Palladium(0) and Brønsted Acid Co-Catalyzed Enantioselective Hydro-Cyclization of 2,4-Dienyl Hydrazones and Oximes

The transition metal-catalyzed asymmetric hydro-functionalization of 1,3-dienes has been well explored, but most reactions focus on electron-neutral substrates in an intermolecular manner. Here we first demonstrate that readily available 2,4-dienyl hydrazones and oximes can be efficiently utilized in the hydro-cyclization reaction under co-catalysis of a Brønsted acid and a chiral palladium complex, furnishing multifunctional dihydropyrazones and dihydroisoxazoles, respectively. Diverse substitution patterns for both types of electron-deficient diene compounds are tolerated, and corresponding heterocycles were generally constructed with moderate to excellent enantioselectivity, which can be elaborated to access products with higher molecular complexity and diversity. Control experiments and density functional theory calculations support that α-regioselective protonation of dienyl substrates by acid and concurrent π-Lewis base activation of Pd0 complex is energetically favoured in the formation of active π-allylpalladium intermediates, and an outer-sphere allylic amination or etherification mode is adopted to deliver the observed cyclized products enantioselectively.

Microwave-assisted protocol towards synthesis of heterocyclic molecules: a comparative analysis with conventional synthetic methodologies (years 2019-2023): a review

Microwave-assisted protocols have become extensively accepted across various scientific and technological domains because of their numerous advantages, shorter reaction times, higher yields, and often milder reaction conditions. In this review, we focus on the synthesis of N, O, and S-containing heterocyclic structural cores, crucial in the development of pharmaceuticals, agrochemicals, and materials science following through conventional and microwave method via eliminating the side products and enhances the product yield that is nowadays the biggest barrier for a synthetic chemist. The major findings emphasizes the substantial advantages of microwave-assisted techniques over conventional synthetic protocols. This comparative study underscores the potential of microwave-assisted techniques to revolutionize heterocyclic compound synthesis, providing insights into optimizing reaction conditions and expanding the scope of chemical synthesis in industrial applications.

Catalytic enantioselective synthesis of 2-pyrazolines via one-pot condensation/6π-electrocyclization: 3,5-bis(pentafluorosulfanyl)-phenylthioureas as powerful hydrogen bond donors

A new conjugate-base-stabilized carboxylic acid (CBSCA) containing a 3,5-bis(pentafluorosulfanyl)phenylthiourea functionality catalyses challenging one-pot condensations/6π-electrocyclizations of hydrazines and α,β-unsaturated ketones under mild conditions. Structurally diverse N-aryl 2-pyrazolines are obtained in good yields and enantioselectivities. The superior performance of 3,5-bis(SF5)phenylthioureas over the widely used 3,5-bis(CF3)phenylthioureas is further demonstrated in the Michael addition of dimethyl malonate to nitrostyrene, using a new Takemoto-type catalyst.

Green synthesis of novel spiropyrazoline-indolinones in neutral deep eutectic solvents and DFT studies

Novel spiropyrazoline-indolinones (4a-t) have been synthesized successfully in neutral deep eutectic solvents by reacting 5-Cl/Br-isatin (1a-b) with aromatic ketones (2a-b) and a variety of substituted hydrazines (3a-e) in good to excellent yields. This eco-friendly straightforward synthetic protocol discloses good functional group compatibility. The conventional synthetic approach was compared with the greener route of microwave-assisted synthesis of spiropyrazolines using ethanol. This approach utilized mild reaction conditions which furnished high yields in short reaction time employing one pot two-step multicomponent. All new compounds were structurally confirmed by detailed spectroscopic analysis and density functional theory calculations. This method provides efficient access to spiropyrazole derivatives using biodegradable and green solvent.

Cu(II)-Catalyzed Aminocyclization of N-Propargyl Hydrazones to Substituted Pyrazolines

An efficient route for the copper(II)-catalyzed synthesis of substituted pyrazolines from readily accessible N-propargyl hydrazones has been reported under open flask conditions via intramolecular C-N bond formation. N-acyl and N-tosyl-substituted pyrazolines have been prepared in moderate to excellent yields. Mechanistic investigations using NMR, high-resolution mass spectrometry (HRMS), and Hammett analyses suggest that the Cu(II) catalyst generally acts as a Lewis acid to form an iminium-ion intermediate via cyclization, which afforded the desired pyrazolines upon hydrolysis. One progesterone receptor antagonist has also been synthesized utilizing this reaction methodology.

Electrochemical Synthesis of Pyrazolines and Pyrazoles via [3+2] Dipolar Cycloaddition

Pyrazolines and pyrazoles are common and important motifs of pharmaceutical agents and agrochemicals. Herein, the first electrochemical approach for their direct synthesis from easily accessible hydrazones and dipolarophiles up to decagram scale is presented. The application of a biphasic system (aqueous/organic) even allows for the conversion of highly sensitive alkenes, wherein inexpensive sodium iodide is employed in a dual role as supporting electrolyte and mediator. In addition, mechanistic insight into the reaction is given by the isolation of key step intermediates. The relevance of the presented reaction is underlined by the synthesis of commercial herbicide safener mefenpyr-diethyl in good yields.

Enantioselective Synthesis of Functionalized Tetrahydropyridines through Iridium-Catalyzed Formal [5+1] Annulation

An efficient iridium-catalyzed asymmetric formal [5+1] annulation by in situ generation of enamines as N-nucleophiles for the synthesis of tetrahydropyridine derivatives is disclosed. The methodology offers direct access to a wide variety of chiral tetrahydropyridine derivatives in moderate to good yields and excellent enantioselectivity.