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.     

Cp*Co(III) catalyzed annulation of N-Cbz hydrazones for the redox-neutral synthesis of isoquinolines via C–H/N–N bond activation

Abstract

A new cascade oxidative cyclization reaction of N-Cbz hydrazones with internal alkynes has been explored for the preparation of isoquinoline derivatives using Cp*Co^III-catalyst through C–H and N–N bond functionalization. N-Cbz hydrazones are rarely explored as directing group for redox-neutral [4?+?2] cyclization reaction through the cyclometallation and this catalyst system does not require any external oxidizing agent, as well as, silver or antimony salt. The current efficient approach has been utilized for the synthesis of different isoquinoline derivatives with good regioselectivity and yields.     

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.     

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 C–H functionalization of C-alkenyl azoles with sulfoxonium ylides for the synthesis of bridgehead N-fused [5,6]-bicyclic heterocycles

The synthesis of bridgehead N-fused [5,6]-bicyclic heterocycles via rhodium(III)-catalyzed C–H functionalization of C-alkenyl azoles with sulfoxonium ylides is disclosed. Reactions proceeded in good to high yields for a range of aryl, heteroaryl and alkyl sulfoxonium ylides. In addition, 2-alkenyl imidazoles with different substitution patterns as well as C-alkenyl triazoles were effective inputs. The reaction could also be performed under straightforward bench top conditions.     

Synthesis of Indazolo[2,1-a]Cinnolines via Rhodium (III)-Catalyzed C–H activation/annulation under mild conditions

A simple, robust and efficient Rh(III)-catalyzed synthesis of novel 12H-indazolo[2,1-a]cinnolin-12-ones has been developed via tandem C–H activation/annulation of 2-phenylindazolones with diazo compounds by using less developed secondary amine as an intrinsic directing group. Notably, a series of nondiscriminating conditions were obtained with excellent test yield. Also, this reaction is highly regioselective with broad substrate scope and both electron-donating and electron-withdrawing groups gave satisfactory yield under very mild conditions (room temperature to 40 °C). It is scalable, compatible with air and H₂O, and the only by-products are N₂ and H₂O. Moreover, the synthesized products represent a new class of fluorophores and their initial spectroscopic characterization has been conducted.     

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.     

Rhodium catalyzed regioselective arene homologation of aryl urea via double C–H bond activation and migratory insertion of alkyne

A convenient rhodium catalyzed oxidative arene homologation of aniline derivatives with symmetrical or unsymmetrical alkynes using Cu(OAc)_2 as oxidant is described. Urea group is shown to be effective as a directing group for initial ortho C–H activation. Two migratory insertion events of alkyne into Rh–C bond occur successively, both with complete regioselectivity. This method is particularly useful for synthesis of polyarenes with different substituents, which has not been reported with conventional protocol. A mechanism has been proposed to explain the observed data.     

I2–DMSO catalyzed synthesis of 1,2,2-triarylethanones via dual C–H activation of aryl methyl ketones

An I₂–DMSO promoted domino protocol was developed for the synthesis of 1,2,2-triarylethanones from readily available aromatic methyl ketones and methoxybenzenes. The 1,2,2-triarylethanones are important precursors for the synthesis of Tamoxifen and Droloxifene. The use of mild reaction conditions, high yield, and single step synthesis are the advantages of the present protocol.     

Synthesis of tetrahydroindolones and tetrahydrocarbazolones via palladium catalyzed C–H activation

The treatment of bromo homoallyl pyrrolyl/indolyl ketone derivatives with Pd(OAc)₂ in the presence of PPh₃ and Cs₂CO₃ in DMF resulted in the formation of tetrahydroindolones and tetrahydrocarbazolones in moderate to good isolated yields.     

Ruthenium catalyzed desymmetrization of diazabicyclic olefins to access heteroaryl substituted cyclopentenes through C–H activation of phenylazoles

The first ruthenium catalyzed redox-neutral C–H activation strategy for the ring-opening of diazabicyclic olefins via C–H bond cleavage of phenyl azoles is reported. The developed method offers a novel route to functionalized cyclopentenes by employing less-expensive ruthenium catalyst and readily accessible biologically significant heteroarenes. The present protocol is a merger of the C–H activation of phenyl substituted heteroaromatics and subsequent β-nitrogen elimination of diazabicyclic olefins.     

Synthesis of xanthones through the palladium-catalyzed carbonylation/C–H activation sequence

A series of xanthones with the dibenzo-γ-pyrone framework were synthesized through the palladium-catalyzed carbonylation/C–H activation sequence from the ortho diazonium salts of diaryl ethers in moderate to excellent yields. After screening the reaction condition, the optimal condition was 5 mol % tetrakis (triphenylphosphine) palladium (0) as the catalyst, potassium carbonate as the base, and toluene as the solvent in the presence of catalytic amount of tetrabutyl ammonium bromide under carbon monoxide atmosphere.     

Solvent-free rhodium(III)-catalyzed synthesis of 2-aminoanilides via C−H amidation of N-nitrosoanilines under ball-milling conditions

A solvent-free rhodium(III)-catalyzed C?H amidation of N-nitrosoanilines with 1,4,2-dioxazol-5-ones has been successfully developed under ball-milling conditions. This protocol provides an efficient and green access to a variety of 2-aminoanilide derivatives with low catalyst loading, remarkable functional group compatibility and excellent yields. In addition, the products allow convenient access to pharmaceutically valuable benzimidazole derivatives through a one-pot two-step synthesis.     

Synthesis of indenoquinolinone through aryne-mediated Pd(II)-catalysed remote C–H activation

Research on Chemical Intermediates - Indenoquinolinones have been synthesized from 2-haloquinoline-3-carbaldehyde through Pd-mediated simultaneous C–H (aldehyde) and C–X bond...     

Rhodium(III)-Catalyzed C−H/N−H Functionalization with Hydrogen Evolution

A rhodium(III)-catalyzed C−H/N−H bond functionalization of benzimidates with α-chloroaldehydes to afford isoquinolin-3-ol derivatives is reported. No external oxidants are needed in this process, and interestingly, evolution of hydrogen gas is observed.     

Synthesis of fused heterocycles via Pd-catalyzed multiple aromatic C–H activation reactions

Recent developments in the synthesis of fused heterocycles using Pd-catalyzed multiple aromatic C–H activation is discussed in this highlight.     

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.     

Oxygen as an oxidant in rhodium(III) catalyzed oxidative C–H/C–H cross-coupling between indoles and oxazoles

A Rh(III)-catalyzed oxidative C–H/C–H cross-coupling between indoles and oxazoles with molecular oxygen as an oxidant has been developed to construct a variety of 2-(indol-2-yl)oxazoles. The chelation-assisted strategy endows this catalytic system with an excellent C2-site selectivity by introducing a pyrimidyl directing group. In addition, polycyclic pyridinium salts are also obtained via Rh(III)-catalyzed C–H activation/cyclization of 2-(indol-2-yl)oxazoles with alkynes.     

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.     

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.