Asymmetric Conjugate Addition Research Articles

Organocatalyzed Asymmetric Conjugate Addition of Alcohols to β-Fluoroalkyl Vinylsulfones by Bifunctional Phosphonium Salt Catalyst.

Chiral secondary alcohols, serving as essential structural motifs, hold significant potential for diverse applications. The exploration of effective synthetic strategies toward these compounds is both attractive and challenging. Herein, we present an asymmetric oxa-Michael reaction involving aliphatic alcohols as nucleophiles and β-fluoroalkyl vinylsulfones catalyzed by bifunctional phosphonium salt (BPS), achieving high yields and excellent enantioselectivities (up to 98% yield and 98% ee). Additionally, a sequential process including asymmetric oxa-Michael and debenzylation, facilitated by BPS/Lewis acid cooperation, was revealed for synthesizing diverse chiral secondary alcohol compounds in high yields (81-88%) with consistent stereoselectivities. Furthermore, mechanistic explorations and subsequent results unveiled that the enantioselectivity originates from hydrogen-bonding and ion-pair interactions between the BPS catalyst and the substrates.

Cellulose supported TiO2/Cu2O for highly asymmetric conjugate addition of α,β-unsaturated compounds in aqueous phase

A series of new kind green cellulose-supported bimetallic TiO2/Cu2O (Cell@TiO2/Cu2O) catalytic materials were obtained by in-situ reduction method employing cellulose as the carrier. The effects of metal percentage composition on the morphology and construction of the catalytic materials were systematically investigated. The Cell@TiO2/Cu2O were characterized by FT-IR, TG, XPS, SEM, TEM, EDS, and the element content was obtained by elemental analysis. Then, the achieved catalytic materials were applied to the chiral borylation reaction of α,β-unsaturated compounds, including nitrile compounds, esters, and α,β-unsaturated ketones. Remarkably, this approach provides an efficient strategy to gain an important class of chiral organic boron compounds with target chiral products in high yields as well as enantioselectivities. Besides, the Cell@TiO2/Cu2O could be easily recycled and effectively reused. This work constructed bimetallic TiO2/Cu2O on cellulose as a newly catalyst to obtain chiral boron compounds in aqueous phase.

Organocatalytic Asymmetric Conjugate Addition of Fluorooxindoles to Quinone Methides.

Fluorooxindoles undergo asymmetric Michael addition to para-quinone methides under phase-transfer conditions with 10 mol% of a readily available cinchona alkaloid ammonium catalyst. This reaction affords sterically encumbered, multifunctional fluorinated organic compounds displaying two adjacent chirality centers with high yields, ee's and dr's.

Catalytic Asymmetric Synthesis of Vicinal Quaternary Stereocenters Enabled by Alkylation of α,α-Disubstituted Aldehydes with 3-Bromooxindoles.

An organocatalytic enantioselective alkylation of α,α-disubstituted aldehydes with 3-bromooxindoles is reported. Enantioenriched oxindoles with vicinal quaternary stereocenters are accessed by an asymmetric conjugate addition process of branched aldehydes with o-azaxylylene intermediates (indol-2-ones). Key to the success of highly diastereo- and enantioselective transformations is the combined use of a triphenylsilyl-protected β-amino alcohol catalyst derived from the spiropyrrolidine scaffold and 3,5-dinitrobenzoic acid. This study also presents a rare example of aldehyde alkylation with the formation of consecutive quaternary stereocenters.

Regio‐ and Enantioselective Synthesis of Succinimides Bearing All‐Carbon Quaternary Centers Using a Chiral Phenanthroline‐Palladium Catalyst

AbstractThe synthesis of chiral succinimides bearing all‐carbon quaternary stereocenters at the C3 position is important, but remains challenging. The present work demonstrates conjugate additions with catalysis by a chiral 1,10‐phenanthroline‐Pd complex (L1‐PdCl2) that exhibit complete regioselectivity with a high degree of enantioselectivity. These reactions afford chiral 3,3‐disubstituted succinimides having all‐carbon quaternary stereocenters with 40–99% ee. Importantly, chiral 3,3‐diaryl succinimides could also be obtained in 35–98% yields and 40–99% ee. Moreover, the present L1‐PdCl2‐catalyzed asymmetric conjugate addition could be performed on the gram scale and was also used to introduce such stereocenters into bioactive molecules.

Persistent guaiazulene arylmethylium ions as electrophilic traps for metal enolates.

Guaiazulene-stabilized cations reacted with metal enolates affording carbonyl compounds with an azulene moiety. Metal enolates generated by asymmetric conjugate addition of organometallic reagents led to enantioenriched products. Additionally, guaiazulene-substituted cations efficiently react with silyl enol ethers. DFT calculations allowed estimation of the electrophilicities of the carbocations. Reaction progress was monitored by a decrease in the reactant's Vis-light absorption and an increase in the product's anti-Kasha emission.

Catalytic asymmetric conjugate addition of coumarins to unsaturated ketones catalyzed by a chiral-at-metal Rh(III) complex.

The first catalytic asymmetric vinylogous Michael addition of coumarins to unsaturated ketones catalyzed by chiral rhodium catalysts has been established. This strategy allowed the synthesis of a variety of highly enantioenriched compounds containing coumarin skeletons in 41-99% yields and 84-99% ee. The developed reaction enriches the chemistry of catalytic asymmetric vinylogous Michael additions of 3-cyano-4-methylcoumarins. Furthermore, the protocol showed obvious advantages in reaction enantioselectivity. When the chiral rhodium catalyst was reduced to 0.06 mol%, a Gram-level reaction was still achieved to provide the desired products with 99% ee.

Organocatalyzed Synthesis of Bicyclic γ‐Lactam Derivatives via Asymmetric Conjugate Addition of Cyclic β‐Keto Esters to Benzoyl Acrylonitriles

AbstractA diaminomethylenemalononitrile organocatalyst efficiently promoted the asymmetric conjugate addition of cyclic β‐keto esters to benzoyl acrylonitriles to afford the corresponding adducts. This was followed by the transformation of the nitrile group into an amide group, resulting in bicyclic γ‐lactam derivatives in good yields with excellent enantioselectivities (up to 99 % ee). As far as we know, this is the first study to synthesize chiral bicyclic γ‐lactam derivatives from cyclic β‐keto esters and benzoyl acrylonitriles.

Expedient recycling of peptide organocatalysts using a nanocrystalline hydroxyapatite catching system

Peptide catalysts represent versatile alternatives to metal-based catalysts and other organocatalysts, due to facile synthesis, modularity, and often high activity and stereoselectivity. In particular, in tripeptides the distance between the catalytic groups is expected to promote 1,4-addition reactions. Immobilization of the catalyst allows easy recovery, but this strategy requires the modification of the sequence by introducing a suitable linker, with the risk of losing catalyst effectiveness. Another option is the inclusion or adsorption on an appropriate material. Herein, we describe the expedient use of nanocrystalline hydroxyapatite (HAp) as a biocompatible, inert peptide-catching system. In contrast to the immobilization strategy, HAp can be utilized with the native sequences. Encouraging results have been obtained in the capture and recycle of the peptide-acid Pro-Pro-AspNH2, utilized for the asymmetric conjugate addition of aldehydes to nitrostyrene.

Organocatalytic Michael Addition of Unactivated α-Branched Nitroalkanes to Afford Optically Active Tertiary Nitrocompounds.

The direct, asymmetric conjugate addition of unactivated α-branched nitroalkanes is developed based on the combined use of chiral amine/ureidoaminal bifunctional catalysts and a tunable acrylate template to provide tertiary nitrocompounds in 55-80% isolated yields and high enantioselectivity (e.r. up to 96:4). Elaboration of the ketol moiety in thus obtained adducts allows a fast entry to not only carboxylic and aldehyde derivatives but also nitrile compounds and enantioenriched 5,5-disubstituted γ-lactams.

Catalytic Asymmetric Vinylogous Conjugate Addition of Butenolide to 2-Ester-Substituted Chromones: Access to Chiral Chromanone Lactones via Trapping of a Copper(I) Enolate by Trimethyl Borate.

A copper-catalyzed asymmetric vinylogous conjugate addition of butenolide to 2-ester-substituted chromones is described, and it delivers syn- or anti-chromanone lactones with high stereoselectivities. The enantioselectivity-determining step varied with the use of B(OMe)3 as an additive, resulting in enhanced stereoselectivities, as revealed by density functional theory calculations, which also provided theoretical insight into the origin of the ligand-dependent diastereodivergence.

Cage-Shaped Phosphites Having C3 -Symmetric Chiral Environment: Steric Control of Lewis Basicity and Application as Chiral Ligands in Rhodium-Catalyzed Conjugate Additions.

Designing chiral ligands with an axial symmetry higher than C2 -rotational symmetry is one of the most crucial approaches to improving enantioselectivity in asymmetric synthesis. Herein, C3 -symmetric chiral cage-shaped phosphites are reported. Their Lewis basicity and chiral environment are precisely controlled by the tethered group. The cage-shaped phosphites successfully worked as chiral ligands in Rh-catalyzed asymmetric conjugate additions, realizing acceptable yields with excellent enantioselectivity, and were used to synthesize a pharmacologically important molecule.

Catalytic Enantioselective Alkylation of Aldehydes with 3-Bromooxindoles.

An asymmetric conjugate addition of aldehydes with o-azaxylylene intermediates (indol-2-ones) from 3-bromooxindoles has been developed. The use of a novel spiro-pyrrolidine (SPD)-derived bifunctional N-sulfonylated amide catalyst is essential for a highly diastereo- and enantioselective transformation to provide a wide array of enantioenriched C3 quaternary oxindoles with structurally diverse β-aldehyde appendages. Further application of this synthetic methodology enables the construction of the tricyclic cores of akuammiline-type alkaloids.

Recent Advancement on the Organocatalyzed Asymmetric Conjugate Addition using Maleimide as a Potential Substrate

AbstractAsymmetric organocatalytic 1,4‐conjugate addition is considered as an effective, powerful and sustainable method for carbon‐carbon bond formation. The core structure of maleimides or succinimides were found in several bioactive molecules as well as in natural products. Unprotected and protected maleimides were employed as an electrophile to construct novel succinimide based organic molecules. The rapid development of several new effective and prominent methodologies encourages us to write a review by summarizing the recent updates. In this review, we present an overview of the past 10 years progress on the asymmetric organocatalytic 1,4 conjugate additions using unprotected and protected maleimides. Special attention has been paid classifying the types of nucleophiles as well as catalysts employed for this asymmetric transformation along with substrate scope, limitation, mechanism, and synthetic utilities for each reaction. Knowing the versatility of maleimides, this review will encourage active researchers around the globe to put more efforts on the development of new methodologies including asymmetric photo redox catalysis for further functionalization of maleimides.

Copper(I)‐Catalyzed Asymmetric Hydrophosphination of 3,3‐Disubstituted Cyclopropenes

AbstractHerein, a copper(I)‐catalyzed asymmetric hydrophosphination of 3,3‐disubstituted cyclopropenes is reported. It provides a series of phosphine derivatives in high to excellent diastereo‐ and enantioselectivities. The methodology enjoys broad substrate scope on both 3,3‐disubstituted cyclopropenes and diarylphosphines. The high stereoselectivity is attributed to both the high stability of the Cu(I)‐(R,R)‐QUINOXP* complex in the presence of stoichiometric HPPh2 and the produced phosphines, and the high‐performance asymmetric induction of the Cu(I)‐(R,R)‐QUINOXP* complex. Finally, the method is used for the synthesis of new chiral phosphine‐olefin compounds built on a cyclopropane skeleton, one of which serves as a wonderful ligand in Rh‐catalyzed asymmetric conjugate addition of phenylboronic acid to various α,β‐unsaturated compounds.

Copper(I)-Catalyzed Asymmetric Hydrophosphination of 3,3-Disubstituted Cyclopropenes.

Herein, a copper(I)-catalyzed asymmetric hydrophosphination of 3,3-disubstituted cyclopropenes is reported. It provides a series of phosphine derivatives in high to excellent diastereo- and enantioselectivities. The methodology enjoys broad substrate scope on both 3,3-disubstituted cyclopropenes and diarylphosphines. The high stereoselectivity is attributed to both the high stability of the Cu(I)-(R,R)-QUINOXP* complex in the presence of stoichiometric HPPh2 and the produced phosphines, and the high-performance asymmetric induction of the Cu(I)-(R,R)-QUINOXP* complex. Finally, the method is used for the synthesis of new chiral phosphine-olefin compounds built on a cyclopropane skeleton, one of which serves as a wonderful ligand in Rh-catalyzed asymmetric conjugate addition of phenylboronic acid to various α,β-unsaturated compounds.

Palladium-Catalyzed Asymmetric Conjugate Addition of Arylboronic Acids to α,β-Unsaturated Lactams: Enantioselective Construction of All-Carbon Quaternary Stereocenters in Saturated Nitrogen-Containing Heterocycles.

Stereogenic nitrogen-containing heterocycles are ubiquitous in natural products and pharmaceutical compounds, but methods for their enantioselective construction have remained elusive. We report a general method for the asymmetric conjugate addition of arylboronic acids to β-alkyl/aryl α,β-unsaturated lactams that affords chiral β,β-disubstituted lactams. The transformation is operationally simple and air- and moisture-tolerant and uses a commercially available (S)-t-Bu-PyOx ligand. The method is high-yielding (up to 95% yield) and enantioselective (up to 97% ee) for a wide range of arylboronic acids and α,β-unsaturated lactams, including those with different ring sizes.

Advances in Catalytic Asymmetric Construction of Quaternary Stereogenic Centers by Conjugate Addition to β,β‐Disubstituted Nitroalkenes

AbstractQuaternary stereogenic centers are of great importance because of their prevalence in a series of bioactive natural products and pharmaceuticals. Although the catalytic asymmetric construction of these highly congested centers poses a formidable challenge, this field has been extensively explored in the past few decades, and several elegant strategies, such as the asymmetric conjugate addition to β,β‐disubstituted nitroalkenes, have been developed. The resultant β‐nitro quaternary stereocenters can be conveniently transformed into the corresponding β‐amino quaternary stereocenters commonly found in bioactive compounds. This review summarizes the recent advances in the construction of all‐carbon quaternary or hetero‐carbon quaternary stereocenters via metal‐catalyzed and organocatalyzed asymmetric conjugate addition to β,β‐disubstituted nitroalkenes, focusing on the scope, applications, and mechanisms of these reactions.

Novel Strategies for Enantio- and Site-Selective Molecular Transformations.

A strategy for symmetric synthesis based on dynamic chirality of enolates (memory of chirality) has been developed. Asymmetric alkylation, conjugate addition, aldol reaction, and arylation via C-N axially chiral enolate intermediates are described. Asymmetric alkylation and conjugate addition via C-O axially chiral enolate intermediates with a half-life of racemization as short as approx. 1 s. at -78 °C have been accomplished. Organocatalysts for asymmetric acylation and site-selective acylation have been developed. Kinetic resolution of racemic alcohols via remote asymmetric induction by the catalyst is shown. Catalyst-controlled site-selective acylation of carbohydrates and its application to total synthesis of natural glycoside are described. Chemo-selective monoacylation of diols and selective acylation of secondary alcohols with reversal of inherent reactivity are also discussed. Geometry-selective acylation of tetrasubstituted alkene diols is achieved, where acylation takes place independent from the steric environments of the substrates.

Asymmetric tandem conjugate addition and reaction with carbocations on acylimidazole Michael acceptors.

We present here a stereoselective tandem reaction based on the asymmetric conjugate addition of dialkylzinc reagents to unsaturated acylimidazoles followed by trapping of the intermediate zinc enolate with carbocations. The use of a chiral NHC ligand provides chiral zinc enolates in high enantiomeric purities. These enolates are reacted with highly electrophilic onium compounds to afford densely substituted acylimidazoles. DFT calculations helped to understand the reactivity of the zinc enolates derived from acylimidazoles and allowed their comparison with metal enolates obtained by other conjugate addition reactions.