Palladium-catalyzed cross-coupling of styrenes with aryl methyl ketones in ionic liquids: direct access to cyclopropanes

Redox Approaches to Carbene Generation in Catalytic Cyclopropanation Reactions

Transition metal-catalyzed carbene transfer reactions have a century-old history in organic chemistry and are a primary method for the synthesis of cyclopropanes. Much of the work in this field has focused on the use of diazo compounds and related precursors, which can transfer a carbene fragment to a catalyst with concomitant loss of a stable byproduct. Despite the utility of this approach, there are persistent limitations in the scope of viable carbenes, most notably those lacking stabilizing substituents. By coupling carbene transfer chemistry with two-electron redox cycles, it is possible to expand the available starting materials that can be used as carbene precursors. In this Minireview, we discuss emerging catalytic reductive cyclopropanation reactions using either gem-dihaloalkanes or carbonyl compounds. This strategy is inspired by classic stoichiometric transformations, such as the Simmons-Smith cyclopropanation and the Clemmensen reduction, but instead entails the formation of a catalytically generated transition metal carbene or carbenoid. We also present recent efforts to generate carbenes directly from methylene (CR2H2) groups via a formal 1,1-dehydrogenation. These reactions are currently restricted to substrates containing electron-withdrawing substituents, which serve to facilitate deprotonation and subsequent oxidation of the anion.

Ionic Liquids in Metal, Photo-, Electro-, and (Bio) Catalysis

Ionic liquids (ILs) have unique physicochemical properties that make them advantageous for catalysis, such as low vapor pressure, non-flammability, high thermal and chemical stabilities, and the ability to enhance the activity and stability of (bio)catalysts. ILs can improve the efficiency, selectivity, and sustainability of bio(transformations) by acting as activators of enzymes, selectively dissolving substrates and products, and reducing toxicity. They can also be recycled and reused multiple times without losing their effectiveness. ILs based on imidazolium cation are preferred for structural organization aspects, with a semiorganized layer surrounding the catalyst. ILs act as a container, providing a confined space that allows modulation of electronic and geometric effects, miscibility of reactants and products, and residence time of species. ILs can stabilize ionic and radical species and control the catalytic activity of dynamic processes. Supported IL phase (SILP) derivatives and polymeric ILs (PILs) are good options for molecular engineering of greener catalytic processes. The major factors governing metal, photo-, electro-, and biocatalysts in ILs are discussed in detail based on the vast literature available over the past two and a half decades. Catalytic reactions, ranging from hydrogenation and cross-coupling to oxidations, promoted by homogeneous and heterogeneous catalysts in both single and multiphase conditions, are extensively reviewed and discussed considering the knowledge accumulated until now.

Chalcone-Based Synthesis of Tetrahydropyridazines via Cloke-Wilson-Type Rearrangement-Involved Tandem Reaction between Cyclopropyl Ketones and Hydrazines

A facile and efficient approach for the synthesis of multisubstituted tetrahydropyridazines starting from cyclopropyl ketones and hydrazines has been developed. The transformation is chalcone-based and takes place via a Cloke-Wilson-type rearrangement-involved tandem reaction catalyzed by TfOH in HFIP.

Cu(ii)-mediated direct intramolecular cyclopropanation of distal olefinic acetate: access to cyclopropane-fused γ-lactones

Cyclopropane-fused ring scaffolds represent one of the most appealing structural motifs in organic chemistry due to their wide presence in bioactive molecules and versatility in organic synthesis. These skeletons are typically prepared from olefinic diazo compounds via transition-metal catalysed intramolecular carbenoid insertion, which suffers from prefunctionalization of starting materials and limited substrate scope. Herein, we disclose a practical copper-mediated direct intramolecular cyclopropanation of distal olefinic acetate to synthesize cyclopropane-fused γ-lactones and lactams. This cascade reaction is postulated to proceed through a hydrogen atom transfer event induced radical cyclization and copper-mediated cyclopropanation sequence. The protocol features high atom- and step-economy, excellent diastereoselectivity, broad tolerance of functional groups, and operational simplicity.

Diastereoselective Synthesis of Cyclopropanes from Carbon Pronucleophiles and Alkenes

Cyclopropanes are desirable structural motifs with valuable applications in drug discovery and beyond. Established alkene cyclopropanation methods give rise to cyclopropanes with a limited array of substituents, are difficult to scale, or both. Herein, we disclose a new cyclopropane synthesis through the formal coupling of abundant carbon pronucleophiles and unactivated alkenes. This strategy exploits dicationic adducts derived from electrolysis of thianthrene in the presence of alkene substrates. We find that these dielectrophiles undergo cyclopropanation with methylene pronucleophiles via alkenyl thianthrenium intermediates. This protocol is scalable, proceeds with high diastereoselectivity, and tolerates diverse functional groups on both the alkene and pronucleophile coupling partners. To validate the utility of this new procedure, we prepared an array of substituted analogs of an established cyclopropane that is en route to multiple pharmaceuticals.

Direct Synthesis of 3-Aryl Substituted Isocoumarins and Phthalides through Palladium Acetate Catalyzed C(sp2 )-H Activation in Ionic Liquids

A novel Pd-catalysed oxidative coupling between benzoic acids and vinylarenes or acrylates to furnish isocoumarins and phthalides is reported. The reaction proceeds smoothly in molten tetrabutylammonium acetate via a selective C-H bond activation, with very low percentage of ligand-free palladium acetate as the catalyst, under atmospheric pressure of oxygen. Sub-stoichiometric amount of copper acetate is also required as a reoxidant for the palladium.

Visible-Light-Induced Photocatalytic Oxidative Decarboxylation of Cinnamic Acids to 1,2-Diketones

A concerted metallophotoredox catalysis has been realized for the efficient decarboxylative functionalization of α,β-unsaturated carboxylic acids with aryl iodides in the presence of perylene bisimide dye to afford 1,2-diketones.

β-Diazocarbonyl Compounds: Synthesis and their Rh(II)-Catalyzed 1,3 C-H Insertions

Herein, we describe the first electrophilic diazomethylation of ketone silyl enol ethers with diazomethyl-substituted hypervalent iodine reagents that gives access to unusual β-diazocarbonyl compounds. The potential of this unexplored class of diazo compounds for the development of new reactions was demonstrated by the discovery of a rare Rh-catalyzed intramolecular 1,3 C-H carbene insertion that led to complex cyclopropanes with excellent stereocontrol.

Acyl radical to rhodacycle addition and cyclization relay to access butterfly flavylium fluorophores

Transition metal-catalyzed C–H activation and radical reactions are two versatile strategies to construct diverse organic skeletons. Here we show the construction of a class of flavylium fluorophores via the merge of radical chemistry and C–H activation starting from (hetero)aryl ketones and alkynes. This protocol is not only applicable to aryl ketones but also to heteroaryl ketones such as thiophene, benzothiophene and benzofuran, thus leading to structural diversity. Mechanism studies, including control experiments, intermediate separation, radical trapping, EPR and ESI-HRMS experiments, demonstrate that the key step lies in the addition of the acyl radical generated by the copper-catalyzed C–C bond cleavage of aryl ketone to the rhodacycle formed via the C–H activation of aryl ketone. The flavylium fluorophores feature butterfly symmetrical configuration, nearly planar skeleton and delocalized positive charge, and exhibit intriguing photophysical properties, such as tunable absorption and emission wavelengths and high quantum yields.

Structural diversity of organic fluorophores is of importance for several applications (fluorescent markers, photosensitizers, etc.). Here the authors report a method to merge radical chemistry with C–H activation to construct a brand-new class of flavylium fluorophores starting from (hetero)aryl ketones and alkynes.

A transition-metal-free & diazo-free styrene cyclopropanation

An operationally simple and broadly applicable novel cyclopropanation of styrenes using gem-diiodomethyl carbonyl reagents has been developed. Visible-light triggered the photoinduced generation of iodomethyl carbonyl radicals, able to cyclopropanate a wide array of styrenes with excellent chemoselectivity and functional group tolerance. To highlight the utility of our photocyclopropanation, we demonstrated the late-stage functionalization of biomolecule derivatives.

Recent advances in the synthesis of cyclopropanes

Cyclopropanes, one of the most important strained rings, have gained much attention for more than a century because of their interesting and unique reactivity. They not only exist in many natural products, but have also been widely used in the fields of organic synthesis, medicinal chemistry and materials science as versatile building blocks. Based on the sustainable development in this area, this review mainly focuses on the recent advances in the synthesis of cyclopropanes classified by the type of catalytic system, including regio-, diastereo-, and enantio-selective reactions.

Concise Synthesis of Polysubstituted Carbohelicenes by a C-H Activation/Radical Reaction/C-H Activation Sequence

Disclosed herein is the merging of C-H activation and radical chemistry, enabling rapid access to a structurally diverse family of fused carbohelicenes through the fusion of α-acetylnaphthalenes with alkynes under oxidative conditions. This cascade process exhibits exquisite chemoselectivity and regioselectivity. The reaction pathway was analyzed by intermediate separations, control experiments, radical trapping, EPR, MALDI-TOF-MS, and ESI-HRMS experiments, and shown to involve a C2-H activation/radical reaction/C8-H activation relay.

Radical asymmetric intramolecular α-cyclopropanation of aldehydes towards bicyclo[3.1.0]hexanes containing vicinal all-carbon quaternary stereocenters

The development of a general catalytic method for the direct and stereoselective construction of cyclopropanes bearing highly congested vicinal all-carbon quaternary stereocenters remains a formidable challenge in chemical synthesis. Here, we report an intramolecular radical cyclopropanation of unactivated alkenes with simple α-methylene group of aldehydes as C1 source via a Cu(I)/secondary amine cooperative catalyst, which enables the single-step construction of bicyclo[3.1.0]hexane skeletons with excellent efficiency, broad substrate scope covering various terminal, internal alkenes as well as diverse (hetero)aromatic, alkenyl, alkyl-substituted geminal alkenes. Moreover, this reaction has been successfully realized to an asymmetric transformation, providing an attractive approach for the construction of enantioenriched bicyclo[3.1.0]hexanes bearing two crucial vicinal all-carbon quaternary stereocenters with good to excellent enantioselectivity. The utility of this method is illustrated by facile transformations of the products into various useful chiral synthetic intermediates. Preliminary mechanistic studies support a stepwise radical process for this formal [2 + 1] cycloaddition.

N-Tosylhydrazone directed annulation via C–H/N–N bond activation in Ru(ii)/PEG-400 as homogeneous recyclable catalytic system: a green synthesis of isoquinolines

A green and sustainable methodology for the synthesis of isoquinolines using Ru(ii)/PEG-400 as homogeneous recyclable catalytic system and N-tosylhydrazone, a rarely explored directing group has been reported.

Recent advances in the synthesis of cyclopropanes

Cyclopropanes have gained much attention by virtue of their interesting structure and unique reactivity. This review discusses the recent advances in the synthesis of cyclopropanes, and some of the related applications will be discussed.

One-Pot Conversion of Epoxidized Soybean Oil (ESO) into Soy-Based Polyurethanes by MoCl₂O₂ Catalysis

An innovative and eco-friendly one-pot synthesis of bio-based polyurethanes is proposed via the epoxy-ring opening of epoxidized soybean oil (ESO) with methanol, followed by the reaction of methoxy bio-polyols intermediates with 2,6-tolyl-diisocyanate (TDI). Both synthetic steps, methanolysis and polyurethane linkage formation, are promoted by a unique catalyst, molybdenum(VI) dichloride dioxide (MoCl₂O₂), which makes this procedure an efficient, cost-effective, and environmentally safer method amenable to industrial scale-up.

A Valuable Upgrade to the Portfolio of Cycloaddition Reactions

Knitting: Recently Antonchick and Manna described a unique annulation that knits together three acetophenones to construct cyclopropanes. The cascade is mediated by organocopper and free radical species, and amounts to the first known [1+1+1] cyclotrimerization. It works well for ketones having electron-deficient or electron-rich substituents in their aryl rings. DTBP=di-tert-butylperoxide.

One-Pot Synthesis of Esters of Cyclopropane Carboxylic Acids via Tandem Vicarious Nucleophilic Substitution-Michael Addition Process

α-Chlorocarbanions generated via base-induced vicarious nucleophilic substitution reaction of alkyl dichloroacetates with nitroarenes react with Michael acceptors to give esters of cyclopropane carboxylic acids substituted with p-nitroaromatic rings.