An Enantioselective Recyclable Polystyrene-Supported Threonine-Derived Organocatalyst for Aldol Reactions

Primary amines as heterogeneous catalysts in an enantioselective [2,3]-Wittig rearrangement reaction

A series of heterogeneous catalysts anchored to different polystyrene-based supports has been prepared and applied in an asymmetric [2,3]-Wittig rearrangement reaction of cyclohexanone derivatives. Among them, primary amino acid-derived (aminomethylated)polystyrene-supported catalysts showed excellent reactivity leading to the formation of rearranged products in good enantioselectivities of both diastereomers. Reusability issues connected to the deactivation of the catalyst were proved to be dependent on the end-capping strategy chosen for the blocking of the unreacted active sites of the resin. This issue of end-capping has not previously been in focus. Using bulkier pivaloyl end-capping moiety, we were able to recycle the catalyst in six consecutive cycles with only marginal deceleration of the reaction. Moreover, the epimerization of the product that occurred while conducting a rearrangement reaction in the presence of a homogeneous catalyst was almost fully eliminated by switching the catalytic system to heterogeneous.

An expeditious synthesis of 6,7-dihydrodibenzo[b,j][4,7] phenanthroline derivatives as fluorescent materials

A rapid and efficient method has been developed for the synthesis of 13,14-dimethyl-6,7-dihydrodibenzo[b,j][4,7]phenanthroline derivatives (3a–d) through the Friedländer condensation of 2-aminoarylketone with 1,4-cyclohexanedione under solvent-free conditions using p-toluenesulphonic acid. The synthetic utility of compounds 3a, 3b, and 3c was demonstrated by synthesizing compounds 6a–kvia Suzuki coupling, 8 by Buchwald–Hartwig amination, and 9a–bvia NBS bromination. Significantly, the emission band corresponding to the π–π* electronic transition of compounds 3a, 6a, 6d, 6f, and 8 showed a redshift with increasing polarity of the solvents. Molar extinction coefficient (ε), Stoke's shift (Δ Created by potrace 1.16, written by Peter Selinger 2001-2019 ]]> ), and quantum yield (Φ_f) were calculated for all these compounds.

A rapid method has been developed for the synthesis of 13,14-dimethyl-6,7-dihydrodibenzo[b,j][4,7]phenanthroline derivatives (3a–d) through the Friedländer condensation of 2-aminoarylketone with 1,4-cyclohexanedione under neat conditions using p-TSA.

Amino Acylguanidines as Bioinspired Catalysts for the Asymmetric Aldol Reaction

The binding and stabilizing effect of arginine residues in certain aldolases served as inspiring source for the development of a family of amino acylguanidine organocatalysts. Screening and optimization led to identify the threonine derivative as the most suitable catalyst for the asymmetric aldol addition of hydroxyacetone, affording the syn diastereomer in high ee. In contrast, the proline derivative yielded the anti diasteromer. MMFF models suggest the presence of an extensive hydrogen bonding network between the acylguanidinium group and the reaction intermediates.

Stereospecific diaza-Cope rearrangement as an efficient tool for the synthesis of DPEDA pyridine analogs and related C2-symmetric organocatalysts

Enantiomerically pure trans-1,2-di-(pyridinyl)ethylene-1,2-diamines were synthesized via a stereospecific diaza-Cope rearrangement and converted to prolinamide-based recyclable organocatalysts.

A polystyrene-supported 9-amino(9-deoxy)epi quinine derivative for continuous flow asymmetric Michael reactions

A polystyrene (PS)-supported 9-amino(9-deoxy)epi quinine derivative catalyzes Michael reactions affording excellent levels of conversion and enantioselectivity using different nucleophiles and structurally diverse enones. The highly recyclable, immobilized catalyst has been used to implement a single-pass, continuous flow process (residence time: 40 min) that can be operated for 21 hours without significant decrease in conversion and with improved enantioselectivity with respect to batch operation. The flow process has also been used for the sequential preparation of a small library of enantioenriched Michael adducts.

Iron/acetic acid mediated synthesis of 6,7-dihydrodibenzo[b,j][1,7]phenanthroline derivatives via intramolecular reductive cyclization

An efficient iron/acetic acid mediated intramolecular reductive cyclization protocol was developed for the synthesis of novel 6,7-dihydrodibenzo[b,j][1,7]phenanthroline derivatives.

Polymer-immobilized chiral catalysts

This review illustrates the current strategies and potential of polymer-immobilized chiral catalysts for highly enantioselective asymmetric synthesis.