Alkene radical arylations

Oxoesters are oxidized with Mn(OAc)3 to the corresponding radicals that can add intermolecularly or intramolecularly (eq Ib)" to generate alkyl radicals. In the presence of Cu(OAc)2 the latter are rapidly quenched and oxidized to give alkenes. Radical arylation with alkyl iodides can be induced with dibenzoyl peroxide the yield of the reaction can be improved using a catalytic amount of Cu(0Ac)2-H20, which minimizes hydrogen abstraction by the intermediate radical but introduces a competitive electron-transfer oxidation of the intermediate radical. The oxidative addition of disulfides to alkenes (Trost hydroxysulfenylation ) can be promoted by catalytic amounts ofCu(OAc)2. ... 

In designing multicomponent coupling reactions, the nature of the individual components is obviously a key factor. Generally speaking, carbon radical species, such as alkyl radicals, aryl radicals, vinyl radicals, and acyl radicals are all classified as nucleophilic radicals, which exhibit high reactivity toward electron-deficient alkenes [2]. To give readers some ideas about this, kinetic results on the addition of tert-butyl and pivaloyl radicals are shown in Scheme 6.2. These radicals add to acrylonitrile with rate constants of 2.4 x 106 M-1 s 1 and 5 x 105 M-1 s-1 at... 

Lipshutz and colleagues presented recently palladium-catalyzed direct coupling reactions of alkyl iodides and vinyl bromides or iodides catalyzed by 1 mol% Pd(amphos)Cl2 in the presence of zinc and TMEDA in a biphasic aqueous/poly-(ethylene glycol tocopheryl sebacate) reaction medium [198], Internal olefins were obtained in 51-95% yield. For aryl-substituted (Aj-vinyl bromides, retention of double bond geometry was observed, while different degrees of isomerization occurred for (Z)-isomers, which may indicate the intervention of a radical addition process in the course of the coupling process. Alkyl-substituted (Z)-vinyl halides were transformed in contrast with retention of alkene geometry. Aryl halides also reacted [199],... 

This general mechanistic scheme readily explains a number of experimental observations. For instance it is very clear why such ester shifts only ever take place between vicinal carbons [1], as it is only this arrangement that permits the formation of an alkene radical cation as intermediate. Intermolecular ester shifts are excluded for the same reason. Rearrangements of o-(acyloxy)aryl radicals (Scheme 7) [13, 14] and their vinyl counterparts would require the intermediacy of very high energy benzyne radical cations, as such no examples are known. Failed migrations between two secondary radicals (Scheme 8) may now be seen as being due not so... 

As noted above, most kinetic studies of alkene radical cations in solution have focused on aryl-substituted systems since they have convenient optical propenies and have been extensively studied by other techniques. Radical cations are frequently identified on the basis of their characteristic UV-visible absorptions and the comparison of their spectra to those obtained in matrices at low temperature." However, a number of other diagnostic tests are also commonly employed to identify these intermediates. For example, their kinetic behavior as a function of solvent nucleophilicity or added nucleophiles is analogous to that of other electrophilic species. Thus, reaction with nucleophiles such as azide and halide ions provides support for the assignment of a transient to a radical cation, although it will not serve to eliminate a carbocation intermediate. More useful in the latter respect is the method of generation of the transient since PET does not in general lead to the formation of carbocations. Quenching of the observed transient with a more easily oxidized... 

The photoinduced anti-Markovnikov addition of methanol to 1,1-diphenylethene reported by Arnold and co-workers in 1973 provides the first example of the addition of a nucleophile to an arylalkene radical cation. There are now a number of studies that demonstrate the generality of nucleophilic addition of alcohols, amines, and anions such as cyanide to aryl- and diaryl-alkene radical cations. Product studies and mechanistic work have established that addition occurs at the 3-position of I-aryl or 1,1 -diarylalkene radical cations to give arylmethyl or diaryl-methyl radical-derived products as shown in Scheme I for the addition of methanol to 1,1-diphenylethene. For neutral nucleophiles, such as alcohols and amines, radical formation requires prior deprotonation of the 1,3-distonic radical cation formed in the initial addition reaction. The final product usually results from reduction of the radical by the sensitizer radical anion to give an anion that is then protonated, although other radical... 

The following three sections discuss recent time-resolved experiments on inter- and intramolecular cycloadditions of aryl-alkene radical cations. These studies address some of the mechanistic issues raised by the earlier studies and also provide kinetic data for the cycloadditions of a number of aryl and diaryl-alkene radical cations. Such kinetic data are essential for the development of this chemistry as a useful synthetic strategy and as a mechanistic probe for radical cation chemistry. 

The types of compounds that can be polymerized readily by the radical-chain mechanism are the same types that easily undergo free-radical addition reactions. Alkenes with aryl, ester, nitrile, or halide substituent groups that can stabilize the intermediate radical are most susceptible to radical polymerization. Terminal alkenes are generally more reactive toward radical-chain polymerization than more highly substituted isomers. The dominant mode of addition in radical-chain polymerization is head-to-tail. The reason for this orientation is that each successive addition of monomer takes place in such a way that the most stable possible radical intermediate is formed. For example, the addition to styrene occurs to give the phenyl-substituted radical to acrylonitrile, to give the cyano-substituted radical ... 

The arylation of alkenes with aryl iodines/brotnines over a radical pathway is mediated by bathophenanthroline (30 mol %) and K(yBu in benzene for 36 h (eq 10). The yield of different substituents R ranges from 44% to 90%. Only trace amounts of biaryl are found. Different getninal aryl substituents induce a mixture of two stereoisomers in moderate yield, which could not be separated. ... 

Another three components synthesis is involved in the extensively investigated NOCAS process (nucleophile olefin combination aromatic substitution, path h) [89,90], In this case, a nucleophile adds to an alkene radical cation and again the interaction between the resulting radical and the radical anion of the aromatic nitrile may follow two paths. The first is electron transfer, which results in sensitized anti-Markovnikov addition onto the alkene, and is favored with stabilized, reducible radicals such as the benzyl radicals obtained from aryl olefins. The latter one is... 

Other limitations of the reaction are related to the regioselectivity of the aryl radical addition to double bond, which is mainly determined by steric and radical delocalization effects. Thus, methyl vinyl ketone gives the best results, and lower yields are observed when bulky substituents are present in the e-position of the alkene. However, the method represents complete positional selectivity because only the g-adduct radicals give reductive arylation products whereas the a-adduct radicals add to diazonium salts, because of the different nucleophilic character of the alkyl radical adduct. ... 

AA sec acrylic acid abstraction sec hydrogen atom transfer abstraction v,v addition and micleophilicity 35 by aikoxy radicals 34-5, 124-5, 392 by alkoxycarbonyloxy radicals 103,127-8 by alkyl radicals 34 5, 113, 116 by f-amyloxy radicals 124 by arenethiyl radicals 132 by aryl radicals 35, 118 by benzovloxy radicals 35, 53, 120, 126 wilh MM a" 53, 120 by /-butovy radicals 35, 53, 55, 124 solvent effects 54, 55. 123 with alkenes 122 3 with ally I acrylates 122 wilh AMS 120, 123 wilh BMA 53, 123 with isopropenvl acetate 121 with MA 120 with MAN 121 with MMA 53, 55, 120.419 with VAc 121 with vinyl ethers 123... 

In addition, Suehiro et al. (1987) were able to accurately determine the rates of dediazoniation of twenty substituted aryldiazenyl radicals formed from aryl(aryl-thio)diazenes (Scheme 8-36), in cyclopropane and in alkenic solvents at -48 to — 117°C, using a time-resolved ESR method. 

Doubts against a radical mechanism were based mainly on the observation that no polymers of the alkene used for the arylation were found for a long time. It was not until the 1970s that Kopylova et al. (1971, 1972) and Ganushchak et al. (1972) obtained telomers with the general formula Ar(CH2CHZ)nCl in low yields under conditions of high vinyl monomer concentration. 

Kochi (1956a, 1956b) and Dickerman et al. (1958, 1959) studied the kinetics of the Meerwein reaction of arenediazonium salts with acrylonitrile, styrene, and other alkenes, based on initial studies on the Sandmeyer reaction. The reactions were found to be first-order in diazonium ion and in cuprous ion. The relative rates of the addition to four alkenes (acrylonitrile, styrene, methyl acrylate, and methyl methacrylate) vary by a factor of only 1.55 (Dickerman et al., 1959). This result indicates that the aryl radical has a low selectivity. The kinetic data are consistent with the mechanism of Schemes 10-52 to 10-56, 10-58 and 10-59. This mechanism was strongly corroborated by Galli s work on the Sandmeyer reaction more than twenty years later (1981-89). 

Arenesulphinylation 264 Arenesulphinylethanes, synthesis of 267 2-Arenesulphonyl-3-aryloxaziridines, as oxidizing agents 254 Arenesulphonyl azides, reactions of 641 At-Arenesulphonyliminopyridinium betaines, mass spectra of 160-162 Arenesulphonyl radicals 215, 1091 Aryldiazonium salts, reactions of 280, 281 Aryl(methylsulphinyl)(methylthio)alkenes, synthesis of 614... 

In these reactions a new carbon-carbon bond is formed, and they may be given the collective title coupling reactions. In each case, an aUcyl or aryl radical is generated and then combines with another radical (a termination process) or attacks an aromatic ring or alkene to give the coupling product. ... 

Free-radical addition of an aryl group and a hydrogen has been achieved by treatment of activated alkenes with a diazonium salt and TiCls. ... 

The compound ArX can be added across double bonds, in a free-radical process, by treatment of alkenes with diazonium salts, although Meerwein arylation (substi-... 

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