Reverse Regioselective Cp*Co(III)-Catalyzed [4 + 2] C-H Annulation of N-Chloroamides with Vinylsilanes: Synthesis of 4-Silylated Isoquinolones and Their Synthetic Utilities

We have developed a Cp*Co(III)-catalyzed reverse regioselective [4 + 2] annulation of N-chlorobenzamides/acrylamides with vinylsilanes for the synthesis of 4-silylated isoquinolones. The reaction was performed at ambient temperature under redox-neutral conditions. The reaction utilized the N-Cl bond as an internal oxidant, furnished the required products with excellent regioselectivities, and demonstrated high functional group tolerance. The synthetic utility of 4-silylated isoquinolones has been demonstrated for the preparation of 4-heteroarylated and 4-alkylated isoquinolones via metal-free C-C couplings. Additionally, 3,4-dihydroisoquinolones were synthesized via protodesilylation of 4-silylated isoquinolones, thus making vinylsilane an ethylene surrogate.

Harnessing Vinyl Acetate as an Acetylene Equivalent in Redox-Neutral Cp*Co(III)-Catalyzed C-H Activation/Annulation for the Synthesis of Isoquinolones and Pyridones

We have developed Cp*Co(III)-catalyzed redox-neutral synthesis of 3,4-unsubstituted isoquinoline 1(2H)-ones at ambient temperature using N-chloroamides as a starting material. The reaction utilizes vinyl acetate as an inexpensive and benign acetylene surrogate. The N-Cl bond of the N-chlorobenzamides plays the role of an internal oxidant and hence precludes the need for an external oxidant. The reaction works with a wide range of substrates having various functional groups and a substrate containing a heterocyclic ring. Notably, the reaction is extended to the N-chloroacrylamides in which vinylic C-H activation occurs to furnish the 2-pyridone derivatives. Preliminary mechanistic studies were also conducted to shed light on the mechanism of this reaction.

Redox-neutral C-H annulation strategies for the synthesis of heterocycles via high-valent Cp*Co(III) catalysis

A variety of biologically active molecules, pharmaceuticals, and natural products consist of a nitrogen-containing heterocyclic backbone. The majority of them are isoquinolones, indoles, isoquinolines, etc.; thereby the synthesis and derivatization of such heterocycles are synthetically very relevant. Also, certain naphthol derivatives have high synthetic utility as agrochemicals and in dye industries. Previous approaches have utilized ruthenium, rhodium, or iridium which may not be desirable due to the high toxicity, low abundance, and high cost of such 4d and 5d metals. Moreover, the need for an external oxidant during the reaction also adds by-products to the system. A high-valent cobalt-catalyzed redox-neutral C-H functionalization strategy has emerged to be a far better alternative in this regard. The use of the non-noble metal cobalt allows for selectivity and specificity in product formation. Also, the redox-neutral concept avoids the use of an external oxidant either due to the presence of a metal in a non-variable oxidation state throughout the catalytic cycle or due to the presence of an oxidizing directing group or an oxidizing coupling partner. Such an oxidizing directing group not only directs the catalyst to a specific reaction site by chelation but also regenerates the catalyst at the end of the cycle. Certain bonds such as N-O, N-N, N-Cl, N-S, and C-S are the main game-players behind the oxidizing property of such directing groups. In the other case, the directing group only chelates the catalyst to a reaction center, whereas the oxidation is carried out by the upcoming group/coupling partner. Overall, merging the redox-neutral concept with the high-valent cobalt catalysis is paving the way forward toward a sustainable and environmentally friendly approach. This review critically describes the mechanistic understanding, scope, limitations, and synthesis of various biologically relevant heterocycles via the redox-neutral concept in the high-valent Cp*Co(III)-catalyzed C-H functionalization chemistry domain.

Ru(II)-Catalyzed Regioselective Redox-Neutral [4 + 2] Annulation of N-Chlorobenzamides with 1,3-Diynes at Room Temperature for the Synthesis of Isoquinolones

Herein, we report Ru(II)-catalyzed C-H/N-H bond functionalization of N-chlorobenzamides with 1,3-diynes via regioselective (4 + 2) annulation for the synthesis of isoquinolones under redox-neutral conditions at room temperature. This represents the first example of C-H functionalization of N-chlorobenzamides using an inexpensive and commercially available [Ru(p-cymene)Cl₂]₂ catalyst. The reaction is operationally simple, works in the absence of any silver additives, and is also applicable to a broad range of substrates with good functional group tolerance. The synthetic utility of the isoquinolone is demonstrated for the synthesis of bis-heterocycles consisting of isoquinolone-pyrrole and isoquinolone-isocoumarin scaffolds.

Cobalt(III)-Catalyzed Regioselective [4 + 2]-Annulation of N-Chlorobenzamides with Substituted Alkenes

A Co(III)-catalyzed redox-neutral [4 + 2] annulation of N-chlorobenzamides/acrylamides with substituted alkenes at ambient temperature is demonstrated. Using this protocol, pharmaceutically important 3,4-dihydroisoquinolinone derivatives were synthesized in good yields. Intriguingly, the synthetically useful functional group of allylic coupling partners such as sulfonyl, carbonate, acetate, phosphate, amide, nitrile, and silane were retained in the final cyclized product. The present annulation reaction was compatible with various substituted benzamides and allylic coupling partners. To support the proposed reaction mechanism, competition experiments, deuterium labeling studies, and kinetic isotope effect studies were performed.

Cobalt(III)-Catalyzed Regio- and Chemoselective [4 + 2]-Annulation of N-Chlorobenzamides/Acrylamides with 1,3-Dienes at Room Temperature

Co(III)-catalyzed regio- and chemoselective redox-neutral C-H annulation of arylamides/acrylamides with 1,3-dienes is described. The present annulation reaction was well suited with a less-reactive 1,3-butadiene. By employing this protocol, pharmaceutically important 3,4-dihydroisoquinolinones were synthesized in good yields. Furthermore, the prepared 3,4-dihydroisoquinolinones were converted into highly important oxirane derivatives in good yields. A plausible mechanistic cycle is proposed and supported by a competition experiment and kinetic isotopic effect (KIE) studies.

Recent advances in transition metal-catalyzed heteroannulative difunctionalization of alkenes via C-H activation for the synthesis of heterocycles

Heterocyclic compounds are the fundamental structural motifs distributed in natural products, pharmaceuticals and biologically active compounds. Thus, there is increasing interest in the development of novel synthetic strategies for the...

Cobalt-Catalyzed 2-(1-Methylhydrazinyl)pyridine-Assisted C-H Alkylation/Annulation: Mechanistic Insights and Rapid Access to Cyclopenta[c]isoquinolinone Derivatives

We have developed cobalt-catalyzed, bidentate 2-(1-methylhydrazinyl)pyridine (MHP)-directed C(sp²)-H alkylation/annulation of benzoic hydrazides with various alkenes. Notably, diverse cyclopenta[c]isoquinolinones and dihydroisoquinolinones were obtained via this functional group-tolerant protocol. The reaction can be performed on a gram scale while maintaining an excellent yield, and the directing group can be removed efficiently under mild conditions. Furthermore, density-functional theory (DFT) calculations provide an incisive understanding of the observed regioselectivities for different olefins.

C-H bond functionalization by high-valent cobalt catalysis: current progress, challenges and future perspectives

Over the last decade, high-valent cobalt catalysis has earned a place in the spotlight as a valuable tool for C-H activation and functionalization. Since the discovery of its unique reactivity, more and more attention has been directed towards the utilization of cobalt as an alternative to noble metal catalysts. In particular, Cp*Co(III) complexes, as well as simple Co(II) and Co(III) salts in combination with bidentate chelation assistance, have been extensively used for the development of novel transformations. In this review, we have demonstrated the existing trends in the C-H functionalization methodology using high-valent cobalt catalysis and highlighted the main challenges to overcome, as well as perspective directions, which need to be further developed in the future.

Ruthenium-catalyzed room-temperature coupling of α-keto sulfoxonium ylides and cyclopropanols for δ-diketone synthesis

Previous transition metal-catalyzed synthesis processes of δ-diketones are plagued by the high cost of the rhodium catalyst and harsh reaction conditions. Herein a low-cost, room temperature ruthenium catalytic method is developed based on the coupling of α-keto sulfoxonium ylides with cyclopropanols. The mild protocol features a broad substrate scope (47 examples) and a high product yield (up to 99%). Mechanistic studies argue against a radical pathway and support a cyclopropanol ring opening, sulfoxonium ylide-derived carbenoid formation, migratory insertion C-C bond formation pathway.

Palladium-Catalyzed Regioselective C-H Functionalization/Annulation Reaction of Amides and Allylbenzenes for the Synthesis of Isoquinolinones and Pyridinones

A regioselective C-H functionalization/annulation reaction of N-sulfonyl amides and allylbenzenes through a palladium-catalyzed C(sp²)-H allylation/aminopalladation/β-H elimination/isomerization sequence has been reported. Various aryl and alkenyl carboxamides are found to be efficient substrates to construct isoquinolinones and pyridinones in up to 96% yield. Using ambient air as the terminal oxidant is another advantage regarding environmental friendliness and operational simplicity.

Rhodium(iii)-catalyzed C–H activation/annulation of N-iminopyridinium ylides with alkynes and diazo compounds

Rh(iii)-Catalyzed C–H activation/annulation of N-iminopyridinium ylides with alkynes and diazo compounds has been realized for the synthesis of isoquinolones and isocoumarins.

Spiro[indene-1,4'-oxa-zolidinones] Synthesis via Rh(III)-Catalyzed Coupling of 4-Phenyl-1,3-oxazol-2(3H)-ones with Alkynes: A Redox-Neutral Approach

Transition-metal-catalyzed C-H activation synthesis of heterocyclic spiro[4,4]nonanes has persistently witnessed the use of additional stoichiometric transition-metal oxidant when employing C═C bond as the spiro ring closure site. Herein, we have addressed the issue by reporting a redox-neutral strategy for spiro[indene-1,4'-oxa-zolidinones] synthesis via Rh(III)-catalyzed coupling of 4-phenyl-1,3-oxazol-2(3H)-ones with alkynes. The synthesis features a broad substrate scope and high regiospecificity.

Silver-mediated radical phosphorylation/cyclization of N-allylbenzamides to access phosphoryl-substituted dihydroisoquinolones

A silver-mediated phosphorylation/cyclization of N-allylbenzamides with phosphine oxides for the synthesis of phosphoryl-substituted dihydroisoquinolones was developed.

3d Transition Metals for C-H Activation

C-H activation has surfaced as an increasingly powerful tool for molecular sciences, with notable applications to material sciences, crop protection, drug discovery, and pharmaceutical industries, among others. Despite major advances, the vast majority of these C-H functionalizations required precious 4d or 5d transition metal catalysts. Given the cost-effective and sustainable nature of earth-abundant first row transition metals, the development of less toxic, inexpensive 3d metal catalysts for C-H activation has gained considerable recent momentum as a significantly more environmentally-benign and economically-attractive alternative. Herein, we provide a comprehensive overview on first row transition metal catalysts for C-H activation until summer 2018.