Efficient Palladium(II) Catalysis under Air. Base-Free Oxidative Heck Reactions at Room Temperature or with Microwave Heating

Ruthenium-Catalyzed Oxidative Cross-Coupling Reaction of Activated Olefins with Vinyl Boronates for the Synthesis of (E,E)-1,3-Dienes

An oxidative cross-coupling reaction between activated olefins and vinyl boronate derivatives has been developed for the highly stereoselective construction of synthetically useful (E,E)-1,3-dienes. The highlight of this reaction is that exclusive stereoselectivity (only E,E-isomer) was achieved from a base-free, ligand-free, and mild catalytic condition with a less expensive [RuCl₂(p-cymene)]₂ catalyst.

Can Donor Ligands Make Pd(OAc)2 a Stronger Oxidant? Access to Elusive Palladium(II) Reduction Potentials and Effects of Ancillary Ligands via Palladium(II)/Hydroquinone Redox Equilibria

Palladium(II)-catalyzed oxidation reactions represent an important class of methods for selective modification and functionalization of organic molecules. This field has benefitted greatly from the discovery of ancillary ligands that expand the scope, reactivity, and selectivity in these reactions; however, ancillary ligands also commonly poison these reactions. The different influences of ligands in these reactions remain poorly understood. For example, over the 60-year history of this field, the Pd^II/0 redox potentials for catalytically relevant Pd complexes have never been determined. Here, we report the unexpected discovery of (L)Pd^II(OAc)₂-mediated oxidation of hydroquinones, the microscopic reverse of quinone-mediated oxidation of Pd⁰ commonly employed in Pd^II-catalyzed oxidation reactions. Analysis of redox equilibria arising from the reaction of (L)Pd(OAc)₂ and hydroquinones (L = bathocuproine, 4,5-diazafluoren-9-one), generating reduced (L)Pd species and benzoquinones, provides the basis for determination of (L)Pd^II(OAc)₂ reduction potentials. Experimental results are complemented by density functional theory calculations to show how a series of nitrogen-based ligands modulate the (L)Pd^II(OAc)₂ reduction potential, thereby tuning the ability of Pd^II to serve as an effective oxidant of organic molecules in catalytic reactions.

Microwave-Assisted Palladium-Catalyzed Cross-Coupling Reactions: Generation of Carbon–Carbon Bond

Cross-coupling reactions furnishing carbon–carbon (C–C) bond is one of the most challenging tasks in organic syntheses. The early developed reaction protocols by Negishi, Heck, Kumada, Sonogashira, Stille, Suzuki, and Hiyama, utilizing palladium or its salts as catalysis have, for decades, attracted and inspired researchers affiliated with academia and industry. Tremendous efforts have been paid to develop and achieve more sustainable reaction conditions, such as the reduction in energy consumption by applying the microwave irradiation technique. Chemical reactions under controlled microwave conditions dramatically reduce the reaction time and therefore resulting in increase in the yield of the desired product by minimizing the formation of side products. In this review, we mainly focus on the recent advances and applications of palladium catalyzed cross-coupling carbon–carbon bond formation under microwave technology.

From biotechnology principles to functional and low-cost metallic bionanocatalysts

Chemical, physical and mechanical methods of nanomaterial preparation are still regarded as mainstream methods, and the scientific community continues to search for new ways of nanomaterial preparation. The major objective of this review is to highlight the advantages of using green chemistry and bionanotechnology in the preparation of functional low-cost catalysts. Bionanotechnology employs biological principles and processes connected with bio-phase participation in both design and development of nano-structures and nano-materials, and the biosynthesis of metallic nanoparticles is becoming even more popular due to; (i) economic and ecologic effectiveness, (ii) simple one-step nanoparticle formation, stabilisation and biomass support and (iii) the possibility of bio-waste valorisation. Although it is quite difficult to determine the precise mechanisms in particular biosynthesis and research is performed with some risk in all trial and error experiments, there is also the incentive of understanding the exact mechanisms involved. This enables further optimisation of bionanoparticle preparation and increases their application potential. Moreover, it is very important in bionanotechnological procedures to ensure repeatability of the methods related to the recognised reaction mechanisms. This review, therefore, summarises the current state of nanoparticle biosynthesis. It then demonstrates the application of biosynthesised metallic nanoparticles in heterogeneous catalysis by identifying the many examples where bionanocatalysts have been successfully applied in model reactions. These describe the degradation of organic dyes, the reduction of aromatic nitro compounds, dehalogenation of chlorinated aromatic compounds, reduction of Cr(VI) and the synthesis of important commercial chemicals. To ensure sustainability, it is important to focus on nanomaterials that are capable of maintaining the important green chemistry principles directly from design inception to ultimate application.

Ligand-Promoted Palladium-Catalyzed Aerobic Oxidation Reactions

Palladium-catalyzed aerobic oxidation reactions have been the focus of industrial application and extensive research efforts for nearly 60 years. A significant transition occurred in this field approximately 20 years ago, with the introduction of catalysts supported by ancillary ligands. The ligands play crucial roles in the reactions, including promotion of direct oxidation of palladium(0) by O₂, bypassing the typical requirement for Cu salts or related redox cocatalysts to facilitate oxidation of the reduced Pd catalyst; facilitation of key bond-breaking and bond-forming steps during substrate oxidation; and modulation of chemo-, regio-, or stereoselectivity of a reaction. The use of ligands has contributed to significant expansion of the scope of accessible aerobic oxidation reactions. Increased understanding of the role of ancillary ligands should promote the development of new synthetic transformations, enable improved control over the reaction selectivity, and improve catalyst activity and stability. This review surveys the different ligands that have been used to support palladium-catalyzed aerobic oxidation reactions and, where possible, describes mechanistic insights into the role played by the ancillary ligand.

Thiophene-Alkyne-Based CMPs as Highly Selective Regulators for Oxidative Heck Reaction

Thiophenes containing an adjacent C≡C group as ligands for Pd^II-promoted organic reactions are reported for the first time. These ligands were utilized as catalytic sites and integrated into the skeleton of conjugated microporous polymers. By employing these CMP materials as selective regulators, oxidative Heck reactions between arylboronic esters and electronically unbiased alkenes provide highly selective linear products.

Mononuclear palladium(ii) and platinum(ii) complexes of P,C-donor ligands: synthesis, crystal structures, cytotoxicity, and mechanistic studies of a highly stereoselective Mizoroki–Heck reaction

Pallada- and platinacycle complexes exhibit potential orientations in stereoselective transformations and anticancer metallodrugs.

A versatile approach to flavones via a one-pot Pd(ii)-catalyzed dehydrogenation/oxidative boron-Heck coupling sequence of chromanones

A variety of flavones were expediently synthesized from readily accessible chromanones via a one-pot sequence involving Pd(ii)-catalyzed dehydrogenation and oxidative boron-Heck coupling with arylboronic acid pinacol esters.

Regioselective aerobic oxidative Heck reactions with electronically unbiased alkenes: efficient access to α-alkyl vinylarenes

Branched-selective oxidative Heck coupling reactions have been developed between arylboronic acids and electronically unbiased terminal alkenes. The reactions exhibit high catalyst-controlled regioselectivity favoring the less common branched isomer. The reactions employ a catalyst composed of Pd(TFA)2/dmphen (TFA = trifluoroacetate, dmphen = 2,9-dimethyl-1,10-phenanthroline) and proceed efficiently at 45-60 °C under 1 atm of O2 without requiring other additives. A broad array of functional groups, including aryl halide, allyl silane and carboxylic acids are tolerated.

Nickel-catalyzed oxidative cross-coupling of arylboronic acids with olefins

A novel and efficient nickel-catalyzed oxidative cross-coupling of arylboronic acids with olefins to synthesize 1,2-diarylalkenes has been developed. By employing Ni(acac)2 as the catalyst, TEMPO (2,2,6,6-tetramethylpiperidine 1-oxyl) as the oxidant, a variety of arylboronic acids and styrene derivatives could be cross-coupled efficiently to afford the corresponding 1,2-diarylalkenes in moderate-to-good yields. Notably, high E-selectivity of 1,2-diarylalkenes was obtained with the aid of a high temperature of 120°C. Moreover, boric acid esters also proved to be efficient coupling partners. Initial mechanistic studies suggest that this reaction proceeds through a radical pathway.

Boron-Heck reaction of cyclic enaminones: regioselective direct arylation via oxidative palladium(II) catalysis

An oxidative boron-Heck reaction of cyclic enaminones with arylboronic acids is reported. This protocol provides a regioselective arylation at the C6 position of cyclic enaminones. When an N-carbamylated cyclic enaminone was employed, a switch to a conjugate addition reaction occurred in the presence of acid.

Oxidative Heck vinylation for the synthesis of complex dienes and polyenes

We introduce an oxidative Heck reaction for selective complex diene and polyene formation. The reaction proceeds via oxidative Pd(II)/sulfoxide catalysis that retards palladium-hydride isomerizations which previously limited the Heck manifold's capacity for furnishing stereodefined conjugated dienes. Limiting quantities of nonactivated terminal olefins (1 equiv) and slight excesses of vinyl boronic esters (1.5 equiv) that feature diverse functionality can be used to furnish complex dienes and polyenes in good yields and excellent selectivities (generally E:Z = >20:1; internal:terminal = >20:1). Because this reaction only requires prior activation of a single vinylic carbon, improvements in efficiency are observed for synthetic sequences relative to ones featuring reactions that require activation of both coupling partners.

Aerobic oxidative Heck/dehydrogenation reactions of cyclohexenones: efficient access to meta-substituted phenols

A new dicationic PdII catalyst, employing a 6,6'-dimethylbipyridine ligand, has been identified that is capable of promoting both aerobic oxidative Heck coupling and dehydrogenation reactions of cyclohexenones. These reactions may be combined in a one-pot sequence to enable the straightforward synthesis of meta -substituted phenols.

Polystyrene-Immobilised Phenanthroline Palladium(II) Complex: A Highly Efficient and Recyclable Catalyst for Oxidative Heck Reactions

Oxidative Heck coupling reactions of arylboronic acids with olefins were carried out in the presence of the polystyrene-immobilised phenanthroline palladium(II) complex using air as a co-oxidant to afford the Heck coupling products in high yields under base-free conditions. This heterogeneous palladium catalyst can be recovered by simple filtration and used for several times without loss of activity or selectivity.

Catalyst-controlled regioselectivity in the synthesis of branched conjugated dienes via aerobic oxidative Heck reactions

Pd-catalyzed aerobic oxidative coupling of vinylboronic acids and electronically unbiased alkyl olefins provides regioselective access to 1,3-disubstituted conjugated dienes. Catalyst-controlled regioselectivity is achieved by using 2,9-dimethylphenanthroline as a ligand. The observed regioselectivity is opposite to that observed from a traditional (nonoxidative) Heck reaction between a vinyl bromide and an alkene. DFT computational studies reveal that steric effects of the 2,9-dimethylphenanthroline ligand promote C-C bond formation at the internal position of the alkene.

Palladium(II)-catalyzed oxidative cyclization of allylic tosylcarbamates: scope, derivatization, and mechanistic aspects

A highly selective oxidative palladium(II)-catalyzed (Wacker-type) cyclization of readily available allylic tosylcarbamates is reported. This operationally simple catalytic reaction furnishes tosyl-protected vinyl-oxazolidinones, common precursors to syn-1,2-amino alcohols, in high yield and excellent diasteroselectivity (>20:1). It is demonstrated that both stoichiometric amounts of benzoquinone (BQ) as well as aerobic reoxidation (molecular oxygen) is suitable for this transformation. The title reaction is shown to proceed through overall trans-amidopalladation of the olefin followed by β-hydride elimination. This process is scalable and the products are suitable for a range of subsequent transformations such as: kinetic resolution (KR) and oxidative Heck-, Wacker-, and metathesis reactions.

Transmetallation versus β-hydride elimination: the role of 1,4-benzoquinone in chelation-controlled arylation reactions with arylboronic acids

The formation of an atypical, saturated, diarylated, Heck/Suzuki, domino product produced under oxidative Heck reaction conditions, employing arylboronic acids and a chelating vinyl ether, has been investigated by DFT calculations. The calculations highlight the crucial role of 1,4-benzoquinone (BQ) in the reaction. In addition to its role as an oxidant of palladium, which is necessary to complete the catalytic cycle, this electron-deficient alkene opens up a low-energy reaction pathway from the post-insertion σ-alkyl complex. The association of BQ lowers the free-energy barrier for transmetallation of the σ-alkyl complex to create a pathway that is energetically lower than the oxidative Heck reaction pathway. Furthermore, the calculations showed that the reaction is made viable by BQ-mediated reductive elimination and leads to the saturated diarylated product.

Sequential allylic C-H amination/vinylic C-H arylation: a strategy for unnatural amino acid synthesis from α-olefins

Tandem reaction sequences that selectively convert multiple C-H bonds of abundant hydrocarbon feedstocks to functionalized materials enable rapid buildup of molecular complexity in an economical way. A tandem C-H amination/vinylic C-H arylation reaction sequence is described under Pd(II)/sulfoxide-catalysis that furnishes a wide range of α- and β-homophenylalanine precursors from commodity α-olefins and readily available aryl boronic acids. General routes to enantiopure amino acid esters and densely functionalized homophenylalanine derivatives are demonstrated.

Palladium-catalyzed oxidative Heck coupling reaction for direct synthesis of 4-arylcoumarins using coumarins and arylboronic acids

An efficient protocol for the direct synthesis of 4-arylcoumarins via palladium-catalyzed oxidative Heck coupling reaction of coumarins and arylboronic acids was developed. 4-Arylcoumarins were obtained in moderate to excellent yields, and the reaction also showed tolerance toward functional groups such as hydro, methoxy, diethylamino, nitro, and chloro groups.

Development of Stereocontrolled Palladium(II)-Catalyzed Domino Heck/Suzuki β,α-Diarylation Reactions with Chelating Vinyl Ethers and Arylboronic Acids

A stereoselective and 1,4-benzoquinone-mediated palladium(II)-catalyzed Heck/Suzuki domino reaction involving metal coordinating cyclic methylamino vinyl ethers and a number of electronically diverse arylboronic acids has been developed and studied. Diastereomeric ratios up to 39:1 and 78 % isolated yields were obtained. The stereoselectivity of the reaction was found to be highly dependent on the nature of the arylboronic acid and the amount of water present in the reaction mixture. Thus, a domino β,α-diarylation-reduction of chelating vinyl ethers can now be accomplished and stereochemically controlled, given that optimized conditions and an appropriate chiral auxiliary are used. To the best of our knowledge, this represents the first example of a stereoselective, oxidative Heck/Suzuki domino reaction in the literature.

Microwave-assisted synthesis of small molecules targeting the infectious diseases tuberculosis, HIV/AIDS, malaria and hepatitis C

The unique properties of microwave in situ heating offer unparalleled opportunities for medicinal chemists to speed up lead optimisation processes in early drug discovery. The technology is ideal for small-scale discovery chemistry because it allows full reaction control, short reaction times, high safety and rapid feedback. To illustrate these advantages, we herein describe applications and approaches in the synthesis of small molecules to combat four of the most prevalent infectious diseases; tuberculosis, HIV/AIDS, malaria and hepatitis C, using dedicated microwave instrumentation.