Magnetically Recyclable Borane Lewis Acid Catalyst for Hydrosilylation of Imines and Reductive Amination of Carbonyls.

Fluorinated arylborane-based Lewis acid catalysts have shown remarkable activity and serve as ideal examples of transition metal-free catalysts for diverse organic transformations. However, their homogeneous nature poses challenges in terms of recyclability and separation from reaction mixtures. This work presents an efficient technique for the heterogenization of boron Lewis acid catalysts by anchoring Piers' borane to allyl-functionalized iron oxide. This catalyst demonstrates excellent activity in the hydrosilylation of imines and the reductive amination of carbonyls using various silanes as reducing agents under mild reaction conditions. The catalyst exhibits broad tolerance towards a wide range of functional substrates. Furthermore, it exhibits good recyclability and can be easily separated from the products using an external magnetic field. This work represents a significant advance in the development of sustainable heterogenous metal-free catalysts for organic transformations.

Carbonylative Self-Coupling of Aryl Boronic Acids Using a Confined Pd Catalyst within Melamine Dendron and Fibrous Nano-Silica: A CO Surrogate Approach.

Development of heterogeneous catalysts with tunable activity and selectivity has posed a persistent challenge. This study addresses this challenge by fabricating a hybrid environment through the combination of mesoporous silica and N-rich melamine dendron via covalent grafting, allowing for controllable growth and encapsulation of Pd NPs. This catalyst presented an excellent catalytic activity for the oxidative carbonylative self-coupling of aryl boronic acids to afford symmetric biaryl ketones using N-formyl saccharin as a sustainable solid CO source and Cu as a co-catalyst.

Single-Atom Catalysis in Organic Synthesis.

Single-atom catalysts hold the potential to significantly impact the chemical sector, pushing the boundaries of catalysis in new, uncharted directions. These materials, featuring isolated metal species ligated on solid supports, can exist in many coordination environments, all of which have shown important functions in specific transformations. Their emergence has also provided exciting opportunities for mimicking metalloenzymes and bridging the gap between homogeneous and heterogeneous catalysis. This Review outlines the impressive progress made in recent years regarding the use of single-atom catalysts in organic synthesis. We also illustrate potential knowledge gaps in the search for more sustainable, earth-abundant single-atom catalysts for synthetic applications.

Recent Advances in Carbon Dioxide Adsorption, Activation and Hydrogenation to Methanol using Transition Metal Carbides.

The inevitable emission of carbon dioxide (CO2 ) due to the burning of a substantial amount of fossil fuels has led to serious energy and environmental challenges. Metal-based catalytic CO2 transformations into commodity chemicals are a favorable approach in the CO2 mitigation strategy. Among these transformations, selective hydrogenation of CO2 to methanol is the most promising process that not only fulfils the energy demands but also re-balances the carbon cycle. The investigation of CO2 adsorption on the surface of heterogeneous catalyst is highly important because the formation of various intermediates which determines the selectivity of product. Transition metal carbides (TMCs) have received considerable attention in recent years because of their noble metal-like reactivity, ceramic-like properties, high chemical and thermal stability. These features make them excellent catalytic materials for a variety of transformations such as CO2 adsorption and its conversion into value-added chemicals. Herein, the catalytic properties of TMCs are summarize along with synthetic methods, CO2 binding modes, mechanistic studies, effects of dopant on CO2 adsorption, and carbon/metal ratio in the CO2 hydrogenation reaction to methanol using computational as well as experimental studies. Additionally, this Review provides an outline of the challenges and opportunities for the development of potential TMCs in CO2 hydrogenation reactions.

Recent advances in transition metal-free catalytic hydroelementation (E = B, Si, Ge, and Sn) of alkynes.

Catalytic hydroelementation of alkynes mainly with hydroboranes and hydrosilanes gives a straightforward and atom-economical access to a wide range of vinylmetalloids, which are used as synthetically useful and/or reactive species in both synthetic and materials chemistry. Thus far, although numerous transition metal catalysts with well-defined ligand systems have been developed for alkyne hydroelementation, the employed catalysts are mainly based on expensive and potentially toxic metals such as Rh, Pt, and Ir, and their conventional inner-sphere hydride transfer pathways are susceptible to reaction systems, often making it difficult to control the selectivity. In this regard, transition metal-free catalysts for hydroelementation (E = B, Si, etc.) have intensively been reported as an alternative to the conventional metal catalytic regimes over the last decade. In this review, we describe the recent advances in transition metal-free catalytic procedures for alkyne hydroelementation using hydrides based on Si, B, Sn, and Ge with strong emphasis on the variation in the catalytic working mode depending on the intrinsic nature of the reaction systems.

Amine-Functionalized Graphene Oxide-Stabilized Pd Nanoparticles (Pd@APGO): A Novel and Efficient Catalyst for the Suzuki and Carbonylative Suzuki-Miyaura Coupling Reactions.

Palladium nanoparticles (NPs) are decorated on the surface of an amine-functionalized graphene oxide (Pd@APGO) and characterized by using various analytical techniques. In this methodology, the surface of graphene oxide is modified using the amine functional groups which help stabilization and distribution of Pd NPs very well and increases the surface electron density of NPs by electron donating from amine groups. This developed catalyst shows a high catalytic activity toward the Suzuki coupling and carbonylative Suzuki-Miyaura coupling reactions at mild reaction conditions. The amine on the graphene oxide plays a very crucial role to stabilize and increase the electron density of Pd NPs and prevents the leaching of Pd metals. The Pd@APGO catalyst showed excellent catalytic activity (>90%) with a large range of substrates for both of the reactions and provides five recycle runs without the loss of its activity.

Recent Advances Utilized in the Recycling of Homogeneous Catalysis.

Homogeneous catalysts often show high activity and selectivity towards the various chemical transformations. Most of the transition metal-based active catalysts are expensive, rare, and have strict regulations for their use in pharmaceutical products. Hence, there is a requirement to develop suitable technologies for the practical separation and recycling of metal complex catalysts along with the sustainability of the process. This review focuses on the recent techniques used for the catalyst separation, their recovery, and recyclability of the homogeneous form of catalysts based on their economic compatibility and industrial applications. Various homogeneous catalysts have been reviewed on the basis of their support or media, active centres and recyclability aspects of the catalysts. This review gives brief insights into the varied examples of different recycling techniques utilized in the past 6-7 years.

Bifunctional Ionic Liquids Derived from Biorenewable Sources as Sustainable Catalysts for Fixation of Carbon Dioxide.

A series of highly efficient, bifunctional ionic liquids containing a quaternary alkyl ammonium cation and an amine anion were prepared from choline and amino acids, respectively. Nine ILs were synthesized, characterized, and applied as organocatalysts for the chemical fixation of carbon dioxide to form cyclic carbonates and quinazoline-2,4(1 H,3 H)-diones. A binary mixture of an IL and a co-catalysts generates deep eutectic solvents (DESs) and accelerates the rate of the cycloaddition reaction at atmospheric pressure and low temperature (70 °C). The presence of the hydroxyl functional group of choline and the free amine group of the amino acids in the ILs has a synergistic effect on the activation of the epoxide and carbon dioxide towards the cycloaddition reactions. These ILs are biodegradable and are synthesized from easily available biorenewable sources. Additionally, this catalytic method demonstrates ultimate environmental benignity because of the mild metal- and solvent-free conditions as well as the recyclability of the catalyst and co-catalyst.

N-Heterocyclic Olefins as Robust Organocatalyst for the Chemical Conversion of Carbon Dioxide to Value-Added Chemicals.

In this report, the activity of N-heterocyclic olefins (NHOs) as a newly emerging class of organocatalyst is investigated for the chemical fixation of carbon dioxide through reactions with aziridines to form oxazolidinones and the N-formylation of amines with polymethylhydrosiloxane (PMHS) or 9-borabicyclo[3.3.1]nonane (9-BBN) as the reducing agent under mild conditions. The exocyclic carbon atoms of NHOs are highly nucleophilic owing to the electron-donating ability of the two nitrogen atoms. This high nucleophilicity of the NHOs activates CO2 molecules to form zwitterionic NHO-carboxylate (NHO-CO2 ) adducts, which are active in formylation reactions as well as the carboxylation of aziridines to oxazolidinones.

Hybrid Amine-Functionalized Graphene Oxide as a Robust Bifunctional Catalyst for Atmospheric Pressure Fixation of Carbon Dioxide using Cyclic Carbonates.

An environmentally-benign carbocatalyst based on amine-functionalized graphene oxide (AP-GO) was synthesized and characterized. This catalyst shows superior activity for the chemical fixation of CO2 into cyclic carbonates at the atmospheric pressure. The developed carbocatalyst exhibits superior activity owing to its large surface area with abundant hydrogen bonding donor (HBD) capability and the presence of well-defined amine functional groups. The presence of various HBD and amine functional groups on the graphene oxide (GO) surface yields a synergistic effect for the activation of starting materials. Additionally, this catalyst shows high catalytic activity to synthesize carbonates at 70 °C and at 1 MPa CO2 pressure. The developed AP-GO could be easily recovered and used repetitively in up to seven recycle runs with unchanged catalyst activity.