Palladium-Catalyzed Aerobic Oxidative Spirocyclization of Alkyl Amides with Maleimides via β-C(sp3)-H Activation

Palladium-Catalysed Spiro-Cyclisation of Substituted Amino Acids with Maleimides via γ-C(sp3)-H Bond Activation

Achieving the site-selective functionalization of unactivated C(sp3)-H bonds remains a major challenge in organic synthesis. Herein, we report an efficient method for the synthesis of spiro pyrrolidine molecules via the γ-C(sp3)-H bond activation of substituted amino acids. A variety of amino acid and amino alcohol derivatives, as well as dipeptides, were functionalized using this method. Also, spirocyclization of optically active substrates provided a potential route for preparing separable diastereomers in pure enantiomeric form. Moreover, some mechanistic insights have been conducted to propose a feasible reaction mechanism for the present spirocyclization reaction.

Synthesis of γ-Lactams via Palladium-Catalyzed C(sp3)-H Bond Activation of Alkyl Sulfonamides with Substituted Alkenes

A methodology for the γ-butyrolactam scaffolds via ligand-enabled C(sp3)-H bond functionalization of sulfonamides with olefins has been demonstrated. The protocol has been found to be compatible with several activated and unactivated olefins, and the desired lactams were formed in excellent yields. A plausible mechanism has been described to account for the desired lactamization reaction as well as supported by mechanistic investigation including a 1H NMR study and isolation of a palladacycle intermediate.

Palladium-Catalyzed C(sp3)-H Activation in A Monoterpene-Based Compound Under Mild Conditions: A Combined Experimental and Theoretical Mechanistic Study

A rare example of the palladium-catalyzed sp3 C-H bond activation in a monoterpene-based compound has been observed in the reaction of PdCl2 with a (+)-3-carene-based ligand HL (HL=N-((1aS,3S,7bR)-1,1,3-trimethyl-7-phenyl-5-(pyridin-2-yl)-1a,2,3,7b-tetrahydro-1H-cyclopropa[f]quinolin-3-yl)acetamide), which yielded the [PdLCl] complex. In contrast to the vast majority of C(sp3)-H activation reactions which require prolonged heating and mixing due to the inert character of the corresponding bond, the reaction reported herein proceeds rapidly under mild conditions. A theoretical insight into the ligand deprotonation has been performed by DFT calculations. The mechanism of the C-H activation involves (i) simultaneous coordination of the CH3 group of HL to the Pd2+ ion and decoordination of the Cl- anion with consequent formation of a Cl⋅⋅⋅H-N hydrogen bond with the amide group, (ii) approximation of the out-of-sphere Cl- anion to one of the hydrogen atoms of the CH3 group mediated by the crane motion of the amide group and (iii) the ejection of the HCl molecule, which increases the entropy of the system and serves as a driving force for the reaction.

Maleimide-Dependent Rh(III)-Catalyzed Site-Selective Mono and Dual C-H Functionalization of 2-Arylbenzo[d]thiazole and Oxazole Derivatives

The site-selective functionalization of aromatic compounds via C-H activation has emerged as a popular tool in organic synthesis. In this study, we report a regioselective coupling of maleimide to 2-arylbenzo[d]thiazoles in the presence of a rhodium(III) catalyst. Depending upon the nature of the substituent (R2-group) present in the maleimide substrate, either mono- or bis-1,4-addition products were observed in this methodology. In the case of R2 = aryl, cyclohexyl, and tert-butyl, mono coupling was observed, whereas substituents, such as methyl, ethyl, benzyl, and methyl thiophene, provided bis coupling as the major products. Similar selectivity was also observed in the case of 2-arylbenzo[d]oxazoles.

Ligand-Enabled Pd-Catalyzed sp3 C-H Macrocyclization: Synthesis and Evaluation of Macrocyclic Sulfonamide for the Treatment of Parkinson's Disease

The development of simplified synthetic strategy to create structurally and functionally diverse pseudo-natural macrocyclic molecules is highly appealing but poses a marked challenge. Inspired by natural scaffolds, herein, we describe a practical and concise ligand-enabled Pd(II)-catalyzed sp3 C-H alkylation, olefination and arylation macrocyclization, which could offer a novel set of pseudo-natural macrocyclic sulfonamides. Interestingly, the potential of ligand acceleration in C-H activation is also demonstrated by an unprecedented enantioselective sp3 C-H alkylation macrocyclization. Moreover, a combination of in silico screening and biological evaluation led to the identification of a novel spiro-grafted macrocyclic sulfonamide 2 a, which showed a promising efficacy for the treatment of Parkinson's disease (PD) in a mouse model through the activation of silent information regulator sirtuin 3 (SIRT3).

RhIII-Catalyzed Direct Heteroarylation of Unactivated C(sp3)-H with N-Heteroaryl Boronates

Disclosed herein is a rhodium(III)-catalyzed direct heteroarylation reaction between unactivated aliphatic C(sp3)-H bonds in 2-alkylpyridines and heteroaryl organoboron reagents. This catalytic protocol is compatible with various heterocyclic boronates containing ortho- and meta-pyridine, pyrazoles, furan, and quinoline with strong coordination capability. The achievement of this methodology provides an efficient route to build new C(sp3)-heteroaryl bonds.

Recent advances in spirocyclization of maleimides via transition-metal catalyzed C-H activation

In recent years, the maleimide scaffold has received a great deal of attention in C-H activation. Several types of products can be constructed using maleimides as a coupling partner. Alkylation, alkenylation, annulation, dehydrogenative annulation and spirocyclization are various reactions shown by maleimides in C-H activation. Thus, the maleimide scaffold has been extensively studied in the last few years in C-H activation owing to its unique reactivity. Among the diverse class of reactions of maleimides, spirocyclization is a less explored reaction. The spirocycles, in particular the spirosuccinimides are interesting candidates in drug discovery and materials chemistry. Therefore the method of spirocyclization of maleimides via C-H activation becomes an important strategy for the synthesis of a diverse array of spirosuccinimides. This review summarizes the reports available in this field from 2015-2023 and also highlights the scopes and prospects of this method.

C-H activation-initiated spiroannulation reactions and their applications in the synthesis of spirocyclic compounds

Spirocyclic skeletons are prevalent in natural products, pharmaceuticals and organic functional materials. Meanwhile, transition-metal-catalyzed C-H activation reactions have demonstrated unparalleled advantages such as high efficiency, excellent atom-economy, good chemoselectivity and regioselectivity for the formation of target organic molecules. In recent years, C-H activation reactions have been creatively utilized in the synthesis of spirocyclic compounds. This review summarizes the most recent progress made in C-H activation-initiated spiroannulation reactions and their applications in the construction of structurally diverse and biologically valuable spirocyclic scaffolds by using alkynes, diazo compounds, maleimides, alkenes, quinones and cyclopropenones as the coupling partners.

Palladium-Catalyzed (3 + 2) Annulation of Aromatic Acids by C(sp3)-H Olefination and Decarboxylative Cross-Coupling Reaction

A palladium-catalyzed (3 + 2) annulation of 2-methylbenzoic acid with maleimide using Ac-Leu-OH as a powerful ligand has been reported. Through a site-selective γ-C(sp3)-H olefination reaction and a sequential decarboxylative cross-coupling reaction, a five-membered cyclic ring was obtained as the final product. This novel reaction features great site selectivity and reactivity to generate various cyclic products in moderate to good yields.