Iodine-Catalyzed Room-Temperature Aerobic Oxidation of C(sp3)-H Bonds and Its Application in the Synthesis of Quinoxaline Derivatives

An iodine-catalyzed aerobic oxidation reaction of C(sp3)-H bonds was established at room temperature. In this transformation, iodine acts as a Lewis acid catalyst, and the pyridine moiety in the substrate plays a crucial role. Under the optimum reaction conditions, picolyl ketone substrates were smoothly transformed into the corresponding 1,2-dicarbonyl compounds, and the subsequent introduction of 1,2-diaminobenzenes into the reaction mixture led to the formation of various quinoxaline derivatives. This synthetic process does not use transition metals and also features mild reaction conditions, operational simplicity, and gram-scale synthesis.

Base-controlled regio-divergent C-H bond functionalization

The choice of appropriate directing groups, catalysts, ligands, bases, and solvents to give different regioisomers from the same precursors can improve the efficiency and atom economy in synthetic organic chemistry, while minimizing costs and expanding know chemical space. Base-controlled regio-divergent C-H bond functionalization is an important control strategy in organic synthesis, with the advantages of being operationally simple, and using cheap and available bases with low-toxicity that are easily separable. This highlight is classified into three sections: base-controlled, base-controlled transition metal-catalysed and base-controlled photocatalytic regio-divergent C-H bond functionalization. By summarizing the C-H bond activation model, the mechanisms underlying the regio-divergence and the influencing factors, this highlight aims to deepen the understanding of selective C-H bond functionalization at a molecular level. We expect that this highlight article will provide valuable information for understanding the mechanism of C-H bond functionalization, whilst providing a reference for developing green, efficient C-H bond transformation strategies.

C-H functionalization through benzylic deprotonation with π-coordination or cation-π-interactions

Benzylic C-H functionalization is a valuable tool to make complex aromatic molecules from simple, readily available alkylbenzenes. While methods that involve benzylic radicals or cations generated by hydrogen atom transfer or oxidation have been well demonstrated, they often require oxidative conditions. In contrast, deprotonation methods offer a complementary approach to transform benzylic C-H bonds through a benzylic carbanion generated by deprotonation. Electrophilic transition metal complexes acidify benzylic protons upon π-coordination to the phenyl ring of substrates, facilitating deprotonation by stabilizing the corresponding benzylic carbanion. Cation-complexes with group(I) metals also acidify benzylic C-H bonds. These approaches enable a significant expansion of the scope and diversity of alkylarenes with various electrophilic reagents. In this review, we discuss the development of benzylic functionalization through deprotonation of η6-arene complexes of transition-metals and cation-π interactions with group(I) metals, as well as progress made in catalysis through reversible arene-metal interactions.

Substrate-Controlled Divergent Reductive Cyclization of 2-Arylanilines Using CO2 as a Switching Reagent

Capturing CO2 is highly valued in the field of organic synthesis, especially underdeveloped dual-CO2 conversion. In this study, we detail a novel reductive cyclization of 2-indolylanilines with dual CO2 as a difunctional reagent in the presence of PMHS [poly(methylhydrosiloxane)], delivering methyl-substituted quinoxalines. Furthermore, another chemoselective cyclization with 2-pyrrolylanilines is also realized by converting mono-CO2. Mechanistic investigations shed light upon the fact that this substrate-controlled divergence mainly depends on the formation of N-diacylative intermediates.

Masters of Mediation: MN(SiMe3)2 in Functionalization of C(sp3)-H Latent Nucleophiles

Organoalkali compounds have undergone a far-reaching transformation being a coupling partner to a mediator in unusual organic conversions which finds its spot in the field of sustainable synthesis. Transition-metal catalysis has always been the priority in C(sp3)-H bond functionalization, however alternatively, in recent times this has been seriously challenged by earth-abundant alkali metals and their complexes arriving at new sustainable organometallic reagents. In this line, the importance of MN(SiMe3)2 (M=Li, Na, K & Cs) reagent revived in C(sp3)-H bond functionalization over recent years in organic synthesis is showcased in this minireview. MN(SiMe3)2 reagent with higher reactivity, enhanced stability, and bespoke cation-π interaction have shown eye-opening mediated processes such as C(sp3)-C(sp3) cross-coupling, radical-radical cross-coupling, aminobenzylation, annulation, aroylation, and other transformations to utilize readily available petrochemical feedstocks. This article also emphasizes the unusual reactivity of MN(SiMe3)2 reagent in unreactive and robust C-X (X=O, N, F, C) bond cleavage reactions that occurred alongside the C(sp3)-H bond functionalization. Overall, this review encourages the community to exploit the untapped potential of MN(SiMe3)2 reagent and also inspires them to take up this subject to even greater heights.

Classes of Amides that Undergo Selective N-C Amide Bond Activation: The Emergence of Ground-State Destabilization

Ground-state destabilization of the N-C(O) linkage represents a powerful tool to functionalize the historically inert amide bond. This burgeoning reaction manifold relies on the availability of amide bond precursors that participate in weakening of the nN → π*C=O conjugation through N-C twisting, N pyramidalization, and nN electronic delocalization. Since 2015, acyl N-C amide bond activation through ground-state destabilization of the amide bond has been achieved by transition-metal-catalyzed oxidative addition of the N-C(O) bond, generation of acyl radicals, and transition-metal-free acyl addition. This Perspective summarizes contributions of our laboratory in the development of new ground-state-destabilized amide precursors enabled by twist and electronic activation of the amide bond and synthetic utility of ground-state-destabilized amides in cross-coupling reactions and acyl addition reactions. The use of ground-state-destabilized amides as electrophiles enables a plethora of previously unknown transformations of the amide bond, such as acyl coupling, decarbonylative coupling, radical coupling, and transition-metal-free coupling to forge new C-C, C-N, C-O, C-S, C-P, and C-B bonds. Structural studies of activated amides and catalytic systems developed in the past decade enable the view of the amide bond to change from the "traditionally inert" to "readily modifiable" functional group with a continuum of reactivity dictated by ground-state destabilization.

Synthesis of N-alkoxycarbonyl Pyrroles from O-Substituted Carbamates: A Synthetically Enabling Pyrrole Protection Strategy

The condensation of readily available O-substituted carbamates with 2,5-dimethoxytetrahydrofuran gives N-alkoxycarbonyl pyrroles in a single step and in good yield. By this method, several common amine protecting groups can be introduced on the pyrrole nitrogen. With the exception of N-Boc, N-alkoxycarbonyl groups have seen only minimal use for protection of the pyrrole nitrogen to date. Here, we show that N-alkoxycarbonyl protection can endow pyrrole with distinct reactivity in comparison with N-sulfonyl protection, for example, in a pyrrole acylation protocol employing carboxylic acids with a sulfonic acid anhydride activator.

Alkali-Amide Controlled Selective Synthesis of 7-Azaindole and 7-Azaindoline through Domino Reactions of 2-Fluoro-3-methylpyridine and Aldehydes

Azaindoles and azaindolines are important core structures in pharmaceuticals and natural products, which have found wide applications in the field of medicinal chemistrty. In this study, we developed a novel...

Benzylic Aroylation of Toluenes Mediated by a LiN(SiMe3)2/Cs+ System

Chemoselective deprotonative functionalization of benzylic C-H bonds is challenging, because the arene ring contains multiple aromatic C(sp²)-H bonds, which can be competitively deprotonated and lead to selectivity issues. Recently it was found that bimetallic [MN(SiMe₃)₂ M = Li, Na]/Cs⁺ combinations exhibit excellent benzylic selectivity. Herein, is reported the first deprotonative addition of toluenes to Weinreb amides mediated by LiN(SiMe₃)₂/CsF for the synthesis of a diverse array of 2-arylacetophenones. Surprisingly, simple methyl benzoates also react with toluenes under similar conditions to form 2-arylacetophenones without double addition to give tertiary alcohol products. This finding greatly increases the practicality and impact of this chemistry. Some challenging substrates with respect to benzylic deprotonations, such as fluoro and methoxy substituted toluenes, are selectively transformed to 2-aryl acetophenones. The value of benzylic deprotonation of 3-fluorotoluene is demonstrated by the synthesis of a key intermediate in the preparation of Polmacoxib.