Nitrogen ligands effects in the palladium-catalyzed carbonylation reaction of nitrobenzene to give N-methyl phenylcarbamate

We previously reported an enhancement in the catalytic activity of a palladium-phenanthroline system towards the carbonylation of nitroarenes to N-methyl arylcarbamates when non-symmetric phenanthrolines were used as ligands. In particular, best results were obtained when an electron donating group was present on just one of the pyridinic rings of the ligand. Here we report the effect of stronger donor groups on the catalytic activity and on the selectivity of the system, even in the presence of different phosphorus acids used as promoters. High catalytic activities were reached but the high basicity of the ligands can negatively affect the product selectivity. The presence of steric hindrance near one or both the nitrogen donor atoms was shown to be detrimental for the activity of the system both with phenanthroline ligands and a quinoline-guanidine hybrid ligand

Transition Metal Catalyzed Reductive Cyclization Reactions of Nitroarenes and Nitroalkenes

Nitroarenes are the entry point for the production of most nitrogen-containing aromatic compounds. Thus, any transformation that leads directly from them to the final product allows saving one or more synthetic steps. This review deals with homogeneously catalyzed reactions leading to the formation of N-heterocyclic compounds from nitroarenes or nitroalkenes in one pot. Reactions that lead to the intermediate formation of amines are not considered. Carbon monoxide is the most often employed reductant because it allows selective reactions, is cheap, and only produces CO2 as stoichiometric byproduct. However, the difficulty in handling pressurized CO has stimulated in recent years the development of CO-surrogates, that is molecules able to liberate CO during the reaction. The use of phosphines and diols has also been developed in conjunction with molybdenum catalysts. The review focusses in more detail on the literature in the period 2006\u20132018, but reference to earlier work is made when necessary to put recent results in a more general context

Palladium-Catalyzed Intramolecular Cyclization of Nitroalkenes : Synthesis of Thienopyrroles

In the presence of carbon monoxide, the palladium/phenanthroline system catalyzes the intramolecular amination of thiophene rings following the reduction of a nitroalkene moiety directly attached to the S-heterocyclic ring. Optimization of the ligand and reaction conditions allowed the synthesis of a series of thienopyrroles aryl/alkyl-substituted at either the 2- or 3-position of the pyrrole ring. By using low pressures of carbon monoxide (5 bars), high yields of fused bicyclic compounds have been obtained (up to 98 % yield)

Use of amido Grignard reagents in inorganic chemistry : Synthesis and crystal structure of anti-[Pd(Cl)(py)(mu-2,6-Pri2C6H3NH)](2)

Treating a pyridine (py) solution of PdCl2(py)(2) with a tetrahydrofuran or diethyl ether solution of the amido Grignard reagent 2,6-(Pr2C6H3NH)-C-1(MgCl) afforded a dimeric palladium complex, containing two bridging amido groups, which has been structurally characterised

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

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

Synthesis of 3,6-Dihydro-2H-[1, 2]-Oxazines from Nitroarenes and Conjugated Dienes, Catalyzed by Palladium/Phenanthroline Complexes and Employing Phenyl Formate as a CO Surrogate

Palladium/phenanthroline catalyzed reduction of nitroarenes by in situ-generated carbon monoxide, from the decomposition of phenyl formate, affords the corresponding nitrosoarenes. The latter are trapped by conjugated dienes to give the corresponding 3,6-dihydro-2H-[1, 2]-oxazines (hetero Diels-Alder adducts). Many functional groups are well tolerated. Yields are higher than those obtainable by any previously reported method, including the direct reaction of the diene with the pure nitrosoarene. The reaction can be performed in a single standard glass pressure tube, without the need for autoclaves or high-pressure CO lines

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