modes of the C-C bonds with the C₃ symmetry axis (lel/ob), geometric isomers (fac/mer), and configurational diastereomers (R/S) arising from carbon stereocenters. These analyses demonstrate a profound stereochemical diversity that can be applied in catalyst optimization. Efforts are made to bridge the often orthogonal nomenclature systems inorganic and organic chemists employ to describe these phenomena.

Metal-Templated Design: Enantioselective Hydrogen-Bond-Driven Catalysis Requiring Only Parts-per-Million Catalyst Loading

Based on a metal-templated approach using a rigid and globular structural scaffold in the form of a bis-cyclometalated octahedral iridium complex, an exceptionally active hydrogen-bond-mediated asymmetric catalyst was developed and its mode of action investigated by crystallography, NMR, computation, kinetic experiments, comparison with a rhodium congener, and reactions in the presence of competing H-bond donors and acceptors. Relying exclusively on weak forces, the enantioselective conjugate reduction of nitroalkenes can be executed at catalyst loadings as low as 0.004 mol% (40 ppm), representing turnover numbers of up to 20 250. A rate acceleration by the catalyst of 2.5 × 10(5) was determined. The origin of the catalysis is traced to an effective stabilization of developing charges in the transition state by carefully orchestrated hydrogen-bonding and van der Waals interactions between catalyst and substrates. This study demonstrates that the proficiency of asymmetric catalysis merely driven by hydrogen-bonding and van der Waals interactions can rival traditional activation through direct transition metal coordination of the substrate.

One-Carbon Homologation of Primary Alcohols to Carboxylic Acids, Esters, and Amides via Mitsunobu Reactions with MAC Reagents

A method is reported for the one-carbon homologation of an alcohol to the extended carboxylic acid, ester, or amide. The process involves the Mitsunobu reaction with an alkoxymalononitrile, followed by unmasking in the presence of a suitable nucleophile. The homologation and unmasking can even be performed in a one-pot process in high yield.