C–C Activation by Retro-Aldol Reaction of Two β-Hydroxy Carbonyl Compounds: Synergy with Pd-Catalyzed Cross-Coupling To Access Mono-α-arylated Ketones and Esters

Copper/O2-Mediated Oxidative C-C Activation of Nitriles for Selective Acylation-Bromination of Anilines

This study reports selective dual amino acylation and C-H bromination of aniline compounds enabled by Cu/O2 catalyst systems. This method involves crucial oxidation-induced C-CN bond cleavage of α-methylene nitriles to generate an acylcyanide intermediate that is facilely intercepted by anilines. After amino acylation, the Cu(II) precatalyst in combination with NBS generates Cu(III)-Br in situ that engages in selective electrophilic para- or ortho-C-H bromination. The substrate scope, mechanistic aspects, and late-stage functionalization of biologically active anilines are studied. This study shows the synthetic potential of oxidative C-CN bond activation of nitriles for the development of valuable reactions.

An oxidative photocyclization approach to the synthesis of Securiflustra securifrons alkaloids

Securines and securamines are cytotoxic alkaloids that contain reactive alkene and heterocyclic residues embedded in skeletons comprising four to six oxidized rings. This structural complexity imparts a rich chemistry to the isolates but has impeded synthetic access to the structures in the nearly three decades since their isolation. We present a flexible route to eight isolates that exemplify the three skeletal classes of metabolites. The route proceeds by the modular assembly of the advanced azides 38 and 49 (13 steps, 6 to 10% yield), sequential oxidative photocyclizations, and late-stage functional group manipulations. With this approach, the targets were obtained in 17 to 19 steps, 12 to 13 purifications, and 0.5 to 3.5% overall yield. The structure of an advanced intermediate was elucidated by microcrystal electron diffraction (MicroED) analysis. The route will support structure-function and target identification studies of the securamines.

Pd-Catalyzed Formal [2 + 2]-Retrocyclization of Cyclobutanols via 2-Fold Csp3-Csp3 Bond Cleavage

In this work, we describe the unexpected 2-fold Csp3-Csp3 bond cleavage suffered by cyclobutanols in the presence of a catalytic amount of Pd(OAc)2 and promoted by the bulky biaryl JohnPhos ligand. Overall, the sequential cleavage of a strained and an unstrained Csp3-Csp3 bond leads to the formal [2 + 2]-retrocyclization products, namely, styrene and acetophenone derivatives. This procedure might enable the use of cyclobutanols as masked acetyl groups, resisting harsh conditions in organic synthesis.

Multifaceted role of silver salts as ligand scavengers and different behavior of nickel and palladium complexes: beyond halide abstraction

The reaction of [NiArBr(PPh₃)₂] with AgBF₄ brings about the abstraction of both the halide and phosphine from the nickel center by silver. When the reaction is carried out in CH₂Cl₂/toluene a mixture of the cationic aquo derivatives [NiAr(H₂O)(PPh₃)₂]BF₄ (2) and [NiAr(H₂O)₂(PPh₃)]BF₄ (3) is formed, along with AgBr and [Ag(PPh₃)_n]BF₄. When the same reaction is carried out in acetone as the solvent, it leads to the completely different complex [NiAr(κ²-O, O-MeC(O)CH₂C(OH)Me₂)(PPh₃)] (5), bearing a chelating ligand formed by the aldol self-condensation of acetone. Phosphine abstraction by silver is less favorable for the analogous palladium(II) complexes and only occurs if a large excess of AgBF₄ is used. Thus, silver salts can be safely used as halide scavengers for palladium derivatives. However, the generation of cationic Ni complexes from neutral precursors by halide extraction with a silver salt may produce naked species, different than those expected, and highly reactive in certain media.

Water-Soluble Pd Nanoparticles for the Anti-Markovnikov Oxidation of Allyl Benzene in Water

The catalytic activity and selectivity of two different water-soluble palladium nanoparticles capped with 5-(trimethylammonio)pentanethiolate and 6-(carboxylate)hexanethiolate ligands are investigated using the catalytic reaction of allyl benzene. The results show that the regioselective transformation of allyl benzene to 3-phenylpropanal occurs at room temperature and under atmospheric pressure in neat water via a Tsuji-Wacker type oxidation. Conventionally, the Tsuji-Wacker oxidation promotes the Markovnikov oxidation of terminal alkenes to their respective ketones in the presence of dioxygen. Water-soluble Pd nanoparticles, however, catalyze the anti-Markovnikov oxidation of allyl benzene to 3-phenylpropanal in up to 83% yields. Catalytic results of other aromatic alkenes suggest that the presence of benzylic hydrogen is a key to the formation of a p-allyl Pd intermediate and the anti-Markovnikov addition of H₂O. The subsequent b-H elimination and tautomerization contribute to the formation of aldehyde products. Water-soluble Pd nanoparticles are characterized using nuclear magnetic resonance (NMR), UV-vis spectroscopy, thermogravimetric analysis (TGA), and transmission electron microscopy (TEM). Catalysis results are examined using ¹H NMR and/or GC-MS analyses of isolated reaction mixtures.

Copper-catalyzed transfer methylenation via C(sp3)–C(sp3) bond cleavage of alcohols

Transfer Methylenation: A copper-catalyzed transfer methylenation via the cleavage of unstrained C(sp3)-C(sp3) bonds is developted. This is a de novo report for transfer hydrocarbylation between alcohols and carbonyl compounds.

Lateral flow immunoassay for the simultaneous detection of fipronil and its metabolites in food samples

We developed a sensitive and rapid lateral flow immunochromatographic (LFI) assay for the simultaneous detection of fipronil and its metabolites in eggs and cucumbers using gold nanoparticle (GNP)-labeled monoclonal antibodies (mAbs). Anti-fipronil mAbs (1B6) were produced using two haptens and identified by heterologous indirect competitive enzyme-linked immunosorbent assay (icELISA) with half maximal inhibitory concentration (IC₅₀) and limit of detection (LOD) values of 0.46 ± 0.07 and 0.05 ± 0.01 ng mL^-1, respectively. The developed LFI strip showed high sensitivity and specificity in the detection of fipronil with cut-off and visual limit of detection (vLOD) values of 10 and 0.25 ng mL^-1, respectively. Furthermore, the application of LFI in the detection of fipronil-spiked egg and cucumber samples was validated by liquid chromatography tandem mass spectrometry (LC-MS/MS). Our developed LFI assay is suitable for detection of fipronil and its metabolites in real samples.

An aerobic and green C-H cyanation of terminal alkynes

This study describes a benign C-H cyanation of terminal alkynes with α-cyanoesters serving as a nontoxic cyanide source. In situ generation of the key copper cyanide intermediate is proposed by a sequence of α-C-H oxidation and copper-mediated β-carbon elimination of α-cyanoesters, releasing the α-ketoester byproduct observed experimentally. The ensuing reaction of copper cyanide with terminal alkynes delivers preferentially cyanoalkynes and surpasses the possible Glaser type dimerization of terminal alkynes or the undesired accumulation of HCN under protic conditions. The presence of the co-oxidant K2S2O8 is crucial to this selectivity, probably by promoting oxidative transmetalation and the resulting formation of the Cu(iii)(acetylide)(CN) intermediate. All the reagents and salts used are commercially available, cheap and nontoxic, avoiding the use of highly toxic cyanide salts typically required in cyanation studies. The scope of this reaction is demonstrated with a set of alkynes and α-cyanoesters. The application of this method to late-stage functionalization of the terminal alkyne group in an estrone derivative is also feasible, showing its practical value for drug design.

Palladium-Catalyzed Cross-Coupling of gem-Bis(boronates) with Aryl Halides: An Alternative To Access Quaternary α-Aryl Aldehydes

The compounds with a quaternary α-aryl aldehyde skeleton are important units in organic chemistry. Previously, the aryl group and carbonyl group are introduced in a stepwise manner. Herein, a novel route is developed to construct the quaternary α-aryl aldehydes with gem-bis(boronates) as precursors, in which the two groups are installed simultaneously. The gem-bis(boronates) are readily available from ketones; as a result, this methodology provides a more general strategy to produce the quaternary α-aryl aldehydes with broad scopes and synthetic convenience.

Retro-metal-ene versus retro-Aldol: mechanistic insight into Rh-catalysed formal [3+2] cycloaddition

Theoretical calculations have been performed to investigate the mechanism and stereoselectivity of rhodium-catalysed intramolecular [3+2] cycloaddition for construction of a substituted hexahydropentalene complex. A new C-C bond cleavage mechanism, retro-Aldol-type, is proposed and verified for this Rh-catalysed [3+2] cycloaddition reaction.

Diels-Alder and Stille Coupling Approach for the Short Protecting-Group-Free Synthesis of Mycophenolic Acid, Its Phenylsulfenyl and Phenylselenyl Analogues, and Reactive Oxygen Species (ROS) Probing Capacity in Water

A short, protecting-group-free synthesis is achieved. The synthesis is step-efficient and general. A Diels-Alder and Stille cross-coupling approach includes key transformations, allowing for a competitive synthesis which involves a rare halophenol Stille cross-coupling study. The phenylselenyl and phenylsulfenyl analogues were prepared as novel compounds in good overall yield. The applicability of one of the intermediates as a potential probe for reactive oxygen species (ROS) in water is investigated.

Synthesis of quinolines and naphthyridines via catalytic retro-aldol reaction of β-hydroxyketones with ortho-aminobenzaldehydes or nicotinaldehydes

A Cu(i)-catalyzed retro-aldol reaction of β-hydroxyketones with ortho-aminobenzaldehydes and nicotinaldehydes is reported that produces a range of quinolines and naphthyridines with high efficiency and selectivity. This reaction uses β-hydroxyketones as a regiospecific ketone-protected enolate source via copper-catalyzed retro-aldol C_α-C_β bond cleavage. The in situ generated copper enolate undergoes kinetically favorable cyclization with ortho-amino aryl aldehydes to produce quinolines and naphthyridines in a chemo- and regioselective manner. The mild and weakly basic reaction conditions also suppress possible side reactions of benzaldehydes under strongly basic conditions, resulting in improved reaction yields.

Phosphine-assisted C–H bond activation in Schiff bases and formation of novel organo cobalt complexes bearing Schiff base ligands

The sp² C–H bond activation of the HCN moiety in diphenylphosphino benzalimines was realized using CoMe(PMe₃)₄ with the elimination of methane.

Catalytic activations of unstrained C–C bond involving organometallic intermediates

In this review, the transition-metal-catalyzed C–C bond activation of relatively stable and unstrained molecules involving organometallic intermediates is discussed in detail.

Ligand effect on the rhodium porphyrin catalyzed hydrogenation of [2.2]paracyclophane with water: key bimetallic hydrogenation

Rhodium porphyrin catalyzed hydrogenation of the aliphatic carbon-carbon σ-bond of [2.2]paracyclophane with water has been examined with a variety of tetraarylporphyrins and axial ligands. Mechanistic investigations show that Rh^III(ttp)H, which can be derived from the reaction of [Rh^II(ttp)]₂ with water without a sacrificial reductant, plays an important role in promoting bimetallic reductive elimination to give the hydrogenation product.

Aerobic oxidative α-arylation of furans with boronic acids via Pd(ii)-catalyzed C–C bond cleavage of primary furfuryl alcohols: sustainable access to arylfurans

Aerobic oxidative α-arylation of furans with boronic acids via Pd(ii)-catalyzed C–C bond cleavage of primary furfuryl alcohol provides sustainable access to arylfurans.

Silver-catalyzed carbon–selenium cross-coupling using N-(phenylseleno)phthalimide: an alternate approach to the synthesis of organoselenides

Silver(I) catalyzed phenylselenylation of terminal alkynes and organoboronic acids has been demonstrated using N-(phenylseleno)phthalimide as an electrophilic SePh donor. A wide variety of terminal alkynes and organoboronic acids are selenylated efficiently to produce the corresponding alkynyl and diaryl selenides, respectively, in good yields. Silver(I) acts as a Lewis acid in this process.

Chemoselective C(α)–C(β) bond cleavage of saturated aryl ketones with amines leading to α-ketoamides: a copper-catalyzed aerobic oxidation process with air

An unprecedented chemoselective C(α)–C(β) bond cleavage of simple saturated aryl ketones with amines leading to α-ketoamidesvia a copper-catalyzed aerobic oxidation process with air has been disclosed.

Mechanism of Pd-catalyzed acylation/alkenylation of aryl iodide: a DFT study

The detailed mechanism of the Pd(0)-catalyzed cross-coupling of aryl iodide, benzoic anhydride and ethyl acrylate was clarified by theoretical methods.

HOTf-Catalyzed, Solvent-Free Oxyarylation of Ynol Ethers and Thioethers

A novel HOTf-catalyzed oxyarylation of ynol ethers and thioethers has been realized with aryl sulfoxides as the oxyarylating reagents, providing α-arylated esters or thioesters in good to excellent yields. Notably, all atoms of the starting materials were incorporated in the product (100% atom economy) and the reaction proceeded under very mild conditions. It was found that the reaction can be ran under air and that the best yields are obtained under solvent-free conditions.

The molecular mechanism of palladium-catalysed cyanoesterification of methyl cyanoformate onto norbornene

Potential energy surface mapping was completed for the entire catalytic cycle of palladium-catalysed cyanoesterification onto norbornene (NBE) using density functional theory calculations. We found that after the oxidative addition step of the reagent methyl cyanoformate, the reaction proceeds through an insertion of olefin into a Pd(II)-COOMe bond first. Subsequently, reductive elimination occurs by transferring the cyanide group from the Pd center to NBE. This rearrangement is triggered by the rotation of the ester group into a π-interaction with the Pd(II) centre. The regioselectivity of olefin insertion is controlled by ionic and covalent interactions in the precursor π-complex formation step. Importantly, all of the intermediates and transition states along the exo pathway were found to be more stable than the corresponding structures of the endo pathway without any sign of crossing over between the two surfaces via isomerization. The rate-determining step is the reductive elimination despite the fact that the corresponding activation barrier is reduced by conformational changes via the rotation of a MeOOC-C(NBE) bond.

Copper-catalyzed retro-aldol reaction of β-hydroxy ketones or nitriles with aldehydes: chemo- and stereoselective access to (E)-enones and (E)-acrylonitriles

A mild and efficient copper-catalyzed retro-aldol reaction of β-hydroxy ketones or nitriles with benzaldehydes produces chemo-, regio- and stereoselectively (E)-enones and (E)-acrylonitriles.

Synthesis and reductive elimination of arylPd(ii) trifluoromethyl complexes: a remarkable concentration effect on chemoselectivity

A remarkable and unprecedented concentration effect on chemoselectivity is observed for the thermal decomposition of Pd(ii) aryl trifluoromethyl complexes. As the concentration increases, the Ar–CF₃ cross-coupling is suppressed gradually whilst the Ar–Ar homocoupling increases steadily.

Divergent synthesis of indoles, oxindoles, isocoumarins and isoquinolinones by general Pd-catalyzed retro-aldol/α-arylation

Divergent synthesis of indoles, oxindoles, isocoumarins and isoquinolinones is achieved by a general Pd-catalyzed retro-aldol reaction of β-hydroxy ketones/esters with aryl halides bearing an ortho-nitro, -ester or -nitrile group. DFT insights have been presented to rationalize the experimental observations and the chemoselectivity.