Zn-ProPhenol Catalyzed Enantioselective Mannich Reaction of 2H-Azirines with Alkynyl Ketones

Game of Aliphatics: A Density Functional Theory Study of Base-Catalyzed Substrate-Controlled Dimerizations of Aliphatic Alkynones

The present work focuses on a comprehensive density functional theory (DFT) study of newly discovered base-catalyzed substrate-controlled dimerizations of aliphatic alkynones. In order to understand the origin of selectivity of the cascade assemblies of 6-methylene-5-oxaspiro[2.4]heptanones and 2-alkenylfurans, structural and electronic properties of neutral and deprotonated alkynone molecules, thermodynamic and kinetic characteristics of the deprotonation of alkynones having diverse C-H active substituents at the carbonyl function under the action of a base, and thermodynamic and kinetic characteristics of possible mechanisms of the discussed cascade reactions were theoretically assessed. The obtained computational results have confirmed and clarified an early qualitative assumption on the key role of the nature of the aliphatic substituent. Apart from fully rationalizing the experimental results, the theoretical DFT data give valuable details and data for predicting the outcome of related base-catalyzed reactions between various electrophilic substrates and nucleophilic species formed from C-H active aliphatic alkynones.

Copper-Catalyzed Highly Enantioselective Addition of a Silicon Nucleophile to 3-Substituted 2H-Azirines Using an Si-B Reagent

3-Substituted 2H-azirines can be considered strained cyclic ketimines, and highly enantioselective addition reactions of silicon nucleophiles to either acyclic or cyclic ketimines have been elusive so far. The present work closes this gap for those azirines by means of a copper-catalyzed silylation using a silyl boronic ester as a latent silicon nucleophile. The resulting C-silylated, unprotected (N-H) aziridines are obtained in high yields and with excellent enantioselectivities and can be further converted into valuable compounds with hardly any erosion of the enantiomeric excess.

Regio- and stereoselective base-catalyzed assembly of 6-methylene-5-oxaspiro[2.4]heptanones from alkynyl cyclopropyl ketones

6-Methylene-5-oxaspiro[2.4]heptanones have been synthesized via base-catalyzed dimerization of available alkynyl cyclopropyl ketones. The reaction proceeds effectively in the presence of the t-BuOK/t-BuOH/THF catalytic system at room temperature to afford the desired spirocycles in a regio- and stereoselective manner. A wider synthetic utility of alkynyl cyclopropyl ketones as novel building blocks was demonstrated by the synthesis of diverse spirocyclopropanes.

Intramolecular Hydrogen-Bond Activation: Strategies, Benefits, and Influence in Catalysis

The activation of molecules through intramolecular hydrogen-bond formation to promote chemical reactions appears as a suitable strategy in organic synthesis, especially for the preparation of chiral compounds under metal and organocatalytic conditions. The use of this interaction has enabled reactivity enhancement of reagents, as well as stabilization of the chemical species and enantiocontrol of the processes.

Microporous hierarchically Zn-MOF as an efficient catalyst for the Hantzsch synthesis of polyhydroquinolines

A three-dimensional walnut-like Zn-based MOF microsphere system was designed and synthesized via hydrothermal reaction of zinc salt with 4,6-diamino-2-pyrimidinethiol as a tridentate ligand. Besides, Zn ions were coordinated to the functional groups of the ligand to give a novel Zn-MOF microsphere material. Afterward, the resultant material was thoroughly characterized using various analysis and physico-chemical methods; including, FT-IR, XRD, TGA, EDX, X-ray mapping, SEM, TEM, and BET analysis. The Zn-MOF microspheres were utilized in the Hantzsch reaction for a selective synthesis of asymmetric polyhydroquinolines, using various aromatic aldehydes. Our strategy aims at providing a controlled synthesis of hierarchically nanoporous Zn-MOF microspheres with a well-defined morphology, structure, and excellent catalytic properties. Besides, it would result in having a promising heterogeneous catalyst for a selective synthesis with good yields, short reaction time, a low limit of steric hindrance and electronic effects. Moreover, the heterogeneity of the catalyst is further tested with hot filtration and also the reusability results point.

Asymmetric synthesis of tetrasubstituted cyclic amines via aza-Henry reaction using cinchona alkaloid sulfonamide/zinc(II) catalysts

The first enantioselective aza-Henry reaction of non-activated cyclic iminoesters, derived from cyclic amino acids, has been developed. Good yields and enantioselectivities were observed for the reaction using our original cinchona alkaloid sulfonamide/zinc(II) catalyst. The transition state was proposed to explain the stereoselectivity based on experiments and DFT calculations.

Asymmetric Catalytic Ketimine Mannich Reactions and Related Transformations

The catalytic enantioselective ketimine Mannich and its related reactions provide direct access to chiral building blocks bearing an α-tertiary amine stereogenic center, a ubiquitous structural motif in nature. Although ketimines are often viewed as challenging electrophiles, various approaches/strategies to circumvent or overcome the adverse properties of ketimines have been developed for these transformations. This review showcases the selected examples that highlight the benefits and utilities of various ketimines and remaining challenges associated with them in the context of Mannich, allylation, and aza-Morita-Baylis-Hillman reactions as well as their variants.

Enantioselective Reaction of 2H-Azirines with Oxazol-5-(4H)-ones Catalyzed by Cinchona Alkaloid Sulfonamide Catalysts

The enantioselective reaction of 2H-azirines with oxazol-5-(4H)-ones (oxazolones) using a cinchona alkaloid sulfonamide catalyst has been developed. The reaction proceeded at the C-2 position of oxazolones to afford products with consecutive tetrasubstituted stereogenic centers in high yield with high diastereo- and enantioselectivity. The obtained aziridines were converted into various chiral compounds without loss of enantiopurity.

Recent advances and prospects in the Zn-catalysed Mannich reaction

Zn-catalysed reactions are ubiquitously important due to their inexpensive, generally less toxic and atom-economic nature. According to the modern criteria of sustainability, their use in a catalytic manner is a highly desirable goal, especially when using chiral ligands. Considering the relevance of well-established zinc-mediated C-C bond formation reactions, it is relatively surprising that the use of Zn as a catalyst is still underdeveloped, especially in comparison with other transition metals. The vast majority of natural molecules, including proteins, nucleic acids and most biologically active compounds, contain nitrogen. Consequently, developing new synthetic methods for the construction of nitrogenous molecules receives great attention from organic chemists. The Mannich reaction is a very basic and very useful platform for the development of several such nitrogen-containing molecules. In this review, we summarise the recent advancements in the Zn-catalysed Mannich reaction, covering the literature from 2011 to 2020.