Vicinal C-Functionalization of Alkenes. Pd/Light-Induced Multicomponent Coupling Reactions Leading to Functionalized Esters and Lactones

Visible-Light-Promoted Radical Cascade Sulfone Alkylation/Cyclization of 2-Isocyanoaryl Thioethers Enabled by Electron Donor-Acceptor Complex Formation

A photo-induced cascade sulfone alkylation/cyclization of 2-isocyanoaryl thioethers is explored. This visible-light-triggered reaction not only occurs under extremely mild reaction conditions but also does not require the presence of a photosensitizer. The photocatalytic process is triggered by the photochemical activity of in situ-generated electron donor-acceptor complexes, arising from the association of 2-isocyanoaryl thioethers and α-iodosulfones. The radical pathway was confirmed by UV-vis spectroscopy, radical trapping, Job's plot, and on/off irradiation experiments.

Copper-Catalyzed 1,2-Dicarbonylative Cyclization of Alkenes with Alkyl Bromides via Radical Cascade Process

Herein, we developed a new procedure on 1,2-dicarbonylative cyclization of 4-aryl-1-butenes with alkyl bromides. Using simple copper catalyst, two molecules of carbon monoxide were introduced into the double bond with the formation of four new C-C bonds and a new ring. Various α-tetralones and 2,3-dihydroquinolin-4-ones were formed in moderate to good yields.

Visible Light-Induced Transition Metal Catalysis

In recent years, visible light-induced transition metal catalysis has emerged as a new paradigm in organic photocatalysis, which has led to the discovery of unprecedented transformations as well as the improvement of known reactions. In this subfield of photocatalysis, a transition metal complex serves a double duty by harvesting photon energy and then enabling bond forming/breaking events mostly via a single catalytic cycle, thus contrasting the established dual photocatalysis in which an exogenous photosensitizer is employed. In addition, this approach often synergistically combines catalyst-substrate interaction with photoinduced process, a feature that is uncommon in conventional photoredox chemistry. This Review describes the early development and recent advances of this emerging field.

Copper-Catalyzed 1,2-Trifluoromethylation Carbonylation of Unactivated Alkenes: Efficient Access to β-Trifluoromethylated Aliphatic Carboxylic Acid Derivatives

A novel copper-catalyzed carbonylative trifluoromethylation of unactivated alkenes has been developed. A broad range of β-trifluoromethylated carboxylic acid derivatives were prepared in moderate to excellent yields from simple alkenes with excellent regioselectivity. It is noteworthy that ethylene gas, as the simplest olefin, can also be applied directly to obtain β-trifluoromethylated amides and ester. This transformation presents the first example on carbonylative trifluoromethylation of alkenes.

Palladium-Catalyzed Carbonylative Four-Component Synthesis of β-Perfluoroalkyl Amides

Transition-metal-catalyzed multicomponent carbonylation is one of the most efficient strategies to construct carbonyl-containing compounds. Herein, a palladium-catalyzed four-component perfluoroalkylation/aminocarbonylation of unactivated alkenes with perfluoroalkyl halides, and amines was developed. A wide range of substrates, including anilines, alkylamines, sulfonamides, and hydrazines are all suitable reaction partners for this catalyst system, resulting in various β-perfluoroalkyl amides with good functional-group tolerance and excellent chemoselectivity. Furthermore, not only alkyl olefins, but also aliphatic alkynes, and even alkyl allenes can all be employed. Notably, several medical and bioactive related molecules are compatible here as well.

Palladium-catalyzed difunctionalization/dearomatization of N-benzylacrylamides with α-carbonyl alkyl bromides: facile access to azaspirocyclohexadienones

An efficient palladium-catalyzed difunctionalization/dearomatization of N-benzylacrylamides with α-carbonyl alkyl bromides as alkyl radical precursors has been described. Various α-carbonyl alkyl bromides, including α-bromoalkyl esters and ketones, reacted smoothly to provide important azaspirocyclohexadienones in moderate to excellent yields. In addition, mechanistic studies suggested that the reaction proceeded via a radical pathway.

Advances in Visible-Light-Mediated Carbonylative Reactions via Carbon Monoxide (CO) Incorporation

The abundant and inexpensive carbon monoxide (CO) is widely exploited as a C1 source for the synthesis of both fine and bulk chemicals. In this context, photochemical carbonylation reactions have emerged as a powerful tool for the sustainable synthesis of carbonyl-containing compounds (esters, amides, ketones, etc.). This review aims at giving a general overview on visible light-promoted carbonylation reactions in the presence of metal (Palladium, Iridium, Cobalt, Ruthenium, Copper) and organocatalysts as well, highlighting the main features of the presented protocols and providing useful insights on the reaction mechanisms.

Light-Promoted Organic Transformations Utilizing Carbon-Based Gas Molecules as Feedstocks

Carbon-based gas molecules are readily available feedstocks and are widely used in industry as building blocks or fuels. However, their application in the synthesis of fine chemicals has been hampered due to operational complexity, poor reaction efficiency and selectivity. Recent development of photoredox-promoted transformations using such gaseous reagents has received considerable attention from the synthetic community. In this review, efforts in developing light-promoted organic transformations using carbon-based natural gases as C1 or C2 feedstocks and to overcome the associated challenges are briefly summarized.

Nickel-Catalyzed Four-Component Carbocarbonylation of Alkenes under 1 atm of CO

Transition-metal-catalyzed carbonylation is one of the most straightforward strategies to prepare carbonyl compounds. However, compared to well-established noble-metal-catalyzed carbonylation reactions, analogue coupling via base-metal, nickel catalysis has received less attention because of the easy formation of highly toxic and unreactive Ni(CO)₄ species between Ni(0) and CO. To date, the use of inexpensive and widely available carbon monoxide (CO) gas for nickel-catalyzed carbonylation reaction remains challenging, and nickel-catalyzed four-component carbonylative reaction has not been reported yet. Here, we report a highly selective nickel-catalyzed four-component carbocarbonylation of alkenes under 1 atm (1 atm) of CO gas to efficiently achieve an array of complex carbonyl compounds, including fluorinated amino acids and oligopeptides of great interest in medicinal chemistry and chemical biology. This reaction relies on a nickel-catalyzed one-pot cascade process to assemble CO, arylboronic acids, and difluoroalkyl electrophiles across the carbon-carbon double bond of alkenes, paving a new way for base-metal-catalyzed carbonylative cascade reaction.

The Persistent Radical Effect in Organic Synthesis

Radical-radical couplings are mostly nearly diffusion-controlled processes. Therefore, the selective cross-coupling of two different radicals is challenging and not a synthetically valuable transformation. However, if the radicals have different lifetimes and if they are generated at equal rates, cross-coupling will become the dominant process. This high cross-selectivity is based on a kinetic phenomenon called the persistent radical effect (PRE). In this Review, an explanation of the PRE supported by simulations of simple model systems is provided. Radical stabilities are discussed within the context of their lifetimes, and various examples of PRE-mediated radical-radical couplings in synthesis are summarized. It is shown that the PRE is not restricted to the coupling of a persistent with a transient radical. If one coupling partner is longer-lived than the other transient radical, the PRE operates and high cross-selectivity is achieved. This important point expands the scope of PRE-mediated radical chemistry. The Review is divided into two parts, namely 1) the coupling of persistent or longer-lived organic radicals and 2) "radical-metal crossover reactions"; here, metal-centered radical species and more generally longer-lived transition-metal complexes that are able to react with radicals are discussed-a field that has flourished recently.

Enantioselective Radical Addition/Cross-Coupling of Organozinc Reagents, Alkyl Iodides, and Alkenyl Boron Reagents

A hybrid transition-metal/radical process is described that results in the addition of organozinc reagents and alkyl halides across alkenyl boron reagents in an enantioselective catalytic fashion. The reaction can be accomplished both intermolecularly and intramolecularly, providing useful product yields and high enantioselectivities in both manifolds.

N-Heterocyclic Carbene-Catalyzed Radical Relay Enabling Vicinal Alkylacylation of Alkenes

The N-heterocyclic carbene-catalyzed radical relay enables the vicinal alkylacylation of styrenes, acrylates and acrylonitrile using aldehydes and tertiary alkyl carboxylic acid-derived redox-active esters. This protocol introduces tertiary alkyl groups and acyl groups to C-C double bonds with complete regioselectivity to produce functionalized ketone derivatives. The radical relay mechanism involves single electron transfer from the enolate form of a Breslow intermediate and radical addition of the resultant alkyl radical to the alkene followed by radical-radical coupling.

Intermolecular selective carboacylation of alkenes via nickel-catalyzed reductive radical relay

The development of catalytic carboacylation of simple olefins, which would enable the rapid construction of ketones with high levels of complexity and diversity, is very challenging. To date, the vast majority of alkene carboacylation reactions are typically restricted to single- and two-component methodologies. Here we describe a three-component carboacylation of alkenes via the merger of radical chemistry with nickel catalysis. This reaction manifold utilizes a radical relay strategy involving radical addition to an alkene followed by alkyl radical capture by an acyl-nickel complex to forge two vicinal C-C bonds under mild conditions. Excellent chemoselectivity and regioselectivity have been achieved by utilizing a pendant weakly chelating group. This versatile protocol allows for facile access to a wide range of important β-fluoroalkyl ketones from simple starting materials.

Transition Metal-Catalyzed Dicarbofunctionalization of Unactivated Olefins

Transition metal (TM)-catalyzed difunctionalization of unactivated olefins with two carbon-based entities is a powerful method to construct complex molecular architectures rapidly from simple and readily available feedstock chemicals. While dicarbofunctionalization of unactivated olefins has a long history typically with the use of either carbon monoxide to intercept C(sp³ )-[M] (alkyl-TM) species or substrates lacking in β-hydrogen (β-Hs), development of this class of reaction still remains seriously limited due to complications of β-H elimination arising from the in situ-generated C(sp³ )-[M] intermediates. Over the years, different approaches have been harnessed to suppress β-H elimination, which have led to the development of various types of olefin dicarbofunctionalization reactions even in substrates that generate C(sp³ )-[M] intermediates bearing β-Hs with a wide range of electrophiles and nucleophiles. In this review, these developments will be discussed both through the lens of historical perspectives as well as the strategies scrutinized over the years to address the issue of β-H elimination. However, this review article by no means is designed to be exhaustive in the field, and is merely presented to provide the readers an overview of the key reaction developments.

Manganese-Catalyzed Carboacylations of Alkenes with Alkyl Iodides

A manganese-catalyzed carboacylation of alkenes with alkyl iodides and carbon monoxide is described. This carbonylative difunctionalization uses both primary and secondary alkyl iodides in reactions with a diverse array of cyclic and acyclic substrates. Examples of successful applications to the synthesis of five-, six-, and seven-membered rings are provided. The inexpensive, first-row catalytic system and mild reaction conditions are expected to facilitate applications in complex synthesis.

Palladium-Catalyzed 6-Endo Selective Alkyl-Heck Reactions: Access to 5-Phenyl-1,2,3,6-tetrahydropyridine Derivatives

A new type of palladium-catalyzed 6-endo-selective alkyl-Heck reaction of unactivated alkyl iodides has been described. This strategy provides efficient access to a variety of 5-phenyl-1,2,3,6-tetrahydropyridine derivatives, which are important structural motifs for bioactive molecules. This process displays a broad substrate scope with excellent 6-endo selectivity. Mechanistic investigations reveal that this alkyl-Heck reaction performs via a hybrid palladium-radical process.

Iron-Catalyzed Convergent Radical Cyclization of Aldehydes with Two Alkenes to 3,4-Dihydropyrans

A novel convergent radical cyclization of an aldehyde with two alkenes has been developed. With this method, polyfunctionalized 3,4-dihydropyrans are built in an efficient and selective manner. The iron-catalyzed redox radical recombinations are proposed for the formation of a 3,4-dihydropyran skeleton.

Synthesis of aromatic β-keto esters via a carbonylative Suzuki–Miyaura coupling reaction of α-iodo esters with arylboronic acids

Aromatic β-keto esters were synthesized via a carbonylative cross-coupling reaction of alkyl iodides and arylboronic acids in the presence of a catalytic amount of Pd catalyst.

Carbonylation reactions of alkyl iodides through the interplay of carbon radicals and Pd catalysts

Numerous methods for transition metal catalyzed carbonylation reactions have been established. Examples that start from aryl, vinyl, allyl, and benzyl halides to give the corresponding carboxylic acid derivatives have all been well documented. In contrast, the corresponding alkyl halides often encounter difficulty. This is inherent to the relatively slow oxidative addition step onto the metal center and subsequent β-hydride elimination which causes isomerization of the alkyl metal species. Radical carbonylation reactions can override such problems of reactivity; however, carbonylation coupled to iodine atom transfer (atom transfer carbonylation), though useful, often suffers from a slow iodine atom transfer step that affects the outcome of the reaction. We found that atom transfer carbonylation of primary, secondary, and tertiary alkyl iodides was efficiently accelerated by the addition of a palladium catalyst under light irradiation. Stereochemical studies support a mechanistic pathway based on the synergic interplay of radical and Pd-catalyzed reaction steps which ultimately lead to an acylpalladium species. The radical/Pd-combined reaction system has a wide range of applications, including the synthesis of carboxylic acid esters, lactones, amides, lactams, and unsymmetrical ketones such as alkyl alkynyl and alkyl aryl ketones. The design of unique multicomponent carbonylation reactions involving vicinal C-functionalization of alkenes, double and triple carbonylation reactions, in tandem with radical cyclization reactions, has also been achieved. Thus, the radical/Pd-combined strategy provides a solution to a longstanding problem of reactivity involving the carbonylation of alkyl halides. This novel methodology expands the breadth and utility of carbonylation chemistry over either the original radical carbonylation reactions or metal-catalyzed carbonylation reactions.