Cobalt(II)-Catalyzed 1,4-Addition of Organoboronic Acids to Activated Alkenes: An Application to Highlycis-Stereoselective Synthesis of Aminoindane Carboxylic Acid Derivatives

Cobalt-Catalyzed Chemo- and Stereoselective Arylative Carbocyclization of 1,6-Allenynes

Different from the well-investigated enynes, transition-metal-catalyzed carbocyclization reactions of allenynes are more attractive as a result of the unique structure and versatile reactivity of allenes. Herein, we report the first cobalt-catalyzed highly chemo- and stereoselective arylative carbocyclization of 1,6-allenynes with arylboronic acids, affording five-membered carbocycles and heterocycles with moderate to high yields, broad substrate scope, and wide functional group compatibility. Moreover, several mechanistic experiments were conducted to gain insight into the reaction process.

Cobalt-Catalyzed Highly Regioselective Three-Component Arylcarboxylation of Acrylate with Aryl Bromides and Carbon Dioxide

Cobalt-catalyzed regioselective three-component arylcarboxylation of acrylate with aryl bromides and carbon dioxide has been developed. The reaction is carried out by using cobalt chloride as a precatalyst and zinc powder as a reducing reagent under CO₂ (1 atm) at 40 °C. A range of aryl bromides are used for this reaction, leading to a series of valuable carboxylic acids with high regioselectivity and functional-group compatibility. Mechanistic experiments and DFT calculations indicate that this arylcarboxylation reaction involves the reaction of CO₂ with a cobalt enolate intermediate to form the C-C bond.

Theoretical Studies of Rare-Earth-Catalyzed [3 + 2] Annulation of Aromatic Aldimine with Styrene: Mechanism and Origin of Diastereoselectivity

The synthesis of multisubstituted 1-aminoindanes through catalyst-controlled diastereodivergent [3 + 2] annulation of aromatic imines with alkenes is of great interest and importance. An understanding of the exact reaction mechanism, especially for the origin of diastereoselectivity, is an essential aspect for further development of such reactions. In this study, density functional theory calculations have been carried out on the rare-earth-catalyzed diastereodivergent [3 + 2] annulation of benzaldimine with styrene. The results show that the reaction mainly involves generation of active species, olefin insertion, cyclization, and protonation steps. The noncovalent interactions, such as C-H···π and metal···π interactions, play an important role in stabilizing the key transition state or intermediate. Both steric and electronic factors account for the diastereoselectivity. The preferred cis-diastereoselectivity could be ascribed to more efficient orbital interaction, while the crowded space will induce the formation of a C-H···π interaction between the N^tBu group and benzene ring in a trans-diastereoselectivity manner, thus stabilizing the trans-selective transition state. Therefore, the stereospecific product could be obtained by fine-tuning the ligand/metal combination of the catalysts.

Iron-Catalyzed Conjugate Addition of Aryl Iodides onto Activated Alkenes under Air in Water

The combination of commercially available FeCl3·6H2O with a water-soluble cationic 2,2′-bipyridyl catalytic system was found to enable the direct conjugate addition of aryl iodides onto activated alkenes, such as an α,β-unsaturated ester and a ketone, in a weakly acidic aqueous solution. This operationally simple protocol was carried out at 80 °C under air atmosphere in a potassium acetate-buffered aqueous solution for 12 h in the presence of Zn dust as a reductant to provide the desired 1,4-adducts in good yields.

Ni(II)-catalyzed asymmetric alkenylations of ketimines

Chiral allylic amines are not only present in many bioactive compounds, but can also be readily transformed to other chiral amines. Therefore, the asymmetric synthesis of chiral allylic amines is highly desired. Herein, we report two types of Ni(II)-catalyzed asymmetric alkenylation of cyclic ketimines for the preparation of chiral allylic amines. When ketimines bear alkyl or alkoxycarbonyl groups, the alkenylation gives five- and six-membered cyclic α-tertiary allylic amine products with excellent yields and enantioselectivities under mild reaction conditions. A variety of ketimines can be used and the method tolerates some variation in alkenylboronic acid scope. Furthermore, with alkenyl five-membered ketimine substrates, an alkenylation/rearrangement reaction occurs, providing seven-membered chiral sulfamide products bearing a conjugated diene skeleton with excellent yields and enantioselectivities. Mechanistic studies reveal that the ring expansion step is a stereospecific site-selective process, which can be catalyzed by acid (Lewis acid or Brønsted acid).

Chiral allylic amines are often encountered in bioactive compounds and can be conveniently transformed into other chiral amines. Here, the authors report nickel-catalyzed enantioselective addition and addition/ring-expansion of alkenylboronic acids to ketimines providing a range of valuable chiral azaheterocycles.

Nickel-catalyzed, ligand-free, diastereoselective synthesis of 3-methyleneindan-1-ols

Nickel-catalyzed, highly diastereoselective annulations between activated allenes and 2-acetylarylboronic acid or 2-formylarylboronic acids to give 3-methyleneindan-1-ols are reported.

A simple route to 1,4-addition reactions by Co-catalyzed reductive coupling of organic tosylates and triflates with activated alkenes

An efficient Co-catalyzed 1,4-addition reaction of alkyl/aryl triflates and tosylates with activated alkenes is described.

Ni(ii)-catalyzed asymmetric addition of arylboronic acids to cyclic imines

A Ni(ii)-catalyzed asymmetric addition of arylboronic acids to cyclic imines has been realized using a tropos biphenyl phosphine-oxazoline ligand.

Synthesis of 2,3-disubstituted indenones by cobalt-catalyzed [3+2] annulation of o-methoxycarbonylphenylboronic acid with alkynes

Treatment of alkynes with o-methoxycarbonylphenylboronic acid in the presence of a cobalt catalyst resulted in the corresponding 2,3-disubstituted indenones.

Cobalt catalysis involving π components in organic synthesis

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.