Rhodium(iii)-catalyzed alkylation of primary C(sp(3))-H bonds with α-diazocarbonyl compounds

C(sp3)-H Bond Functionalization of 8-Methylquinolines

Quinolines have emerged as essential components in various medicinal agents, playing a key role in treating various ailments. Numerous drugs with a quinoline core have been recognized for their antimalarial, antibacterial, and anticancer activities and have been successfully commercialized, including chloroquine, ciprofloxacin, topotecan, etc. Over the past two decades, a tremendous expansion in the C-H bond functionalization of quinoline scaffolds to widen this chemical space for drug discovery have been witnessed. This review article summarizes the efforts toward C(sp3)-H functionalization of 8-methylquinolines for C(sp3)-C/X bond formation under metal and metal-free strategies. Each section briefly overviews the C(sp3)-H functionalization of 8-methylquinoline, highlighting the metal and metal-free approaches.

Palladium catalyzed C(sp3)-H alkylation of 8-methylquinolines with aziridines: access to functionalized γ-quinolinylpropylamines

Palladium-catalyzed directed site-selective C(sp3)-H alkylation of 8-methylquinolines has been accomplished using aziridine as the alkylating source via a sequential C-H and C-N bond activation process. The site selectivity, functional group tolerance and possible late-stage modifications are important practical features of this reaction.

Rhodium-catalyzed transformations of diazo compounds via a carbene-based strategy: recent advances

Diazo compounds are known to be good coupling partners in the synthesis of heterocycles, carbocycles and functionalized molecules via a rhodium carbene-based strategy. Many heterocyclic and carbocyclic compounds, including isoquinolones and isocoumarins, quinoxalines, indoles, pyrrones, benzothazines, enaminones, benzenes and seven-membered rings, can be constructed using this rhodium-catalyzed system. The reaction mechanism involves C-H activation, carbene insertion and an annulation/functionalization sequence. This review describes the progress made in the last five years in rhodium-catalyzed transformations of diazo compounds as easily accessible precursors in organic chemistry.

Solvent-Controlled Rh(III)-Catalyzed Mono- and Dual Functionalization of Quinolyl Aldoximes with Diazo Compounds

A solvent-controlled Rh(III)-catalyzed mono- and dual-C-H bond activation/carbene migratory insertion with diazo compounds as a single coupling partner was demonstrated. The reaction proceeded under mild conditions, yielding products in good to excellent yields. These results are significant for the development of the domino multiple functionalization of C(sp2)-H bonds via a carbenoid insertion approach.

Transition Metal-Catalyzed C-H Activation/Functionalization of 8-Methylquinolines

8-Methylquinoline is regarded as an ideal substrate to participate in diversely C(sp3)-H functionalization reactions. The presence of the chelating nitrogen atom enables 8-methylquinoline to easily form cyclometallated complexes with various transition metals, leading to the selective synthesis of functionalized quinolines. Considering the great importance of quinoline cores in medicinal chemistry, in this review article, we have covered the publications related to the C-H activation and functionalization of 8-methylquinoline under transition metal catalysis during the last decade.

Rhodium-catalyzed three-component C(sp3)/C(sp2)-H activation enabled by a two-fold directing group strategy

Rh-catalyzed three-component C(sp3)/C(sp2)-H activation has been achieved through a two-directing group strategy. This protocol provides a convenient and efficient pathway for the construction of diverse 8-alkyl quinoline derivatives in one-pot. Furthermore, mechanistic studies revealed that the first C-H amidation was significantly faster than the sequential C-H alkylation.

Palladium-Catalyzed Regioselective Insertion of Carbenes into γ-C(sp3)-H Bonds of Aliphatic Amines

A migratory insertion of carbenes into distal γ-C(sp3)-H bonds of aliphatic amines has been successfully developed. The synergistic interplay among a palladium catalyst, picolinamide directing group, a carefully selected base additive, and an essential ligand proved crucial in achieving high yields. These findings hold significant value for advancing the exploration of regioselective carbene insertions into nonactivated C(sp3)-H bonds.

Selective benzylic Csp3-H bond activations mediated by a phosphorus-nitrogen PN3P-nickel complex

In contrast to the typical C_sp2-H activation, a PN³P-Nickel complex chemoselectively cleaved the benzylic C_sp3-H bond of toluene in the presence of KHMDS, presumably via an in situ generated potassium benzyl intermediate. Under similar conditions, CO underwent deoxygenation to afford the corresponding nickel cyano complex, and ethylbenzene was dehydrogenated to give styrene and a nickel hydride compound. 2,6-Xylyl isocyanide was transformed into an unprecedented indolyl complex, likely by trapping the activated benzyl species with an isocyanide moiety.

Palladium-Catalyzed Migratory Insertion of Carbenes and C-C Cleavage of Cycloalkanecarboxamides

A palladium catalyzed reaction of cycloalkanecarboxamides and diazomalonates or bis(phenylsulfonyl)diazomethane has been developed. The reaction proceeds via carbene migratory insertion and cascade C-C cleavage pathways. Cycloalkanecarboxamides with four to seven membered rings are applicable in the transformation. A series of ring opening products were prepared with moderate yields. The finding provides valuable clues for the development of new reactions involving carbene migratory insertion and the cleavage of unstrained C(sp³)-C(sp³) bonds.

Rh(iii)-Catalyzed C–C coupling of unactivated C(sp3)–H bonds with iodonium ylides for accessing all-carbon quaternary centers

Rhodium-catalyzed inert C(sp3)–H activation/carbene insertion has been realized, leading to the construction of all-carbon quaternary centers.

Rh(III)-Catalyzed mild straightforward synthesis of quinoline-braced cyclophane macrocycles via migratory insertion

An efficient Rh(III)-catalyzed straightforward strategy is developed for the synthesis of quinoline braced cyclophane macrocycles via methyl (sp³) C-H functionalization. The method is mild, simple and regioselective with various ring sizes and has good functional group tolerance. The method proceeds via C8-methyl metalation, metal-carbene formation and a subsequent migratory insertion. High dilution is not necessary for this macrocyclization and the only byproduct is nitrogen. A preliminary investigation shows that the C-H metalation step is the rate-determining step.

Pd(II)-Catalyzed Arylation/Oxidation of Benzylic C-H of 8-Methylquinolines: Access to 8-Benzoylquinolines

An efficient access to 8-benzoylquinoline was developed by a sequential arylation/oxidation of 8-methylquinolines with aryl iodides in the presence of Pd(OAc)₂. This transformation demonstrates good tolerance of a wide range of functional groups on aryl iodides, providing good to excellent yields of 8-benzoylquinolines.

Transition-Metal-Catalyzed C-H Bond Functionalization of Arenes/Heteroarenes via Tandem C-H Activation and Subsequent Carbene Migratory Insertion Strategy

The past decade has witnessed tremendous developments in transition-metal-catalyzed C-H bond activation and subsequent carbene migratory insertion reactions, thus assisting in the construction of diverse arene/heteroarene scaffolds. Various transition-metal catalysts serve this purpose and provide efficient pathways for an easy access to substituted heterocycles. A brief introduction to metal-carbenes has been provided along with key mechanistic pathways underlying the coupling reactions. The purpose of this review is to provide a concise knowledge about diverse directing group-assisted coupling of varied arenes/heteroarenes and acceptor-acceptor/donor-acceptor diazo compounds. The review also highlights the synthesis of various carbocycles and fused heterocycles through diazo insertion pathways, via C-C, C-N and C-O bond forming reactions. The mechanism usually involves a C-H activation process, followed by diazo insertion leading to subsequent coupling.

Cp*Rh(III)-Catalyzed Regioselective C(sp3)-H Electrophilic Trifluoromethylthiolation of 8-Methylquinolines

Rh(III)-catalyzed regioselective trifluoromethylthiolation of the unactivated C(sp³)-H bond of 8-methylquinolines with bench-stable electrophilic trifluoromethylthiolating reagent via C(sp³)-H activation is explored. Various substituted 8-methylquinolines provided the products in good yields with high regioselectivity. Current reaction conditions are also applicable for the late-stage functionalization of natural molecule santonin and caffeine-substituted 8-methylquinoline.

Diastereoselective and Reversed Regioselective Annulations of N-Alkyl Anilines to Julolidines and Lilolidines

A three-component annulation reaction of N-alkyl anilines, cyclic 1,3-dicarbonyl compounds, and aryl aldehydes to julolidines and lilolidines is reported. The 6π-electrocyclization enabled the annulation to proceed with reversed regioselectivity as compared with the annulation that occurs in the Povarov reaction. Both cyclic and acyclic N-alkyl anilines participated in the reaction to provide a wide range of julolidines and lilolidines as the single regio- and diastereoisomers in good to excellent yields.

Recent trends in catalytic sp3 C-H functionalization of heterocycles

Heterocycles are a ubiquitous substructure in organic small molecules designed for use in materials and medicines. Recent work in catalysis has focused on enabling access to new heterocycle structures by sp³ C-H functionalization on alkyl side-chain substituents-especially at the heterobenzylic position-with more than two hundred manuscripts published just within the last ten years. Rather than describing in detail each of these reports, in this mini-review we attempt to highlight gaps in existing techniques. A semi-quantitative overview of ongoing work strongly suggests that several specific heterocycle types and bond formations outside of C-C, C-N, and C-O have been almost completely overlooked.

Rhodium(iii)-catalyzed diamidation of olefins via amidorhodation and further amidation

Rh(iii)/Co(iii)-catalyzed diamidation of amide-tethered olefins has been realized using dioxazolones and arylsulfonamides as different classes of amidating reagents.

Dithiane-directed Rh(iii)-catalyzed amidation of unactivated C(sp3)-H bonds

An oxidant-free Rh(iii)-catalyzed direct amidation of alkyl dithianes via C(sp3)-H bond activation utilizing diverse and robust dioxazolone reagents is reported. The reaction hinges on use of a Cp*Rh(iii) complex in combination with an essential amino-carboxylate additive to generate usefully protected 1,3-aminoaldehyde derivatives. The scalability of the reaction was demonstrated as was a series of downstream product functionalizations, including dithiane deprotection, anion alkylation and reductive desulfurization, highlighting the general applicability of this transformation in the synthesis of novel scaffolds and building blocks.

Rh(iii)-Catalyzed straightforward arylation of 8-methyl/formylquinolines using diazo compounds

A straightforward Rh(iii)-catalyzed general strategy was developed for the introduction of naphthol/phenol moieties to 8-methyl and formylquinolines using diazonaphthalen-2(1H)-ones/quinone diazides.

Rhodium(iii)-catalyzed sulfonamide directed ortho C–H carbenoid functionalization via metal carbene migratory insertion

A rhodium(iii)-catalyzed sulfonamide directed ortho C–H carbenoid functionalization has been developed with good yields.

A comprehensive overview of directing groups applied in metal-catalysed C-H functionalisation chemistry

The present review is devoted to summarizing the recent advances (2015-2017) in the field of metal-catalysed group-directed C-H functionalisation. In order to clearly showcase the molecular diversity that can now be accessed by means of directed C-H functionalisation, the whole is organized following the directing groups installed on a substrate. Its aim is to be a comprehensive reference work, where a specific directing group can be easily found, together with the transformations which have been carried out with it. Hence, the primary format of this review is schemes accompanied with a concise explanatory text, in which the directing groups are ordered in sections according to their chemical structure. The schemes feature typical substrates used, the products obtained as well as the required reaction conditions. Importantly, each example is commented on with respect to the most important positive features and drawbacks, on aspects such as selectivity, substrate scope, reaction conditions, directing group removal, and greenness. The targeted readership are both experts in the field of C-H functionalisation chemistry (to provide a comprehensive overview of the progress made in the last years) and, even more so, all organic chemists who want to introduce the C-H functionalisation way of thinking for a design of straightforward, efficient and step-economic synthetic routes towards molecules of interest to them. Accordingly, this review should be of particular interest also for scientists from industrial R&D sector. Hence, the overall goal of this review is to promote the application of C-H functionalisation reactions outside the research groups dedicated to method development and establishing it as a valuable reaction archetype in contemporary R&D, comparable to the role cross-coupling reactions play to date.

Rhodium-Catalyzed Cyclization of O,ω-Unsaturated Alkoxyamines: Formation of Oxygen-Containing Heterocycles

O,ω-Unsaturated N-tosyl alkoxyamines undergo unexpected Rh^III -catalyzed intramolecular cyclization by oxyamination to produce oxygen-containing heterocycles. Mechanistic studies show that an aziridine intermediate seems to be responsible for the formation of the heterocycles, possibly via a Rh^V species.

Iridium-catalyzed [4 + 2] annulation of 1-arylindazolones with α-diazo carbonyl compounds: access to indazolone-fused cinnolines

An Ir-catalyzed tandem strategy for the synthesis of indazolone-fused cinnolines by [4 + 2] annulation of 1-arylindazolones with α-diazo carbonyl compounds.

Rh-Catalyzed C–H bond alkylation of indoles with α,α-difluorovinyl tosylate via indolyl group migration

An unprecedented C–H alkylation of indoles was achieved with α,α-difluorovinyl tosylate through an indolyl group shift process enabled by fluorine substituents.

Divergent synthesis of α-aryl ketones/esters via rhodium-catalyzed selective deesterification and decarbonylation of diazo compounds

A rhodium-catalyzed selective deesterification, decarbonylation and dual C–H activation of diazo compounds is presented.

Cp*Co(iii)-catalysed selective alkylation of C–H bonds of arenes and heteroarenes with α-diazocarbonyl compounds

Cp*Co(iii)-catalysed selective alkylation of directed C–H bonds of arenes and heteroarenes has been accomplished employing donor–acceptor carbenes, derived from α-diazocarbonyl compounds.