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1. Non-Classical Amide Bond Formation: Transamidation and Amidation of Activated Amides and Esters by Selective N–C/O–C Cleavage

   https://doi.org/10.1055/s-0040-1707101  

   Li, Guangchen; Szostak, Michal (January 2020, Synthesis)

   In the past several years, tremendous advances have been made in non-classical routes for amide bond formation that involve transamidation and amidation reactions of activated amides and esters. These new methods enable the formation of extremely valuable amide bonds via transition-metal- catalyzed, transition-metal-free or metal-free pathways by exploiting chemoselective acyl C–X (X = N, O) cleavage under mild conditions. In a broadest sense, these reactions overcome the formidable challenge of activating C–N/C–O bonds of amides or esters by rationally tackling nN→π*C=O delocalization in amides and nO→π*C=O donation in esters. In this account, we summarize the recent remarkable advances in the development of new methods for the synthesis of amides with a focus on (1) transition-metal/NHC- catalyzed C–N/C–O bond activation, (2) transition-metal-free highly selective cleavage of C–N/C–O bonds, (3) the development of new acyl-transfer reagents, and (4) other emerging methods. 

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2. Synthesis of Amides by Transamidation and Amidation of Activated Amides and Esters

   Li, Guangchen; Szostak, Michal (January 2020, Science of synthesis)

   This chapter provides a summary of the recent advances in direct transamidation and amidation reactions of activated amides and esters via transition- metal-catalyzed and transition-metal-free C(acyl)–N and C(acyl)–O bond cleavage as a new disconnection for the synthesis of amide bonds. 

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3. Metal-assisted selective recognition of biothiols by a synthetic receptor array

   https://doi.org/10.1039/c8cc07220e  

   Liu, Yang; Duan, Yaokai; Gill, Adam D.; Perez, Lizeth; Jiang, Qiaoshi; Hooley, Richard J.; Zhong, Wenwan (November 2018, Chemical Communications)

   A synergistic combination of a deep cavitand host, fluorophore guests and transition metal ions can be used to sense small molecule thiols of biological interest with good efficiency and selectivity in complex aqueous media. 

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4. Conversion of esters to thioesters under mild conditions

   https://doi.org/10.1039/D1OB00187F  

   Shi, Yijun; Liu, Xuejing; Cao, Han; Bie, Fusheng; Han, Ying; Yan, Peng; Szostak, Roman; Szostak, Michal; Liu, Chengwei (April 2021, Organic & Biomolecular Chemistry)

   null (Ed.)

   We report conversion of esters to thioesters via selective C–O bond cleavage/weak C–S bond formation under transition-metal-free conditions. The method is notable for a general and practical transition-metal-free system, broad substrate scope and excellent functional group tolerance. The strategy was successfully deployed in late-stage thioesterification, site-selective cross-coupling/thioesterification/decarbonylation and easy-to-handle gram scale thioesterification. Selectivity and computational studies were performed to gain insight into the formation of weak C–S bonds by C–O bond cleavage, which contrasts with the traditional trend of nucleophilic additions to carboxylic acid derivatives. 

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5. Iterative Exponential Growth of Oxygen-Linked Aromatic Polymers Driven by Nucleophilic Aromatic Substitution Reactions

   https://doi.org/10.3389/fchem.2021.620017  

   Jaynes, Tyler J.; Sharafi, Mona; Campbell, Joseph P.; Bocanegra, Jessica; McKay, Kyle T.; Little, Kassondra; Osadchey Brown, Reilly; Gray, Danielle L.; Woods, Toby J.; Li, Jianing; et al (April 2021, Frontiers in Chemistry)

   null (Ed.)

   This work presents the first transition metal-free synthesis of oxygen-linked aromatic polymers by integrating iterative exponential polymer growth (IEG) with nucleophilic aromatic substitution (S N Ar) reactions. Our approach applies methyl sulfones as the leaving groups, which eliminate the need for a transition metal catalyst, while also providing flexibility in functionality and configuration of the building blocks used. As indicated by 1) 1 H- 1 H NOESY NMR spectroscopy, 2) single-crystal X-ray crystallography, and 3) density functional theory (DFT) calculations, the unimolecular polymers obtained are folded by nonclassical hydrogen bonds formed between the oxygens of the electron-rich aromatic rings and the positively polarized C–H bonds of the electron-poor pyrimidine functions. Our results not only introduce a transition metal-free synthetic methodology to access precision polymers but also demonstrate how interactions between relatively small, neutral aromatic units in the polymers can be utilized as new supramolecular interaction pairs to control the folding of precision macromolecules. 

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