Palladium‐Catalyzed Formation of <i>N</i>‐Heteroarenes from Nitroarenes using Molybdenum Hexacarbonyl as the Source of Carbon Monoxide

Zhou, Fei; Wang, Duo‐Sheng; Driver, Tom G.

doi:10.1002/adsc.201500700

Cited by 59 publications

(34 citation statements)

References 92 publications

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“…As a comparison, if the previously investigated protocol based on n ‐butyl formate was applied, lower yields were achieved (see compounds 2 a , 2 h , 2 n , 2 q , and 2 t ), which thus confirmed the effectiveness of phenyl formate as a CO source . Notably, in several cases the achieved yields were higher than those previously reported in the literature by using pressurized CO. For example, the best yields obtained in the cases of compounds 2 a , 2 h , 2 e , and 2 q were 83, 63, 87, and 55 %, respectively. This fact clearly demonstrates that our protocol is a virtuous alternative to those based on the use of pressurized CO.…”

Section: Methodsmentioning

confidence: 98%

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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Section: Methodsmentioning

confidence: 98%

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

2017

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“…[22] Thesecond role of palladacycle 7 is to trigger the CÀHb ond activation of the substrate (1a)t op roduce biscyclometallated 10. Alternatively,t he amide could be formed from the reaction of palladacycle 10 with arylisocyanate, [4,5] which could be formed from the [Mo(CO) 6 ]-mediated reduction of the nitroarene, [17] or from the reaction of aniline or Nhydroxy-aniline with acyl-Pd species 11.T he lack of amide formation when isocyanate,a niline,o rN-hydroxyaniline are used as the nitrogen source,h owever, strongly suggests that bond formation does not occur through these two potential pathways. [24] Reduction of the NÀObond in 14 by molybdenum or palladium then produces the product amide.This reduction could occur before or after dissociation of the palladacycle catalyst.…”

Section: Angewandte Chemiementioning

confidence: 99%

“…We have shown that Pd-catalyzed intermolecular C À H bond aminocarbonylation of 2-aryl-or 2-heteroarylpyridines can be achieved using nitroarenes as the nitrogen source and [Mo(CO) 6 ]a st he carbonyl source.O ur mechanistic studies revealed that the active catalyst is apalladacycle,which serves to both reduce the nitroarene to an itrosoarene as well as activate the aryl C À Hb ond. Our future studies will build on these results by exploring the reactivity of palladacycles towards nitroarenes to trigger intermolecular CÀNb ondforming reactions to rapidly and efficiently build complexity of simple heteroarenes.…”

Section: Angewandte Chemiementioning

confidence: 99%

“…[1] Although significant progress has been achieved, [2] the development of transition metal-catalyzed processes that transform CÀHbonds into amides has lagged and often requires the use of isocyanates or other reactive reagents to effect bond construction. During the course of our investigations into the reactivity of nitroarenes towards [Mo(CO) 6 ]a nd palladium, [6] we were curious if we could achieve intermolecular CÀHb ond functionalization reactions,areactivity pattern of electrophilic metal N-aryl catalytic intermediates,which had to date eluded our studies into aryl azides (Scheme 1). During the course of our investigations into the reactivity of nitroarenes towards [Mo(CO) 6 ]a nd palladium, [6] we were curious if we could achieve intermolecular CÀHb ond functionalization reactions,areactivity pattern of electrophilic metal N-aryl catalytic intermediates,which had to date eluded our studies into aryl azides (Scheme 1).…”

mentioning

confidence: 99%

“…[c] [Pd(OAc) 2 ]2 0C O1 .5 atm 1.5 n.r. Thei dentity of the reductant was also critical to the success of this reaction:n or eaction was observed using 20 mol %o f[ Mo(CO) 6 ]a nd CO,o ri f alternative sources of CO were used in place of [Mo(CO) 6 ] (entries 5-8). Changing the identity of the palladium counterion had ad eleterious effect on the success of the reaction (entries 3-4).…”

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confidence: 99%

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Nitroarenes as the Nitrogen Source in Intermolecular Palladium‐Catalyzed Aryl C–H Bond Aminocarbonylation Reactions

et al. 2017

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A three-component palladium-catalyzed aminocarbonylation of aryl and heteroaryl sp C-H bonds using nitroarenes as the nitrogen source was achieved using Mo(CO) as the reductant and origin of the CO. This intermolecular C-H bond functionalization does not requires any exogenous ligand to be added, and our mechanism experiments indicate that the palladacycle catalyst serves two roles in the aminocarbonylation reaction: reduce the nitroarene to a nitrosoarene and activate the sp C-H bond.

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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 Formation of N‐Heteroarenes from Nitroarenes using Molybdenum Hexacarbonyl as the Source of Carbon Monoxide

Cited by 59 publications

References 92 publications

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

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

Nitroarenes as the Nitrogen Source in Intermolecular Palladium‐Catalyzed Aryl C–H Bond Aminocarbonylation Reactions

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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