Synthesis of unsymmetrical ketones via dual catalysed cross-coupling of α,β-unsaturated carboxylic acids with aryldiazonium salts

A visible light-enabled synthesis of unsymmetrical ketones has been accomplished by the cross-coupling of α,β-unsaturated carboxylic acids and aryldiazonium salts embracing a synergistic eosin Y and Co(OAc)2·4H2O catalysis. The reaction involves decarboxylative aerobic CC bond cleavage, and is endowed with the creation of new C-C and C-O bonds with good substrate scope.

Photocatalyzed oxidative cleavage of CC bond to carbonyl compounds by a recyclable homogeneous carbon nitride semiconductor/aqueous system

The development of methods based on direct oxidative cleavage of alkynes to carbonyls is an extremely challenging task. In this work, we describe a UV light-driven, potassium/sodium poly(heptazine imide) (K,Na-PHI)-catalyzed protocol for the selective oxidative cleavage of alkynes to acids under an O₂ atmosphere using water as a solvent. Various aromatic alkynes can be selectively cleaved to afford aromatic acids in good to high yield under clean, mild conditions. Mechanistic investigation indicates that the photogenerated hydroxyl radicals in situ act as green oxidation active species. Moreover, this protocol could be further developed as a sequential oxidative cleavage/esterification process for the preparation of aryl esters. Notably, the KNaPHI-II/water catalyst system could be used several times without significant loss of activity.

Reactivity of Vinyl Epoxides/Oxetanes/Cyclopropanes toward Arynes: Access to Functionalized Phenanthrenes

The reactivity of vinyl epoxides/oxetanes/cyclopropanes toward arynes has been demonstrated under mild conditions to give the corresponding phenanthrenes in moderate to good yields. This transition-metal-free cascade process involves a series of Diels-Alder reaction, ring-opening aromatization, and ene reaction. Various functionalized phenanthrenes could be synthesized utilizing the versatile hydroxy group. Interestingly, vinyl epoxides/oxiranes experience preferentially the Diels-Alder reaction toward arynes over nucleophilic attack of epoxides/oxiranes.

Traceless directing groups: a novel strategy in regiodivergent C-H functionalization

The use of functional groups as internal ligands for assisting C-H functionalization, termed the chelation assisted strategy, is emerging as one of the most powerful tools for construction of C-C and C-X bonds from inert C-H bonds. However, there are various directing groups which cannot be either removed after functionalization or require some additional steps or reagents for their removal, thereby limiting the scope of structural diversity of the products, and the step and atom economy of the system. These limitations are overcome by the use of the traceless directing group (TDG) strategy wherein functionalization of the substrate and removal of the directing group can be carried out in a one pot fashion. Traceless directing groups serve as the most ideal chelation assisted strategy with a high degree of reactivity and selectivity without any requirement for additional steps for their removal. The present review overviews the use of various functional groups such as carboxylic acids, aldehydes, N-oxides, nitrones, N-nitroso amines, amides, sulfoxonium ylides and silicon tethered directing groups for assisting transition metal catalyzed C-H functionalization reactions in the last decade.

Rh-Catalyzed C-H Amination/Annulation of Acrylic Acids and Anthranils by Using -COOH as a Deciduous Directing Group: An Access to Diverse Quinolines

A method for the synthesis of diverse polysubstituted quinolines from readily available acrylic acids and anthranils has been developed. The weakly coordinating -COOH directing group, which can be tracelessly removed in the cascade cyclization, is essential for this reaction. Diverse polysubstituted quinolines were obtained under mild reaction conditions with simple H₂O and CO₂ as byproducts. More importantly, 1,2,3,4-tetrahydroacridine, which is the core skeleton of tacrine (an Alzheimer's disease drug), was conveniently synthesized.

C3N4-Photocatalyzed aerobic oxidative cleavage of CC bonds in alkynes with diazonium salts leading to two different aldehydes or esters in one pot

C₃N₄-Photocatalyzed oxidative cleavage of CC bonds in alkynes with diazonium salts to obtain two different aldehydes or esters.

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.

Cu-catalyzed oxygenation of alkene-tethered amides with O2 via unactivated C[double bond, length as m-dash]C bond cleavage: a direct approach to cyclic imides

An efficient aerobic unactivated C Created by potrace 1.16, written by Peter Selinger 2001-2019 ]]> C bond cleavage process was achieved, in which the succinimide or glutarimide derivatives could be prepared directly from alkenyl amides.

The transformations of unactivated alkenes through C Created by potrace 1.16, written by Peter Selinger 2001-2019 ]]> C bond double cleavage are always attractive but very challenging. We report herein a chemoselective approach to valuable cyclic imides by a novel Cu-catalyzed geminal amino-oxygenation of unactivated C Created by potrace 1.16, written by Peter Selinger 2001-2019 ]]> C bonds. O₂ was successfully employed as the oxidant as well as the O-source and was incorporated into alkenyl amides via C Created by potrace 1.16, written by Peter Selinger 2001-2019 ]]> C bond cleavage for the efficient preparation of succinimide or glutarimide derivatives. Moreover, the present strategy under simple conditions can be used in the late-stage modification of biologically active compounds and the synthesis of pharmaceuticals, which demonstrated the potential application.

A convergent synthesis of vinyloxyimidazopyridine via Cu(i)-catalyzed three-component coupling

The synthesis of vinyloxyimidazopyridine with complete regio- and stereoselectivity has been achieved by the Cu(i)-catalyzed three-component coupling of 2-aminopyridine, 2-oxoaldehyde and alkyne. This protocol is operationally very simple and has much potential for the synthesis of heteroarylated vinyl ethers from basic chemicals. Steroidal imidazopyridinyl vinyl ether was obtained successfully from ethynylestradiol.

Iridium-Catalyzed α-Selective Arylation of Styrenes by Dual C-H Functionalization

An Ir^I‐system modified with a ferrocene derived bisphosphine ligand promotes α‐selective arylation of styrenes by dual C−H functionalization. These studies offer a regioisomeric alternative to the Pd‐catalyzed Fujiwara–Moritani reaction.

Palladium-Catalyzed Decarboxylative ortho-Amidation of Indole-3-carboxylic Acids with Isothiocyanates Using Carboxyl as a Deciduous Directing Group

Palladium-catalyzed ortho-amidation of indole-3-carboxylic acids with isothiocyanates by using the deciduous directing group nature of carboxyl functionality to afford indole-2-amides is demonstrated. Both C-H functionalization and decarboxylation took place in one pot, and hence, this carboxyl group served as a unique, deciduous (or traceless) directing group. This reaction offers a broad substrate scope as demonstrated for several other heterocyclic carboxylic acids like chromene-3-carboxylic acid, imidazo[1,2- a]pyridine-2-carboxylic acid, benzofuran-2-carboxylic acid, pyrrole-2-carboxylic acid, and thiophene-2-carboxylic acid. In the reaction using 2-naphthoic acid, of the two possible isomers, only one isomer of the amide was exclusively formed. The indole-2-amide product underwent palladium-catalyzed C-H functionalization to afford the diindole-fused 2-pyridones by combining two molecules of the indole moiety, with the elimination of an amide group from one of them, attached at the C3-position for the C-C/C-N bond formation. The structures of key products are confirmed by X-ray crystallography.

Synthesis of 1,1'-Diarylethanes and Related Systems by Displacement of Trichloroacetimidates with Trimethylaluminum

Benzylic trichloroacetimidates are readily displaced by trimethylaluminum under Lewis acid promoted conditions to provide the corresponding methyl substitution product. This method is a convenient way to access 1,1'-diarylethanes and related systems, which play a significant role in medicinal chemistry, with a number of systems owing their biological activity to this functionality. Most benzylic substrates undergo ready displacement, with electron deficient systems being the exception. The use of an enantiopure imidate showed significant racemization, implicating the formation of a cationic intermediate.

Regioselective synthesis of unsymmetrical biheteroaryls via copper(ii)-catalyzed cascade annulation

A facile and efficient copper-catalyzed cascade annulation of an imidazoheterocycle with salicylaldehyde, piperidine, and terminal alkyne was developed to afford unsymmetrical biheteroaryl derivatives in high yields.

Synthesis of 3-Substituted 2-Arylpyridines via Cu/Pd-Catalyzed Decarboxylative Cross-Coupling of Picolinic Acids with (Hetero)Aryl Halides

A decarboxylative cross-coupling of 3-substituted picolinic acids with (hetero)aryl halides is presented. In the presence of catalytic Cu₂O and Pd(1,5-cyclooctadiene)Cl₂ with 2-dicyclohexylphosphino-2'-(N,N-dimethylamino)biphenyl as the ligand, both electron-rich and electron-deficient aryl bromides and chlorides as well as heteroaryl bromides were successfully coupled with various picolinate salts under mild conditions in yields up to 96%. This protocol provides an efficient entry to 2-(hetero)arylpyridines, an attractive substance class in drug discovery.

The use of carboxylic acids as traceless directing groups for regioselective C–H bond functionalisation

This feature article focuses on the use of carboxylic acids as traceless directing groups and the application of this concept to achieve regiochemical control in organic chemistry.

One-pot synthesis of N-heterocycles and enimino carbocycles by tandem dehydrative coupling–reductive cyclization of halo-sec-amides and dehydrative cyclization of olefinic sec-amides

We report a versatile approach for the one-pot synthesis of substituted pyrrolidine, piperidine, indolizidine, and quinolizidine ring systems, and enimino carbocycles.

A deciduous directing group approach for the addition of aryl and vinyl nucleophiles to maleimides

A Rh(iii)-catalyzed C–H activation followed by conjugate addition to maleimides, using carboxylic acid as a traceless/deciduous directing group, to formally furnish a C_sp2–C_sp3 bond is presented.