Remarkable electronic effect on rhodium-catalyzed carbonyl additions and conjugated additions with arylmetallic reagents

Rh(III)-Catalyzed Tandem Bicyclization of 2-Arylimidazo[1,2-a]pyridines with Cyclic Enones for the Construction of Bridged Scaffolds

An efficient Rh(III)-catalyzed bicyclization of 2-arylimidazo[1,2-a]pyridine with cyclic enones has been developed for the synthesis of bridged imidazopyridine derivatives in excellent yields up to 95%. The reaction proceeds through a sequential conjugate addition of ortho-C-H bond of aryl group followed by an intramolecular C3-alkylation of imidazopyridine ring in a highly regioselective manner.

En route to metal-mediated and metal-catalysed reactions in water

This perspective report presents the key approaches for the development of various organometallic reactions in aqueous media. In view of future sustainability, the efficient use of natural resources, such as renewable biomass-based feedstocks, constitutes an important aspect for sustainable chemical industry. The exploration and discovery of efficient organometallic reactions or equivalents in water enrich the toolbox of organic chemists for the direct conversion of biomass-derived feedstocks into high-valued chemicals and the direct modification of biomolecules in their native aqueous environment, which contributes to future sustainability.

Synthesis, characterization of active Sn(0), and its application in selective propargylation of aldehyde at room temperature in water

Active Sn(0) particles are synthesized in high yields by the chemical reduction of the blue–black stannous oxide using freshly prepared sodium stannite solution as reducing agent at 40 °C and 60 °C. The Sn(0) particles are characterized using powder XRD, SEM, and DSC. The as-synthesized Sn(0) particles are applied as reagent for the regioselective synthesis of homopropargyl alcohols from propargyl bromide and aldehydes in distilled water at room temperature (in 50%–84% yields). No assistance of heat, microwave, ultrasound, organic co-solvent, co-reagent, or inert atmosphere is required for this reaction. The propargylation reaction is highly chemoselective towards aldehyde over other less electrophilic carbonyl functional groups such as ketone, amide, and carboxylic acid. Our in-house synthesized homopropargyl alcohols can be used to synthesize conjugated 1,3-diynes.

Direct conjugate additions using aryl and alkyl organic halides in air and water

The direct aryl-conjugate addition to electron-deficient alkenes without prior stoichiometric formation of organometallic reagents in water catalyzed by copper represents an important but unresolved challenge.

Water opens the door to organolithiums and Grignard reagents: exploring and comparing the reactivity of highly polar organometallic compounds in unconventional reaction media towards the synthesis of tetrahydrofurans

It has always been a firm conviction of the scientific community that the employment of both anhydrous conditions and water-free reaction media is required for the successful handling of organometallic compounds with highly polarised metal-carbon bonds. Herein, we describe how, under heterogeneous conditions, Grignard and organolithium reagents can smoothly undergo nucleophilic additions to γ-chloroketones, on the way to 2,2-disubstituted tetrahydrofurans, "on water", competitively with protonolysis, under batch conditions, at room temperature and under air. The reactivity of the above organometallic reagents has also been investigated in conventional anhydrous organic solvents and in bio-based eutectic and low melting mixtures for comparison. The scope and limitations of this kind of reaction are discussed.

The Barbier-Grignard-type arylation of aldehydes using unactivated aryl iodides in water

Carbon-carbon bond formation is the essence of organic synthesis. One of the most important methods for forming carbon-carbon bonds is the Barbier-Grignard-type reaction, which was discovered over a century ago. However, it is still highly desirable to further improve this process. In this article, we describe a Barbier-Grignard-type direct arylation of aldehydes by using unactivated iodides mediated by zinc and catalysed by rhodium in water. This method bypasses a number of challenges encountered by the conventional Barbier-Grignard reaction, such as strict exclusion of moisture and air, protection-deprotection of various acidic hydrogens in the substrates, and so forth. It thereby creates a safer, more convenient and more environmentally benign strategy to access the diarylmethanols and aryl alkyl alcohols, ubiquitous skeletons found in fine chemicals, biologically active molecules and pharmaceuticals. Importantly, the same reaction performed in an organic solvent proceeded sluggishly to give much inferior yields.

Synthesis and structures of monomeric group 14 triols and their reactivity

The first stable stannanetriol and germanetriol bearing sterically congested aryl groups were synthesized by hydrolysis of the corresponding trichloro-stannane and -germane. The stannanetriol is monomeric in solution as well as in the crystalline state, as evidenced by X-ray diffraction analysis and CP-MAS NMR spectroscopy. The stannanetriol reacted with silicone grease to afford a cagelike compound having three Sn–O–Si–O–Sn linkages in the molecule. All the group 14 triols can be converted to the corresponding trihalo compounds in good yields.

A homogeneous, recyclable polymer support for Rh(I)-catalyzed C-C bond formation

A robust and practical polymer-supported, homogeneous, recyclable biphephos rhodium(I) catalyst has been developed for C-C bond formation reactions. Control of polymer molecular weight allowed tuning of the polymer solubility such that the polymer-supported catalyst is soluble in nonpolar solvents and insoluble in polar solvents. Using the supported rhodium catalysts, addition of aryl and vinylboronic acids to the electrophiles such as enones, aldehydes, N-sulfonyl aldimines, and alkynes occurs smoothly to provide products in high yields. Additions of terminal alkynes to enones and industrially relevant hydroformylation reactions have also been successfully carried out. Studies show that the leaching of Rh from the polymer support is low and catalyst recycle can be achieved by simple precipitation and filtration.

Alternatives to organoboron reagents in rhodium-catalyzed conjugate additions

The rhodium-catalyzed conjugate addition of organoboron reagents to alkene acceptors has emerged as an important methodology in organic synthesis. The process results in the formation of new carbon-carbon bonds, and in principle, can create new stereocenters at both the alpha and beta carbon atoms of the acceptor. Other organometallics are known to participate in the key transmetalation to rhodium and are becoming more established in stereoselective synthesis (notably arylzinc reagents). This Focus Review will describe recent developments in the use of alternative organometallic donors to organoboron reagents in rhodium-catalyzed conjugate addition reactions and their growing applications in synthesis.

A homogeneous, recyclable rhodium(I) catalyst for the hydroarylation of Michael acceptors

A robust and practical polymer-supported, recyclable biphephos rhodium(I) catalyst has been developed. Control of polymer molecular weight allowed the tuning of solubility such that the polymer-supported catalyst is soluble in nonpolar solvents and not soluble in polar solvents. Thus, catalytic addition of aryl- and vinylboronic acids to enones occurs under completely homogeneous conditions and catalyst recycle can be achieved by simple precipitation and filtration.

1,4-Additions of arylboron, -silicon, and -bismuth compounds to α,β-unsaturated carbonyl compounds catalyzed by dicationic palladium(II) complexes

An enantioselective synthesis of cyclic and acyclic β-aryl ketone and aldehydes via Pd(II)-catalyzed 1,4-addition of Ar-m [m = B(OH)2, BF3K, Si(OMe)3, SiF3, BiAr2] to α,β-unsaturated ketones or aldehydes is described. The catalytic cycle involves transmetallation between Ar-m and Pd complexes as a key process, the mechanism of which is discussed on the basis of characterization of the transmetallation intermediate and electronic effect of the substituents. The enantioselection mechanism and efficiency of a chiraphos ligand for structurally planar α,β-unsaturated ketones are discussed on the basis of the X-ray structure of the catalyst and results of density functional theory (DFT) computational studies on the model of coordination of the substrates to the phenylpalladium(II)/(S,S)-chiraphos intermediate.

Sn(3) and Sn(10) sulfonate-oxide-hydroxide clusters with two different sulfonate binding modes

A tin sulfonate-oxide-hydroxide tetracation with two different sulfonate bindings, an electrostatic and a monohapto covalent one, and a tin sulfonate-oxide-hydroxide monocation with two other kinds of sulfonate bindings, an electrostatic and a dihapto bridging one are described here.