Heterocycle Synthesis through Pd-Catalyzed Carbonylative Coupling

Preparation of pyridopyrazines through tandem Pd-catalyzed C-N/C-C coupling reactions of Ugi adducts

A Pd-catalyzed selective tandem cyclization of the Ugi adduct via Buchwald-Hartwig/C-H bond functionalization reactions has been reported. This sequence offers an interesting approach for synthesizing a wide range of pyrido[1,2-a]pyrazine-3,6-dione scaffolds under mild reaction conditions in moderate to excellent yields. The scope and limitations of the protocol are discussed.

Direct synthesis of phthalimides via palladium-catalysed double carbonylation of o-dihaloarenes with nitroarenes

The direct carbonylation of readily available nitro compounds is more attractive and straightforward than the use of traditional amines as nucleophiles. Herein, a practical palladium-catalysed double carbonylation of nitroarenes with o-dihaloarenes has been developed for the construction of various N-aryl phthalimides. Key to the success of this transformation is the use of Mo(CO)6, which acts as both a reducing agent and a solid carbonyl source. A wide range of nitroarenes and o-dihaloarenes as well as o-iodobenzoic acids reacted smoothly to give phthalimides in 27-94% yields.

Palladium Iodide Catalyzed Multicomponent Carbonylative Synthesis of 2-(4-Acylfuran-2-yl)acetamides

2-Propargyl-1,3-dicarbonyl compounds have been carbonylated under oxidative conditions and with the catalysis of the PdI2/KI catalytic system to selectively afford previously unreported 2-(4-acylfuran-2-yl)acetamides in fair to good yields (54-81%) over 19 examples. The process takes place under relatively mild conditions and occurs via a mechanistic pathway involving Csp-H activation by oxidative monoamincarbonylation of the terminal triple bond of the substrates with formation of 2-ynamide intermediates, followed by 5-exo-dig O-cyclization (via intramolecular conjugate addition of the in situ formed enolate to the 2-ynamide moiety) and aromative isomerization.

Palladium-Catalyzed Reductive Double Carbonylation of Nitroarenes with Aryl Halides Using Mo(CO)6 as a Reductant and Carbonyl Source

A new palladium-catalyzed reductive double carbonylation of nitroarenes with aryl halides for the synthesis of benzoxazin-4-ones has been reported. The key to success was the use of Mo(CO)₆ as a reductant and bench-stable solid carbonyl sources. Various aryl iodides, bromides, and trifluoromethanesulfonates are suitable reaction partners and produce corresponding benzoxazin-4-one derivatives in moderate to good yields. Preliminary mechanistic studies indicate that nitrosoarene was first generated as the key intermediate through nitro reduction. Remarkably, this method avoids the use of toxic CO gas and is further applied to the late-stage modification of estrone.

Transition metal-catalysed carbene- and nitrene transfer to carbon monoxide and isocyanides

Transition metal-catalysed carbene- and nitrene transfer to the C1-building blocks carbon monoxide and isocyanides provides heteroallenes (i.e. ketenes, isocyanates, ketenimines and carbodiimides). These are versatile and reactive compounds allowing in situ transformation towards numerous functional groups and organic compounds, including heterocycles. Both one-pot and tandem processes have been developed providing valuable synthetic methods for the organic chemistry toolbox. This review discusses all known transition metal-catalysed carbene- and nitrene transfer reactions towards carbon monoxide and isocyanides and in situ transformation of the heteroallenes hereby obtained, with a special focus on the general mechanistic considerations.

Synthesis of Benzothiophene-3-carboxylic Esters by Palladium Iodide-Catalyzed Oxidative Cyclization-Deprotection-Alkoxycarbonylation Sequence under Aerobic Conditions

A palladium-catalyzed carbonylative approach to benzothiophene-3-carboxylic esters, starting from simple and readily available building blocks [2-(methylthio)phenylacetylenes, CO, an alcohol, and O₂ (from air)], is reported. The process is catalyzed by the simple PdI₂/KI catalytic system to give the desired products in fair to high yields (57-83%). Interestingly, the reaction also works nicely in the ionic liquid BmimBF₄ as the solvent, with the possibility to recycle the catalytic system several times without appreciable loss of activity.

A general strategy for the synthesis of α-trifluoromethyl- and α-perfluoroalkyl-β-lactams via palladium-catalyzed carbonylation

β-Lactam compounds play a key role in medicinal chemistry, specifically as the most important class of antibiotics. Here, we report a novel one-step approach for the synthesis of α-(trifluoromethyl)-β-lactams and related products from fluorinated olefins, anilines and CO. Utilization of an advanced palladium catalyst system with the Ruphos ligand allows for selective cycloaminocarbonylations to give diverse fluorinated β-lactams in high yields.

Isoindolinone Synthesis via One-Pot Type Transition Metal Catalyzed C-C Bond Forming Reactions

Isoindolinone structure is an important privileged scaffold found in a large variety of naturally occurring as well as synthetic, biologically and pharmaceutically active compounds. Owing to its crucial role in a number of applications, the synthetic methodologies for accessing this heterocyclic skeleton have received significant attention during the past decade. In general, the synthetic strategies can be divided into two categories: First, direct utilization of phthalimides or phthalimidines as starting materials for the synthesis of isoindolinones; and second, construction of the lactam and/or aromatic rings by different catalytic methods, including C-H activation, cross-coupling, carbonylation, condensation, addition and formal cycloaddition reactions. Especially in the last mentioned, utilization of transition metal catalysts provides access to a broad range of substituted isoindolinones. Herein, the recent advances (2010-2020) in transition metal catalyzed synthetic methodologies via formation of new C-C bonds for isoindolinones are reviewed.

Recent developments in the synthesis of azaindoles from pyridine and pyrrole building blocks

The azaindole framework is ubiquitous in bioactive natural products and pharmaceuticals. This review highlights the synthetic approaches to azaindoles with advantages and limitations, mechanistic pathways and biological importance.

Palladium-catalyzed [2 + 2 + 1] annulation: access to chromone fused cyclopentanones with cyclopropenone as the CO source

A variety of chromone fused cyclopentanones are efficiently generated in good to high yields via palladium-catalyzed [2 + 2 + 1] annulation, in which cyclopropenone was utilized for the first time as the sole CO surrogate in the carbonylation process.

Cyclization of Thiopropargyl Benzimidazoles by Combining Iron(III) Chloride and Diorganyl Diselenides

A practical synthetic approach to the synthesis of 3-(organoselenyl)-imidazothiazines was developed. The methodology involved the regioselective 6-endo-dig cyclization of thiopropargyl benzimidazoles promoted by diorganyl diselenides and iron(III) chloride. The investigation to determine the best reaction conditions indicated the use of thiopropargyl benzimidazoles (0.25 mmol) with diorganyl diselenides (1.0 equiv) and iron(III) chloride (2.0 equiv) in dichloromethane at 40 °C for 30 min to be optimal. Under these conditions, the scope of the substrates was evaluated varying the structures of thiopropargyl benzimidazoles and diorganyl diselenides giving 28 3-(organoselenyl)-imidazothiazines in moderate to good yields. The reaction conditions were also applicable to diorganyl ditellurides; however, they did not work for diorganyl disulfides. The mechanism studies were carried out indicating that the cyclization proceeds via a cooperative action of diorganyl diselenides and iron(III) chloride, but a direct electrophilic cyclization, promoted by the in situ formed electrophilic organoselenium species, cannot be ruled out.