Mild C−H/C−C Activation byZ-Selective Cobalt Catalysis

Facile Preparation of (2Z,4E)-Dienamides by the Olefination of Electron-deficient Alkenes with Allyl Acetate

Direct cross-coupling between two alkenes via vinylic C-H bond activation represents an efficient strategy for the synthesis of butadienes with high atomic and step economy. However, this functionality-directed cross-coupling reaction has not been developed, as there are still limited directing groups in practical use. In particular, a stoichiometric amount of oxidant is usually required, producing a large amount of waste. Due to our interest in novel 1,3-butadiene synthesis, we describe the ruthenium-catalyzed olefination of electron-deficient alkenes using allyl acetate and without external oxidant. The reaction of 2-phenyl acrylamide and allyl acetate was chosen as a model reaction, and the desired diene product was obtained in 80% isolated yield with good stereoselectivity (Z,E/Z,Z = 88:12) under optimal conditions: [Ru(p-cymene) Cl2]2 (3 mol %) and AgSbF6 (20 mol %) in DCE at 110 ºC for 16 h. With the optimized catalytic conditions in hand, representative α- and/or β-substituted acrylamides were investigated, and all reacted smoothly, regardless of aliphatic or aromatic groups. Also, differently N-substituted acrylamides have proven to be good substrates. Moreover, we examined the reactivity of different allyl derivatives, suggesting that the chelation of acetate oxygen to the metal is crucial for the catalytic process. Deuterium-labeled experiments were also conducted to investigate the reaction mechanism. Only Z-selective H/D exchanges on acrylamide were observed, indicating a reversible cyclometalation event. In addition, a kinetic isotope effect (KIE) of 3.2 was observed in the intermolecular isotopic study, suggesting that the olefinic C-H metalation step is probably involved in the rate-determining step.

Direct Functionalization of C-H Bonds by Iron, Nickel, and Cobalt Catalysis

Non-precious-metal-catalyzed reactions are of increasing importance in chemistry due to the outstanding ecological and economic properties of these metals. In the subfield of metal-catalyzed direct C-H functionalization reactions, recent years have shown an increasing number of publications dedicated to this topic. Nickel, cobalt, and last but not least iron, have started to enter a field which was long dominated by precious metals such as palladium, rhodium, ruthenium, and iridium. The present review article summarizes the development of iron-, nickel-, and cobalt-catalyzed C-H functionalization reactions until the end of 2016, and discusses the scope and limitations of these transformations.

Manganese-catalyzed allylation via sequential C-H and C-C/C-Het bond activation

Manganese-catalyzed sequential C-H and C-C/C-Het bond activation to synthesize allylic alcohols, allylated arenes, functionalized cyclopentenes and skipped dienes is reported. This protocol can be readily scaled up and various coupling partners are applied in manganese catalysis for the first time. Moreover, manganese-catalyzed alkenyl C(sp²)-H activation is also shown. Complimentary to the standard solution-based protocols, these reactions also proceed efficiently under neat conditions, which is unprecedented for abundant metal catalyzed C-H activation reactions.

Additive-Free Pd-Catalyzed α-Allylation of Imine-Containing Heterocycles

An additive-free Pd-catalyzed α-allylation of different imino-group-ontaining heterocycles is reported. The activation of α-CH pronucleophiles (pKa (DMSO) > 25) occurs without the addition of strong bases or Lewis acids using only the Pd/Xantphos catalyst system. The reaction scope has been studied for various 5- and 6-membered nitrogen-containing heterocycles (yields up to 96%). Mechanistic investigations suggest an initial allylation of the imine-N followed by a Pd-catalyzed formal aza-Claisen rearrangement.

Rhodium-catalyzed C2 and C4 C–H activation/annulation of 3-(1H-indol-3-yl)-3-oxopropanenitriles with internal alkynes: a facile access to substituted and fused carbazoles

Rhodium-catalyzed C2 and C4 C–H activation/annulation of 3-(1H-indol-3-yl)-3-oxopropanenitriles with internal alkynes has been developed. Substituted and fused carbazole derivatives were obtained through formal rhodium-catalyzed (4+2) or tandem (4+2) and (5+2) cycloaddition reactions under mild reaction conditions in good yields.

Cobalt(iii)-catalyzed cross-coupling of enamides with allyl acetates/maleimides

Cp*Co(iii)-catalyzed direct allylation of enamides has been accomplished with the exclusive formation of allylated Z-enamides with high efficiency.

Cp*CoIII-catalyzed directed C–H trifluoromethylthiolation of 2-phenylpyridines and 6-arylpurines

Cp*Co^III-catalyzed directed C–H trifluoromethylthiolation using N-trifluoromethylthiodibenzenesulfonimide as an electrophilic SCF₃ source is described.

Efficient syntheses of 3-hydroxyimino-1-isoindolinones and 3-methylene-1-isoindolinones via Cu-promoted C–H activation–nitroalkylation–intramolecular cyclization tandem processes

Efficient and selective syntheses of 3-hydroxyimino-1-isoindolinones and 3-methylene-1-isoindolinones were achieved via 8-aminoquinoline assisted Cu-promoted coupling of benzamides with nitroalkanes.

Recent advances using [Cp*Co(CO)I2] catalysts as a powerful tool for C–H functionalisation

This perspective highlights recent applications of Cp*Co^III catalysts in C–H functionalisation protocols, exemplifying both terminal couplings and heterocycle formation.

Cp*Co(iii)-catalyzed ortho-amidation of azobenzenes with dioxazolones

Herein we have reported a decarboxylative ortho-amidation of azobenzene via cobalt catalyzed C–H amidation with dioxazolone.

Manganese-catalyzed synthesis of monofluoroalkenes via C–H activation and C–F cleavage

A redox-neutral manganese-catalyzed α-fluoroalkenylation of arenes via C–H activation and C–F cleavage has been described.

Cobalt-Catalyzed C(sp2)-C(sp3) Cross-Coupling Reactions of Diarylmanganese Reagents with Secondary Alkyl Iodides

A cobalt-catalyzed cross-coupling of diarylmanganese reagents with secondary alkyl iodides using the THF-soluble salt CoCl₂·2LiCl, which leads to the cross-coupling products in up to 92% yield, is reported. High diastereoselectivities can be reached in these cross-couplings (dr up to 99:1). Remarkably, rearrangement of secondary alkyl iodides to unbranched products was not observed in these C-C forming reactions.

Rhodium(III)-Catalyzed Regio- and Stereoselective C-H Allylation of Arenes with Vinyl Benzoxazinanones

Vinyl benzoxazinanone was applied as an electrophilic allylating reagent for a series of arenes under redox-neutral Rh(III) catalysis. This reaction occurs in high efficiency under mild conditions to afford allylarenes bearing a sulfonamide functionality in exclusively E-selectivity. This allylation system combines C-H activation of arenes and scission of an unstrained six-membered ring.

Cobalt-catalyzed chelation assisted C–H allylation of aromatic amides with unactivated olefins

The reaction of aromatic amides with unactivated olefins in the presence of Co(OAc)₂ gave allylated products, not vinylated products.

Transition metal-catalysed couplings between arenes and strained or reactive rings: combination of C–H activation and ring scission

Transition metal-catalysed systems of C–H activation of arenes in combination with opening of strained or reactive three, four, and five membered rings are summarized in this review.

Cobalt-promoted selective arylation of benzamides and acrylamides with arylboronic acids

A novel cobalt-promoted arylation of aryl C–H bonds with arylboronic acids has been realized by using 8-aminoquinoline as the directing group. A notable feature of this newly developed protocol is that acrylamides, which cannot be arylated using copper salts as the promoter, can also be efficiently arylated.