Into the Blue: Ketene Multicomponent Reactions under Visible Light

Synthesis of spiroindolenines through a one-pot multistep process mediated by visible light

Spiro-heterocyclic indolenines are privileged scaffolds widely present in numerous indole alkaloids. Here, we develop a novel approach for the one-pot multistep synthesis of different spiro[indole-isoquinolines]. The protocol proposed involves the visible light mediated oxidation of N-aryl tertiary amines using bromochloroform with the generation of a reactive iminium species, which reacts with an isocyanide and an electron-rich aniline in a three-component Ugi-type reaction to give an α-aminoamidine. This compound might undergo an additional visible light-mediated oxidation to furnish a second iminium intermediate, which acts as electrophile in an intramolecular electrophilic aromatic substitution giving the final spiro-indolenine. The scope of the process has been investigated with respect to all three components. Simple operations, mild conditions, and good yields make this strategy a convenient and sustainable way to obtain novel spiro-indolenine derivatives.

Catalyst-Free [4+1] Annulation of α-Imidoyl Sulfoxonium Ylides and Diazo Compounds Enabling the Modular Synthesis of 2-Indanones and 3(2H)-Furanones

A novel substrate-regulated [4+1] annulation of α-imidoyl sulfoxonium ylides with diazoketones under catalyst-free conditions is described. The reaction proceeds through a coupling of sulfoxonium ylides and in situ-generated ketenes to form the key reactive zwitterionic intermediates, followed by selective formation of C-C or C-O bonds to achieve five-membered ring systems. The cascade reaction permits the direct synthesis of synthetically useful 2-indanones and 3(2H)-furanones, which expands the reaction pattern of sulfoxonium ylides in annulation transformation.

Visible Light-Induced Reactions of Diazo Compounds and Their Precursors

In recent years, visible light-induced reactions of diazo compounds have attracted increasing attention in organic synthesis, leading to improvement of existing reactions, as well as to the discovery of unprecedented transformations. Thus, photochemical or photocatalytic generation of both carbenes and radicals provide milder tools toward these key intermediates for many valuable transformations. However, the vast majority of the transformations represent new reactivity modes of diazo compounds, which are achieved by the photochemical decomposition of diazo compounds and photoredox catalysis. In particular, the use of a redox-active photocatalysts opens the avenue to a plethora of radical reactions. The application of these methods to diazo compounds led to discovery of transformations inaccessible by the classical reactivity associated with carbenes and metal carbenes. In most cases, diazo compounds act as radical sources but can also serve as radical acceptors. Importantly, the described processes operate under mild, practical conditions. This Review describes this subfield of diazo compound chemistry, particularly focusing on recent advancements.

Enantioselective Synthesis of Chiral Amides by a Phosphoric Acid Catalyzed Asymmetric Wolff Rearrangement

The enantioselective addition of potent nucleophiles to ketenes poses challenges due to competing background reactions and poor stereocontrol. Herein, we present a method for enantioselective phosphoric acid catalyzed amination of ketenes generated from α-aryl-α-diazoketones. Upon exposure to visible light, the diazoketones undergo Wolff rearrangement to generate ketenes. The phosphoric acid not only accelerates ketene capture by amines to form a single configuration of aminoenol intermediates but also promotes an enantioselective proton-transfer reaction of the intermediates to yield the products. Mechanistic studies elucidated the reaction pathway and explained how the catalyst expedited the transformation and controlled the enantioselectivity.

Visible-Light-Induced Synthesis of Branched Ethers via Multicomponent Reactions

The Spin-Center Shift (SCS) elimination is a specific way for the generation of radicals with relevance in synthetic and biochemical pathways. The combination of SCS-mediated radical chemistry and atom-transfer radical addition (ATRA) offers new directions in diversity-oriented chemical synthesis. Herein, we report a photoredox three-component reaction of α-acyloxy-N-heterocycles as radical precursors, styrene derivatives as radical trapping agents, and alcohols as nucleophilic quenchers. The novel radical-polar crossover reaction provides access to a diverse set of branched ethers possessing high structural complexity. The utility of the transformation was also demonstrated by the synthesis of a complex drug derivative and it was easily scalable to the multigram level. The scope and limitations were also explored and a plausible mechanism was proposed.

Isocyanide-Based Multicomponent Reactions Promoted by Visible Light Photoredox Catalysis

Isocyanide-based multicomponent reactions claim a one century-old history of flourishing developments. On the other hand, the enormous impact of recent progresses in visible light photocatalysis has boosted the identification of new straightforward and green approaches to both new and known chemical entities. In this context, the application of visible light photocatalytic conditions to multicomponent processes has been promoting key stimulating advancements. Spanning from radical-polar crossover pathways, to photoinduced and self-catalyzed transformations, to reactions involving the generation of imidoyl radical species, the present literature analysis would provide a general and critical overview about the potentialities and challenges of exploiting isocyanides in visible light photocatalytic multicomponent reactions.

Visible-Light-Driven Solventylation Strategy for Olefin Functionalization

Amphiphilic aryl radicals generated upon visible light irradiation of arylazo sulfones have been exploited in the development of a solventylation strategy via hydrogen atom transfer (HAT). The present protocol succeeded in the versatile functionalization of various olefins with carbon-centered radicals deriving from acetone, acetonitrile, chloroform, methylene chloride, nitromethane, methyl acetate, and methyl formate under metal- and photocatalyst-free conditions. The direct addition of the aryl radicals onto the olefin substrates was suppressed under high dilution conditions.

Visible-Light-Promoted Synthesis of 1,3-Dicarbonyl Sulfoxonium Ylides

A novel visible-light-promoted coupling of diazoketones with sulfoxonium ylides, employing a violet light-emitting diode, is described under both batch and continuous flow conditions. This transformation permits the direct synthesis of synthetically useful 1,3-dicarbonyl sulfoxonium ylides (33 examples, 21-85% yields), by means of an acylation from the in situ and selective generation of ketenes. The reaction performed under flow conditions proved to be very efficient, providing the 1,3-dicarbonyl sulfoxonium ylides with higher yields and shorter reaction times.

Study and application of graphene oxide in the synthesis of 2,3-disubstituted quinolines via a Povarov multicomponent reaction and subsequent oxidation

The carbocatalyzed synthesis of 2,3-disubstituted quinolines is disclosed. This process involved a three-component Povarov reaction of anilines, aldehydes and electron-enriched enol ethers, which gave the substrate for the subsequent oxidation. Graphene oxide (GO) was exploited as a heterogeneous, metal-free and sustainable catalyst for both transformations. The multicomponent reaction proceeded under simple and mild reaction conditions, exhibited good functional group tolerance, and could be easily scaled up to the gram level. A selection of tetrahydroquinolines obtained was subsequently aromatized to quinolines. The multistep synthesis could also be performed as a one-pot procedure. Investigation of the real active sites of GO was carried out by performing control experiments and a by full characterization of the carbon material by X-ray photoelectron spectroscopy (XPS) and solid-state nuclear magnetic resonance (ssNMR).

The 100 facets of the Passerini reaction

This perspective aims at celebrating the 100th anniversary of the discovery of the Passerini three component reaction. After being nearly neglected for many years, now this reaction has become quite popular, thanks to the achievements of the last 30 years, which have revealed several chances of exploitation in organic synthesis. Though not being comprehensive, this review means to show the various ways that have been used in order to expand the utility of the Passerini reaction. Post-MCR transformations to give heterocycles or peptidomimetics, variants through single component replacement, stereochemical issues, and applications in total syntheses will be especially covered.

Chiral spiro phosphoric acid-catalysed enantioselective reaction of ketenes with N-H pyrroles

In the presence of a chiral spiro phosphoric acid catalyst, the asymmetric reaction of disubstituted ketenes with N-H pyrroles occurred to afford enantioenriched C-acylated pyrroles bearing α-stereogenic carbon centres. The described reaction constitutes a rare example of a catalytic asymmetric reaction of ketenes with carbon-based nucleophiles.