Palladium‐Catalyzed Intramolecular Cyclization of Nitroalkenes: Synthesis of Thienopyrroles

El‐Atawy, Mohamed A.; Ferretti, Francesco; Ragaini, Fabio

doi:10.1002/ejoc.201700165

Cited by 31 publications

(33 citation statements)

References 50 publications

(33 reference statements)

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“…Many different phenanthrolines were tested and best results were obtained with the strongly electrondonating 4,7‐dimethoxyphenanthroline (Scheme . See also paragraph 2.4 and Scheme ), in accord with the idea that a nitro group connected to the electrondonating thiophene ring requires a more strongly donating ligand on the metal to be activated …”

Section: Synthesis Of N‐heterocycles By Intra‐molecular Cyclization Rmentioning

confidence: 78%

“…Among the most useful ligands, 3,4,7,8‐tetramethyl‐phenanthroline has been employed in nitroarene cyclization reactions for many years . More recently, 4,7‐dimethoxy‐phenanthroline was found to give even better results . Phenanthrolines are easily displaced from palladium at high temperature and an excess of the ligands (typically from 2 fold to 16 fold) is always necessary to keep the catalytic system stable.…”

Section: General Trends In the Reduction Of Nitroarenesmentioning

confidence: 99%

“…[46] More recently, 4,7-dimethoxy-phenanthroline was found to give even better results. [23,47] Phenanthrolines are easily displaced from palladium at high temperature and an excess of the ligands (typically from 2 fold to 16 fold) is always necessary to keep the Scheme 1. Typical ligands employed in transition metal-catalyzed reductive cyclization reactions of nitroarenes by CO. catalytic system stable.…”

Section: Ligandsmentioning

confidence: 99%

“…[48][49] This appears to be due to the enhanced hemilability of the non-symmetric ligands, which is important in the carbonylation reaction. [50][51] However, no improvement was observed by using non-symmetric phenanthroline when they were employed as ligands in a reductive cyclization reaction, [47] indicating that hemilability is not important for these reactions.…”

Section: Ligandsmentioning

confidence: 99%

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Transition Metal Catalyzed Reductive Cyclization Reactions of Nitroarenes and Nitroalkenes

2019

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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 2006–2018, but reference to earlier work is made when necessary to put recent results in a more general context.

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Section: Synthesis Of N‐heterocycles By Intra‐molecular Cyclization Rmentioning

confidence: 78%

Section: General Trends In the Reduction Of Nitroarenesmentioning

confidence: 99%

Section: Ligandsmentioning

confidence: 99%

Section: Ligandsmentioning

confidence: 99%

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Transition Metal Catalyzed Reductive Cyclization Reactions of Nitroarenes and Nitroalkenes

2019

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“…Ru 3 (CO) 12 and [Pd(Phen) 2 ][BF 4 ] 2 (Phen=1,10‐phenanthroline) were chosen as catalysts for preliminary studies. Indeed, Ru 3 (CO) 12 was extensively employed in decarbonylation reactions of alkyl formates, whereas [Pd(Phen) 2 ][BF 4 ] 2 was shown to be active in the reductive cyclization of aromatic nitro compounds with gaseous CO as the reductant ,,. The results are summarized in Table S1 (Supporting Information).…”

Section: Methodsmentioning

confidence: 99%

Synthesis of N‐Heterocycles by Reductive Cyclization of Nitro Compounds using Formate Esters as Carbon Monoxide Surrogates

2017

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As eries of alkyl and aryl formate esters were evaluated as CO sourcesi nt he Pd-and Pd/Ru-catalyzed reductivec yclization of substituted nitro compounds to afford heterocycles, especially indoles.P henyl formate was found to be the most effective, and it allowed the desired products to be obtained in excellent yields, often highert han those previously reported with pressurized CO. Through detailed experimentsa nd kinetics tudies, we were able to discern between metal-catalyzed and basemediated activation of phenyl formate and confirmed that just the base waseffective in catalyzing its decarbonylation.The catalytic reductivec arbonylation of aromatic nitro compounds is an efficient method for the preparation of al arge variety of bulk [1] and fine [2] chemicals. In particular,d uring the years, ap lethora of nitrogen-containing heterocyclesh ave been prepared from suitably ortho-substituted nitroarenes by using CO as as toichiometricr eductant (Scheme 1) and examples of intermolecularr eactions and of reactions employing bnitrostyrenes and relatedm olecules as reagents are also known. [3] Despite the high efficiency of several of these reactions, their use has not spread outside the limited number of groups that reported them. This is likely mostly due to the fact that they generally involve the use of pressurized CO, which requires safetym easures that are not availablei nm ost synthetic organic laboratories. To overcome this limitation andt o allow more researchers to use this chemistry, several years ago we startedi nvestigating the use of molecules capable of releasing CO in situ, thus avoidingt he need for high-pressure equipmenta nd CO lines. Among several suitabler eagents, [4] we chose to employ formate esters because of their commercial availability,l ow cost, and low toxicity and selected the Pdcatalyzed synthesis of indoles from o-nitrostyrenes (Scheme 1, upper left) as the first reaction to test. We also chose to operate in as ingle glass pressure tube because this equipment is cheap and availablei nd ifferents izes. As teel autoclave can also be used, but without the need forp ressurized CO. Some of the aforementioned CO-releasing reagents require the use of two-chamber reactors to separatet he CO-releasing reaction from the carbonylation one. When we startedt his study,n oa lternative hadb een reported to the use of gaseous CO in nitroarene cyclization reactions. Very recently,Driver and co-workers reportedt he use of Mo(CO) 6 to this aim. [5] However, the toxicity of this CO source is remarkable and could create complications if used on al arger scale.Owing to its very low cost, our initial investigation was devoted to proving if n-butyl formate could be as uccessful CO surrogate in the reductive cyclization of methyl (E)-2-nitrocinnamate (1a).Ru 3 (CO) 12 and [Pd(Phen) 2 ][BF 4 ] 2 (Phen = 1,10-phenanthroline) were chosen as catalysts for preliminary studies. Indeed, Ru 3 (CO) 12 was extensively employed in decarbonylation reactions of alkyl formates, [6] whereas [Pd(Phen) 2 ][BF 4

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Reductive Cyclization of 2‐Nitro‐ and β‐Nitrostyrenes, 2‐Nitrobiphenyls, and 1‐Nitro‐1,3‐Dienes to Indoles, Carbazoles, and Pyrroles

Söderberg

Berkowitz

2022

Organic Reactions

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This chapter presents a comprehensive review of reductive cyclizations of 2‐nitro‐ and β‐nitrostyrenes, 2‐nitrobiphenyls, and 1‐nitro‐1,3‐dienes to furnish indoles, carbazoles, and pyrroles, respectively, as well as heteroatom analogs thereof. Two variations of the reductive cyclization are discussed: the Cadogan–Sundberg reaction, which is mediated by ternary phosphorus compounds, and the Watanabe–Cenini–Söderberg reaction, which employs a palladium catalyst in the presence of carbon monoxide. A few closely related reductive cyclizations are also presented, as are comparisons with other cyclizations that form the nitrogen–carbon bond of indoles and derivatives.

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Palladium‐Catalyzed Intramolecular Cyclization of Nitroalkenes: Synthesis of Thienopyrroles

Cited by 31 publications

References 50 publications

Transition Metal Catalyzed Reductive Cyclization Reactions of Nitroarenes and Nitroalkenes

Transition Metal Catalyzed Reductive Cyclization Reactions of Nitroarenes and Nitroalkenes

Synthesis of N‐Heterocycles by Reductive Cyclization of Nitro Compounds using Formate Esters as Carbon Monoxide Surrogates

Reductive Cyclization of 2‐Nitro‐ and β‐Nitrostyrenes, 2‐Nitrobiphenyls, and 1‐Nitro‐1,3‐Dienes to Indoles, Carbazoles, and Pyrroles

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