Palladium-Catalyzed α-Ketone Arylation under Mild Conditions

Electronically flexible PYA ligands for efficient palladium-catalyzed α-arylation of ketones

Palladium-catalyzed cross-coupling chemistry and in particular ketone α-arylation has been relying on a rather narrow range of supporting ligands with almost no alternatives to phosphines and N-heterocyclic carbenes. Here we introduce a class of well-defined palladium(II) complexes supported by N,N'-chelating and electronically flexible pyridylidene amide (PYA)-pyridyl ligands as catalysts for efficient α-arylation of ketones. Steric and electronic variations of the N,N'-bidentate ligand indicate that the introduction of an ortho-methyl group on the pyridinum heterocycle of the PYA ligand enhances the arylation rate and prevents catalyst deactivation, reaching turnover numbers up to 7300 and turnover frequencies of almost 10 000 h-1, which is similar to that of the best phosphine complexes known to date. Introducing a shielding xylyl substituent accelerates catalysis further, however at the expense of lower selectivity towards arylated ketones. Substrate scope investigations revealed that both electron-rich and -poor aryl bromides as well as a broad range of electronically and sterically modified ketones are efficiently converted, including aliphatic ketones. Mechanistic investigations using Hammett and Eyring analyses indicated that both, oxidative addition and reductive elimination are relatively fast, presumably as a consequence of the electronic flexibility of the PYA ligand, while enolate coordination was identified as the turnover-limiting step.

Iron-Catalyzed Oxidative α-Amination of Ketones with Primary and Secondary Sulfonamides

We report the iron-catalyzed α-amination of ketones with sulfonamides. Using an oxidative coupling approach, ketones can be directly coupled with free sulfonamides, without the need for prefunctionalization of either substrate. Primary and secondary sulfonamides are both competent coupling partners, with yields from 55% to 88% for deoxybenzoin-derived substrates.

N-Heterocyclic carbene complexes enabling the α-arylation of carbonyl compounds

The considerable importance of α-arylated carbonyl compounds, which are widely used as final products or as key intermediates in the pharmaceutical industry, has prompted numerous research groups to develop efficient synthetic strategies for their preparation in recent decades. In this context, the α-arylation of carbonyl compounds catalyzed by transition-metal complexes have been particularly helpful in constructing this motif. As illustrated in this contribution, tremendous advances have taken place using palladium- and nickel-NHC (NHC = N-heterocyclic carbene) complexes as pre-catalysts for the arylation of a wide range of ketones, aldehydes, esters and amides with electron-rich, electron-neutral, electron-poor, and sterically hindered aryl halides or pseudo-halides. Despite significant progress, especially in asymmetric α-arylations promoted by chiral NHC ligands, there are numerous challenges which have and continue to encourage further studies on this topic. Some of these are presented in this report.

Different effects of metal-NHC bond cleavage on the Pd/NHC and Ni/NHC catalyzed α-arylation of ketones with aryl halides

Fundamental differences in the behavior of Pd/NHC and Ni/NHC catalytic systems in ketones α-arylation were elucidated and exploited.

Palladium-Catalyzed Intramolecular Insertion of Alkenes into the Carbon-Nitrogen Bond of β-Lactams

The carbon-nitrogen bond of β-lactams is cleaved by palladium(0), and an alkene is intramolecularly inserted therein. The following reductive elimination produces nitrogen-containing benzo-fused tricycles in good to high yields.

Synthesis, structure and catalysis of a NHC–Pd(ii) complex based on a tetradentate mixed ligand

NHC–Pd(ii) complex 1 has been prepared. The catalytic activity of 1 in C–C coupling reactions was studied.

N-heterocyclic carbene–palladium(ii)-1-methylimidazole complex catalyzed α-arylation reactions of tetralones with aryl chlorides and further transformation of the products

NHC–Pd(ii)–Im complex-catalyzed α-arylation of tetralones with aryl chlorides and further transformation of the products was reported.

Catalyst-free cross-coupling of N-tosylhydrazones with chromium(0) Fischer carbene complexes: a new approach to diarylethanone

Chromium Fischer carbene complexes react with the in situ generated diazo compounds to afford methyl enolate products which are hydrolyzed to give diarylethanones.

Copper-catalyzed hydrophosphinations of styrenes in water at room temperature

Copper-catalyzed hydrophosphinations of styrenyl systems in water, at room temperature is herein reported, enabled by our ‘designer’ surfactant TPGS-750-M.

Modular isoquinoline synthesis using catalytic enolate arylation and in situ functionalization

A methyl ketone, an aryl bromide, an electrophile, and ammonium chloride were combined in a four-component, three-step, and one-pot coupling procedure to furnish substituted isoquinolines in overall yields of up to 80%. This protocol utilizes the palladium catalyzed α-arylation reaction of an enolate, followed by in situ trapping with an electrophile, and aromatization with ammonium chloride. tert-Butyl cyanoacetate participated in a similar protocol; after functionalization and decarboxylation, 3-amino-4-alkyl isoquinolines were prepared in high yield.

Visible-light-mediated α-arylation of enol acetates using aryl diazonium salts

Visible light mediates efficiently the α-arylation of enol acetates by aryl diazonium salts under mild conditions using [Ru(bpy)(3)]Cl(2) as a photoredox catalyst. The broad scope of the reaction toward various diazonium salts and enol acetates was explored. The application of this reaction in the concise synthesis of 2-substituted indoles was demonstrated.

Synthesis of Anovel Sterically Hindered Imidazolinium Chloride: An N-Heterocyclic Carbene Precursor

A hindered imidazolinium chloride which is a bulky, strongly donating N-heterocyclic carbene precursor has been synthesised from pinacolone in six steps. The chlorides can be easily converted to its corresponding hexafluorophosphate salt in near quantitative yield. The two new compounds were characterised by 1H and 13C NMR, IR, and elemental analyses.

Synthesis of a Series of Pyridine-Functionalised Benzoimidazole-Based N-Heterocyclic Carbene Ligands

A series of pyridine-functionalised bis(benzoimidazolium) dichlorides, dibromides and dihexafluorophosphate with different alkyl bridged length ( n = 2–4) were synthesised in high yield. All the compounds are new and fully characterised by 1H and 13C NMR, IR and elemental analyses.

Efficient Synthesis of Alkane-Bridged N,N’-Diaryl Bisimidazolium Chlorides under Solvent-Free Conditions

A series of N, N’-diaryl bisimidazolium dichlorides with different alkyl bridged length ( n = 2–6, 10) was synthesised from N-arylimidazoles with the less expensive, but less reactive dialkyl chlorides effectively in solvent-free conditions at ambient pressure. Compared with conventional methods, this protocol features a short reaction time, good yields and an eco-friendly process.