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As an earth-abundant first-row transition metal, cobalt catalysts offer a broad range of economical methods for organic transformations via C–H activation. One of the transformations is the addition of C–H to C–X multiple bonds to afford alkylation, alkenation, amidation, and cyclization products using low- or high-valent cobalt catalysts. This hydroarylation is an efficient approach to build new C–C bonds in a 100% atom-economical manner. In this review, the recent developments of Co-catalyzed hydroarylation reactions and their mechanistic studies are summarized.

An efficient method for the synthesis of cis-2-alkyl- or allyl-1,2-dihydronaphythalenes through a nickel-catalyzed highly regio- and stereoselective ring-opening addition of alkyl- or allylzirconium reagents to 7-oxabenzonorbornadienes is described. The alkylative ring-opening products from 7-oxabenzonorbornadienes underwent dehydration readily with HCl to afford the corresponding naphthalene derivatives in excellent yields.

A new and efficient method for silaboration of allenes catalyzed by palladium complexes and initiated by organic iodides is demonstrated. The method provides a convenient and general route to various 2-silylallylboronates in good yields with excellent regio- and stereoselectivity.

Research has been conducted on the electron-deficient diynes. Results demonstrate that these compounds undergo [2+2+2] cycloaddition reactions with allenes in the presence of zinc powder.

Studies reveal that nickel and palladium-catalyzed reactions of oxa and azabicyclic olefins with benzynes, alkynes, propiolates, zirconium reagents and o-halobenzene derivatives take place with proper selectivity and yields.

Research has been conducted on the organoboronic acids. The method of constructing carbon-carbon bonds in a regio- and stereoselective fashion via palladium-catalyzed assembling of allenes, organic halides and arylboronic acids has been developed and is discussed.

Research has been conducted on the bicyclic alkenes. The [2+2] cycloaddition of these alkenes with different alkynes has been investigated in the presence of Co(PPh (sub)3) (sub)2 I (sub)2, PPh (sub)3 and Zn powder in toluene in producing the corresponding exo-cyclobutene derivatives and the details are reported.

Research has been conducted on the pyridyl carboxylates. Results indicate the additional chemistry of 2-pyridyl carboxylates with benzynes and demonstrate the 1,4-addition of 2-pyridyl carboxylates to benzyne at 1,2-positions.

Research demonstrates that aryl and vinylic halides react with substituted allenes and potassium carbonate, mediated by palladium complex, to form substituted 1,3-dienes. Results reveal that catalytic reactions can be designed around carbopalladation of allenes.

Research concerning reaction of phosphites with dimethyl acetylenedicarboxylate and C (sub)60 to produce phosphite ylides has been conducted. Hydrolysis of these ylides used to introduce phosphonate ester, phosphine oxide and phosphonic acid to a fullerene moiety is described.

Research has been conducted on the C (sub)60 and acetylenes reaction with phosphines. Introduction of phosphorus substituents onto C (sub)60 is discussed.

An analysis of a synthesized series of trans bis(maleimade) and bis(maleic anhydride) molybdenum (0) complexes reveals the form of a large class of weak interligand interaction displaying metal complexes. The nitrogen and oxygen atoms of the maleimide and anhydride ligands, respectively, point at a carbonyl group, indicating interligand interactions. X-ray and NMR spectroscopic analysis of the Mb(0) complexes reveal that of the three possible conformations possible, the ligands have an orthogonal olefin-aligned with a N-Mo-CO vector.

Research has been conducted on the aromatic ketones. The reaction of aryl iodides with aryl aldehydes producing biaryl ketones in the presence of Zn and Ni(dppe)Br (sub)2 is described.

A palladium-catalyzed chelation-assisted C-H activation of alkyl aryl ketones and their reaction with aryl iodides is described that has produced ortho-arylated products. A possible reaction mechanism is proposed that has involved directed dual C-H bond activation and enolate cyclization for the formation of 10,10-dialkylphenanthrone derivatives.

A convenient and highly regioselective method is described for the preparation of 5-methylenepyrrolidinone derivatives from various nitriles and acrylamides by a cobalt-catalyzed reductive coupling reaction. A possible mechanism is described that has involved the formation of a cobaltaazacyclopentene intermediate from nitrile and acrylamide, protonation, keto-amide cyclization and dehydration.

A convenient and highly regioselective synthesis of substituted isoquinoline derivatives from various aromatic ketoximes and alkynes via a one-pot, rhodium-catalyzed C-H bond activation is described. A probable mechanism that involves chelation-assisted C-H activation via oxidative addition of Rh(I) to an ortho-C-H bond, insertion of the alkyne, reductive elimination, intramolecular electrocyclization, and aromatization is also described.

The reaction of dimethyl acetylenedicarboxylate (DMAD) with [C.sub.60] in the presence of hexamethylphosphorous triamide (HMPT) or hexaethylphosphorus triamide (HEPT) has resulted in fullerene derivatives incorporating HMPT or HEPT yields. A phosphoramidate side product, which has resulted from the addition of HMPT or HEPT to [C.sub.60] followed by hydrolysis, has displayed a featureless absorption spectrum in the visible region and a more negative first reductive potential relative to that of [C.sub.60].

A highly chemo-, regio- and stereoselective reaction for assembling an alkene and alkyne via a cobalt-catalyzed reductive-coupling mode is presented. The methodology could also be used to synthesize six- and seven-membered cyclic lactones and lactams.

The ring-opening addition of methyl 2,3-dimethoxy-6-iodobenzoate to oxabenzonorbornadienes followed by cyclization in the presence of Ni[Br.sub.2](dppe) and Zn metal powder in acetonitrile to give the corresponding benzocoumarin derivatives is described. The method is also used for the preparation of natural product arnottin I by employing tert-butyldimethylsilyl-protected oxabenzonorbornadiene 11 as the substrate for reaction with o-iodobenzoate.

An efficient utilization of a cobalt catalyst for the reductive coupling of primary, secondary and tertiary alkyl bromides and iodides with a wide variety of conjugated alkenes such as acrylates, acrylonitrile, vinyl ketone and vinyl sulfone in good to excellent yields is reported. This methodology employs a cobalt complex CoI2dppe and it is also noted that the reaction appears to undergo an oxidative addition driven route rather than the expected radical route.

The organic compounds consisting of a 4-amino-1,8-naphthalic anhydride group was successfully synthesized and these compounds were employed as orange and red emitters in organic electoluminescent (EL) devices. A simple three-layer red-emitting device with good luminous efficiency and colour purity was achieved by using ANA-1 as the host emitter.

An efficient method for the cocyclotrimerization of bicyclic alkenes and benzynes catalyzed by palladium phosphine complexes is described to give the corresponding norbornane anellated 9,10-dihydro-phenanthrene derivatives. For the synthesis of polyaromatics, substituted benzo[b]triphenylenes (8a-f), the [2+2+2] cocyclotrimerization products from oxa- and azabicyclic alkenes can be applied via a simple Lewis acid mediated deoxyaromatization in good yields.

An efficient cobalt-catalyzed carbocylization for the synthesis of indenols and indenes and a new method for reductive decyanation is explained. The Co-(dppe)I2/Zn system successfully catalyzed the carbocyclization reaction of o-iodobenzenzaldehydes and o-iodophenyl ketones with alkynes to afford indenols in high regioselectivity and excellent yields.

Research has been conducted on substituted 2-acylallylmetal reagents. The authors describe the synthesis of these reagents carried out via the use of allenes, bimetallic reagents and acyl chlorides.

Research has been conducted on allenes, aryl halides, and alkenylzirconium reagents. The authors describe the nickel-catalyzed regio- and stereoselective assembly of these reagents.

Research has been conducted on nickel-catalyzed carbocyclization reaction. The mechanism of this reaction has been investigated and discussed.

Research has been conducted on methanofullerenes with alpha-ylidic ester. The authors report the study of acid-catalyzed ring-expansion of these compounds.

A study is conducted to demonstrate that there are three basic reactions for the alkynes with arynes, namely dehydro Diels-Alder reaction (DDAR), ene, and acetylenic C-H addition reactions. These provide simple and efficient routes for the synthesis of various phenanthrene, phenylallene, and phenylalkyne derivatives.

The study reports a novel regio- and stereoselective cyclization of iodobenzyl alcohol with alkyl propiolates. This catalysis paves a new way for the synthesis of seven-membered lactones.

A concise approach to the synthesis of oxindoles and phenanthridones from anilides is described. In the presence of catalytic amount of Pd(OAc)2, 2-iodoacetates and 2-iodobenzoates can be used to functionalize ortho C-H bond of anilides, which subsequently undergo intramolecular cyclization to give the products. A possible reaction mechanism that involves a PdII/PdIV catalytic cycle is proposed with the support of detailed mechanistic studies. [ABSTRACT FROM AUTHOR]

Conspectus: Over the last three decades, transition-metal-catalyzed organic transformations have been shown to be extremely important in organic synthesis. However, most of the successful reactions are associated with noble metals, which are generally toxic, expensive, and less abundant. Therefore, we have focused on catalysis using the abundant first-row transition metals, specifically cobalt. In this Account, we demonstrate the potential of cobalt catalysis in organic synthesis as revealed by our research. We have developed many useful catalytic systems using cobalt complexes. Overall, they can be classified into several broad types of reactions, specifically [2 + 2 + 2] and [2 + 2] cycloadditions; enyne reductive coupling; reductive [3 + 2] cycloaddition of alkynes/allenes with enones; reductive coupling of alkyl iodides with alkenes; addition of organoboronic acids to alkynes, alkenes, or aldehydes; carbocyclization of o-iodoaryl ketones/aldehydes with alkynes/electron-deficient alkenes; coupling of thiols with aryl and alkyl halides; enyne coupling; and C-H bond activation. Reactions relying on π components, specifically cycloaddition, reductive coupling, and enyne coupling, mostly afford products with excellent stereo- and regioselectivity and superior atom economy. We believe that these cobalt-catalyzed π-component coupling reactions proceed through five-membered cobaltacyclic intermediates formed by the oxidative cyclometalation of two coordinated π bonds of the substrates to the low-valent cobalt species. The high regio- and stereoselectivity of these reactions are achieved as a result of the electronic and steric effects of the π components. Mostly, electron-withdrawing groups and bulkier groups attached to the π bonds prefer to be placed near the cobalt center of the cobaltacycle. Most of these transformations proceed through low-valent cobalt complexes, which are conveniently generated in situ from air-stable Co(II) salts by Zn- or Mn-mediated reduction. Overall, we have shown these reactions to be excellent substitutes for less desirable noble-metal systems. Recent successes in cobalt-catalyzed C-H activation have especially advanced the applicability of cobalt in this field. In addition to the more common low-valent-cobalt-catalyzed C-H activation reactions, an in situ-formed cobalt(III) five-membered complex with a 1,6-enyne effectively couples with aromatic ketones and esters through ortho C-H activation, opening a new window in this research area. Interestingly, this reaction proceeds under milder reaction conditions with broad substrate scope. Furthermore, many of the reactions we have developed are highly enantioselective, including enantioselective reductive coupling of enones and alkynes, addition of organoboronic acids to aldehydes, and the cyclization of 2-iodobenzoates with aldehydes. Overall, this Account demonstrates the versatility and utility of cobalt catalysis in organic synthesis. [ABSTRACT FROM AUTHOR]

Pd-catalyzed selective ortho-olefination of arenes assisted by an allylic C-C double bond at room temperature using O2 as a terminal oxidant is described. A possible mechanism involving the initial coordination of allylic C-C bond to Pd followed by selective o-C-H bond metalation is proposed. [ABSTRACT FROM AUTHOR]

Fluorescent 2,3,6,7,9,10-hexaphenylanthracene-based derivatives (HPhAn) were synthesized to serve as deep-blue dopants in organic electroluminescent (EL) devices. The fluorescent emissions of HPhAnfluorophores, fine-tuned from 462 to 449 nm with varied fluorinated phenyl rings attached to the central carbons of the anthracene core, produced photoluminescence quantum yields of 82–88% in the solid state. Unlike the known monostyrylamine deep-blue dopant, these blue dopants in dilute solutions showed no pronounced spectral red shift with increasing solvent polarity; the emission color of HPhAndopants is thus insensitive to the polarity of the medium. The highest-occupied molecular orbital (HOMO) and lowest-unoccupied molecular orbital (LUMO) energy levels calculated from the oxidation potentials and optical energy gaps are 5.61–5.70 eV and 2.73–2.89 eV, respectively. These thermally robust compounds undergo thermal decomposition at temperatures greater than 300 °C. The relationships among current density, voltage and luminance, the EL efficiency and the operational stability of devices based on the blue HPhAndopants were examined. These HPhAn-doped EL devices are activated at voltages less than 3.5 V and attained an external quantum efficiency as much as 5.3% in the deep-blue light region with Commission Internationale d'Éclairage (CIE) coordinates (0.14, 0.10), near the National Television System Committee (NTSC) standard blue coordinates (0.14, 0.08). [ABSTRACT FROM AUTHOR]