Oxidative N-Heterocyclic Carbene Catalysis

N-Heterocyclic carbene (NHC) catalysis is a by now consolidated organocatalytic platform for a number of synthetic (asymmetric) transformations via diverse reaction modes/intermediates. In addition to the typical umpolung processes involving acyl anion/homoenolate equivalent species, implementation of protocols under oxidative conditions greatly expands the possibilities of this methodology. Oxidative NHC-catalysis allows for oxidative and oxygenative transformations through specific manipulations of Breslow-type species depending upon the oxidant used (external oxidant or O2 /air), the derived NHC-bound intermediates paving the way to non-umpolung processes through activation of carbon atoms and heteroatoms. This review is intended to update the state of the art in oxidative NHC-catalyzed reactions that appeared in the literature from 2014 to present, with a strong focus to crucial intermediates and their mechanistic implications.

A Semicrystalline Furanic Polyamide Made from Renewable Feedstocks

Semi-aromatic polyamides (SAPs) synthesized from petrochemical diacids and diamines are high-performance polymers that often derive their desirable properties from a high degree of crystallinity. Attempts to develop partially renewable SAPs by replacing petrochemical diacids with biobased furan-2,5-dicarboxylic acid (FDCA) have resulted in amorphous materials or polymers with low melting temperatures. Herein, we report the development of poly(5-aminomethyl-2-furoic acid) (PAMF), a semicrystalline SAP synthesized by the polycondensation of CO₂ and lignocellulose-derived monomer 5-aminomethyl-2-furoic acid (AMF). PAMF has glass-transition and melting temperatures comparable to that of commercial materials and higher than that of any previous furanic SAP. Additionally, PAMF can be copolymerized with conventional nylon 6 and is chemically recyclable. Molecular dynamics (MD) simulations suggest that differences in intramolecular hydrogen bonding explain why PAMF is semicrystalline but many FDCA-based SAPs are not.