Stereoselective Michael Additions of Arylacetic Acid Derivatives by Asymmetric Organocatalysis

Because of the versatility of chiral 1,5-dicarbonyl structural motifs, the development of stereoselective Michael additions of arylacetic acid derivatives to electron-deficient alkenes is an important challenge. Over recent decades, an array of enantio- and diastereoselective methods of this type have been developed through the use of chiral organocatalysts. In this article, three distinct strategies in this research area are highlighted. Catalytic generation of either a chiral iminium electrophile (iminium catalysis) or a chiral enolate nucleophile (Lewis­ base catalysis) has allowed the efficient construction of stereogenic C–C bonds. We also introduce a synergistic catalytic approach involving the merger of these two catalytic cycles that provides selective access to all four stereoisomers of products with vicinal stereocenters.1 Introduction2 Iminium Catalysis3 Lewis Base Catalysis4 Synergistic Organocatalysis5 Summary

A Versatile Method for Preparing Polysaccharide Conjugates via Thiol-Michael Addition

Polysaccharide conjugates are important renewable materials. If properly designed, they may for example be able to carry drugs, be proactive (e.g., with amino acid substituents) and can carry a charge. These aspects can be particularly useful for biomedical applications. Herein, we report a simple approach to preparing polysaccharide conjugates. Thiol-Michael additions can be mild, modular, and efficient, making them useful tools for post-modification and the tailoring of polysaccharide architecture. In this study, hydroxypropyl cellulose (HPC) and dextran (Dex) were modified by methacrylation. The resulting polysaccharide, bearing α,β-unsaturated esters with tunable DS (methacrylate), was reacted with various thiols, including 2-thioethylamine, cysteine, and thiol functional quaternary ammonium salt through thiol-Michael addition, affording functionalized conjugates. This click-like synthetic approach provided several advantages including a fast reaction rate, high conversion, and the use of water as a solvent. Among these polysaccharide conjugates, the ones bearing quaternary ammonium salts exhibited competitive antimicrobial performance, as supported by a minimum inhibitory concentration (MIC) study and tracked by SEM characterization. Overall, this methodology provides a versatile route to polysaccharide conjugates with diverse functionalities, enabling applications such as antimicrobial activity, gene or drug delivery, and biomimicry.

Origin of Enantioselectivity Reversal in Lewis Acid-Catalysed Michael Additions Relying on the Same Chiral Source

Enantiodivergence is an important concept in asymmetric catalysis that enables access to both enantiomers of a product relying on the same chiral source as reagent. This strategy is particularly appealing as an alternate approach when only one enantiomer of the required chiral ligand is readily accessible but both enantiomers of the product are desired. Despite the potential significance, general catalytic methods to effectively reverse enantioselectivity by changing an achiral reaction parameter remain underdeveloped. Herein we report our studies focused on elucidating the origin of metal-controlled enantioselectivity reversal in Lewis acid-catalysed Michael additions. Rigorous experimental and computational investigations reveal that specific interactions between the substrate and ligand depending on the choice of metal catalyst are a key factor responsible for the observed enantiodivergence. This holds potential to further our understanding of and facilitate the design of future enantiodivergent transformations.

Electron Rich Triarylphosphines as Nucleophilic Catalysts for Oxa-Michael Reactions

Herein, we study the activity of methoxysubstituted arylphosphines (4-methoxy-phenyl)diphenylphosphine (MMTPP) and tris(4-trimethoxyphenyl)phosphine (TMTPP) in catalyzing oxa-Michael additions in comparison to commonly used triphenylphosphine (TPP). Acrylonitrile, acryl amide and divinyl sulfone are used as Michael acceptors and propargyl alcohol, allyl alcohol, n-propanol and i-propanol are assessed as Michael donors. In many cases, catalyst loadings of only 1 mol% in respect to the Michael acceptor are sufficient to provide full conversion towards the Michael adduct in 24 h at room temperature. Generally, TMTPP is the most active catalyst in all cases. The experimental activity trend was rationalized by calculating the Michael acceptor affinities of all phosphine – Michael acceptor combinations. Besides this parameter, the acidity of the alcohol has a strong impact on the reaction speed. The oxidation stability of the phosphines was evaluated and electron richest TMTPP was found to be only slightly more sensitive to oxidation than TPP. Finally, the catalysts were employed in the oxa-Michael polymerization of 2-hydroxyethyl acrylate. With TMTPP polymers characterized by number average molar masses of about 1200 g/mol at room temperature are accessible. Polymerizations carried out at 80 °C resulted in macromolecules containing a considerable share of Rauhut-Currier type repeat units and consequently lower molar masses were obtained.

Electron Rich Triarylphosphines as Nucleophilic Catalysts for Oxa-Michael Reactions

Herein, we study the activity of methoxysubstituted arylphosphines (4-methoxy-phenyl)diphenylphosphine (MMTPP) and tris(4-trimethoxyphenyl)phosphine (TMTPP) in catalyzing oxa-Michael additions in comparison to commonly used triphenylphosphine (TPP). Acrylonitrile, acryl amide and divinyl sulfone are used as Michael acceptors and propargyl alcohol, allyl alcohol, n-propanol and i-propanol are assessed as Michael donors. In many cases, catalyst loadings of only 1 mol% in respect to the Michael acceptor are sufficient to provide full conversion towards the Michael adduct in 24 h at room temperature. Generally, TMTPP is the most active catalyst in all cases. The experimental activity trend was rationalized by calculating the Michael acceptor affinities of all phosphine – Michael acceptor combinations. Besides this parameter, the acidity of the alcohol has a strong impact on the reaction speed. The oxidation stability of the phosphines was evaluated and electron richest TMTPP was found to be only slightly more sensitive to oxidation than TPP. Finally, the catalysts were employed in the oxa-Michael polymerization of 2-hydroxyethyl acrylate. With TMTPP polymers characterized by number average molar masses of about 1200 g/mol at room temperature are accessible. Polymerizations carried out at 80 °C resulted in macromolecules containing a considerable share of Rauhut-Currier type repeat units and consequently lower molar masses were obtained.