Asymmetric catalytic synthesis of functionalized tetrahydroquinolines in supercritical fluids

Investigating the efficacy of green solvents and solvent-free conditions in hydrogen-bonding mediated organocatalyzed model reactions

In this study, we have delved into various reactions conducted using green solvents or under solvent-free conditions, employing hydrogen bonding organocatalysis to advance more sustainable practices in chemical synthesis. The outcomes suggest that cyclopentyl methyl ether could potentially replace non-polar organic solvents such as hexane and toluene with comparable enantioselectivity and yields. The non-polar nature of liquefied or supercritical CO2 restricts its application to reactions that require non-polar solvents. Furthermore, pursuing solvent-free conditions, even without liquid substrates, might result in similar conversion rates with reduced catalyst loading. These findings highlight the potential of exploring solvent-free conditions when enantioselectivity is not of concern. Based on the results, solvent-free conditions and bio-based solvents can serve as viable alternatives to conventional organic solvents without compromising performance. This is expected to influence the way chemists approach reaction optimisation within method development in the field, fostering a broader adoption of environmentally friendly approaches.

Recent Advances in Greener Asymmetric Organocatalysis Using Bio-Based Solvents

Efficient synthetic methods that avoid the extensive use of hazardous reagents and solvents, as well as harsh reaction conditions, have become paramount in the field of organic synthesis. Organocatalysis is notably one of the best tools in building chemical bonds between carbons and carbon-heteroatoms; however, most examples still employ toxic volatile organic solvents. Although a portfolio of greener solvents is now commercially available, only ethyl alcohol, ethyl acetate, 2-methyltetrahydrofuran, supercritical carbon dioxide, ethyl lactate, and diethyl carbonate have been explored with chiral organocatalysts. In this review, the application of these bio-based solvents in asymmetric organocatalytic methods reported in the last decade is discussed, highlighting the proposed mechanism pathway for the transformations.

Enantioselective Synthesis of 4-Cyanotetrahydroquinolines via Ni-Catalyzed Hydrocyanation of 1,2-Dihydroquinolines

A Ni-catalyzed asymmetric hydrocyanation that enables the formation of 4-cyanotetrahydroquinolines in good yields with excellent enantioselectivities is presented herein. A variety of functional groups are well-tolerated, and a gram-scale reaction supports the synthetic potential of the transformation. Additionally, several crucial intermediates for pharmaceutically active agents, including a PGD2 receptor antagonist, are now accessible through asymmetric synthesis using this new protocol.

Green Asymmetric Organocatalysis

Asymmetric organocatalysis is becoming one of the main tools for the synthesis of chiral compounds that are needed as medicines, crop protection agents, and other bioactive molecules. It can be effectively combined with various green chemistry methodologies. Intensification techniques, such as ball milling, flow, high pressure, or light, bring not only higher yields, faster reactions, and easier product isolation, but also new reactivities. More sustainable reaction media, such as ionic liquids, deep eutectic solvents, green solvent alternatives, and water, also considerably enhance the sustainability profile of many organocatalytic reactions.

Enantioselective Synthesis of 4-Allyl Tetrahydroquinolines via Copper(I) Hydride-Catalyzed Hydroallylation of 1,2-Dihydroquinolines

CuCl/(R,R)-Ph-BPE-catalyzed asymmetric hydroallylation of 1,2-dihydroquinolines, prepared from readily available quinolines, was developed. The optically active tetrahydroquinolines (THQs) bearing an allylic functionality at position 4 were obtained in good yields and excellent enantioselectivity. The introduced allylic groups are amenable to diverse transformations, thus offering chances to rapidly expand the THQ libraries.

Progress in the Chemistry of Tetrahydroquinolines

Tetrahydroquinoline is one of the most important simple nitrogen heterocycles, being widespread in nature and present in a broad variety of pharmacologically active compounds. This Review summarizes the progress achieved in the chemistry of tetrahydroquinolines, with emphasis on their synthesis, during the period from mid-2010 to early 2018.

Asymmetric synthesis of highly functionalized spirothiazolidinone tetrahydroquinolines via a squaramide-catalyzed cascade reaction

A bifunctional squaramide-catalyzed asymmetric cascade aza-Michael/Michael addition reaction for the synthesis of chiral spirothiazolidinone tetrahydroquinolines with three contiguous stereocenters has been developed. This cascade reaction proceeded well under mild conditions, and afforded the desired products in high to excellent yields (up to >99% yield) with excellent diastereoselectivities (>25 : 1 dr) and high enantioselectivities (up to 96% ee). More importantly, both the amplification and the derivation experiments do not affect the stereoselectivity.

Recent advances in the asymmetric synthesis of pharmacology-relevant nitrogen heterocycles via stereoselective aza-Michael reactions

In this review, recent applications of a stereoselective aza-Michael reaction for asymmetric synthesis of naturally occurring N-containing heterocyclic scaffolds and their usefulness to pharmacology are summarized.

Prospective Symbiosis of Green Chemistry and Energetic Materials

A global increase in environmental pollution demands the development of new "cleaner" chemical processes. Among urgent improvements, the replacement of traditional hydrocarbon-derived toxic organic solvents with neoteric solvents less harmful for the environment is one of the most vital issues. As a result of the favorable combination of their unique properties, ionic liquids (ILs), dense gases, and supercritical fluids (SCFs) have gained considerable attention as suitable green chemistry media for the preparation and modification of important chemical compounds and materials. In particular, they have a significant potential in a specific and very important area of research associated with the manufacture and processing of high-energy materials (HEMs). These large-scale manufacturing processes, in which hazardous chemicals and extreme conditions are used, produce a huge amount of hard-to-dispose-of waste. Furthermore, they are risky to staff, and any improvements that would reduce the fire and explosion risks of the corresponding processes are highly desirable. In this Review, useful applications of almost nonflammable ILs, dense gases, and SCFs (first of all, CO₂ ) for nitration and other reactions used for manufacturing HEMs are considered. Recent advances in the field of energetic (oxygen-balanced and hypergolic) ILs are summarized. Significant attention is paid to the SCF-based micronization techniques, which improve the energetic performance of HEMs through an efficient control of the morphology and particle size distribution of the HEM fine particles, and to useful applications of SCFs in HEM processing that makes them less hazardous.

Immobilization of CALB on activated chitosan: Application to enzymatic synthesis in supercritical and near-critical carbon dioxide

•

Lipase B from Candida antarctica immobilized on activated chitosan.

•

The loading capacity of the new biocatalyst is around 20 mg/g of support.

•

Biocatalyst was higher than that of the CALB-Octyl (by a 53-fold factor).

•

Enzymatic esterification reaction in supercritical or near-critical CO2.

•

Molecular sieves promoted 16.0% increase in enzymatic esterification reaction.

The objective of this new paper was to evaluate the enzymatic esterification reaction conducted in supercritical or near-critical CO₂, catalyzed by immobilized lipase B from Candida antarctica (CALB). The biocatalyst was prepared through the immobilization of CALB by covalent attachment using chitosan sequentially activated with Glycidol, ethylenediamine (EDA) and glutaraldehyde as support. In order to determine the best operational conditions of the esterification reaction (1: 1 (alcohol–acid); biocatalyst content, 10% (by substrate mass); 45 °C), an experimental design (2³) was conducted to evaluate the effects of the following parameters: alcohol to oil molar ratios, reaction time and temperature. The maximum loading of chitosan was 20 mg protein/g support, and the thermal and solvent stability of the new biocatalyst was higher than that of the CALB-GX (by a 26-fold factor), CALB-OC (by a 53-fold factor) and Novozym 435 (by a 3-fold factor). The maximum conversion was 46.9% at a temperature of 29.9 °C, ethanol to oleic acid molar ratio equal to 4.50:1, and a reaction time of 6.5 h. Additionally, the removal of water from the medium, by using molecular sieves, promoted a 16.0% increase in the conversion of oleic acid into ethyl esters.