Catalytic C–H activation has emerged as a powerful tool for sustainable syntheses. In the recent years, notable success was achieved with the development of cobalt-catalyzed C–H functionalizations with either in situ generated or single-component cobalt-complexes under mild reaction conditions. Herein, recent progress in the field of organometallic cobalt-catalyzed C–H activation is reviewed until November 2015.

Cobalt-Catalyzed C―H Activation

The development of new methods for the direct functionalization of unactivated C–H bonds is ushering in a paradigm shift in the field of retrosynthetic analysis. In particular, the catalytic enantioselective functionalization of C–H bonds represents a highly atom- and step-economic approach toward the generation of structural complexity. However, as a result of their ubiquity and low reactivity, controlling both the chemo- and stereoselectivity of such processes constitutes a significant challenge. Herein we comprehensively review all asymmetric transition-metal-catalyzed methodologies that are believed to proceed via an inner-sphere-type mechanism, with an emphasis on the nature of stereochemistry generation. Our analysis serves to document the considerable and rapid progress within in the field, while also highlighting limitations of current methods.

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Catalytic Enantioselective Transformations Involving C-H Bond Cleavage by Transition-Metal Complexes.

Several researchers conducted studies to demonstrate metal catalyzed alkene and alkyne hydrocylation. A team of researchers developed a method for the synthesis of cyclooctenones using the intramolecular hydrogenation. Eight-membered ring formation was achieved by the incorporation of a cyclopropane ring in the substrate to demonstrate metal catalyzed alkene and alkyne hydrocylation. The key step of the method involved the fragmentation and isomerization of rhodacycle 25 into ring-expanded rhodacycle 26. The researchers investigated the reactions of the two isomers of a deuterium-labeled substrate to explore the mechanism in operation. Another team of researchers prepared a series of medium-ring sulfur heterocycles using intramolecular hydroacylation to demonstrate the investigations.

Transition Metal Catalyzed Alkene and Alkyne Hydroacylation

Molecular syntheses largely rely on time- and labour-intensive prefunctionalization strategies. In contrast, C-H activation represents an increasingly powerful approach that avoids lengthy syntheses of prefunctionalized substrates, with great potential for drug discovery, the pharmaceutical industry, material sciences, and crop protection, among others. The enantioselective functionalization of omnipresent C-H bonds has emerged as a transformative tool for the step- and atom-economical generation of chiral molecular complexity. However, this rapidly growing research area remains dominated by noble transition metals, prominently featuring toxic palladium, iridium and rhodium catalysts. Indeed, despite significant achievements, the use of inexpensive and sustainable 3d metals in asymmetric C-H activations is still clearly in its infancy. Herein, we discuss the remarkable recent progress in enantioselective transformations via organometallic C-H activation by 3d base metals up to April 2019.

Enantioselective C-H Activation with Earth-Abundant 3d Transition Metals.

Cobalt–chiral diphoshine catalytic systems promote intramolecular hydroacylation reactions of 2-acylbenzaldehydes and 2-alkenylbenzaldehydes to afford phthalide and indanone derivatives, respectively, in moderate to good yields with high enantioselectivities. The ketone hydroacylation did not exhibit a significant H/D kinetic isotope effect (KIE) with respect to the aldehyde C–H bond, indicating that C–H activation would not be involved in the rate-limiting step.

Cobalt-Catalyzed Enantioselective Intramolecular Hydroacylation of Ketones and Olefins

Unusual type of catalysis by paramagnetic cobalt(0) complexes. Isolation of catalytically active 17-electron intermediate, (Ph3P)2Co(CH2CHCH2CH2CHO), in the 4-pentenal intramolecular hydroacylation

1.

By the example of cyclization of 4-pentenal into cyclopentanone it has been shown that the complexes Co(PPh3)4 and Co2(N2)(PPh3)6 catalyze the reaction of intramolecular hydroacylation.




2.

It has been established that side reactions of 4-pentenal leading to 4-pentene-1-diol, 2-propenyl-2,6-pentadienal, and mono(4-pentenoates) of 2-propenyl-6-heptene-1,3-diol are caused by cobalt hydride complexes formed as intermediates.

Intramolecular hydroacylation catalyzed by cobalt complexes

1.

The efficiency of the catalytic system containing Co(O) and various ligands (phosphines, diphosphines, phosphites, and diamines) in the intramolecular hydroacylation reaction is greatest when using chelating diphosphines 1,2-bis(diphenylphosphino)ethane and 1,2-bis(diphenylphosphino)propane (ligand:Co=1∶:1).




2.

For monodentate phosphorus-containing ligands, the activity and selectivity of the catalytic system increase with increasing donor capacity and decreasing bulk of the ligands.




3.

In the presence of the Co2(μ-N2)(PPh3)6 + 1,2-bis(diphenylphosphino)ethane catalytic system, 4-pentenal cyclizes to cyclopentanone in acetonitrile at 70°C with about 100% selectivity at complete conversion (20 moles per g-at Co).

Intramolecular hydroacylation reaction catalyzed by Co2(μ-N2)(PPh3)6 in the presence of various ligands

Catalytic cyclization of γ,σ-unsaturated aldehydes (intramolecular hydroacylation) in the presence of (PPh3)2Co(Ph2PCH2CH2PPh2) gives four-membered and five-membered cycloalkanones. Depending on aldehyde structure the selectivity is 90–97% at 10–100% aldehyde conversion.

Cyclization of unsaturated aldehydes catalyzed by (PPh3)2Co(Ph2PCH2CH2PPh2)

1.

Reductive coupling of butyryl chloride with CoCl(PPh3)3 affords 4-heptanone, 4,5-octanedione, 4-hydroxy-5-octanone, 4-butyryloxy-5-octanone, and 4,5-dibutyryloxy-4-octene.




2.

A mechanism for the reductive coupling of butyryl chloride is proposed, involving the intermediate formation of an acylcobalt(III) dichloride complex.

A. B. Tuzikov

Papers

Unusual type of catalysis by paramagnetic cobalt(0) complexes. Isolation of catalytically active 17-electron intermediate, (Ph3P)2Co(CH2CHCH2CH2CHO), in the 4-pentenal intramolecular hydroacylation

Intramolecular hydroacylation catalyzed by cobalt complexes

Intramolecular hydroacylation reaction catalyzed by Co2(μ-N2)(PPh3)6 in the presence of various ligands

Cyclization of unsaturated aldehydes catalyzed by (PPh3)2Co(Ph2PCH2CH2PPh2)

Reductive coupling of butyryl chloride in the presence of CoCl(PPh3)3