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1. Transamidation of Amides and Amidation of Esters by Selective N–C(O)/O–C(O) Cleavage Mediated by Air- and Moisture-Stable Half-Sandwich Nickel(II)–NHC Complexes

   https://doi.org/10.3390/molecules26010188  

   Buchspies, Jonathan ; Rahman, Md. Mahbubur ; Szostak, Michal ( January 2021 , Molecules)

   The formation of amide bonds represents one of the most fundamental processes in organic synthesis. Transition-metal-catalyzed activation of acyclic twisted amides has emerged as an increasingly powerful platform in synthesis. Herein, we report the transamidation of N-activated twisted amides by selective N–C(O) cleavage mediated by air- and moisture-stable half-sandwich Ni(II)–NHC (NHC = N-heterocyclic carbenes) complexes. We demonstrate that the readily available cyclopentadienyl complex, [CpNi(IPr)Cl] (IPr = 1,3-bis(2,6-diisopropylphenyl)imidazol-2-ylidene), promotes highly selective transamidation of the N–C(O) bond in twisted N-Boc amides with non-nucleophilic anilines. The reaction provides access to secondary anilides via the non-conventional amide bond-forming pathway. Furthermore, the amidation of activated phenolic and unactivated methyl esters mediated by [CpNi(IPr)Cl] is reported. This study sets the stage for the broad utilization of well-defined, air- and moisture-stable Ni(II)–NHC complexes in catalytic amide bond-forming protocols by unconventional C(acyl)–N and C(acyl)–O bond cleavage reactions.
2. Hypervalent iodine-promoted twofold oxidative coupling of amines with amides and thioamides: chemoselective pathway to oxazoles and thiazoles

   https://doi.org/10.1039/D3SC00301A  

   Nan, Jiang ; Ren, Xin ; Yan, Qiang ; Liu, Shilei ; Wang, Jing ; Ma, Yangmin ; Szostak, Michal ( March 2023 , Chemical Science)

   Direct functionalization of the C(O)–N amide bond is one of the most high-profile research directions in the last few decades; however oxidative couplings involving amide bonds and functionalization of thioamide C(S)–N analogues remain an unsolved challenge. Herein, a novel hypervalent iodine-induced twofold oxidative coupling of amines with amides and thioamides has been established. The protocol accomplishes divergent C(O)–N and C(S)–N disconnection by the previously unknown Ar–O and Ar–S oxidative coupling and highly chemoselectively assembles the versatile yet synthetically challenging oxazoles and thiazoles. Employing amides instead of thioamides affords an alternative bond cleavage pattern, which is a result of the higher conjugation in thioamides. Mechanistic investigations indicate ureas and thioureas generated in the first oxidation as pivotal intermediates to realize the oxidative coupling. These findings open up new avenues for exploring oxidative amide and thioamide bond chemistry in various synthetic contexts.
3. Thioesterification and Selenoesterification of Amides via Selective N–C Cleavage at Room Temperature: N–C(O) to S/Se–C(O) Interconversion

   https://doi.org/10.1055/s-0039-1690055  

   Rahman, Md. Mahbubur ; Li, Guangchen ; Szostak, Michal ( March 2020 , Synthesis)

   The direct nucleophilic addition to amides represents an attractive methodology in organic synthesis that tackles amidic resonance by ground-state destabilization. This approach has been recently accomplished with carbon, nitrogen and oxygen nucleophiles. Herein, we report an exceedingly mild method for the direct thioesterification and selenoesterification of amides by selective N–C(O) bond cleavage in the absence of transition metals. Acyclic amides undergo N–C(O) to S/Se–C(O) interconversion to give the corresponding thioesters and selenoesters in excellent yields at room temperature via a tetrahedral intermediate pathway (cf. an acyl metal).
4. Decarbonylative cross-coupling of amides

   https://doi.org/10.1039/c8ob01832d  

   Liu, Chengwei ; Szostak, Michal ( November 2018 , Organic & Biomolecular Chemistry)

   Cross-coupling reactions are among the most powerful C–C and C–X bond forming tools in organic chemistry. Traditionally, cross-coupling methods rely on the use of aryl halides or pseudohalides as electrophiles. In the past three years, decarbonylative cross-couplings of amides have emerged as an attractive method for the construction of a wide variety of carbon–carbon and carbon–heteroatom bonds, allowing for the synthetically-valuable functional group inter-conversion of the amide bond. These previously elusive reactions hinge upon selective activation of the N–C(O) acyl amide bond, followed by CO extrusion, in a formal double N–C/C–C bond activation, to generate a versatile aryl–metal intermediate as an attractive alternative to traditional cross-couplings of aryl halides and pseudohalides. In this perspective review, we present recent advances and key developments in the field of decarbonylative cross-coupling reactions of amides as well as discuss future challenges and potential applications for this exciting field.
5. 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.