Biodegradable Chitosan‐Derived Thioureas as Recoverable Supported Organocatalysts – Application to the Stereoselective Aza‐Henry Reaction

Abstract

Eight different biodegradable chitosan‐supported bifunctional chiral thioureas have been prepared as a greener and more sustainable alternative to those supported on petrochemical‐derived polymers. These organocatalysts promoted an enantioselective aza‐Henry reaction, which afforded good product yields with moderate to high enantioselectivity. The activity and stereodirecting ability of these materials were dependent on the accessibility of the reactants to the active site and increased with the length of the tether that connected the thiourea to the biopolymer. The best performing catalyst was able to be recovered and recycled five times without a loss of activity.

Valorization of Waste: Sustainable Organocatalysts from Renewable Resources

“…In 2017, Andrés and co‐workers prepared eight different chitosan‐supported bifunctional chiral thioureas and evaluated their ability to catalyze the enantioselective aza‐Henry reaction . The catalytic activity turned out to be dependent on the length of the spacer group connecting the thiourea to the biopolymer, which affected the accessibility of the substrates to the active site.…”

Recent Developments on Supported Hydrogen‐bond Organocatalysts

“…Chitosan has been employed itself as organocatalyst in different types of covalent catalysis processes, including Knoevenagel, Henry and aldol reactions . Recently, six novel chitosan‐based catalysts ( IX a – f ) presenting different lengths of the tether linking the chitosan to the thiourea, in the substituent at the chiral carbon and in the position of the stereocenter in the chain, have been prepared. Each catalyst (10 mol %) was tested in the aza‐Henry reaction of N ‐Boc‐benzaldimine ( 6 a ) with 6.0 equivalents of nitromethane ( 7 a ) in absence of solvent at room temperature (Scheme ).…”

Chiral Bifunctional Thioureas and Squaramides and Their Copolymers as Recoverable Organocatalysts. Stereoselective Synthesis of 2-Substituted 4-Amino-3-nitrobenzopyrans and 3-Functionalized 3,4-Diamino-4H-Chromenes

“…The presence of electronwithdrawing groups in the aniline substituent is important because they are responsible for the increasing the acidity, and consequently the activity of the catalysts. 12 As an extension of our interest in the synthesis of supported bifunctional squaramides 13 and thioureas in both synthetic 14 and natural 15 polymers we summarize here the bottom-up preparation of monomeric and polymeric novel catalysts and their use and recovering in a cascade oxa-Michael aza-Henry process directed to the synthesis of trisubstituted benzopyrane and 4H-chromene derivatives with up to three contiguous stereocenters.…”