Chain Reactions in Substitution Processes Which Proceed via Radical-Anion Intermediates

Kornblum, Nathan; Michel, Robert E.; Kerber, Robert C.

doi:10.1021/ja00975a062

Cited by 129 publications

(41 citation statements)

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“…In the 60's, an electron transfer radical mechanism for aliphatic nucleophilic substitution was proposed by two authors, Kornblum 81 and Russell, 82 who independently were working on chain reactions via radical anion intermediates. Bunnett extensively studied radical nucleophilic substitution reactions and expanded these studies to aromatic systems.…”

Section: -80mentioning

confidence: 99%

The mechanism of the modified Ullmann reaction

et al. 2010

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The copper-mediated aromatic nucleophilic substitution reactions developed by Fritz Ullmann and Irma Goldberg required stoichiometric amounts of copper and very high reaction temperatures. Recently, it was found that addition of relatively cheap ligands (diamines, aminoalcohols, diketones, diols) made these reactions truly catalytic, with catalyst amounts as low as 1 mol% or even lower. Since these catalysts are homogeneous, it has opened up the possibility to investigate the mechanism of these modified Ullmann reactions. Most authors agree that Cu(I) is the true catalyst even though Cu(0) and Cu(II) catalysts have also shown to be active. It should be noted however that Cu(I) is capable of reversible disproportionation into Cu(0) and Cu(II). In the first step, the nucleophile displaces the halide in the LnCu(I)X complex forming LnCu(I)ZR (Z = O, NR¢, S). Quite a number of mechanisms have been proposed for the actual reaction of this complex with the aryl halide: 1. Oxidative addition of ArX forming a Cu(III) intermediate followed by reductive elimination; 2. Sigma bond metathesis; in this mechanism copper remains in the Cu(II) oxidation state; 3. Single electron transfer (SET) in which a radical anion of the aryl halide is formed (Cu(I)/Cu(II)); 4. Iodine atom transfer (IAT) to give the aryl radical (Cu(I)/Cu(II)); 5. p-complexation of the aryl halide with the Cu(I) complex, which is thought to enable the nucleophilic substitution reaction. Initially, the radical type mechanisms 3 and 4 where discounted based on the fact that radical clock-type experiments with ortho-allyl aryl halides failed to give the cyclised products. However, a recent DFT study by Houk, Buchwald and co-workers shows that the modified Ullmann reaction between aryl iodide and amines or primary alcohols proceeds either via an SET or an IAT mechanism. Van Koten has shown that stalled aminations can be rejuvenated by the addition of Cu(0), which serves to reduce the formed Cu(II) to Cu(I); this also corroborates a Cu(I)/Cu(II) mechanism. Thus the use of radical clock type experiments in these metal catalysed reactions is not reliable. DFT calculations from Hartwig seem to confirm a Cu(I)/Cu(III) type mechanism for the amidation (Goldberg) reaction, although not all possible mechanisms were calculated.

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Section: -80mentioning

confidence: 99%

The mechanism of the modified Ullmann reaction

et al. 2010

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“…Substitution reactions at an sp 3 carbon atom of the reductive alkylating agents, p-nitrobenzyl chloride or 2-halogeno-2-nitropropane, which proceed via a chain multi-stage sequence involving radical anions and free radicals as intermediates were first proposed independently by Kornblum 1 and Russell 2 in 1966. This pathway has been applied in 1970 to rationalize the substitution of unactivated aromatic halides and named S RN 1 by Bunnett.…”

Section: Introductionmentioning

confidence: 99%

“…178.7 °C. 1 -6-bromoimidazo[1,2-a]pyridine (3d). Following the procedure used for 3c, the bromo derivative 3d was obtained as a white-grey solid in 80% yield from sodium benzenesulfinate and 3b.…”

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confidence: 99%

Synthesis of new 6-halogeno-imidazo[1,2-a]pyridines by SRN1 reactions

Castera¹,

Kaafarani²,

Crozet³

et al. 2003

Arkivoc

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New 2-chloromethyl-6-halogeno-imidazo[1,2-a]pyridines and 2-chloromethyl-6-halogeno-3-nitroimidazo[1,2-a]pyridines were prepared and reacted under experimental conditions of S RN 1 reactions with different sulfur and carbon centered nucleophiles to give new 6-halogeno-2-substituted-imidazo[1,2-a]pyridines and 6-halogeno-3-nitro-2-substituted-imidazo[1,2-a] pyridines in good yields. Only the chloromethyl group was found to be reactive under these experimental conditions.

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“…During the past two decades, the majority of the investigations of the aliphatic process have centered around 2-chloro-2-nitropropane and Ernitrobenzylchloride [13,14,15]. The mechanism was originally proposed to explain the C-alkylation of anions of aliphatic nitro compounds with the previously mentioned substrates.…”

Section: Methodsmentioning

confidence: 99%

“…The formation of 2,3-dimethyl-2,3-dinitro-7-octene was accompanied with a slight amount of 2,3-dimethyl-2,3-dinitrobutane which is consistent with previous reported experiments. 15. Preparation of 2-cyclopropy1-3-meth.yl-2.3-dinitro-7-octene…”

Section: (63%)mentioning

confidence: 99%

Free-radical clocks in the aliphatic S[subscript RN]1 process and reactions of nucleophiles with 1,1-dinitro-2,2-diphenylethylene

Dedolph

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Chain Reactions in Substitution Processes Which Proceed via Radical-Anion Intermediates

Cited by 129 publications

References 1 publication

The mechanism of the modified Ullmann reaction

The mechanism of the modified Ullmann reaction

Synthesis of new 6-halogeno-imidazo[1,2-a]pyridines by SRN1 reactions

Free-radical clocks in the aliphatic S[subscript RN]1 process and reactions of nucleophiles with 1,1-dinitro-2,2-diphenylethylene

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