Regiodivergent CDC reactions of aromatic aldehydes with unactivated arenes controlled by transient directing strategy

Unified approaches in transition metal catalyzed C(sp3)-H functionalization: recent advances and mechanistic aspects

In organic synthesis, C(sp3)-H functionalization is a revolutionary method that allows direct alteration of unactivated C-H bonds. It can obviate the need for pre-functionalization and provides access to streamlined and atom economical routes for the synthesis of complex molecules starting from simple starting materials. Many strategies have evolved, such as photoredox catalysis, organocatalysis, non-directed C-H activation, transiently directed C-H activation, and native functionality directed C-H activation. Together these advances have reinforced the importance of C(sp3)-H functionalization in synthetic chemistry. C(sp3)-H functionalization has direct applications in pharmacology, agrochemicals, and materials science, demonstrating its ability to transform synthetic approaches by creating new retrosynthetic disconnections and boost the efficiency of chemical processes. This review aims to provide an overview of current state of C(sp3)-H functionalization, focusing more on recent breakthroughs and associated mechanistic insights.

Ligand-Promoted Palladium-Catalyzed β-C(sp3)–H Arylation of Ketones Using Acetohydrazide as a Transient Directing Group

A palladium-catalyzed β-C(sp3)–H arylation of aliphatic ketones by using acetohydrazide as a transient directing group has been developed. The reaction proceeds through a less-favored [5,5]-bicyclic palladacycle intermediate and is promoted by a pyridine ligand.

Suzuki-Miyaura Type Regioselective C-H Arylation of Aromatic Aldehydes by a Transient Directing Strategy

Herein, we disclose a common approach for palladium-catalyzed direct coupling of the ortho-C-H bond of aromatic aldehydes with various organoboronic reagents by a transient directing strategy. In contrast to widely used cross-coupling reactions of C-H bonds with aryl halides, which generally need silver salt as a halide removal reagent, the method which used BQ/TFA as weak oxidation system for the PdII/Pd0 redox cycle is cost-effective, ecofriendly, and more aligned with green catalysis. This broadly applicable method opens up a new and efficient Suzuki-Miyaura coupling route for the direct formation of carbon-carbon bonds by C-H bond activation.

ortho-C(sp3)-H arylation of aromatic aldehydes using 2-amino-N-methyl-acetamide as a L,L-type transient directing group

Pd-catalyzed ortho-C(sp³)-H arylation of aromatic aldehydes using 2-amino-N-methyl-acetamide as a simple, efficient and commercially available L,L-type transient directing group (TDG) is reported. The reaction exhibited excellent substrate compatibility and generated the desired products in moderate-to-high yields up to 78%. Further acid-catalyzed cyclization and dehydrative aromatization were also tested, and furnished some polycyclic aromatic hydrocarbons with excellent yields up to 96%. The X-ray crystal structure of a 2-methylbenzaldehyde ortho-C(sp³)-H palladation intermediate was obtained. Then, a plausible reaction mechanism involving the formation of a [5,6]-fused palladacycle was proposed. This approach offers valuable insights for exploiting novel L,L-type TDGs.

Chemodivergent Synthesis of Indeno[1,2-b]indoles and Isoindolo[2,1-a]indoles via Mn(III)-Mediated or Electrochemical Intramolecular Radical Cross-Dehydrogenative Coupling

Chemodivergent synthesis of indeno[1,2-b]indoles and isoindolo[2,1-a]indoles from the same starting materials involving radical cross-dehydrogenative couplings have been developed. Mn(OAc)₃·2H₂O selectively promoted an intramolecular radical C-H/C-H dehydrogenative coupling reaction to provide indeno[1,2-b]indoles, while an intramolecular radical C-H/N-H dehydrogenative coupling reaction could proceed via electrochemistry to deliver isoindolo[2,1-a]indoles. Plausible mechanisms of the chemodivergent reactions were proposed.

PdII-Catalyzed Site-selective β- and γ-C(sp3)-H Arylation of Primary Aldehydes Controlled by Transient Directing Groups

Pd(II)-catalyzed site-selective β- and γ-C(sp³)-H arylation of primary aldehydes is developed by rational design of L,X-type transient directing groups (TDG). External 2-pyridone ligands are identified to be crucial for the observed reactivity. By minimizing the loading of acid additives, the ligand effect is enhanced to achieve high reactivities of the challenging primary aldehyde substrates. Site selectivity can be switched from the proximate to the relatively remote position by changing the bite angle of TDG to match the desired palladacycle size. Experimental and computational investigations support this rationale for designing TDG to potentially achieve remote site-selective C(sp³)-H functionalizations.