Asymmetric Conjugate Addition Reactions Research Articles

Stereodivergent Construction of 1,3‐Chiral Centers via Tandem Asymmetric Conjugate Addition and Allylic Substitution Reaction

AbstractHerein, we report a synthesis of cyclohexanones bearing multi‐continuous stereocenters by combining copper‐catalyzed asymmetric conjugate addition of dialkylzinc reagents to cyclic enones with iridium‐catalyzed asymmetric allylic substitution reaction. Good to excellent yields, diastereoselectivity and enantioselectivity can be obtained. Unlike the stereodivergent construction of adjacent stereocenters (1,2‐position) reported in the literature, the current reaction can achieve the stereodivergent construction of nonadjacent stereocenters (1,3‐position) by a proper combination of two chiral catalysts with different enantiomers.

Stereodivergent Construction of 1,3-Chiral Centers via Tandem Asymmetric Conjugate Addition and Allylic Substitution Reaction.

Herein, we report a synthesis of cyclohexanones bearing multi-continuous stereocenters by combining copper-catalyzed asymmetric conjugate addition of dialkylzinc reagents to cyclic enones with iridium-catalyzed asymmetric allylic substitution reaction. Good to excellent yields, diastereoselectivity and enantioselectivity can be obtained. Unlike the stereodivergent construction of adjacent stereocenters (1,2-position) reported in the literature, the current reaction can achieve the stereodivergent construction of nonadjacent stereocenters (1,3-position) by a proper combination of two chiral catalysts with different enantiomers.

Polymer-Supported Chiral Heterogeneous Copper Catalyst for Asymmetric Conjugate Addition of Ketones and Imines under Batch and Flow

Enantioselective conjugate addition reactions are powerful methods in synthesizing optically active molecules. Procedures that employ chiral homogeneous catalysts have been developed extensively, but ones that use chiral heterogeneous catalysts are less explored and remain a challenge. Here we report a polymer-supported chiral heterogeneous copper catalyst that demonstrated high reactivity and enantioselectivity in asymmetric conjugate addition reactions of both ketones and imines. It was found that reduced steric hindrance inside the micro catalytic environment and a 1:1 ligand to metal ratio was crucial for the high efficiency and reusability of the chiral heterogeneous catalyst. Various α,β-unsaturated carbonyl substrates, including pharmaceutical intermediates and natural products, could be transformed in high yields with high enantioselectivities. Successful application of this methodology in a continuous flow fashion further broadened its usage in dealing with sensitive organozinc reagents. In addition, production was successfully scaled up under the flow condition.

Cinchona Derivatives as Bifunctional H‐bonding Organocatalysts in Asymmetric Vinylogous Conjugate Addition Reactions

AbstractAsymmetric vinylogous conjugate addition reactions constitute an essential class of enantioselective C−C bond‐forming transformations characterized by remote functionalization of substrates and the construction of distal stereocenters. After the renaissance of organocatalysis in 2000, vinylogous addition methodologies have seen new horizons of development. Cinchona alkaloids and some of their derivatives have appeared as efficient chiral catalysts in several organic transformations. Their non‐toxicity, low cost, availability in pseudo‐enantiomeric forms, and “open flask” reaction procedures have been the main reasons behind their success in asymmetric synthesis. These organocatalysts have also shown remarkable applications in vinylogous chemistry. This mini‐review covers the developments of asymmetric vinylogous conjugate addition (VCA) reactions under bifunctional cinchona alkaloids/derivatives catalysis.

Reversal of Enantioselectivity in the Conjugate Addition Reaction of Cyclic Enones with the CuOTf/Azolium Catalytic System

Hydroxyamide-functionalized azolium salt (NHC•HI 4) was evaluated for dual enantioselective control in a Cu-catalyzed asymmetric conjugate addition (ACA) reaction. This investigation was based on our previously reported ACA reaction catalyzed using CuOTf combined with NHC•AgI complex 1. It was revealed that the stereocontrol of the catalytic ACA reaction depended on the order of the addition of the substrates. Additionally, the chiral NHC ligand precursors, substrates, the relationship between the catalyst ee (eecat) and product ee (eepro), and halogen counter anion were completely evaluated. These results suggested that the catalytic performance of the CuOTf/4 system was comparable with that of the CuOTf/1 system. Furthermore, to gain knowledge of the Cu species generated using CuOTf and NHC ligand precursor, the reaction of CuOTf with 1 was investigated. Although obtaining the corresponding NHC•CuX species failed, the corresponding NHC•AuCl complex 11 could be synthesized by allowing 1 to react with AuCl•SMe2.

Asymmetric conjugate addition reactions with chiral oxadiazinones: Unusual conformational properties of the oxadiazinones

Abstract A series of Ephedra based oxadiazinones have been prepared, acylated, and examined in the asymmetric conjugate addition reaction with Grignard reagents in the presence of copper(I) bromide-dimethyl sulfide complex. The highest diastereoselectivity that was obtained in the conjugate addition reaction was observed with the (1R,2S)-ephedrine based N4-methyloxadiazinone (5:1 d.r.) favoring the formation of the (S)-configuration of the conjugate addition product. Efforts to enhance the level of diastereoselection via increasing the steric volume of the stereo-directing N4-substituent of the oxadiazinone (N4- = p-methoxyphenyl or -isopropyl) led to an observed decrease in the level of diastereoselection. A computational study was conducted to examine the conformations adopted by the N4-methyloxadiazinone vs. the N4-isopropyl-oxadiazinone that yielded the lower diastereoselectivity. An argument is made for the stereoelectronic properties of the N4-substituent being the cause of both the moderate diastereoselectivity and the unexpected facial preference for the conjugate addition.

Restoration of catalytic activity by the preservation of ligand structure: Cu-catalysed asymmetric conjugate addition with 1,1-diborylmethane.

Reported herein is a novel reaction engineering protocol to enhance the efficiency of a transition metal-catalysed process by strategically preventing ligand degradation. Based on spectroscopic investigations, a decomposition pathway of a chiral phosphoramidite ligand during a Cu-catalysed reaction was identified. The involvement of the destructive process could be minimized under the modified reaction conditions that control the amount of nucleophilic alkoxide base, which is the origin of ligand decomposition. Overall, the strategy has been successfully applied to a new class of asymmetric conjugate addition reactions with bis[(pinacolato)boryl]methane, in which α,β-unsaturated enones are utilised as substrates.

Asymmetric Conjugate Addition of α-Cyanoketones to Benzoyl Acrylonitrile Derivatives Using a Diaminomethylenemalononitrile Organocatalyst

AbstractA diaminomethylenemalononitrile (DMM) organocatalyst was used to efficiently promote asymmetric conjugate addition of various α-cyanoketones to benzoyl acrylonitrile derivatives. The corresponding 1,5-dicarbonyl compounds containing vicinal tertiary and quaternary stereogenic centers are versatile synthetic intermediates and were obtained in good yields and with excellent enantioselectivities (up to 96% ee). The present study describes the first successful examples of asymmetric conjugate addition reactions of α-cyanoketones with benzoyl acrylonitriles. In addition, the DMM organocatalyst can be recovered and reused up to five times without significant loss of either catalytic activity or enantioselectivity.

Total synthesis of (+)-pilosinine via a stereodivergent conjugate addition strategy

In recent work, asymmetric conjugate addition reactions to chiral 4-phenyl-N-enoyl-1,3-oxazolidinones have been shown to give different stereochemical outcomes depending on the conditions employed. Through the application of stereodivergent reaction conditions, the total synthesis of (+)-pilosinine and the formal synthesis of (−)-pilosinine has been completed from a single enantiomer of the 1,3-oxazolidinone auxiliary.

(±)-trans-1,2-Cyclohexanediamine-Based Bis(NHC) Ligand for Cu-Catalyzed Asymmetric Conjugate Addition Reaction

Bis(NHC) ligand precursors, L1, based on trans-1,2-diaminocyclohexane were designed and synthesized. To introduce chirality at the hydroxyamide side arm on the NHC of L1, a chiral β-amino alcohol, such as enantiopure leucinol, was used. Cu-catalyzed asymmetric conjugate addition reactions of cyclic and acyclic enones with Et2Zn were selected to evaluate the performance of L1 as a chiral ligand. For the reaction of cyclic enone, a combination of [bis(trimethylsilyl)acetylene]-(hexafluoroacetylacetonato)copper(I) (Cu(hfacac)(btmsa)) with a (±)-trans-1,2-cyclohexanediamine-based bis(NHC) ligand precursor, (rac; S,S)-L1, which was prepared from (S)-leucinol, was the most effective. Thus, treating 2-cyclohexen-1-one (3) with Et2Zn in the presence of catalytic amounts of Cu(hfacac)(btmsa) and (rac; S,S)-L1 afforded (R)-3-ethylcyclohexanone ((R)-4) with 97% ee. Similarly, use of (rac; R,R)-L1, which was prepared from (R)-leucinol, produced (S)-4 with 97% ee. Conversely, for the asymmetric 1,4-addition reaction of the acyclic enone, optically pure (−)-trans-1,2-cyclohexanediamine-based bis(NHC) ligand precursor, (R,R; S,S)-L1, worked efficiently. For example, 3-nonen-2-one (5) was reacted with Et2Zn using the CuOAc/(R,R; S,S)-L1 catalytic system to afford (R)-4-ethylnonan-2-one ((R)-6) with 90% ee. Furthermore, initially changing the counterion of the Cu precatalyst between an OAc and a ClO4 ligand on the metal reversed the facial selectivity of the approach of the substrates. Thus, the conjugate addition reaction of 5 with Et2Zn using the Cu(ClO4)2/(R,R; S,S)-L1 catalytic system, afforded (S)-6 with 75% ee.

Organocatalytic enantioselective conjugate addition of pyrazolin-5-ones to arylomethylidene malonates.

An efficient asymmetric organocatalytic conjugate addition reaction of pyrazolin-5-ones with arylomethylidene malonates has been successfully developed by employing bifunctional Cinchona derived thiourea organocatalysts. The protocol provides straightforward access to pyrazole substituted scaffolds in good yields with high stereocontrol.

Recent Advances in Metal-Catalyzed Asymmetric 1,4-Conjugate Addition (ACA) of Nonorganometallic Nucleophiles.

The metal-catalyzed asymmetric conjugate addition (ACA) reaction has emerged as a general and powerful approach for the construction of optically active compounds and is among the most significant and useful reactions in synthetic organic chemistry. In recent years, great progress has been made in this area with the use of various chiral metal complexes based on different chiral ligands. This review provides comprehensive and critical information on the enantioselective 1,4-conjugate addition of nonorganometallic (soft) nucleophiles and their importance in synthetic applications. The literature is covered from the last 10 years, and a number of examples from before 2007 are included as background information. The review is divided into multiple parts according to the type of nucleophile involved in the reaction (such as C-, B-, O-, N-, S-, P-, and Si-centered nucleophiles) and metal catalyst systems used.

DFT Study of the Strong Solvent Effects in the Cu‐Catalyzed Asymmetric Conjugate Addition Reaction

Significant solvent effects on enantioselection exist in the reaction of Cu‐catalyzed asymmetric conjugate addition of alkylzinc reagents to α,β‐unsaturated carbonyl compounds. Results of our DFT study suggest that solvent effects on enantioselection can be caused by structural changes in the stereodetermining Transition States (TS). We have seen how unconventional hydrogen bonding of the nucleophilic solvent results in a change of the computed optical yield comparable to that caused by well‐known coordination of the solvent to the electrophilic metal center.

Inversions in asymmetric conjugate addition reaction of cyclic enones catalyzed by the Cu/NHC-AgX system: Factors affecting the stereoselective formation of both enantiomers

A switchable enantioselectivity was achieved in a Cu-catalyzed asymmetric conjugate addition (ACA) reaction. The ethylene-bridged, hydroxyamide-functionalized NHC-AgI complex, readily accessible from a chiral β-amino alcohol, was found to be a versatile chiral ligand precursor for dual enantioselective control. The stereocontrol of the catalytic ACA reaction depended on the order of addition of the substrates. To a THF solution containing CuOTf (6 mol %), NHC-AgI complex (1b, R1 = Et, R2 = Bn, 4 mol %), and 2-cyclohexen-1-one (3) was added Et2Zn to yield (R)-3-ethylcyclohexanone ((R)-4) in 74% enantioselectivity (method A). In stark contrast, when 3 was added as a last component to a mixture of CuOTf (4 mol %), 1b (10 mol %), and Et2Zn in THF (method B), the ACA reaction afforded (S)-4 with 87% ee. In order to obtain an insight into various aspects of the present ACA reaction, the chiral NHC ligand precursors, substrates, and NHC skeletons were fully evaluated. A study of the relationship between the catalyst ee (eecat) and the product ee (eepro) revealed a chiral amplification phenomenon. Moreover, it was found that the ACA reaction using method A was not affected by the halide ion on the NHC-AgX complex, whereas the halide ion on the silver complex showed a significant influence on the enantiocontrol of the ACA reaction using method B. In particular, a Cu/NHC/I catalytic ratio of 1/1/2 was needed for a successful ACA reaction using method B. According to these factors affecting the reversal of enantioselectivity in the ACA reaction, a possible reaction pathway was proposed.

Recent Advances in Asymmetric Organocatalyzed Conjugate Additions to Nitroalkenes.

The asymmetric conjugate addition of carbon and heteroatom nucleophiles to nitroalkenes is a very interesting tool for the construction of highly functionalized synthetic building blocks. Thanks to the rapid development of asymmetric organocatalysis, significant progress has been made during the last years in achieving efficiently this process, concerning chiral organocatalysts, substrates and reaction conditions. This review surveys the advances in asymmetric organocatalytic conjugate addition reactions to α,β-unsaturated nitroalkenes developed between 2013 and early 2017.

ChemInform Abstract: Peptide‐Catalyzed Stereoselective Conjugate Addition Reactions of Aldehydes to Maleimide.

AbstractThe tripeptide H‐dPro‐Pro‐Asn‐NH2 is presented as catalyst for asymmetric conjugate addition reactions of aldehydes to maleimide.

A Multifaceted Directing Group Switching Ynones as Michael Donors in Chemo-, Enantio-, and γ-Selective 1,4-Conjugate Additions with Nitroolefins.

α,β-Unsaturated ynones have historically been used as Michael acceptors in conjugate addition reactions. Herein, we have demonstrated for the first time that ynones can be harnessed as Michael donors for use in catalytic asymmetric conjugate addition reactions by strategically introducing a CO2t-Bu group as a multitasking directing group. Furthermore, this concept has enabled designer ynones as versatile synthetic equivalents of both α' anions of ynones and γ monoanions of 1,3-diketones, which are synthetically valued but difficult to generate. The first catalytic enantioselective conjugate addition of ynones as Michael donors has been realized in good yields with high enantioselectivities. A unified approach to regiospecifically and chemo- and enantioselectively access hitherto elusive γ-Michael adducts of 1,3-diketones has been achieved in a divergent manner. The strategy described here by exploring new reactivity and creating new reagents holds great potential applications in other still unsolved transformations.

Synthetic and Organocatalytic Studies of Quinidine Analogues with Ring-Size Modifications in the Quinuclidine Moiety.

Six highly enantiopure analogues of [2.2.2] were synthesized with five- or seven-membered rings in the (original) quinuclidine skeleton. Five of these compounds were prepared through epoxide opening by a secondary cyclic amine, providing the nor- and homoquinuclidine moieties through five- and six-membered ring formation. This method failed in the case of seven-membered ring formation, so for that particular ring size a different synthetic route starting from 3-quinuclidone was applied. The six novel analogues were examined as organocatalysts in four asymmetric conjugate addition reactions and the results compared with those of known cinchona alkaloid catalysts. This study shows that modification of the quinuclidine ring can have a substantial influence on catalyst activity and enantioselectivity. To acquire more insight into the characteristics of the new catalysts, the pKaH values were determined by means of fluorescence spectroscopy. Furthermore, relative reaction rates of conjugate thiol additions reactions catalyzed by these quinidine analogues were measured through polarimetry.

Peptide‐Catalyzed Stereoselective Conjugate Addition Reactions of Aldehydes to Maleimide

AbstractThe tripeptide H‐dPro‐Pro‐Asn‐NH2 is presented as a catalyst for asymmetric conjugate addition reactions of aldehydes to maleimide. The peptidic catalyst promotes the reaction between various aldehydes and unprotected maleimide with high stereoselectivities and yields. The obtained products were readily derivatized to the corresponding pyrrolidines, lactams, lactones, and peptide‐like compounds. 1H NMR spectroscopic, crystallographic, and computational investigations provided insight into the conformational properties of H‐dPro‐Pro‐Asn‐NH2 and revealed the importance of hydrogen bonding between the peptide and maleimide for catalyzing the stereoselective C−C bond formation.

Peptide-Catalyzed Stereoselective Conjugate Addition Reactions of Aldehydes to Maleimide.

The tripeptide H-dPro-Pro-Asn-NH2 is presented as a catalyst for asymmetric conjugate addition reactions of aldehydes to maleimide. The peptidic catalyst promotes the reaction between various aldehydes and unprotected maleimide with high stereoselectivities and yields. The obtained products were readily derivatized to the corresponding pyrrolidines, lactams, lactones, and peptide-like compounds. (1) H NMR spectroscopic, crystallographic, and computational investigations provided insight into the conformational properties of H-dPro-Pro-Asn-NH2 and revealed the importance of hydrogen bonding between the peptide and maleimide for catalyzing the stereoselective C-C bond formation.