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1. 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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2. Iron-Catalyzed C(sp2)–C(sp3) Cross-Coupling of Aryl Chlorobenzoates with Alkyl Grignard Reagents

   https://doi.org/10.3390/molecules25010230  

   Bisz, Elwira; Szostak, Michal (January 2020, Molecules)

   Aryl benzoates are compounds of high importance in organic synthesis. Herein, we report the iron-catalyzed C(sp2)–C(sp3) Kumada cross-coupling of aryl chlorobenzoates with alkyl Grignard reagents. The method is characterized by the use of environmentally benign and sustainable iron salts for cross-coupling in the catalytic system, employing benign urea ligands in the place of reprotoxic NMP (NMP = N-methyl-2-pyrrolidone). It is notable that high selectivity for the cross-coupling is achieved in the presence of hydrolytically-labile and prone to nucleophilic addition phenolic ester C(acyl)–O bonds. The reaction provides access to alkyl-functionalized aryl benzoates. The examination of various O-coordinating ligands demonstrates the high activity of urea ligands in promoting the cross-coupling versus nucleophilic addition to the ester C(acyl)–O bond. The method showcases the functional group tolerance of iron-catalyzed Kumada cross-couplings. 

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3. Suzuki–Miyaura cross-coupling of amides and esters at room temperature: correlation with barriers to rotation around C–N and C–O bonds

   Lei, Peng; Meng, Guangrong; Shi, Shicheng; Ling, Yun; An, Jie; Szostak, Roman; Szostak, Michal (August 2017, Chemical science)

   The Suzuki-Miyaura cross-coupling has been widely recognized as one of the most important methods for the construction of C–C bonds. However, in contrast to traditional aryl halide or pseudohalide electrophiles, coupling reactions with unactivated C–N and C–O electrophiles have proven significantly more challenging. Here we report the first general palladium-catalyzed Suzuki-Miyaura cross-coupling of both common amides and aryl esters through the selective cleavage of the C–N and C–O bonds under exceedingly mild conditions. Notably, for the first time we demonstrate selective C(acyl)– N and C(acyl)–O cleavage/cross-coupling under the same reaction conditions. The reaction uses a commercially available, bench-stable and operationally-convenient (n3-1-t-Bu-indenyl)Pd(IPr)(Cl) precatalyst. Furthermore, we demonstrate that the reactivity of generic amides and aryl esters can be correlated with barriers to isomerization around the C(acyl)–X (X = N, O) bond, thus providing a blueprint for the development of a broad range of novel coupling reactions of ester and amide electrophiles by the selective activation of C–O and C–N bonds. 

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4. Decarbonylative thioetherification by nickel catalysis using air- and moisture-stable nickel precatalysts

   Liu, Chengwei; Szostak, Michal (February 2018, Chemical communications)

   A general, highly selective method for decarbonylative thioetherification of aryl thioesters by C–S cleavage is reported. These reactions are promoted by a commercially-available, userfriendly, inexpensive, air- and moisture-stable nickel precatalyst. The process occurs with broad functional group tolerance, including free anilines, cyanides, ketones, halides and aryl esters, to efficiently generate thioethers using ubiquitous carboxylic acids as ultimate cross-coupling precursors (cf. conventional aryl halides or pseudohalides). Selectivity studies and site-selective orthogonal cross-coupling/thioetherification are described. This thioester activation/coupling has been highlighted in the expedient synthesis of biorelevant drug analogues. In light of the synthetic utility of thioethers and Ni(II) precatalysts, we anticipate that this user-friendly method will be of broad interest. 

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5. Pd-Catalyzed Suzuki-Miyaura Cross-Coupling of Pentafluorophenyl Esters

   https://doi.org/10.3390/molecules23123134  

   Buchspies, Jonathan; J. Pyle, Daniel; He, Huixin; Szostak, Michal (December 2018, Molecules)

   Although the palladium-catalyzed Suzuki-Miyaura cross-coupling of aryl esters has received significant attention, there is a lack of methods that utilize cheap and readily accessible Pd-phosphane catalysts, and can be routinely carried out with high cross-coupling selectivity. Herein, we report the first general method for the cross-coupling of pentafluorophenyl esters (pentafluorophenyl = pfp) by selective C–O acyl cleavage. The reaction proceeds efficiently using Pd(0)/phosphane catalyst systems. The unique characteristics of pentafluorophenyl esters are reflected in the fully selective cross-coupling vs. phenolic esters. Of broad synthetic interest, this report establishes pentafluorophenyl esters as new, highly reactive, bench-stable, economical, ester-based, electrophilic acylative reagents via acyl-metal intermediates. Mechanistic studies strongly support a unified reactivity scale of acyl electrophiles by C(O)–X (X = N, O) activation. The reactivity of pfp esters can be correlated with barriers to isomerization around the C(acyl)–O bond. 

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