Free Radicals in Organic Chemistry J. Am. Chem. Soc. 1996, 118, 4226

Fossey, Jacques; Lefort, and Daniel; Sorba, Janine

doi:10.1021/ja9654490

Cited by 32 publications

(54 citation statements)

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“…[8][9][10] Radical addition reactions have been an important area for research in free radical chemistry and in chemical kinetics. The interplay of steric, polar, and substituent effects, as well as reaction enthalpy, affect the rate of addition of a radical to a double bond.…”

Section: Introductionmentioning

confidence: 99%

Electron Spin Resonance and Laser Flash Photolysis Study of Radical Addition to Vinyl Acrylate and Related Alkenes

2001

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Addition of two radicals (diphenyl phosphinoyl and 2-hydroxy-2-propyl) to the bifunctional alkene, vinyl acrylate, was studied by both time-resolved (TR) and steady-state (SS) ESR and laser flash photolysis (LFP). The adduct radicals are predominately a result of tail addition (addition to the unsubstituted carbon atom) of the acrylate double bond. Chemical structures of the adducts were established by comparison of the observed ESR spectra with those of adducts of the same reactive radicals to structurally related alkenes, tert-butyl acrylate and vinyl pivalate, which have only one type of double bond. Adducts of bulky phosphinoyl radicals to the acrylates demonstrate hindered rotation and a cis-trans isomerization at room temperature. The structure of the adduct radicals and the reactivity of the two radicals are discussed. Absolute rate constants for the addition of the phosphinoyl radical to the alkenes were measured by LFP in ethyl acetate at 296 K. A rate constant of k add ) (33 ( 1) × 10 6 M -1 s -1 was found for vinyl acrylate. The latter value is ∼1.5 times higher than that for tert-butyl acrylate (k add ) (22 ( 1) × 10 6 M -1 s -1 ) and ∼17 times higher than that for vinyl pivalate (k add ) (2.0 ( 0.1) × 10 6 M -1 s -1 ). These rate constants are consistent with conclusions derived from the ESR data. The results provide some insights into free radical polymerization of vinyl acrylate and vinyl ethers.

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Section: Introductionmentioning

confidence: 99%

Electron Spin Resonance and Laser Flash Photolysis Study of Radical Addition to Vinyl Acrylate and Related Alkenes

2001

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“…The biphenyl compounds 19, 20 and 21 are products of homolytic intermolecular aromatic substitution reactions between the radicals of benzamides 5, 6 and 9 (generated by tri-n-butyltin radicals) and benzene, the solvent. The mechanisms of Bu 3 SnH-mediated homolytic aromatic substitution presented in the literature 31,33 can be applied to explain the formation of the unexpected biphenyl products. These mechanisms are shown in scheme 2.…”

Section: Resultsmentioning

confidence: 99%

“…The first step proceeds by addition of the initial radical of the benzamides 5, 6 and 9 to the benzene (solvent) to yield the π radical A. 31,33 Disproportionation of A or hydrogen-atom transfer from Bu 3 SnH leads to the intermediate B and its isomers, which can undergo oxidation by oxygen on workup (path a). 31 Whilst oxidation during workup may be plausible, dihydro-type systems, such B, are not oxidized rapidly in air 31 and the yield of the biphenyl compound 21 was higher than 50% (67%), as occur frequently in this kind of reaction.…”

Section: Resultsmentioning

confidence: 99%

“…Hydrogen-atom transfer of π radical A to other radicals present in the reaction mixture, 31,33 mainly 2-cyanoprop-2-yl radical furnished by the radical initiator AIBN, 31 appears to be the predominant reaction to yield biphenyl compounds (path b). 31,33 Addition of reactive aryl radical to the solvent may compete effectively with the desired chain-transfer processes for reactions involving very low concentrations of substrate. 30,31,34 However, to the best of our knowledge, in the literature there are no biphenyl compounds obtained as main product by the Bu 3 SnH method, as described here.…”

Section: Resultsmentioning

confidence: 99%

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Tri-n-butyltin Hydride-Mediated radical reactions of ortho-and meta-Iodobenzamides to synthesize benzomacrolactams: surprising formation of biphenyl compounds from meta-regioisomers

Faraco¹,

Oliveira²,

Pinto³

et al. 2009

J. Braz. Chem. Soc.

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em diferentes condições. Dependendo das condições, os três precursores que contêm o átomo de iodo em posição meta conduziram a uma surpreendente quantidade de compostos bifenílicos, nos quais o átomo de iodo foi substituído por um grupo fenila. O substrato 2-iodobenzamida levou apenas ao produto de hidrogenólise. Nenhum produto ciclizado foi isolado de nenhuma das reações. As estruturas dos compostos bifenílicos inéditos foram elucidadas utilizando-se as espectrometrias de massa, de RMN 1 H e 13 C e experimentos DEPT, COSY, HMQC e HMBC. Foram propostos mecanismos de formação dos compostos bifenílicos e hipóteses para explicar os diferentes comportamentos dos isômeros orto-e metaiodobenzamidas na reação radicalar.Reactions of methyl 4-O-allyl-2,3-di-O-benzyl-6-deoxy-6-(3-iodobenzoylamino)-α-Dgalactopyranoside, its gluco epimer, methyl 2,3-di-O-benzyl-6-deoxy-6-(3-iodobenzoylamino)-4-O-(1-pentenyl)-α-D-glucopyranoside and its ortho-regioisomer with tri-n-butyltin hydride were performed in different conditions. Depending on reaction conditions the three meta-iodo isomers gave a surprising amount of biphenyl compounds. The 2-iodo isomer led only to the undesired but expected hydrogenolysis product. No cyclized products were isolated in all the reactions. The structures of the new biphenyl products were elucidated by 1 H and 13 C NMR spectroscopy, DEPT, COSY, HMQC and HMBC experiments and ESI-MS/MS. Mechanisms for the formation of these new biphenyl derivatives and hypotheses to explain the different outcomes for radical reactions of 3-or 2-iodobenzamides were presented. Keywords: biphenyl compounds, aryl radical cyclization, 3-and 2-iodobenzamides IntroductionIn view of the numerous biological activities of macrolactams [1][2][3][4] and the synthetic challenge they present, 3,5 we have studied the Bu 3 SnH-mediated radical cyclization reactions using unsaturated iodides as precursors of macrolactams, 6-12 mainly allyloxy-ortho-iodobenzamides derived from carbohydrates to give benzomacrolactams. [6][7][8][9][10][11] These precursors furnished benzomacrolactams with 11-, 12-and 20-membered ring by regioselective endo aryl radical carbocyclization. [6][7][8][9] The benzamides methyl 4-Oallyl-2,3-di-O-benzyl-6-deoxy-6-(2-iodobenzoylamino)-α-D-galactopyranoside (1) and its gluco epimer 2 were found to furnish the benzomacrolactams 3 and 4 owing to 11-endo cyclization in 32% and 40% yield, respectively 6,7 ( Figure 1).Considering that (i) we have been stimulated to study the Bu 3 SnH-mediated radical reactions to construct Faraco et al. 1505 Vol. 20, No. 8, 2009 macrocycles, since there is a limited number of protocols available for the synthesis of large rings using free radical-mediated macrocyclizations, 13 (ii) the endomacrocyclization mode is favoured over the exo, [6][7][8][9]12,[14][15][16][17][18][19][20] (iii) the carbohydrate moiety impose conformational restriction to favour cyclization, [6][7][8][9]12 and (iv) although the majority of fused aromatic macrocycles have been obtained from ortho-halogenobenzene...

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“…These results show the preference of the pyridyl radical, in this case, to take part in homolytic aromatic substitution. Finally, on the assumption that: 16 (a) alkenylaryl radicals are systems in which the lack of chain flexibility greatly increases the difference in strain energy between the transition states for exo and endo ring closures, (b) when the 3-position is occupied by a heteroatom (N in the present case), both lengths and angles in the transition state are modified, leading to a variation of the rate of cyclization and regioselectivity (exo/ endo), (c) in the well-studied case of the hexenyl radical, the presence of a substituent such as Me has a different effect on the rate and the regioselectivity depending on its position in the chain, additional experiments were carried out using alkenylheteroarylderivatives 10c and 10d. These products have been obtained from salts 8c and 8d, by N,N-reduction (to supply compounds 9c and 9d) and acetylation, respectively.…”

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confidence: 99%

Pyridinium N-2′-pyridylaminide: radical cyclization in the synthesis of annulated 2-aminopyridines

Sánchez

Núñez

Burgos

et al. 2006

Tetrahedron Letters

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Abstract-The synthesis of annulated 2-aminopyridines by intramolecular radical pyridylation of the appropriate substrates, obtained from pyridinium N-2 0 -pyridylaminide, can be performed using TTMSS and AIBN. Ó 2006 Elsevier Ltd. All rights reserved.Substituted 2-aminopyridines and their annulated derivatives constitute an important class of organic compounds, widely represented in the molecules of pharmacological interest, in both therapeutic 1 and recognition agent fields. 2 In recent years, particular attention has been devoted to the development of synthetic methods that provide an entry into this class of compounds. Thus, 7-azaindoline I and 1,2,3,4-tetrahydro [1,8]naphthyridine II ( Fig. 1) derivatives which have been described as therapeutically important compounds, 3 remain a somewhat inaccessible class of derivatives. Simple 7-azaindoline structures have been prepared by the sluggish hydrogenations of azaindoles. 4 More recently, a free radical-mediated aryl-amination has been reported, whereby an aryl radical adds to the nitrogen of an azomethine bond to supply the required compound. 5 Alternatively, Wijtmans et al. 6 have described a new sequence based on Van der Plas's reaction. 1,2,3,4-Tetrahydro [1,8]naphthyridines are usually prepared by the selective catalytic hydrogenations of the corresponding precursors, either prepared by Skraup, 7 Friedländer 1d,e,8 or Friedel-Crafts 6 approaches. Moreover, Palukcki and co-workers. 9 reported the preparation of 1,2,3,4-tetrahydro [1,8]naphthyridine fragments, using two different methods, one of which relied, again, on variations of Friedländer reaction, 9a the other being based in a double Suzuki-Miyaura reaction and Chichibabin cyclization. 9b,c Therefore, in addition to more specific methodologies, a universal synthetic method, which allows entry into these annulated systems would be highly desirable. Accordingly, Davies et al. 10 recently described how the ortho alkylation of Boc-protected 2-aminopyridines with a,xdihaloalkanes, followed by in situ cyclization, results in the corresponding annulated pyridine derivatives in good yields.On the other hand, Zard and coworkers 11 reported the preparation of a series of compounds containing a 2-aminopyridine nucleus fused to a saturated ring (7-azaoxindole, 7-azaindoline, tetrahydro[1,8]naphthyridine and tetrahydro-5H-pyrido[2,3-b]azepin-8-one), starting from various 2,6-dichloropyridines, through a radical xanthate-mediated cyclization. Although this approach is more general and flexible than previous traditional routes, initial studies starting from the suitable 2-aminopyridines produced unwanted reactions on the cyclic nitrogen, as a result of its higher nucleophilicity. In this context, during the past few years our research group has been interested in the chemistry of pyridinium N-2 0 -pyridylaminide 1a (Scheme 1). Compound 1a is a stable heterocyclic betaine in which the exocyclic nitrogen anion is stabilized by the presence of a pyridinium moiety. 12 Moreover, the cyclic nitrogen is partially block...

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Free Radicals in Organic Chemistry J. Am. Chem. Soc. 1996, 118, 4226

Cited by 32 publications

References 0 publications

Electron Spin Resonance and Laser Flash Photolysis Study of Radical Addition to Vinyl Acrylate and Related Alkenes

Electron Spin Resonance and Laser Flash Photolysis Study of Radical Addition to Vinyl Acrylate and Related Alkenes

Tri-n-butyltin Hydride-Mediated radical reactions of ortho-and meta-Iodobenzamides to synthesize benzomacrolactams: surprising formation of biphenyl compounds from meta-regioisomers

Pyridinium N-2′-pyridylaminide: radical cyclization in the synthesis of annulated 2-aminopyridines

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