Development of Organocatalytic Darzens Reactions Exploiting the Cyclopropenimine Superbase

A truly organocatalytic approach to the Darzens reaction affording α,β-epoxy carbonyl compounds in good yields was developed taking advantage of the high basic strength and low nucleophilicity of cyclopropenimine superbases. The catalytic active free base can easily be generated in situ from its hydrochloride salt and maintained in the active deprotonated form by performing the reactions in a heterogeneous reaction system in the presence of excess potassium carbonate as a sacrificial base.

Heterogeneous Organocatalysts for Light-Driven Reactions in Continuous Flow

Within the realm of organic synthesis, photocatalysis has blossomed since the beginning of the last decade. A plethora of classical reactivities, such as selective oxidation of alcohol and amines, redox radical formation of reactive species in situ, and indirect activation of an organic substrate for cycloaddition by EnT, have been revised in a milder and more sustainable fashion via photocatalysis. However, even though the spark of creativity leads scientists to explore new reactions and reactivities, the urgency of replacing the toxic and critical metals that are involved as catalysts has encouraged chemists to find alternatives in the branch of science called organocatalysis. Unfortunately, replacing metal catalysts with organic analogues can be too expensive sometimes; however, this drawback can be solved by the reutilization of the catalyst if it is heterogeneous. The aim of this review is to present the recent works in the field of heterogeneous photocatalysis, applied to organic synthesis, enabled by continuous flow. In detail, among the heterogeneous catalysts, g-CN, polymeric photoactive materials, and supported molecular catalysts have been discussed within their specific sections, rather than focusing on the types of reactions.

Enantio- and Diastereoselective Variations on α-Iminonitriles: Harnessing Chiral Cyclopropenimine-Thiourea Organocatalysts

Chiral 1-pyrrolines containing a nitrile motif serve as crucial structural scaffolds in biologically active molecules and exhibit diversity as building blocks owing to their valuable functional groups; however, the asymmetric synthesis of such compounds remains largely unexplored. Herein, we present an enantio- and diastereoselective method for the synthesis of α-chiral nitrile-containing 1-pyrroline derivatives bearing vicinal stereocenters through the design and introduction of chiral cyclopropenimine-based bifunctional catalysts featuring a thiourea moiety. This synthesis entails a highly stereoselective conjugate addition of α-iminonitriles to a wide array of enones, followed by cyclocondensation, thereby affording a series of cyanopyrroline derivatives, some of which contain all-carbon quaternary centers. Moreover, we demonstrate the synthetic utility of this strategy by performing a gram-scale reaction with 1% catalyst loading, along with a variety of chemoselective transformations of the product, including the synthesis of a vildagliptin analogue. Finally, we showcase the selective synthesis of all four stereoisomers of the cyanopyrroline products through trans-to-cis isomerization, highlighting the versatility of our approach.

Phosphine Catalyzed Michael-Type Additions: The Synthesis of Glutamic Acid Derivatives from Arylidene-α-amino Esters

The reaction of arylidene-α-amino esters with electrophilic alkenes to yield Michael-type addition compounds is optimized using several phosphines as organocatalysts. The transformation is very complicated due to the generation of several final compounds, including those derived from the 1,3-dipolar cycloadditions. For this reason, the selection of the reaction conditions is a very complex task and the slow addition of the acrylic system is very important to complete the process. The study of the variation in the structural components of the starting imino ester is performed as well as the expansion of other electron-poor alkenes. The crude products have a purity higher than 90% in most cases without any purification. A plausible mechanism is detailed based on the bibliography and the experimental results. The synthesis of pyroglutamate entities, after the reduction of the imino group and cyclization, is performed in high yields. In addition, the hydrolysis of the imino group, under acidic media, represents a direct access to glutamate surrogates.

Efficiency in Carbon Dioxide Fixation into Cyclic Carbonates: Operating Bifunctional Polyhydroxylated Pyridinium Organocatalysts in Segmented Flow Conditions

Novel polyhydroxylated ammonium, imidazolium, and pyridinium salt organocatalysts were prepared through N-alkylation sequences using glycidol as the key precursor. The most active pyridinium iodide catalyst effectively promoted the carbonation of a set of terminal epoxides (80 to >95% yields) at a low catalyst loading (5 mol%), ambient pressure of CO₂, and moderate temperature (75 °C) in batch operations, also demonstrating high recyclability and simple downstream separation from the reaction mixture. Moving from batch to segmented flow conditions with the operation of thermostated (75 °C) and pressurized (8.5 atm) home-made reactors significantly reduced the process time (from hours to seconds), increasing the process productivity up to 20.1 mmol_(product) h^-1 mmol_(cat)^-1, a value ~17 times higher than that in batch mode.

Photoredox Cross-Dehydrogenative Coupling of N-Aryl Glycines Mediated by Mesoporous Graphitic Carbon Nitride: An Environmentally Friendly Approach to the Synthesis of Non-Proteinogenic α-Amino Acids (NPAAs) Decorated with Indoles

Indole-decorated glycine derivatives are prepared through an environmentally benign cross-dehydrogenative coupling between N-aryl glycine analogues and indoles (yield of ≤81%). Merging heterogeneous organocatalysis and photocatalysis, C–H functionalization has been achieved by selective C-2 oxidation of N-aryl glycines to afford the electrophilic imine followed by Friedel–Crafts alkylation with indole. The sustainability of the process has been taken into account in the reaction design through the implementation of a metal-free recyclable heterogeneous photocatalyst and a green reaction medium. Scale-up of the benchmark reaction (gram scale, yield of 69%) and recycling experiments (over seven runs without a loss of efficiency) have been performed to prove the robustness of the protocol. Finally, mechanistic studies were conducted employing electron paramagnetic resonance spectroscopy to unveil the roles of the photocatalyst and oxygen in the formation of odd-electron species.

NMR relaxation time measurements of solvent effects in an organocatalysed asymmetric aldol reaction over silica SBA-15 supported proline

The behaviour of solvents in solid-supported proline organocatalysts is explored using NMR relaxation measurements coupled with reaction screening. Solvents with a lower affinity for the solid surface lead to a higher reactivity.