Transition‐Metal‐Catalyzed Direct Arylation of (Hetero)Arenes by C?H Bond Cleavage

The area of transition‐metal‐catalyzed direct arylation through cleavage of C? H bonds has undergone rapid development in recent years, and is becoming an increasingly viable alternative to traditional cross‐coupling reactions with organometallic reagents. In particular, palladium and ruthenium catalysts have been described that enable the direct arylation of (hetero)arenes with challenging coupling partners—including electrophilic aryl chlorides and tosylates as well as simple arenes in cross‐dehydrogenative arylations. Furthermore, less expensive copper, iron, and nickel complexes were recently shown to be effective for economically attractive direct arylations.     

Iron‐Catalyzed Chemoselective ortho Arylation of Aryl Imines by Directed C?H Bond Activation

No Fe‐ar : Iron catalyzes an imine‐directed C? H bond activation to introduce an ortho‐aryl group to an acetophenone‐derived imine using a diarylzinc reagent (see scheme), whereas palladium catalyzes the conventional substitution reaction . The title reaction features mild and selective C? H bond activation in the presence of aryl bromide, chloride, or sulfonate groups, and 1,2‐dichloroisobutane is essential to achieve such selectivity.

     

Palladium‐Catalyzed Direct C?H Arylation of Isoxazoles at the 5‐Position

A palladium‐catalyzed direct arylation of isoxazoles with aryl iodides has been achieved. The C H bond at the 5‐position is activated selectively to give coupling products in moderate to good yields. This direct arylation was applied to the synthesis of a spiro‐type chiral ligand, which proved to be most effective to the palladium‐catalyzed tandem cyclization of a dialkenyl alcohol.     

Palladium‐Catalyzed Dearomative Cyclocarbonylation by C?N Bond Activation

A fundamentally novel approach to bioactive quinolizinones is based on the palladium‐catalyzed intramolecular cyclocarbonylation of allylamines. [Pd(Xantphos)I₂], which features a very large bite angle, has been found to facilitate the rapid carbonylation of azaarene‐substituted allylamines into bioactive quinolizinones in good to excellent yields. This transformation represents the first dearomative carbonylation and is proposed to proceed by palladium‐catalyzed C N bond activation, dearomatization, CO insertion, and a Heck reaction.     

Ruthenium‐Catalyzed Cascade C?H Functionalization of Phenylacetophenones

Three orthogonal cascade C H functionalization processes are described, based on ruthenium‐catalyzed C H alkenylation. 1‐Indanones, indeno indenes, and indeno furanones were accessed through cascade pathways by using arylacetophenones as substrates under conditions of catalytic [{Ru(p‐cymene)Cl₂}₂] and stoichiometric Cu(OAc)₂. Each transformation uses C H functionalization methods to form C C bonds sequentially, with the indeno furanone synthesis featuring a C O bond formation as the terminating step. This work demonstrates the power of ruthenium‐catalyzed alkenylation as a platform reaction to develop more complex transformations, with multiple C H functionalization steps taking place in a single operation to access novel carbocyclic structures.     

Direct ortho‐Arylation of N‐Phenacylpyridinium Bromide by Palladium‐Catalyzed C?H‐Bond Activation

A novel palladium catalyzed direct ortho‐arylation of N‐phenacylpyridinium bromide was developed. The amazing N‐phenacyl group regioselectively activates the C? H bond of pyridine and automatically departs from the arylated products. A kinetic isotope effect study proved that the reaction went through a C? H‐bond activation pathway and 2,6‐diphenylpyridine was produced stepwise from 2‐phenylpyridine.     

Diastereoselective Carbocyclization of 1,6‐Heptadienes Triggered by Rhodium‐Catalyzed Activation of an Olefinic C?H Bond

The use of α,ω‐dienes as functionalization reagents for olefinic carbon–hydrogen bonds has been rarely studied. Reported herein is the rhodium(I)‐catalyzed rearrangement of prochiral 1,6‐heptadienes into [2,2,1]‐cycloheptane derivatives with concomitant creation of at least three stereogenic centers and complete diastereocontrol. Deuterium‐labeling studies and the isolation of a key intermediate are consistent with a group‐directed C H bond activation, followed by two consecutive migratory insertions, with only the latter step being diastereoselective.     

Ruthenium‐Catalyzed C?C Bond Cleavage in Lignin Model Substrates

Ruthenium–triphos complexes exhibited unprecedented catalytic activity and selectivity in the redox‐neutral C C bond cleavage of the β‐O‐4 lignin linkage of 1,3‐dilignol model compounds. A mechanistic pathway involving a dehydrogenation‐initiated retro‐aldol reaction for the C C bond cleavage was proposed in line with experimental data and DFT calculations.     

Palladium‐Catalyzed Direct C?H Arylation of Isoxazoles at the 5‐Position

A palladium‐catalyzed direct arylation of isoxazoles with aryl iodides has been achieved. The C H bond at the 5‐position is activated selectively to give coupling products in moderate to good yields. This direct arylation was applied to the synthesis of a spiro‐type chiral ligand, which proved to be most effective to the palladium‐catalyzed tandem cyclization of a dialkenyl alcohol.