Nitrogen-Enriched Graphene Metal and Metal Oxide Nanoparticles as Innovative Catalysts: New Uses

A new class of catalysts has recently brought the attention of researchers, which is generated by pyrolyzing first transition row metal complexes with nitrogen ligands adsorbed on an inert support, such as carbon, silica. The catalysts have a metal/metal oxide core, surrounded by a few nitrogen-enriched graphene layers (NGr). These materials, which only contain cheap and abundant metals such as iron and cobalt, catalyze reactions for which noble metals are usually required; thus representing a cheaper and more sustainable alternative of the costly noble metals. Until now, such catalysts have been employed mainly in the context of hydrogenation reactions. The objective of this work is to expand the field of applicability of this new class of catalysts. We have used Fe2O3/NGr@C to catalyze olefin cyclopropanation, a reaction for which the use of these catalysts has not previously been investigated. The activity of Fe2O3/NGr@C has been studied by using ethyl diazoacetate and \u3b1-methylstyrene as substrates. Various parameters such as solvents, temperature and time were changed. Fe2O3/NGr@C-catalysts showed best activity in dimethoxyethane at 60 oC, affording high yields of the desired cyclopropanes (mixture of cis and trans isomers) and only 1-2 % of ethyl maleate and fumarate

Synthesis of Carbazoles: Use of Formate Esters as CO Surrogates in the Palladium Catalyzed Reductive Cyclization of 2-Nitrobiphenyls

Palladium complexes with phenanthrolines are so far the most effective catalysts for the reductive cyclization of nitroarenes by carbon monoxide to yield a variety of heterocyclic compounds.[1] Despite the high efficiency and the high atom-economical character of many of these reactions, they have not become of widespread use. This is mainly ascribable to the need for pressurized CO and pressure equipment (including CO safety measures). In the aim of turning this kind of reaction into a as an in situ source of CO. The reaction can be performed in a glass pressure tube, a cheap equipment accessible to every laboratory. Our previous work was mainly focused on the synthesis of indoles by reductive cyclization of o-nitrostyrenes[2] and oxazines by the hetero Diels-Alder condensation of a conjugated diene with a nitrosoarene formed in situ by the reduction of the starting nitroarene.[3] However, the application of the previously developed method to the reductive cyclization of 2-nitrobiphenyls to carbazoles afforded only moderate yields even under harsher conditions and higher catalyst loadings. The result is not totally unexpected since this reductive cyclization is known to be more difficult than the other previously studied. Here we report the results of our investigations on this reaction aimed at both improving the catalytic performance and better understanding the reaction mechanism

Synthesis of Oxazines by Palladium Catalyzed Reductive Cyclization of Nitroarenes and Dienes Using Phenyl Formate as CO Surrogate

The synthesis of hetero-Diels-Alder adducts derived from nitrosoarenes as dienophiles (oxazines) has been the focus of much attention in recent years, since these products have pharmacological activity themselves or can be easily transformed into other products. 1 However, their usual synthesis requires problematic intermediate isolation of nitroso compounds. We have previously reported a method for oxazines synthesis in up to 91% yields in one pot by the reaction of unfunctionalized dienes with nitroarenes and carbon monoxide.2 Despite the high efficiency of this synthetic procedure, it has not become of widespread use by the chemical community. This is mostly because it involves the use of pressurized CO, requiring safety measures that are not available in most synthetic organic laboratories. To overcome this limitation, we started investigating the use of molecules capable of releasing CO in situ, thus avoiding the need for high-pressure equipment and CO lines. Recently, we have reported3 an efficient, convenient and general synthetic procedure to produce nitrogen heterocycles from nitro compounds in presence of a Pd catalyst employing phenyl formate as the CO releasing agent. In this study, we take advantage of this general procedure in the synthesis of oxazines from dienes and nitroarenes (Figure 1). First, due to its high cost, the amount of the diene was optimized down to 1:4 nitroarene to diene ratio. The reaction works well for nitroarenes bearing either electron-donating or electron-withdrawing substituents, a moderate steric hindrance on the nitroarene is well tolerated and yields up to 99% in one pot were reached. In addition, variation in the diene were also investigated using 2,3-dimethoxy-1,3-butadiene, isoprene and myrcene affording the corresponding oxazines in good yields

Palladium Catalyzed Reductive Cyclization of Nitrobiphenyls Using Formate Esters as CO Surrogates

Palladium complexes with phenanthroline ligands are so far the most effective catalysts for the reductive cyclization of nitroarenes by carbon monoxide to yield a variety of heterocyclic compounds.[1] Despite the high efficiency and the high atom-economical character of many of these reactions, they have not become of widespread use. This is mainly attributed to the need for pressurized CO and pressure equipment (requiring CO safety measures). In the aim of turning this kind of reaction into a \u201cgeneral tool\u201d for the synthetic chemist, we developed a procedure based on the use of phenyl formate as an in situ source of CO. The reaction can be performed in a glass pressure tube, a cheap equipment accessible to every laboratory. Our previous work was mainly focused on the synthesis of indoles by reductive cyclization of o-nitrostyrenes[2] and oxazines by the hetero Diels-Alder condensation of a conjugated diene with a nitrosoarene formed in situ by the reduction of the starting nitroarene.[3] However, the application of the previously developed method to the reductive cyclization of 2-nitrobiphenyls to carbazoles (Scheme 1) afforded only moderate yields even under harsher conditions and higher catalyst loadings. The result is not totally unexpected since this reductive cyclization is known to be more difficult than the other previously studied. Here we report the results of our investigations on this reaction aimed at both improving the catalytic performance and better understanding the reaction mechanism

Effect of Iron Cocatalysts on the Palladium-Catalyzed Oxidative Carbonylation of Aniline

Isocyanates are key intermediates for the chemical industry, but their production is actually based on the use of toxic phosgene (path A in Figure 1). Among suitable alternatives, the reductive carbonylation of nitroarenes (Path B) and the oxidative carbonylation of amines (Path C) are the most investigated. Research in our group mostly focused on the reductive carbonylation of nitroarenes, but the connections we could identify between this reaction and the oxidative carbonylation of amines1 recently pushed us to investigate the latter reaction. The most active catalytic system reported to date is based on palladium catalysts with iodide as promoter.2 We decided to investigate this system in more detail and found some surprising results. The most important new findings are: 1) When the reaction is performed directly in a steel autoclave, the reported data are fully reproducible. However, if the reaction is performed in a glass liner or a Teflon lined autoclave, the activity of the catalytic system drops to about 1/3 of the original one or lesser. 2) One essential role of iodide is to etch some iron from the autoclave walls. The high activity can be restore even in a Teflon coated autoclave if small amounts of iron salts are added. 3) Iodide and iron have complex interactions and can even deactivate each other when their molar ratio falls within a certain range. 4) If iron is present, iodide is no longer necessary, but a halide (chloride) is anyway required. 5) The solvent is not completely innocent in the reaction. These and other trends will be discussed

Nitrogen-Enriched Graphene Iron Oxide Nanoparticles as Innovative Catalysts: First Application to Cyclopropanation Reactions

A new class of catalysts having a metal/metal oxide core surrounded by a few nitrogen-enriched graphene layers (NGR) has recently brought immense attention in research. Until now, NGR catalysts have mostly been employed for hydrogenation reactions. In this work, we expand the field of applicability of NGR catalysts to cyclopropanation reactions. The activity of Fe2O3/NGr@C has been studied by using ethyl diazoacetate and \u3b1-methylstyrene as substrates. Various parameters such as solvents, temperature and time were changed. Fe2O3/NGr@C-catalysts showed best activity in dimethoxyethane at 60 oC, affording high yields of the desired cyclopropanes (mixture of cis and trans isomers) and only 1-2 % of ethyl maleate and fumarate (Figure 1). The catalyst gradually deactivates after each recycle, but we were able to reactivate the recovered catalyst by treating it with dilute H2O2 (1:10 with distilled water). Like \u3b1-methylstyrene, several olefins such as 4-chloro-\u3b1-methyl styrene, 4-methylstyrene, 2- methylstyrene, 3-methylstyrene, 4-chlorostyrene, 4-t-butylstyrene, 1-octene etc. have been tested in order to explore the substrate scope. The corresponding cyclopropanes were obtained in high to excellent isolated yields (84-98%). In all cases trans diastereoselectivity was found, but even the minor cis isomer could be isolated in a pure form