Direct synthesis of benzoxazinones via Cp*Co(III)-catalyzed C–H activation and annulation of sulfoxonium ylides with dioxazolones

A highly novel and direct synthesis of benzoxazinones was developed via Cp*Co(III)-catalyzed C–H activation and [3 + 3] annulation between sulfoxonium ylides and dioxazolones. The reaction is conducted under base-free conditions and tolerates various functional groups. Starting from diverse readily available sulfoxonium ylides and dioxazolones, a variety of benzoxazinones could be synthesized in one step in 32%-75% yields.     

Rhodium(III)-catalyzed C-H alkylation of heterocycles with allylic alcohols in water: A reusable catalytic system for the synthesis of β-aryl ketones

A highly efficient, green and sustainable protocol for rhodium(III)-catalyzed C-H alkylation of heterocycles with allylic alcohols has been achieved, which affords a series of β-aryl ketones in high yields. The reaction proceeds smoothly in water under air, and works well with heterocycles such as indoles, indolines, pyrroles and carbazoles. Notably, the expensive rhodium catalyst in water could be easily separated from the organic products, and reused for at least five times without loss of its catalytic activity and selectivity, which is a promising, green and sustainable pathway for the synthesis of β-aryl ketones. To the best of our knowledge, this is the first example for the reuse of expensive rhodium catalyst in water.     

Rhodium(III)-catalyzed ring-opening of strained olefins through C–H activation of O-acetyl ketoximes: an efficient synthesis of trans-functionalized cyclopentenes and spiro[2.4]heptenes

An efficient strategy for the stereoselective synthesis of functionalized cyclopentenes and spiro[2.4]heptenes from strained olefins via C–H activation of aryl ketone O-acetyl ketoximes using [RhCl₂Cp^∗]₂ catalyst is described. The results revealed that a wide range of readily accessible aryl and heteroaryl ketoximes are compatible in this method for the ring opening of bicyclic and spirotricyclic olefins.     

Rhodium(III)-catalyzed chemodivergent annulations between phenyloxazoles and diazos via C–H activation

Acid-controlled, chemodivergent and redox-neutral annulations for the synthesis of isocoumarins and isoquinolinones have been realized via Rh(III)-catalyzed CH activation. Diazo compounds act as a carbene precursor, and coupling occurs in one-pot process, where adipic acid and trimethylacetic acid promote chemodivergent cyclizations.     

Iron(III) catalyzed direct C–H functionalization at the C-3 position of chromone for the synthesis of fused chromeno-quinoline scaffolds

This communication describes an iron-catalyzed route for the synthesis of 6-substituted chromeno[3,2-c]quinolin-7-one. The method developed does not require any pre-functionalization to execute the pivotal coupling reaction at the C-3 position of flavones. The final step involves the consecutive application of imine formation, C_sp²-C_sp² coupling and oxidation reaction, with aromatic aldehydes and 2-(2-aminophenyl)-4H-chromen-4-one as the reactants. Presence of electron donating/withdrawing groups was well tolerated in the aldehydes and the method developed could also be extended to other substituted 2-(2-aminophenyl)-4H-chromen-4-one’s. This is the first report of synthesis of 6-substituted chromeno[3,2-c]quinolin-7-one’s via direct functionalization of the C-3 site of flavones.     

Rhodium(III)-catalyzed benzo[c]azepine-1,3(2H)-dione synthesis via tandem C–H alkylation and intermolecular amination of N-methoxylbenzamide with 3-bromo-3,3-difluoropropene

Here, a rhodium(III)-catalyzed benzo[c]azepine-1,3(2H)-dione synthesis via tandem C–H alkylation and intramolecular amination of N-methoxylbenzamide with 3-bromo-3,3-difluoropropene as the alkylation agent is reported. The substituted benzamides and protected indoles are all tolerated, yielding the corresponding products in moderate to good yields. Further study revealed those bioactive compounds such as piperic acid and a key precursor of Roflumilast all perform well, highlighting the synthetic utility of this method.     

Rhodium(III)-catalyzed synthesis of indoles from 1-alkylidene-2-arylhydrazines and alkynes via C–H and N–N bond cleavages

1-Alkylidene-2-arylhydrazines undergo annulative coupling with internal alkynes in the presence of a rhodium(III) catalyst and a copper(II) salt. The reaction proceeds through cleavage of the C–H and N–N bonds of hydrazines to afford 1,2,3-trisubstituted indole derivatives.     

Synthesis of substituted β-carbolines via gold(III)-catalyzed cycloisomerization of N-propargylamides

Indole-substituted N-propargylamides undergo a gold(III)-catalyzed cyclization to give oxazoles or β-carbolinones, depending on the substitution pattern at the amide nitrogen. Secondary amides furnished oxazoles via a 5-exo-dig cyclization, while tertiary indole-2-carboxamides cycloisomerized to give 2,9-dihydro-β-carbolin-1-ones upon treatment with catalytic amounts of gold(III) chloride. An optimized procedure was developed to prepare several new N-benzyl-β-carbolinones as well as the corresponding β-carbolines, which are of interest as core structures found in various natural products such as the harman, ervolanine, and lavendamycin class of alkaloids.     

Rhodium(I)-catalyzed [3+2] intramolecular cycloaddition of alkylidenecyclopropane–propargylic esters

An interesting rhodium(I)-catalyzed [3+2] intramolecular cycloaddition of alkylidenecyclopropanes 1 containing propargylic esters has been described in this context. A variety of bicyclo[3.3.0]octene derivatives were obtained in moderate to good yields under mild conditions. The alkylidenecyclopropane-enynes 2 could be also converted to the corresponding cycloadducts 3 in good yields.     

Palladium-catalyzed oxidative C–H/C–H cross-couplings of thiazolo[5,4-d]pyrimidine with aromatic (hetero)cycles

A novel Pd(II)-catalyzed dehydrogenative cross-coupling reaction of thiazolo[5,4-d]pyrimidine with unactivated (hetero)arenes via C–H bond activation was achieved. This protocol provides a straightforward and operationally simple method for the synthesis of 2-arylsubstituted thiazolo[5,4-d]pyrimidines of interest in pharmaceutical sciences.     

Synthesis of CF3-substituted isoindolones via rhodium(iii)-catalyzed carbenoid C–H functionalization of aryl hydroxamates

An efficient method for the selective preparation of trifluoro-methyl-substituted isoindolones has been developed via Rh^III catalyzed C-H activation / [4 + 1]-annulation of aryl hydroxamates with functionalized acceptor/acceptor diazo compounds as cross-coupling partners.     

Oxidative C(Sp)–H activation and C–N cleavage of N-methyl amines under transition-metal-free condition for synthesis of methylene-bridged bis-1,3-diketones

A transition-metal-free oxidative methylenation reaction was developed. Methylene-bridged bis-1,3-dicarbonyl compounds were synthesized by oxidative C(Sp³)–H activation and C–N cleavage of N-methyl amines. This novel reaction avoids the use of transition metal catalyst. Furthermore, the reaction are very mild and operational convenient.     

Pd(II)-catalyzed intramolecular C–H activation/C–C cross coupling for the synthesis of carbazoles from diaryl acetamides

Pd(II)-catalyzed intramolecular C–H activation/C–C cross coupling within N,N-diphenyl-acetamides allows the efficient formation of differently substituted carbazoles. In this cross-dehydrogenative coupling, many different functional groups are tolerated and the starting material N,N-diphenyl-acetamides can be easily prepared in one step from commercially available acetanilides and iodobenzenes.     

Pd-catalyzed enantioselective C–H arylation of phosphinamides with boronic acids for the synthesis of P-stereogenic compounds

A Pd-catalyzed enantioselective C–H arylation of phosphinamides with aryl boronic acids is presented. With readily affordable amino acid derivatives as chiral ligands, the reaction proceeded efficiently to afford the P-stereogenic phosphinamides in up to 75% yield and with 99% ee under mild conditions. Most importantly, the reaction could be performed on a large scale for various substrates. This method provides an alternative and reliable way for accessing P-stereogenic phosphinamides.     

Oxidation-promoted synthesis of ferrocenyl planar chiral rhodium(iii) complexes for C–H functionalization catalysis

The chemical oxidation of rhodium(i) complexes [Rh(L)(COD)][BF4], where L is a ferrocenyl phosphine/N-heterocyclic carbene ligand, with 2 equiv. of a triaryl-aminium salt [(4-BrC6H4)3N][BF4] in acetonitrile gave planar chiral, air-stable [Rh(L–H)(MeCN)3][BF4]2 complexes where the ferrocene (C5H4CH2ImR or C5H4CH2BImCH2Mes) ring has been C–H activated at the position 2 in good to excellent yields. An important reactivity difference between our complexes and the ubiquitous [Cp*Rh(MeCN)3]X2 complex has been observed in the Grignard-type arylation of 4-nitrobenzaldehyde.     

Palladium(II)-catalyzed oxidative annulation of alkenylindoles with alkynes initiated by C–H activation

A palladium(II)-catalyzed annulation of alkenylindoles with diarylethynes initiated by C–H bond activation was developed. This method provided a convenient way for the synthesis of dihydrocyclopenta[b]indoles by enyne cyclization initiated by C–H bond activation.     

Regioselective synthesis of C-2 substituted imidazo[4,5-b]pyridines utilizing palladium catalysed C–N bond forming reactions with enolizable heterocycles

In this Letter we report a rapid and facile access to C2-substituted imidazo[4,5-b]pyridine analogues utilizing palladium mediated Buchwald–Hartwig cross-coupling reactions. The use of enolizable heterocycles as cross-coupling partners resulted in a wide range of imidazo[4,5-b]pyridine analogues which are prone to have medicinal relevance. Xantphos and Pd(OAc)₂ were found to be more effective for the coupling of 2-halo imidazo[4,5-b]pyridines with pyridone nucleophiles. A regioselective approach for the synthesis of 2-substituted 3H-imidazo[4,5-b]pyridine and 1H-imidazo[4,5-b]pyridine is also reported.     

Synthesis and biological activity of chromones annelated at the C(7)–C(8) bond with heterocycles (review)

Published data on the synthesis of chromones annelated at the C(7)–C(8) bond with nitrogen-, oxygen-, and sulfur-containing heterocycles of various sizes, including those with two and three heteroatoms, are discussed. Data on the biological activity of chromone derivatives isolated from natural sources and obtained synthetically are presented.