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Chiral diselenide ligands for the asymmetric copper-catalyzed conjugate addition of Grignard reagents to enones
Abstract
The copper-catalyzed conjugate addition of Grignard reagents to enones in the presence of chiral diselenide oxazoline ligands has been studied and found to provide good yields and useful levels of asymmetric induction.
... On the basis of our preliminary results on the racemic version of this reaction and reports from other groups [10,20], we speculated that conformationally rigidified chiral selenium-catalysts might be particularly suitable for the aspired asymmetric allylic alkene imidation [21]. Therefore, we initially tested the literature-known chiral diselenide 4 (Equation (1)) [22], which possesses a Lewis-basic side chain. Such selenium species were frequently reported to adopt significantly stabilized conformations by means of nonbonding interactions between a transiently-formed cationic selenium center and a heteroatom (e.g., oxygen or nitrogen) suitably juxtaposed in the side chain [23]. ...
... On the basis of our preliminary results on the racemic version of this reaction and reports from other groups [10,20], we speculated that conformationally rigidified chiral selenium-catalysts might be particularly suitable for the aspired asymmetric allylic alkene imidation [21]. Therefore, we initially tested the literature-known chiral diselenide 4 (Equation (1)) [22], which possesses a Lewis-basic side chain. Such selenium species were frequently reported to adopt significantly stabilized conformations by means of nonbonding interactions between a transientlyformed cationic selenium center and a heteroatom (e.g., oxygen or nitrogen) suitably juxtaposed in the side chain [23]. ...
A series of unprecedented chiral selenium-π-acid catalysts for the asymmetric, oxidative functionalization of alkenes has been developed. In total, eleven different chiral dihydrodiselenocine and (di-)alkoxyphenyl (di)selenide motifs have been synthesized in a concise, modal, and straightforward fashion. Commercially available, non-racemic alcohols have been predominantly used as chiral building blocks for the facile assembly of the selenium-π-acid catalysts. These species have been exemplarily applied to the enantioselective intermolecular imidation and intramolecular acyloxylation of two olefins using N-fluorobenzenesulfonimide (NFSI) and ambient air, respectively, as terminal oxidants. In part, the catalysts provide very good yields of up to 99% and enantiomeric ratios of up to 83.5:16.5 under aerobic conditions.
... Apart from these reactions, the catalytic amount of chiral organoselenium-based ligands has been utilized in many transition metal-catalyzed reactions [65]. For instance, Braga and Shi's research team independently reported copper-catalyzed conjugate additions of organometallic reagents to cyclic enones using chiral selenium-based ligands [73][74]. Also, a few organoselenium ligands find direct applications in stereoselective allylic alkylation reactions using palladium catalysis [75][76][77][78]. ...
Organoselenium chemistry has developed as an important tool in the field of synthetic and medicinal chemistry. Various organoselenium reagents have been developed and used successfully to achieve different organic transformations such as the selenocyclizations, oxyselenenylations and selenoxide eliminations etc. Additionally, the potential of organoselenium reagents is not limited their use as stoichiometric reagents but they have successfully used as organocatalyst in number of synthetic transformations. Various organic and inorganic oxidants have been identified as terminal oxidants to regenerate the active catalytic specie. In this review article, the recent progress of organoselenium reagents in catalysis is being highlighted along with their asymmetric variants.
... [1] Key to its successful accomplishment relies on the development of sophisticatedly designed chiral ligands for the metal catalysts. [2] For the most established copper-catalyzed asymmetric conjugate addition reaction of Grignard reagents with enones, a chiral amine [1a,b] was the first of the successful chiral sources of heterocuprates and was followed by the zinc complexes of chiral amino alcohols [3] and chiral thiols, [1g] and recently by a chiral selenide [4] and a chiral pyrazole derivative. [5] Other successful approaches for controlling high enantioselectivity rely on the coordination of a phosphorus atom with copper, and a chiral amidophosphane 1 proved to be a good ligand for the copper-catalyzed conjugate addition of Grignard reagents to enones, [6] which was followed by a chiral ferrocenyl-based monophosphane [7] as well as a bisphosphane. ...
Peptidic modification of (S)-2-[(diphenylphosphino)methyl]pyrrolidine gave a dipeptide-connected amidomonophosphane ligand for the highly efficient, copper-catalyzed asymmetric conjugate addition reaction of organozinc reagents with cycloalkenones, giving 3-alkylated cycloalkanones in high enantioselectivity of up to 98 % ee. A model that predicts the stereochemistry of the reaction is discussed.
Up until a few decades ago, chalcogen chemistry was mainly represented by the chemistry of sulfur-containing compounds. However, with the rise in research around selenium and tellurium compound chemistry, the field has developed significantly as it continues to uncover and understand the peculiarities chalcogens exhibit. Taking an introductory approach, this book is the foundation to the subject that has been long needed.
Covering organic and inorganic synthesis, structural properties, coordination chemistry and computational modelling, all key classes of chalcogen compounds are illustrated. Applications across materials science, biology, pharmaceutical science and environmental topics highlight to readers the impact of chalcogen chemistry in many aspects of research.
Edited by international leaders in the field, Chalcogen Chemistry brings together contributions from acclaimed researchers around the world. This book is ideal for newcomers and established researchers, and provides the first building block to uncovering this fascinating field.
Synthesis of a new class of amino phosphinodiselenoic acid ester and β-amino diselenides is described by employing a reaction between Nβ-protected amino alkyl iodide and phosphorus pentaselenide (P2Se5). In the...
The enantioselective dichlorination of alkenes is a continuing challenge in organic synthesis owing to the limitations of selective and independent antarafacial delivery of both electrophilic chlorenium and nucleophilic chloride to an olefin. Development of a general method for the enantioselective dichlorination of isolated alkenes would allow access to a wide variety of polyhalogenated natural products. Accordingly, the enantioselective suprafacial dichlorination of alkenes catalyzed by electrophilic organoselenium reagents has been developed to address these limitations. The evaluation of twenty-three diselenides as precatalysts for enantioselective dichlorination is described, with a maximum e.r. of 76:24 Additionally, mechanistic studies suggest an unexpected Dynamic Kinetic Asymmetric Transformation (DyKAT) process may be operative.
Organoselenium chemistry is now become an important tool in synthetic and medicinal chemistry. Organoselenium reagents are more commonly known as electrophiles but there are few organic transformations where they act as nucleophiles. These reagents have been successfully employed to achieve number of synthetically important transformations such as oxyselenenylations, selenocyclization and selenoxide eliminations etc. In past two decades, another episode of their success is introduced as they have developed as potential catalysts in organic synthesis. Various selenium-catalyzed approaches such as oxidation, reduction, cyclization, rearrangement and stereoselective reactions have been successfully investigated. During these reactions, a number of organic and inorganic oxidants have been employed to regenerate different active catalytic species in situ. In this review article, recently developed selenium-catalyzed reactions are covered including stereoselective reactions.
A simple, efficient and eco-friendly synthetic protocol has been developed for the preparation of diaryl selenium compounds. To the best of our knowledge, this is the first report on the use of selenium dioxide as selenium source for the synthesis of diaryl selenides. Compared with other selenium sources the new source SeO2 gives an environment-friendly and low-cost synthetic route, which may be useful for large-scale synthesis.
The first copper(I)-catalysed conjugate addition of Grignard reagents to α,β-unsaturated carbonyl compounds was reported in 1941. Impressive developments have been made since then, with catalytic asymmetric additions representing the most remarkable achievement. The recent discovery that copper(I) is able to catalyse the asymmetric 1,2-addition of Grignard reagents to α,β-unsaturated, as well as aromatic ketones, was a true revelation. Recent progress in copper(I)-catalysed addition of Grignard reagents is reviewed throughout this chapter, comparing and contrasting the well-established 1,4-selectivity of Cu(I)-ligand complexes with the newly introduced 1,2-selectivity. Mechanistic insights towards the better understanding of the regiodivergence are also discussed.
Introduction
Enantioselective Conjugate Addition to Cyclic Enones
Enantioselective Conjugate Addition to Acyclic Enones
Enantioselective Conjugate Addition to α,β‐Unsaturated Esters and Thioesters
Application of the Conjugate Addition of Grignard Reagents to α,β‐Unsaturated Thioesters in the Synthesis of Natural Products
Mechanistic Studies
Enantioselective Allylic Alkylation with Grignard Reagents
Conclusions
Phenyl or m-tolyl selenoamides reacted with functionalized α-haloacetic acid in 1:1 or 1:2 molar ratio with no catalyst in various alcohols to undergo a selenium transfer reaction. This reaction in different molar ratios furnished the same C-Se-Se-C coupling products with a special structure bis(alkoxycarbonylmethyl) diselenide. A new and facile method for synthesis of such bis(alkoxycarbonylmethyl) diselenides with the advantages of mild reaction conditions, high yields, easy availability of starting materials and good chemical selectivity was provided. In order to investigate the mechanism for this reaction, methyl α-bromoacetate or ethyl α-bromoacetate as starting material was added to the solution of phenyl selenoamide in ethanol in 1:1 or 2:1 molar ratio under the neutral condition at room temperature. The results indicated that the same product bis(methoxycarbonylmethyl) or bis(ethoxycarbonylmethyl) diselenide was formed. Byproduct ethyl benzoate was also separated. A mechanism for this C-Se-Se-C coupling reaction was proposed.
Copper-catalyzed asymmetric 1,4-, 1,6- and 1,2-conjugate addition of Grignard reagents is an important method to build up a carbon chiral center, which exists in the key intermediates of many natural products and chiral drugs. In this paper, the copper-catalyzed asymmetric 1,4-, 1,6- and 1,2-conjugate addition of Grignard reagents to α,β-unsaturated compounds is summarized systematically, and such acceptors as α,β-unsaturated ketones, esters, lactones, aldehydes, thioesters as well as sulfones are involved. In addition, the mechanism of asymmetric 1,4-conjugate addition is also introduced.
A convenient approach has been presented for the synthesis of Nβ-protected amino diselenides from the corresponding amino alkyl iodides using in situ generated NaBH2Se3 as an efficient selenating reagent. All the diselenides are obtained in good yields under very mild conditions, short duration times and the protocol is free from racemization. The methodology has been effectively extended to the synthesis of N-protected l-selenocystine methyl ester. Clean oxidation of Nβ-protected amino diselenides to Nβ-protected amino seleninic acids using 35% aqueous H2O2 has also been accomplished. The present protocol is compatible with all the common urethane protecting groups. This journal is
A novel approach to the synthesis of β-substituted dialkyl diselenides and selenides is described through reaction of bis(trimethylsilyl)selenide with epoxides, thiiranes, and aziridines catalyzed by tetrabutylammonium fluoride. Selective formation of a wide variety of β-hydroxy, β-mercapto, and β-amino diselenides and selenides is achieved by controlling the reaction conditions in the regioselective attack of the silyl selenide onto the ring-strained heterocycles. All reactions occur in a highly regioselective and enantioconservative manner, affording the title compounds with good to high yields. Selenium-77 NMR chemical shifts were measured to verify the selective formation of the β-substituted diselenides and selenides.
A simple and efficient procedure for the synthesis of a new class of selenoesters 4a and 4b was developed. Polarized-light optical microscopy (POM), differential scanning calorimetry (DSC), and X-ray diffraction showed that the selenoester 4a with a shorter alkyl chain displayed a wide nematic range (ΔT = 110.7 °C), while 4b with a longer alkyl chain possesses a rich phase polymorphism: Cr → SmI → SmC → N → I mesophase sequence during heating cycle and I → N → SmC → SmI → SmX → Cr transition on cooling. For 4b, a large temperature range (135.7 °C) from crystal to isotropic phase can be observed. In addition, UV-Vis, steady-state fluorescence emission and excitation spectra in solution were also applied in order to characterize their photophysical behaviour. Compounds 4a and 4b are fluorescent in the blue region and present a Stokes shift at around 80 nm and 39 nm in dichloromethane and dioxane, respectively.
Asymmetric conjugate additions (1,4-additions) of aryl-Grignard reagents to cyclohex-2-enone, currently a more or less unsolved challenge, were investigated. For this purpose, a small library of phenol-derived chiral phosphane-phosphite ligands containing TADDOL- or BINOL-based phosphite moieties was evaluated. These ligands are easily prepared by a short modular scheme previously developed in this laboratory. Two particularly powerful ligands (4a and 4b, both TADDOL-derived and each possessing a bulky tert-butyl substituent ortho to the phosphite group) were identified. Conditions were optimized with use of the addition of (4-methoxyphenyl)magnesium bromide to cyclohexenone as a standard reaction system. Under optimized conditions [CuBr·SMe2 (4 mol-%), ligand 4a (6 mol-%), 2-methyl-THF, –78 °C, slow addition of Grignard reagent] the 1,4-product was obtained with high enantioselectivity (up to 95 % ee) and good regioselectivity (r.r. = 90:10). The scope of the method was probed with different aryl-Grignard reagents. It was found that reagents with electron-donating substituents in meta- or para-positions performed particularly well, whereas the presence of F or CF3 substituents led to decreased ee values. Only ortho-substituted aryl-Grignard reagents did not give rise to useful results. A series of phosphane-phosphite ligands were also tested in the Rh-catalyzed 1,4-addition of phenylboronic acid to cyclohexenone, but enantioselectivities did not exceed 70 % ee in this case.
A new class of bicyclic O,P ligands has been prepared and evaluated in the 1,4-addition to α,β-unsaturated ketones. The best results with the bicyclic ligand were obtained using a lower catalyst loading than what has been reported with similar ligands when adding a Grignard reagent to 2-cyclohexenone.
We have developed an efficient method for the synthesis of diorganyl selenides and β-seleno amines using Zn, catalytic amounts of ZnO nanopowder, as a catalyst and ionic liquid as a recyclable solvent. This ZnO/ionic liquid system shows high efficiency in catalyzing these transformations with the formation of the desired products in high yields.
New organoselenium(ii) halides of the type [RSe](+)X(-) [R = 2,6-(Me2NCH2)2C6H3; X = Cl (), Br (), I ()] were prepared by cleavage of the Se-Se bond in R2Se2 () with SO2Cl2 followed by halogen exchange when organoselenium chloride was treated with NaBr or KI. The reaction between and R'2MCln resulted in new ionic [RSe](+)[R'2MCln+1](-) [R' = 2-(Me2NCH2)C6H4, n = 1, M = Sb (), Bi (); R' = Ph, M = Sb, n = 1 () or n = 3 ()] species. All new compounds were investigated in solution by multinuclear NMR spectroscopy ((1)H, (13)C, (77)Se, 2D experiments) and mass spectrometry. The ionic nature of and the antimonates species was confirmed by conductivity studies. The molecular structures of [{2,6-(Me2NCH2)2C6H3}Se](+)Cl(-)·nH2O (·H2O and ·2H2O) and [{2,6-(Me2NCH2)2C6H3}Se](+)[Ph2SbCl4](-) (), respectively, were established by single-crystal X-ray diffraction, pointing out that the ionic nature of these compounds is also preserved in the solid state, with both nitrogen atoms strongly trans coordinated to the selenium atom of the cation. Theoretical calculations carried out at the DFT level were exploited to investigate the nature of the bonding in compounds and the free cation [RSe](+) (). A topological analysis based on the theory of Atoms-In-Molecules (AIM) and Electron Localization Function (ELF) jointly to a Natural Bond Orbital (NBO) approach was used to shed light on the effect of the nature of the halogen species X on the bonding within the 3c-4e N-Se-N moiety.
Copper (Cu) based nanoparticles mediated organic transformations have been focused worldwide because these methodologies afford industrially important fine chemicals under mild conditions. Moreover, Cu is environment friendly and relatively inexpensive. This review highlights the important developments of Cu based nanocatalysts for organic transformations.
The synthesis and characterization of low-valent organoselenium and -sulfur compounds incorporating sterically unhindered 2-phenyl-2-oxazoline are described. Organylselenenyl halides, RSeX (X=Cl, Br, I) were prepared from diselenide and the benzyl selenide derivative was synthesized by the reaction of in situ generated lithium arylselenolate, OxSe−Li+
(Ox=2-phenyl-2-oxazoline) with benzyl chloride. These compounds in general show strong SeN intramolecular interactions as compared with the substituted oxazoline analogues. Bis[2-(2-oxazolinylphenyl)] disulfide and [2-(2-oxazolinyl)phenyl]benzyl sulfide were synthesized by the ortho-lithiation method and characterized by 1H and 13C NMR spectroscopy. The SN intramolecular interactions were confirmed by single crystal X-ray crystallography.
An efficient and conceptually new protocol for C–Se cross coupling of potassium selenocyanate with aryl halides via copper-catalyzed cascade reaction has been developed in water. Utilizing this protocol, a variety of aryl and heteroaryl halides were reacted with potassium selenocyanate to afford the corresponding diaryl selenides in moderate to good yields.
A straightforward and efficient methodology is described to synthesize structurally diverse diorganyl selenides, sulfides, seleno- and thioesters by using commercially available Zn dust in ionic liquid. Excellent yields were achieved under neutral conditions at room temperature in a short time. The solvent/ionic liquid is reusable and exhibited higher performance as compared with organic solvents.
For Abstract see ChemInform Abstract in Full Text.
A series of alpha-amino acid-derived 1,2,4-oxadiazoles have been synthesized via an easy, inexpensive and one-pot protocol. The heterocycles were obtained in good yields and in relatively short reaction times.
Aziridine sulfides, a new type of chiral S,N-ligand, have been easily synthesized in a straightforward synthetic route, from an inexpensive and easily available chiral pool. They were used in the Pd-catalyzed asymmetric allylic alkylation of 1,3-diphenyl-2-propenyl acetate with dimethymalonate anion, furnishing the alkylated product in excellent yields and stereoselectivity up to 99%.
Herein, we report an eco-friendly synthesis of selenoesters from acyl chlorides catalyzed by recyclable CuO nanopowder in ionic liquid as a recyclable solvent in good to excellent yields. This protocol shows high efficiency in catalyzing this transformation in a greener fashion than previous protocols due to the non-residual methodological design.
Modular chiral thiazolidine derivatives were synthesized in a single step from inexpensive and commercially available starting materials. These ligands catalyzed enantioselective arylation of different aldehydes using aryl boronic acids as a source of transferable aryl groups. The products were obtained in excellent yields and good enantioselectivities.
A set of chiral β-seleno amines were prepared by the ring-opening reaction of unprotected aziridines. Under acid conditions diaryl or dialkyl diselenides were reduced by zinc and treated with unprotected aziridines to produce the desired products in good yields. Chiral β-telluro amine was also obtained using this method.
A simple and efficient procedure for the synthesis of unsymmetrical diorganyl selenides from an one-pot indium(III) catalyzed procedure, in the presence of zinc, has been developed. Various organic halides and even the unreactive organic chlorides underwent the reaction efficiently. Also diaryl and dialkyl diselenides underwent the coupling reaction.
A one-step method for the synthesis of dialkyl diselenides, by reaction of alkyl tosylates with sodium diselenide is described. Three variants of the synthesis, using as an example the preparation of optically active di(cis-myrtanyl) diselenide, are compared.
A new series of modular chiral selenium-containing oxazolines has been synthesized from inexpensive and commercially available l-serine and l-aspartic acid. These new compounds were evaluated as chiral ligands in the palladium-catalyzed asymmetric allylic alkylation reaction, furnishing the product in high enantiomeric excess, using Cs2CO3/CH2Cl2 as the base/solvent system.
An efficient procedure for the oxidative coupling of alkyl, aryl and heteroaryl thiols with atmospheric oxygen is reported. The methodology utilizes BMIM-BF4 as a recyclable solvent and does not require support materials or metal salts. Symmetric disulfides are obtained in excellent yields.
The reaction of sodium diselenide with optically active alkyl tosylates or chlorides is found to be a useful method for the synthesis of optically active dialkyl diselenides. Optically active monoterpene diselenides derived from menthane, carane, pinane, and bornane systems have been obtained. The influence of the terpene fragment of the obtained diselenides on the diastereomeric excess of the methoxyselenylation products has been investigated. The best result for methoxyselenylation was observed for (–)-diisopinocamphyl diselenide. (© Wiley-VCH Verlag GmbH & Co. KGaA, 69451 Weinheim, Germany, 2006)
A set of chiral beta-seleno amines have been efficiently synthesized via the ring-opening reaction of chiral N-acyl oxazolidinones by selenium nucleophiles. These compounds could be transformed into beta-seleno amides by reaction with acid chlorides. The present method is applicable to the synthesis of beta-chalcogeno amides containing selenium, sulfur and tellurium atoms in good yields. Additionally, these new compounds were evaluated as ligands in the palladium-catalyzed asymmetric allylic alkylation, giving the corresponding alkylated products in up to 98% ee.
New chiral sulfides and disulfides were synthesized from readily available and inexpensive cysteine by straightforward methods in order to elucidate the relative importance of the various donor atoms (N, O, S) available in free or alkylated form resulting in co- valent or dative bonds to the metal, respectively. Their application in the addition of diethylzinc to aldehydes provides secondary alcohols with up to 99% ee, and S-configuration, when catalytic amounts of disulfide ligands with the ability to form an S-Zn bond were used. In contrast to this, benzyl alcohols with the opposite absolute configuration R could be achieved, albeit with decreased yield and enantioselectivity, by the use of alkylated sulfide ligands.
A series of modular chiral β-chalcogen amides have been efficiently synthesized from inexpensive and easily available 2-oxazolines. All the selenium, sulfur, and tellurium compounds were evaluated as chiral ligands in the palladium-catalyzed asymmetric allylic alkylation. The corresponding alkylated products were obtained in excellent enantiomeric excess, using BSA/CH2Cl2 as the base/solvent system.
An eco-friendly cross-coupling reaction of aryl and alkyl bromides with diselenides using a catalytic amount of CuO nanopowder as a catalyst and an ionic liquid as a recyclable solvent is reported. The system shows high efficiency to catalyze this transformation, and in a green fashion due to the recyclable approach and the non-residual design protocol. This procedure has been utilized for the synthesis of a variety of diaryl selenides in good to excellent yields from the readily available aryl and alkyl bromides and diselenides.
Chiral selenium-containing compounds have found growing application in asymmetric catalysis over the past few years. The large majority of these ligands are derived from readily available chiral amino alcohols in a few high-yielding synthetic steps. The advantages of using these compounds are described, particularly those regarding ready accessibility, modular nature, formation of strong bonds with soft metals and more rarely with hard ones.
By using an appropriate electrophilic or nucleophilic organoselenium fragment, novel chiral organoselenium catalysts or ligands were designed from chiral aziridines, oxazolines, ferrocenes, etc. High regioselectivity and enantioselectivity were achieved through the four following techniques: the enantioselective copper-catalyzed conjugate addition of organometallic reagents to enones, the diorganozinc addition to aldehydes, palladium-catalyzed enantioselective allylic alkylation and asymmetric hydrosilylation or transfer hydrogenations of ketones.
• 1 Introduction
• 2 Chiral Organoselenium Compounds as Catalysts for Enantioselective Synthesis
• 2.1 Enantioselective Hydrosilylation of Ketones
• 2.2 Enantioselective Addition of Diorganozinc Reagents to Aldehydes
• 2.3 Enantioselective Conjugate Additions to Enones
• 2.4 Palladium-Catalyzed Asymmetric Allylic Alkylations
• 3 Summary and Outlook
Selenium-based methods have developed rapidly over the past few years and certain features of chiral selenium-containing compounds make these reagents particularly valuable for efficient stereoselective reactions. Recent advances in stereoselective transformations involving one-pot selenenylation-deselenylation sequences, which occur using only catalytic amounts of the optically active diselenides in the synthesis of valuable building blocks will be summarized. Additionally, recent results of catalytic reactions using chiral selenides and diselenides such as the enantioselective copper catalyzed conjugate addition of organometallic reagents to enones, diorganozinc addition to aldehydes, palladium-catalyzed enantioselective allylic alkylation, among other topics will also be addressed.
Ammonia and selenoaldehydes are both problematic components in Ugi reactions. Here we report the efficient direct multicomponent synthesis of sensitive selenocysteine peptides without the use of convertible (protected) primary amines, including suitable deprotection protocols for selenols.
In the last decades, organoselenium chemistry has been developed as an important tool for synthetic and medicinal chemistry. The variety of organoselenium reagents has led to many versatile and new approaches in synthetic organic chemistry. Various synthetic moieties can be selectively introduced into complex molecules under very mild reaction conditions. The unique properties of selenium allow the development of novel and highly selective transformations, which can be employed for the synthesis of synthetic and naturally occurring chiral building blocks. This chapter highlights some recent developments of stereoselective reactions using organoselenium reagents including asymmetric catalysis.
A set of chiral β-seleno amides were efficiently synthesized by a simple and efficient procedure involving a ring-opening reaction of 2-oxazoline with diorganyl dichalcogenides mediated by indium(I) iodide. As an application, the synthesis of selenocysteine derivatives was accomplished. (© Wiley-VCH Verlag GmbH & Co. KGaA, 69451 Weinheim, Germany, 2007)
A series of non-natural selenium-containing amino acid derivatives and peptides have been synthesized, in a flexible and modular strategy. The peptide coupling reaction between N-protected amino acids and chiral β-seleno amines afforded the desired products in high yields. (© Wiley-VCH Verlag GmbH & Co. KGaA, 69451 Weinheim, Germany, 2005)
New chiral selenide- and diselenide amino acid derivatives have been synthesized. By a simple and efficient two-step route, these new compounds were obtained from inexpensive and commercially available L-amino acids. The products, with a highly modular character, were obtained in good to excellent yields. Selected examples were also efficiently used as GPx mimics, catalyzing the reduction of H2O2 to water at the expense of PhSH. (© Wiley-VCH Verlag GmbH & Co. KGaA, 69451 Weinheim, Germany, 2009)
The chiral sulfonamide-thiophosphoramide ligand L1, prepared from the reaction of (1R,2R)-(−)-1,2-cyclohexanediamine with diphenylthiophosphoryl chloride and p-toluenesulfonyl chloride, was used as a chiral ligand in Cu(MeCN)4ClO4-promoted catalytic asymmetric addition of diethylzinc to cyclic enones using LiCl as an additive in which up to 90% ee can be realized under mild conditions within 0.5 h. This chiral ligand is stable and recoverable after usual work-up and can be reused in the same catalytic asymmetric reaction. Moreover, it was found that this series of chiral ligands represents a type of S,O-bidentate ligands on the basis of 1H NMR, 31P NMR and 13C NMR spectroscopic investigations. The linear effect of ligand ee and product ee further revealed that the active species is a monomeric Cu(I) complex bearing a single ligand.
An account of the most commonly used reagents for the introduction of the benzeneselenyl (phenyl seleno) group is given. The review focuses on the various methods of its introduction as auxiliary, modifying or protective entity, and its subsequent removal, thereby often promoting other reactions as cyclizations or double bond formation. Less emphasis is laid on reactions of the phenylselenenylated intermediates with the PhSe-group left intact utilizing its stabilizing properties on charged intermediates, on reagents with a modified phenyl group, e.g. chiral derivatives, or on reactions not involving intermediate CSe-bond formation.
A series of chiral enantiomerically pure mercaptoaryl-oxazolines has been synthesized. Copper(I) thiolate complexes derived from these ligands proved to be efficient catalysts for the 1,4-addition of Grignard reagents to cyclic enones. Enantioselectivities increased in the sequence cyclopentenone (16–37% ee) < cyclohexenone (60–72% ee) < cycloheptenone (83–87% ee).
The new chiral compounds, (R,S)-di[2-(1-dimethylaminoethyl)ferrocenyl] dichalcogenides (R,S)-{[EC5H3CHMe(NMe2)]Fe(C5H5)}2(E = S, Se, Te), work effectively as chiral ligands for the rhodium(I)-catalysed asymmetric hydrosilylation of alkyl aryl ketones resulting in high enantiomeric excess and moderate chemical yields.
Three novel selenium-containing binaphthyl compounds (1–3) have been synthesized in optically active form and applied to the asymmetric ring-opening reaction of cyclohexene oxide (4), resulting in enantiomeric excesses of between 16 and 50%.
Four novel optically active bis[(R,S)- and (S,R)-2-(1-dimethylaminoethyl)ferrocenyl] dichalcogenides [abbreviated as (R,S)- and (S,R)-(Fc*E)2(E = S and Te)], each of which possesses two axial and two central elements of chirality, have been prepared by lithiation of commercially available chiral (1-dimethylaminoethyl)ferrocenes, followed by treatment with elemental sulfur or tellurium and air oxidation, in 42–65% isolated yields. The structure of the di-(S,R)-ferrocenyl disulfide has been fully characterized by X-ray crystallography. Ferrocenyl chalcogenide anions (Fc*E–) produced by reduction of the above (Fc*E)2 and the known (Fc*Se)2 with LiAlH4 in tetrahydrofuran are found to act as useful stereoselective nucleophiles for ring opening of meso-epoxides such as cyclohexene oxide, cyclopentene oxide, cyclooctene oxide and cis-stilbene oxide to give the corresponding ferrocenyl β-hydroxyalkyl chalcogenides in high yields with up to 71% diastereoisomeric excess (de). Although the diastereoselectivity depends on the combination of chalcogen atom and meso-epoxide, it is generally higher with E = S and Se than with E = Te. Selenoxide elimination of (S,R)-2-(1-dimethylaminoethyl)ferrocenyl 2-hydroxycyclohexyl selenide (66% de), obtained by the ring opening of cyclohexene oxide with (S,R)-(Fc*Se)2, gives (S)-cyclohex-2-enol with 60% ee. The S-configuration suggests that the ring opening, by the nucleophilic attack of the chiral ferrocenyl chalcogenide anion on the epoxide carbon, mainly occurs via the least hindered transition state considering the interactions between the epoxide, the 1-dimethylaminoethyl moiety and the ferrocene.
A new, easily accessible, chiral disulfide 3 was prepared from l-cysteine in a short synthetic sequence (Scheme. 1) and applied successfully as a highly efficient catalyst for the enantioselective addition of diethyl zinc to aromatic and aliphatic aldehydes to afford the product alcohols in up to more than 99% ee. In contrast to the more common amino alcohols used in similar reactions, catalyst 3 does not have a protic hydrogen in the form of a free hydroxy group.
The enantioselective conjugate addition using a number of scalemic β-amino sulfide ligands to achieve enantiomeric excesses of up to 64% are described together with the X-ray absolute structure analysis of a dimeric copper(I) complex.
Several chiral imidazolidine disulfides 4a–d derived from l-cystine have been synthesized. These ligands have been applied as chiral catalysts in the asymmetric addition of diethylzinc to aldehydes. The best results were obtained by employing 5 mol% of imidazolidine disulfide 4a, and chiral secondary alcohols were obtained in up to 91% e.e.
New chiral diselenides were synthesized from readily available chiral amines. Their application in the catalytic addition of diethylzinc to aldehydes provided the secondary alcohols in up to 98 % ee.
Three types of an external, chiral amide ligand, 2–6, were prepared and examination of their behavior in an asymmetric conjugate addition reaction of lithium dimethylcuprate with chalcone revealed the possibility for steric tuning to realize high selectivity.
Several sulfur-containing optically active C2-symmetrical ligands have been synthesized from (R)-cysteine and applied successfully as chiral catalysts in the asymmetric addition of diethylzinc to aldehydes. The resulting secondary alcohols could be obtained in good yields and excellent enantiomeric excess.
As chiral ligands for transition metal complex-catalyzed asymmetric reactions, a variety of novel chiral ferrocenyl chalcogen compounds, which possess planar chirality due to the 1,2-unsymmetrically disubstituted ferrocene structure, have been prepared from chiral ferrocenes. There are seven diferrocenyl dichalcogenides (4−10), nine alkyl or aryl ferrocenyl chalcogenides (11−19), two bis(ferrocenylseleno)alkanes (20 and 21), two 1-(phenylchalcogeno)-1-[2-(diphenylphosphino)ferrocenyl]ethanes (22 and 24), and two 1-(phenylchalcogeno)-1-[1‘,2-bis(diphenylphosphino)ferrocenyl]ethanes (23 and 25). 2,3-O,O‘-Isopropylidene-2,3-dihydroxy-1,4-bis(phenylchalcogeno)butanes (26−28) are also synthesized. The Rh(I) complex-catalyzed hydrosilylation of ketones with diphenylsilane in the presence of these chiral ligands including the reported [R,S;R,S]-bis[2-[1-(dimethylamino)ethyl]ferrocenyl] dichalcogenides (1−3), followed by hydrolysis with dilute HCl, affords the corresponding chiral alcohols (R-configuration) in moderate to quantitative yield with up to 88% enantiomeric excess (ee). Similar treatment of acetophenone in the presence of diferrocenyl dichalcogenides (1, 2, 3, and 10) and a catalytic amount of Ir(I) complex gives chiral 1-phenylethanol of the opposite configuration (S) compared with the Rh case in high yield with up to 23% ee. The new complex prepared from a cationic rhodium compound and the diferrocenyl diselenide (2) shows an activity for asymmetric hydrosilylation of acetophenone to afford 1-phenylethanol in 60% chemical yield with 60% ee. Asymmetric hydrosilylation of imines and asymmetric hydrogenation of an enamide also proceed smoothly using the Rh(I)−diselenide (2) catalytic system to give the corresponding sec-amines and amide with up to 53% and 69% ee, respectively. A catalytic cycle involving the formation of tetracoordinated rhodium(I)−dichalcogenide complex (two Se and two N atoms to one Rh) followed by oxidative addition of the Si−H bond to Rh(I) and carbonyl addition to the produced rhodium(III) hydride complex is proposed for hydrosilylation of ketones.
Asymmetric methoxyselenenylation of alkenes was studied using some chiral ferrocenylselenium compounds which were prepared from chiral ferrocenyl-substituted amine, sulfoxide, oxazoline, and pyrrolidine. The highest diastereoselectivity was observed using the chiral amino-substituted ferrocenylserenium triflates in the reaction with trans-β-methylstyrene in an excellent yield. The reaction with silyl enol ethers gave chiral α-seleno ketone with moderate to excellent selectivities. The β,γ-unsaturated ester may be converted into the optically active γ-alkoxy α,β-unsaturated ester using ammonium persulfate in the presence of a catalytic amount of the chiral diferrocenyl diselenide in low optical yields.
The reactions of cyclohexenone, cyclopentenone, (E)-3-penten-2-one, and (E)-3-octen-2-one with chiral organo(hetero)cuprates containing Me, n-Bu, or tert-butyl transferable ligands afforded the adducts in optical yields as high as 41-83%. Variation in the chiral nontransferable ligands derived from (S)-proline involves, formally, replacement of the hydroxy group of (S)-prolinol with a methoxy, pyrrolidyl, phenylthio, or methylthio substituent. The extent of asymmetric induction was a function of all the experimental variables while the absolute stereochemistry was dependent upon substrate structure, cuprate composition, and solvent.
The use of nucleophiles bearing chiral centres, the use of α,β-unsaturated systems with the chirality at the γ position and the presence of chiral ligands or other chiral mediators are the more common sources for the control of asymmetry through Michael addition reactions.
A series of new chiral diselenides with a N-atom in the side chain was prepared by a short synthetic sequence (Scheme 1). Only 1 mol-% of these diselenides catalyzed very effectively the diethylzinc addition to various aromatic and α,β-unsaturated aldehydes yielding the secondary alcohols in up to 98% ee (Scheme 2 and Tables 1 and 2). An asymmetric amplification was observed with these catalysts. Detailed NMR studies were performed to characterize the catalytically active species.
The selenyl triflate generated from the reaction of di-2-[(1S)-1-(methylthio)ethyl]phenyl diselenide with silver triflate reacts with various substituted O-allyl oximes to promote ring closure, which affords optically active isoxazolidines in high yields and with good diastereoselectivity (up to 93:7). Enantiomerically enriched 1,3-amino alcohols can be easily obtained by NO bond cleavage of these heterocycles, which was readily effected by treatment with zinc in aqueous acetic acid.
A simple synthesis of two new nitrogen containing diselenides, 7 and 8, is described. These compounds were employed as starting materials to effect the asymmetric methoxyselenenylation, the hydroxyselenenylation as well as the cyclofunctionalization of olefins. Starting from 7 all these addition processes occurred with good facial selectivity. Poor results were however obtained with 8. One-pot selenenylation-elimination sequences using catalytic amounts of the diselenide 7 have also been investigated. (C) 2001 Published by Elsevier Science Ltd.
Short synthetic procedures for the preparation of new chiral, non-racemic nitrogen-containing diselenides are described. Only 1 mol% of these diselenides can very efficiently catalyze the diethylzinc addition to benzaldehydes affording the secondary alcohols in high enantiomeric purities (up to 97% ee).
Selective conjugate addition (0 % enantiomeric excess (e.e.)) of organoarenethiolatocuprates (from methyl lithium and 2-[1-(R)-(dimethylamino)ethyl]phenylthiolatocopper(I), CuSAr*) to benzylideneacetone (BA) is found up to a LiMe/CuSAr* ratio of 21 indicating the potential of the chiral SAr*-anion as non-transferable group; at higher ratios only 1, 2-addition occurs. Reactions of methyl magnesium iodide with BA in the presence of a catalytic amount of CuSAr* (9 mol%) result in exclusive conjugate addition with 57% e.e..
Di[(1R)-2-Oximo-endo-3-bornyl] diselenide (4) and its benzoate derivative 5 were prepared from the corresponding known 2-keto diselenide 1. Treatment of 4 and 5 with bromine, followed by silver triflate in methanol-dichloromethane, generated the corresponding selenenyl triflates 6b and 7b. The latter reagents reacted with a variety of mono-, di-, and trisubstituted alkenes to afford the corresponding 1,2-addition products (beta-methoxy selenides) in a highly diastereoselective manner. The free oxime 6b was particularly effective in such methoxyselenenylations, giving diastereomeric ratios (d.r.s) ranging from 86:14 to > 98:2. Even cis-disubstituted alkenes, which typically give poor d.r.s in similar additions with other chiral selenium electrophiles, underwent highly stereoselective additions with this reagent. Reductive deselenizations of the adducts obtained from styrene and cis- and trans-stilbene provided the corresponding methyl ethers, whose absolute configurations were determined by comparison with authentic samples. As expected, the dominant enantiomers thus obtained from cis- and trans-stilbene, using either 6b or 7b, had opposite configurations. Moreover, each geometrical isomer of stilbene produced methyl ethers with the same configuration when treated with either the oxime 6b or the benzoate 7b. Coordination effects between the substituents at the 2-position of the camphor molecule and the positive selenium atoms in the intermediate seleniranium ions are believed to play an important role in determining the stereochemical outcome of methoxyselenenylations. Selenenyl triflate 6b and selenenyl chloride 7c were also investigated in the electrophilic cyclizations of several unsaturated alcohols and carboxylic acids. However, diastereoselectivities were typically much lower than in the methoxyselenenylations. When the selenenyl bromide 6a, derived from the addition of bromine to the corresponding diselenide 4, was allowed to stand in the absence of an alkene, it underwent intramolecular cyclization with the oxime hydroxyl group, followed by further bromination, to afford the unusual oxaselenazole 11, whose structure was determined by spectroscopic means as well as by X-ray crystallography.
Chiral and achiral organochalcogen compounds bearing a heteroatom in close proximity are easily accessible via the directed aromatic lithiation route. The lithium chalcogenolates prepared by the insertion of selenium or tellurium into the C-Li bond are used to synthesize various chalcogen compounds such as Se/Te, N donor ligands, dichalcogenides, monomeric metal chalcogenolates, and macrocycles. The differences in the stability and reactivity of the organochalcogen compounds derived from various substrates are described in terms of electronic and stereochemical properties of donor atoms.
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