Oxidative NHC‐Catalysis as Organocatalytic Platform for the Synthesis of Polyester Oligomers by Step‐Growth Polymerization

Abstract

The application of N‐heterocyclic carbene (NHC) catalysis to the polycondensation of diols and dialdehydes under oxidative conditions is herein presented for the synthesis of polyesters using fossil‐based (ethylene glycol, phthalaldehydes) and bio‐based (furan derivatives, glycerol, isosorbide) monomers. The catalytic dimethyl triazolium/1,8‐diazabicyclo[5.4.0]undec‐7‐ene couple and stoichiometric quinone oxidant afforded polyester oligomers with a number‐average molecular weight (Mn) in the range of 1.5–7.8 kg mol−1 as determined by NMR analysis. The synthesis of a higher molecular weight polyester (polyethylene terephthalate, PET) by an NHC‐promoted two‐step procedure via oligoester intermediates is also illustrated together with the catalyst‐controlled preparation of cross‐linked or linear polyesters derived from the trifunctional glycerol. The thermal properties (TGA and DSC analyses) of the synthesized oligoesters are also reported.

Exploring Oxidative NHC‐Catalysis as Organocatalytic Polymerization Strategy towards Polyamide Oligomers

“…Initially, we were aware that the direct oxidative N‐acylation with aldehydes [15] may be precluded in the case of primary amines because of the competing imine formation and that this issue can be addressed by the addition of nucleophiles as additives [15a,b,c,f] or by the execution of a two‐step procedure with activated ester intermediates [15d,g] . Application of the reaction conditions previously optimized for synthesis of PEs by the same oxidative strategy [11] (triazolium salt C1 (5 mol %) as the pre‐catalyst, 1,8‐diazabicyclo[5.4.0]undec‐7‐ene (DBU, 12.5 mol %) as the base and quinone 4 (1 equiv) as the external oxidant in anhydrous THF), resulted in no formation of the target oligomer 3 aa because of the formation of a complex reaction mixture containing low molecular weight (LMW) mixed aldehyde and imine derivatives of 2 a (mainly monomers and dimers) as judged by ESI‐MS analysis (entry 1).…”

“…Our group recently contributed in this area of research describing the unprecedented polycondensation of dialdehydes and diols promoted by NHCs under oxidative conditions to access PEs by the step‐growth polymerization technique [11] . Key step of this approach is the oxidation of the Breslow intermediate to generate the corresponding acyl azolium, which is susceptible of nucleophilic attack by the diol for completing the formal aldehyde‐to‐ester conversion in an iterative fashion (Scheme 1).…”

Enantioselective N‐Acylation of Biginelli Dihydropyrimidines by Oxidative NHC Catalysis

“…Recycle of Oxidant 8: The alcohol resulting from oxidant 8 reduction (3,3',5,5'‐tetra‐ tert ‐butyl‐[1,1'‐biphenyl]‐4,4'‐diol) was recovered by column chromatography after each run of Table . The subsequent oxidation to 8 was performed stirring the 3,3',5,5'‐tetra‐ tert ‐butyl‐[1,1'‐biphenyl]‐4,4'‐diol (578 mg, 1.41 mmol) with 11 (80 mg, 0.14 mmol) in THF (10 mL) under air atmosphere (1 atm, balloon) for 16 h. Filtration over a pad of Celite and subsequent concentration under reduced pressure afforded 8 as a dark red amorphous solid (572 mg, 88 %) , …”

“…Organocatalytic strategies by N ‐heterocyclic carbene (NHC) catalysis represent a useful tool for the execution of a broad range of asymmetric transformations including benzoin‐type condensations, Stetter reactions, hydroacylations, annulations, and cycloadditions These processes proceed by normal polarity or umpolung reactivity through well‐established activation modes involving key reactive species, namely the Breslow intermediate, homoenolate, and azolium (di)enolate intermediates (Scheme a) . Additionally, the synthetic opportunities given by NHC organocatalysts are further expanded by the application of redox protocols (internal and external oxidation strategies) leading to the acyl azolium intermediate for the acylation of oxygen‐, sulfur‐, and nitrogen‐nucleophiles in challenging kinetic resolution (KR), macrolactonization, desymmetrization,, and polymerization processes. In particular, the N ‐acylation reaction using aldehydes as acylating agents in place of carboxylic acids/derivatives has proven to possess some practical advantages (mild reaction conditions, chemoselectivity, no need of coupling reagents) and it has been successfully applied to the functionalization of alkylamines, anilines, amides, sulfoximines, azomethine imines, and several nitrogen‐containing heterocycles.…”

“…Optically active N3‐acylated DHPMs are typically obtained from the corresponding enantioenriched substrate by treatment with stoichiometric, highly reactive acid chlorides or anhydrides at elevated temperatures in the presence of a base . As part of our studies on oxidative NHC catalysis,, , we herein present the direct asymmetric N ‐acylation of racemic DHPMs with aldehydes through catalytically generated acyl azolium as a mild reaction protocol, tolerant to substitutional variation around the DHPM scaffold (Scheme c).…”