Nickel(0)-induzierte CC-verknüpfung zwischen kohlendioxid und alkinen sowie alkenen

Photoinduced formation of a platina-α-lactone - a carbon dioxide complex of platinum. Insights from femtosecond mid-infrared spectroscopy

The binding of carbon dioxide to a transition metal is a complex phenomenon that involves a major redistribution of electron density between the metal center and the triatomic ligand. The chemical reduction of the ligand reveals itself unambiguously by an angular distortion of the CO2-molecule as a result of the occupation of an anti-bonding π-orbital and a shift of its antisymmetric stretching vibration, ν3, to lower wavenumbers. Here, we generate a carbon dioxide complex of the heavier group-10 metal, platinum, by ultrafast electronic excitation and cleavage of CO2 from the photolabile oxalate precursor, oxaliplatin, and monitored the ensuing primary dynamics with ultrafast mid-infrared spectroscopy. A neutral and thermally relaxed CO2-molecule is detected in the ν3-region within 60 ps after impulsive excitation with 266 nm light. Concurrently, an induced absorption peaking at 1717 cm-1 is observed, which is distinctly up-shifted relative to the oxalate stretching bands of the precursor and which resembles the CO stretching absorption of organic ketones. Accompanying density functional theory suggests that the 1717 cm-1-absorption arises from a Pt-CO2 product complex featuring a side-on binding mode, which can indeed be regarded as a ketone; specifically, as the metalla-α-lactone, 1-oxa-3-platinacyclopropan-2-one.

Recent developments on the transformation of CO2 utilising ligand cooperation and related strategies

A portfolio of value-added chemicals, fuels and building block compounds can be envisioned from CO₂ on an industrial scale. The high kinetic and thermodynamic stabilities of CO₂, however, present a significant barrier to its utilisation as a C1 source. In this context, metal-ligand cooperation methodologies have emerged as one of the most dominant strategies for the transformation of the CO₂ molecule over the last decade or so. This review focuses on the advancements in CO₂ transformation using these cooperative methodologies. Different and well-studied ligand cooperation methodologies, such as dearomatisation-aromatisation type cooperation, bimetallic cooperation (M⋯M'; M' = main group or transition metal) and other related strategies are also discussed. Furthermore, the cooperative bond activations are subdivided based on the number of atoms connecting the reactive centre in the ligand framework (spacer/linker length) and the transition metal. Several similarities across these seemingly distinct cooperative methodologies are emphasised. Finally, this review brings out the challenges ahead in developing catalytic systems from these CO₂ transformations.

Development of N-Phosphinomethyl-Substituted NHC-Nickel(0) Complexes as Robust Catalysts for Acrylate Salt Synthesis from Ethylene and CO2

By using a nickel complex with an N-phosphinomethyl-N-heterocyclic carbene ligand (NHC-P), the reducing ability and thermal stability of the complex were improved considerably compared to the previously reported bipyridine and bisphosphine complexes, and acrylate salt was prepared from ethylene and CO₂ with the highest TON ever reported for nickel systems even without using metallic zinc. Oxidative cyclization of ethylene and CO₂ on the NHC-P nickel complex was found to proceed very rapidly compared to previous systems.

Advances in the use of CO2 as a renewable feedstock for the synthesis of polymers

Carbon dioxide offers an accessible, cheap and renewable carbon feedstock for synthesis. Current interest in the area of carbon dioxide valorisation aims at new, emerging technologies that are able to provide new opportunities to turn a waste into value. Polymers are among the most widely produced chemicals in the world greatly affecting the quality of life. However, there are growing concerns about the lack of reuse of the majority of the consumer plastics and their after-life disposal resulting in an increasing demand for sustainable alternatives. New monomers and polymers that can address these issues are therefore warranted, and merging polymer synthesis with the recycling of carbon dioxide offers a tangible route to transition towards a circular economy. Here, an overview of the most relevant and recent approaches to CO2-based monomers and polymers are highlighted with particular emphasis on the transformation routes used and their involved manifolds.

How the Coordinated Structures of Ag(I) Catalysts Affect the Outcomes of Carbon Dioxide Incorporation into Propargylic Amine: A DFT Study

Density functional theory calculations have been carried out to explore the detailed mechanisms for carbon dioxide incorporation of N-unsubstituted propargylic amine catalyzed by Ag(I) catalysts. We show that the reaction undergoes substrate adsorption or displacement, isomerization from amine-coordinated species to the alkyne-coordinated species, CO₂ attack, and proton transfer, giving the carbamate intermediate. Subsequently, the reaction would bifurcate at the intermolecular ring-closing step, which produces five-membered ring (5MR) and six-membered ring (6MR) products at the same time, thus raising a regioselectivity issue. Our calculations reveal that the outcomes of the reaction critically depend on the coordination number and the basicity of the ligands. Higher coordinate number and stronger basicity of the ligands would stabilize the 5MR transition state over the 6MR counterpart. Such a preference can be rationalized by using transition state energy decomposition. All of these results could promote the rational design of noble metal/organic base combined catalysts with higher selectivity.

Simple ZnEt2as a catalyst in carbodiimide hydroalkynylation: structural and mechanistic studies

Simple ZnEt₂is an efficient catalyst for the addition of terminal alkynes to carbodiimides, through amidinate complexes, and consecutive isocyanate addition and intramolecular cyclohydroamination.

Synthesis of Carboxylic Acids and Esters from CO2

The achievements in the synthesis of carboxylic acids and esters from CO₂ have been summarized and discussed.

Investigations on the catalytic carboxylation of olefins with CO2 towards α,β-unsaturated carboxylic acid salts: characterization of intermediates and ligands as well as substrate effects

The carboxylation of olefins beyond ethylene towards α,β-unsaturated carboxylic acid salts and a detailed investigation on the critical steps in the catalysis are reported. The influence of two chelating phosphine ligands and several olefins on the elemental steps of the catalysis is shown. The work focusses on the formation of intermediate olefin complexes, lactone formation and base induced elimination of the lactone. The direct carboxylation of olefins is possible using nickel catalysts, which opens a new route towards the desired α,β-unsaturated carboxylic acid salts. The reaction works particularly well for 1,3-dienes and proceeds via the formation of allyl-carboxylates. The ability to form such allyl-type lactone complexes seems in this case to be the most challenging step towards satisfactory turnover numbers.

Nickel-catalyzed double carboxylation of alkynes employing carbon dioxide

The nickel-catalyzed double carboxylation of internal alkynes employing carbon dioxide (CO2) has been developed. The reactions proceed under CO2 (1 atm) at room temperature in the presence of a nickel catalyst, Zn powder as a reducing reagent, and MgBr2 as an indispensable additive. Various internal alkynes could be converted to the corresponding maleic anhydrides in good to high yields. DFT calculations disclosed the indispensable role of MgBr2 in the second CO2 insertion.

Recent advances in carbon dioxide capture, fixation, and activation by using N-heterocyclic carbenes

In the last two decades, CO2 emission has caused a lot of environmental problems. To mitigate the concentration of CO2 in the atmosphere, various strategies have been implemented, one of which is the use of N-heterocyclic carbenes (NHCs) and related complexes to accomplish the capture, fixation, and activation of CO2 effectively. In this review, we summarize CO2 capture, fixation, and activation by utilizing NHCs and related complexes; homogeneous reactions and their reaction mechanisms are discussed. Free NHCs and NHC salts can capture CO2 in both direct and indirect ways to form imidazolium carboxylates, and they can also catalyze the reaction of aromatic aldehydes with CO2 to form carboxylic acids and derivatives. Moreover, associated with transition metals (TMs), NHCs can form NHC-TM complexes to transform CO2 into industrial acid or esters. Non-metal-NHC complexes can also catalyze the reactions of silicon and boron complexes with CO2 . In addition, catalytic cycloaddition of epoxides with CO2 is another effective function of NHC complexes, and NHC ionic liquids perform excellently in this aspect.

Nickelalactones with an allyl subunit – the effect of penta-coordination on structures and stability

Nickelalactone derivatives with an allyl subunit containing different mono- and bidentate ligands were synthesized and characterized. The introduced neutral ligands strongly affect the structures of these versatile molecules and can be used to modulate their stabilities.

Transformation of carbon dioxide with homogeneous transition-metal catalysts: a molecular solution to a global challenge?

A plethora of methods have been developed over the years so that carbon dioxide can be used as a reactant in organic synthesis. Given the abundance of this compound, its utilization in synthetic chemistry, particularly on an industrial scale, is still at a rather low level. In the last 35 years, considerable research has been performed to find catalytic routes to transform CO(2) into carboxylic acids, esters, lactones, and polymers in an economic way. This Review presents an overview of the available homogeneous catalytic routes that use carbon dioxide as a C(1) carbon source for the synthesis of industrial products as well as fine chemicals.

Transition-metal-catalyzed C-C bond formation through the fixation of carbon dioxide

Carbon dioxide is an important carbon source in the atmosphere and is "problematic" toward the activities of human beings. Although carbon dioxide is a cheap, abundant and relatively nontoxic C1 source, its chemical transformations have not been widely developed so far and are still far from synthetic applications, especially in the construction of the C-C bond. This critical review summarizes the recent advances on transition-metal-catalyzed C-C bond formation through the fixation of carbon dioxide and their synthetic applications (124 references).