Palladium‐Catalyzed Isocyanide Insertion with Allylic Esters: Synthesis of N ‐(But‐2‐enoyl)‐ N ‐( tert ‐butyl)benzamide Derivatives via Intramolecular Acyl Transfer Termination

Acyclic Twisted Amides

In this contribution, we provide a comprehensive overview of acyclic twisted amides, covering the literature since 1993 (the year of the first recognized report on acyclic twisted amides) through June 2020. The review focuses on classes of acyclic twisted amides and their key structural properties, such as amide bond twist and nitrogen pyramidalization, which are primarily responsible for disrupting nN to π*C═O conjugation. Through discussing acyclic twisted amides in comparison with the classic bridged lactams and conformationally restricted cyclic fused amides, the reader is provided with an overview of amidic distortion that results in novel conformational features of acyclic amides that can be exploited in various fields of chemistry ranging from organic synthesis and polymers to biochemistry and structural chemistry and the current position of acyclic twisted amides in modern chemistry.

Transition Metal and Inner Transition Metal Catalyzed Amide Derivatives Formation through Isocyanide Chemistry

The synthesis of amides is a substantial research area in organic chemistry because of their ubiquitous presence in natural products and bioactive molecules. The use of easily accessible isocyanides as amidoyl (carbamoyl) synthons in cross-coupling reactions using transition metal and inner transition metöal catalysts is a current trend in this area. Isocyanides, owing to their coordination ability as a ligand and inherent electronic properties for reactions with various partners, have expanded the potential application of these transformations for the preparation of novel synthetic molecules and pharmaceutical candidates. This review gives an overview of the achievements in isocyanide-based transition metal and inner transition metal catalyzed amide formation and discusses highlights of the proposed distinct mechanisms.

1 Introduction

2 Synthesis of Arenecarboxamides

3 Synthesis of Alkanamides

4 Synthesis of Cyclic Amides

5 Formation of Alkynamides

6 Formation of Acrylamide-like Molecules

7 Formation of Ureas and Carbamates

8 Conclusion

Recent Advances in Palladium-Catalyzed Isocyanide Insertions

Isocyanides have long been known as versatile chemical reagents in organic synthesis. Their ambivalent nature also allows them to function as a CO-substitute in palladium-catalyzed cross couplings. Over the past decades, isocyanides have emerged as practical and versatile C1 building blocks, whose inherent N-substitution allows for the rapid incorporation of nitrogeneous fragments in a wide variety of products. Recent developments in palladium catalyzed isocyanide insertion reactions have significantly expanded the scope and applicability of these imidoylative cross-couplings. This review highlights the advances made in this field over the past eight years.

Al/N-based active Lewis pairs: isocyanate insertion products as efficient nucleophiles employed for the facile generation of highly functional molecules

The previously reported active Lewis pair (ALP) i Bu2Al–N(2-Ad)NC5H10 (1) (2-Ad = 2-adamantyl) is readily accessible by hydroalumination of the hydrazone H10C5N–N=(2-C10H14) with H–Al i Bu2. Treatment of 1 with two equivalents of isocyanates R-N=C=O yields six-membered AlC2N2O heterocycles 2 (2a, R = Ph; 2b, R = p-Tol) by dual insertion into the Al–N bonds. 2a reacts as a nucleophile with carboxylic acid chlorides R-C(O)–Cl [R = CH2 t Bu, p-Tol, H2CCH(Me)C6H4(4-CH2CHMe2) (Ibu-profen acid chloride), 0.5 (1,4-C6H4)] to afford by elimination of i Bu2AlCl and hydrolysis new triuret derivatives R-C(O)[N(Ph)C(O)]2–N(2-Ad)NC5H10 (3a to 3d) as colourless, sparingly soluble solids in moderate (3c) to high (3b) yields. The analogous reaction of 2a with (p-Tol)–C(Cl)=N(p-Tol) leads to the imidoyl derivative (p-Tol)N=C(p-Tol)[N(Ph)C(O)]2–N(2-Ad)NC5H10 (4a), which showed a fast exchange of phenyl and tolyl groups to yield a mixture of isomers. The analogous reaction of 2b affords the corresponding compound 4b for which a single isomer is isolated despite the scrambling of substituents.

Synthesis of imides via palladium-catalyzed three-component coupling of aryl halides, isocyanides and carboxylic acids

A palladium-catalyzed three-component synthesis of acyclic imides from feedstock aryl halides, carboxylic acids and isocyanides through the intermediacy of isoimides has been developed. The key to the success of this approach was controlled isocyanide slow addition and organic/aqueous biphasic conditions. This transition-metal-catalyzed approach features readily available starting materials, atom- and step-economy, good functional group compatibility and gram-scale synthetic capability. Utilization of this new method is illustrated in the late-stage functionalization of drugs Carprofen, Loxoprofen and Flurbiprofen. This strategy has also been successfully applied in the synthesis of cyclic imides including phthalimide, homophthalimide, and 2H-2-benzazepine-1,3-dione derivatives.

Metal-Free C-H Functionalization of Allenamides: An Access to Branched Allylic Esters

A regioselective acyloxylation with carboxylic acids at the proximal carbon of allenamides by an N-iodosuccinimide-mediated C-H functionalization is reported. The reaction proceeds rapidly, is scalable to a gram scale, and displays a broad substrate scope, providing an efficient and practical protocol for the synthesis of branched allylic esters. Notably, protected amino acids were tolerated under the reaction conditions and afforded allylic amino acid esters in moderate yields.

Palladium-Catalyzed Diarylation of Isocyanides with Tetraarylleads for the Selective Synthesis of Imines and α-Diimines

Using tetraaryllead compounds (PbAr₄) as arylating reagents, isocyanides undergo selective diarylation in the presence of palladium catalysts such as Pd(OAc)₂ or Pd(PPh₃)₄ to afford imines and/or α-diimines based on the isocyanide employed. With aliphatic isocyanides, imines are obtained preferentially, whereas α-diimines are formed in the case of electron-rich aromatic isocyanides. The differences in imine/α-diimine selectivity can be attributed to the stability of imidoylpalladium intermediates formed in this catalytic reaction. Compared with other arylating reagents, tetraaryllead compounds are excellent candidates for use in the selective transformations to imines and/or α-diimines, especially in terms of inhibiting the oligomerization of isocyanides, which results in a lower product selectivity in many transition-metal-catalyzed reactions of isocyanides.

Palladium-Catalyzed Multicomponent Reaction of Alkynes, Carboxylic Acids, and Isocyanides: A Direct Approach to Captodative Olefins

A palladium-catalyzed multicomponent reaction of alkynes, carboxylic acids, and isocyanides has been developed with the assistance of silver salt under mild conditions. Highly functionalized captodative olefins are synthesized efficiently by this method, which can find many applications as versatile synthons in organic synthesis.

Palladium-Metalated Porous Organic Polymers as Recyclable Catalysts for the Chemioselective Synthesis of Thiazoles from Thiobenzamides and Isonitriles

Two types of thiazole derivatives are synthesized through a multistep cascade sequence with Pd-metalated phosphorus-doped porous organic polymers (POPs) as heterogeneous catalysts. The POPs could be used as both ligands and catalyst supports. No obvious aggregation and loss of any catalytic activity of the catalysts were observed after 10 runs of the reaction. More importantly, imidazo[4,5- d]thiazoles, which are a new class of thiazole derivatives, could be obtained through K₂CO₃-promoted intramolecular cyclization of the synthesized polysubstituted thiazoles. Furthermore, the in vitro anticancer activity of these new compounds were tested with MTT assay, and compound 4b exhibited good antitumor activity toward T-24 and A549 cells with IC₅₀ values of 10.3 ± 0.8 and 11.8 ± 0.5 μM, respectively. In addition, the action mechanism of 4b on tumor cells was determined.