Pd-Catalyzed Decarbonylative sp2 C-H Arylation: Construction of Five- and Six-Membered (Hetero)Cyclic Compounds

The cyclic compounds have wide applications in the design and synthesis of drugs and materials; thus, their efficient construction attracts much attention from the synthetic community. In this letter, we report an efficient method for preparing cyclic compounds starting from the readily available carboxylic acids. This reaction takes place through intramolecular decarbonylative sp2 C-H arylation, enabling efficient synthesis of a wide range of five- and six-membered cyclic compounds. Both carbo- and heterocycles can be produced under the reaction conditions. Moreover, this reaction features a wide substrate scope with high functional group tolerance. The scale-up experiments also show its practicality in organic synthesis. Those experimental results indicate that this reaction would find wide applications in the synthetic community.

Palladium-Catalyzed Decarbonylative Annulation of 2-Arylbenzoic Acids with Internal Alkynes toward Phenanthrenes

A palladium-catalyzed decarbonylative annulation of 2-arylbenzoic acids with internal alkynes via C(sp2)-H activation has been developed. A series of phenanthrenes were produced in moderate to good yield with good functional group tolerance. The mechanism study indicated that the C(sp2)-H activation should be the rate-determining step during the reaction.

Rhodium(III)-catalyzed regioselective C2-alkenylation of indoles with CF3-imidoyl sulfoxonium ylides to give multi-functionalized enamines using a migratable directing group

A rhodium(III)-catalyzed regioselective C2-alkenylation of indoles for the construction of α-CF₃ substituted enamines has been developed, which utilizes CF₃-imidoyl sulfoxonium ylides (TFISYs) as alkenylating agents for the first time. A wide array of indolyl- and trifluoromethyl-decorated enamine derivatives have been assembled in moderate to good yields.

Mechanistic Insights into Cobalt-Catalyzed Regioselective C4-Alkenylation of 3-Acetylindole: A Detailed Theoretical Study

A detailed mechanistic study of Co(III)-catalyzed C4-alkenylation of 3-acetylindole (1a) was done based on calculations at density functional theory (DFT) and correlated wave function levels. The whole catalytic cycle consists of four steps: C-H activation, olefin insertion, β-hydride elimination, and regeneration of the catalyst. The theoretical results support olefin insertion as the rate-determining step leading to the experimentally observed regioselectivity of the C4 site over the C2 site. By the analysis of three-dimensional (3D) geometries and the NCl plot, the preference for the C4 site over the C2 site could be attributed to the weaker repulsive interaction between the indole moiety and olefin in the transition states of the olefin insertion step for the former. The reliability of the theoretical mechanistic results is further confirmed through the DFT calculation of other related indole derivatives and olefin substrates.

Rhodium-Catalyzed Aryl Migratory/Decarbonylation of Diaryl Ketones via the Activation of Unstrained C–C Bonds

A Rh-catalyzed aryl migratory/decarbonylation of unstrained ketones has been developed. This viable transformation provides a complementary decarbonylative method using diaryl ketones to afford a variety of alkenylated heterocycles in good yields with moderate to good stereoselectivity and broad substrate scope. The synthetic utility of this protocol is also demonstrated by the conversion of the alkenylated heterocycles to tetrahydrocarbazole derivatives.

An entry to 2-(cyclobut-1-en-1-yl)-1H-indoles through a cyclobutenylation/deprotection cascade

A transition-metal-free strategy for the synthesis of 2-(cyclobut-1-en-1-yl)-1H-indoles under mild conditions is described herein. A series of substituted 2-(cyclobut-1-en-1-yl)-1H-indoles are accessed by a one-pot cyclobutenylation/deprotection cascade from N-Boc protected indoles. Preliminary experimental and density functional theory calculations suggest that a Boc-group transfer is involved in the underlying mechanism.

Tunable C–H arylation and acylation of azoles with carboxylic acids by Pd/Cu cooperative catalysis

Direct C–H arylation and acylation of azoles with carboxylic acids are achieved selectively through Pd/Cu cooperative catalysis: biaryls are generated selectively with dppp as ligand, while biaryl ketones are obtained with high selectivity using dpph or Ph₂PCy as ligand.

Transition metal-catalyzed C–H functionalizations of indoles

This review summarises a wide range of transformations on the indole skeleton, including arylation, alkenylation, alkynylation, acylation, nitration, borylation, and amidation, using transition-metal catalyzed C–H functionalization as the key step.

Rhodium(i)-catalyzed C6-selective C-H alkenylation and polyenylation of 2-pyridones with alkenyl and conjugated polyenyl carboxylic acids

A versatile Rh(i)-catalyzed C6-selective decarbonylative C-H alkenylation of 2-pyridones with readily available, and inexpensive alkenyl carboxylic acids has been developed. This directed dehydrogenative cross-coupling reaction affords 6-alkenylated 2-pyridones that would otherwise be difficult to access using conventional C-H functionalization protocols. The reaction occurs with high efficiency and is tolerant of a broad range of functional groups. A wide scope of alkenyl carboxylic acids, including challenging conjugated polyene carboxylic acids, are amenable to this transformation and no addition of external oxidant is required. Mechanistic studies revealed that (1) Boc2O acts as the activator for the in situ transformation of the carboxylic acids into anhydrides before oxidative addition by the Rh catalyst, (2) a decarbonylation step is involved in the catalytic cycle, and (3) the C-H bond cleavage is likely the turnover-limiting step.

Ruthenium(II)-Catalyzed Regioselective C-8 Hydroxylation of 1,2,3,4-Tetrahydroquinolines

Ru(II)-catalyzed chelation-assisted highly regioselective C8-hydroxylation of 1,2,3,4-tretrahydroquinolines has been developed. Various 1,2,3,4-tetrahydroquinolines underwent smooth C8-H hydroxylation with cheap and safe K₂S₂O₈ as the oxidant and oxygen source to furnish the corresponding products in good to excellent yields with high tolerance of the functional groups. The choice of a readily installable and removable N-pyrimidyl directing group is the key to catalysis. Mechanistic studies suggest the involvement of a six-membered ruthenacycle intermediate in the catalytic cycle. The method can also be extended to the direct hydroxylation of other (hetero)arene C-H bonds.

Rh(iii)-Catalyzed regio- and stereoselective bisindolylation of vinyl acetate: an efficient approach toward (E)-1,2-bis(2-indolyl)ethenes

An efficient Rh(iii)-catalyzed regio- and stereoselective cross-coupling between vinyl acetate andN-(2-pyridiyl)indoles providing a series of (E)-2,2′-bis(indolyl)ethenes has been developed.

Synthesis of perinaphthenones through rhodium-catalyzed dehydrative annulation of 1-naphthoic acids with alkynes

A convenient method for the synthesis of perinaphthenones via rhodium-catalyzed dehydrative annulation of naphthoic acids with alkynes, which gave good to high yields of perinaphthenones, was developed.

Rhodium(i)-catalysed decarbonylative direct C–H vinylation and dienylation of arenes

Rh(i)-Catalyzed decarbonylative direct C–H bond vinylation and dienylation of arenes with acrylic acid and (E)-penta-2,4-dienoic acid have been developed.

A versatile rhodium(iii) catalyst for direct acyloxylation of aryl and alkenyl C–H bonds with carboxylic acids

Rh(iii)-Catalyzed highly regioselective direct acyloxylation of aryl and alkenyl sp² C–H bonds with various carboxylic acids has been developed.

Iron(ii)-catalyzed C-2 cyanomethylation of indoles and pyrroles via direct oxidative cross-dehydrogenative coupling with acetonitrile derivatives

A novel and efficient approach for the C(sp²)–H/C(sp³)–H oxidative coupling of indoles and pyrroles with acetonitrile derivatives was reported by using the Fe(ii) complex.

Rhodium(iii)-catalyzed C–H activation at the C4-position of indole: switchable hydroarylation and oxidative Heck-type reactions of maleimides

A Rh(iii)-catalyzed C–H activation of indole at the C4-position leading to novel and switchable functionalization has been reported by employing a weakly co-ordinating COCF₃ group as a directing group.

One-pot synthesis of fluorescent 2,4-dialkenylindoles by rhodium-catalyzed dual C–H functionalization

A one-pot synthesis of fluorescent 2,4-dialkenylindoles by rhodium-catalyzed dual C–H bond alkenylation of indoles.

Electronic Nature of Ketone Directing Group as a Key To Control C-2 vs C-4 Alkenylation of Indoles

A novel mode of achieving site selectivity between C-2 and C-4 positions in the indole framework by altering the property of the ketone directing group is disclosed. Methyl ketone, as directing group, furnishes exclusively C-2 alkenylated product, whereas trifluoromethyl ketone changes the selectivity to C-4, indicating that the electronic nature of the directing group controls the unusual choice between a 5-membered and a 6-membered metallacycle. The screening of other carbonyl-derived directing groups reveals that strong and weak directing groups exhibit opposite selectivity. Experimental controls and deuteration experiments lend support to the proposed mechanism.

Manganese catalyzed C–H functionalization of indoles with alkynes to synthesize bis/trisubstituted indolylalkenes and carbazoles: the acid is the key to control selectivity

The Mn-catalyzed C–H alkenylations of indole with terminal- and/or internal-alkynes have been developed to selectively yield indolylalkene and carbazole.