Radicals tert-butyl radical

Of the two extremes experimental studies indicate that the planar sp model describes the bonding m alkyl radicals better than the pyramidal sp model Methyl rad ical IS planar and more highly substituted radicals such as tert butyl radical are flattened pyramids closer m shape to that expected for sp hybridized carbon than for sp ... 

The mechanism of side-chain oxidation is complex and involves reaction of C-J-l bonds at the position next to the aromatic ring to form intermediate ben-zylic radicals, tert- Butyl benzene has no benzylic hydrogens, however, and is therefore inert. 

It was shown that no rearrangement of isobutyl radical to tert-butyl radical (which would involve the formation of a more stable radical by a hydrogen shift) took place during the chlorination of isobutane. 

Comparison of the tert-butyl radical (a 3° radical) and the isobutyl radical (a 1° radical) relative to isobutene ... 

Radical cyclization of acyclic sulfinamides 239 provides easy access to cyclic sulfinamides 241 <06AG(E)633>. Conceivably, the reaction pathway involves thiophilic attack by the aryl radical with a concomitant or successive expulsion of the p-tolyl or tert-butyl radical. 

Few kinetic studies of reactions of alkyl radicals with tin hydrides other than Bu3SnH have been reported. Studies of the reactions of the tert-butyl radical with Me3SnH and Ph3SnH were performed by the rotating sector method,80 but an error in absolute values exists in that method as judged by differences in rate constants for reactions of Bu3SnH with alkyl radicals... 

Stevenson, D.P. Ionization and Dissociation by Electronic Impact. Ionization Potentials and Energies of Formation of Sec-Propyl and Tert-Butyl Radicals. Some Limitations on the Method. Discuss. Faraday Soc. 1951,10,35-45. 

As per this scheme, the tert-butyl radical adds to the initial nitroso trap producing the nitroxyl radical. A triplet in ESR spectra with = 1.5-1.7 mT (depending on the solvent) corresponds to the radical. The triplet is often so intense that it can lap over signals from other spin adducts (Forshult et al. 1969). 

The main chemistry generates some products (HCrO ) reminiscent of the Craq002+/CH3CH0 reaction, but there is clearly another path, responsible for the formation of C02 and isobutene. The latter was formed in comparable yields in air- and 02-saturated solutions, which rules out the disproportionation of tert-butyl radicals (formed by decarbonylation) as the source of this product. It is much more likely that the initially formed intermediate branches into two sets of products, Craq03++ CMe3C(0)0H and (Cr3++ C02 +isobutene, as shown in Scheme 8. 

According to this scheme, kx is the rate constant for Reactions 5 and 6 together, and a represents the fraction of these interactions that terminate. (The step represented by kx can be further broken down to include tert-BU2O4 as intermediate.) Reactions 3, 5, and 6 have also been studied at 25°C. in the gas phase by photogeneration of tert-butyl radicals in the presence of oxygen (24). The competition between Reactions 5 and 6 is demonstrated by product studies on our Experiment 91 and measured as indicated below. 

Another, even a more influential, example is double fragmentation in the cation radical of 9 - / er/ - but yl-.V-methylacridan, which was generated electrochemically or photo-chemically (Anne et al. 1998). In acidic or weakly basic media, the tert-butyl radical is cleaved, with methylacridinium cation formation. This direction is depicted on the left side of Scheme 6-48, where 2,4,6-trimethylpyridine is marked as a base. If a strong base is used,... 

A group of perfluoroalkyl radicals which do exhibit marked increases in reactivity due to enhanced electrophilicity are the branched, 2° and 3° perfluoroalkyl radicals, specifically the perfluoro-iso-propyl and tert-butyl radicals [118]. 

Fischer H (1986) Substituent Effects on Absolute Rate Constants and Arrhenius Parameters for the Addition of Tert-Butyl Radicals to Alkenes. In Viehe HG, Janousek Z, Merenyi R, (eds) Substituent Effects in Radical Chemistry. Reidel, Dordrect, p 123... 

The addition of alkyl radicals to alkenes is important for C-C bond formation. A tert-butyl radical, a typical nucleophilic radical, reacts with acrylonitrile taking a rate constant of 2.4 X 106 M-1 s-1 (27 °C), through a SOMO-LUMO interaction. However, it reacts with 1-methylcyclohexene, an electron-rich alkene, taking a rate constant of 7.4 X 102M-1 s-1 (21 °C). On the other hand, the diethyl malonyl radical, a typical electrophilic radical, shows the opposite reactivity [66-71]. Similarly, the rate constant for the reaction of nucleophilic C2H5 and cyclohexene is2X 102 M 1 s 1, while that of electrophilic C3F7 with cyclohexene is 6.2 X 105 M-1 s 1. 

Radical coupling of organic halides with alkenes or alkynes takes place intermolecularly (Equation (12)).54 Tin hydride-mediated radical additions to a series of a-methyleneglutarates furnish 2,4-dialkyl-substituted glutarates.55 Using MgBr2-OEt2 as an additive, exclusive yy/t-selectivities are achieved upon tert-butyl radical addition at —78 °C. 

The P-addition of alkyl radicals to 4-methyl-2-(arylsulfinyl)-2-cyclopentenone 117 has been shown to occur in a completely stereocontrolled manner. Of a mixture of (4/ )- and (45)-117, only (4R)-117 reacts with t-Bu and i-Pr radicals to give the trans adducts 119a and 119b in 99% yield, while (45)-117 remained entirely unreacted. The stereochemical outcome of the reaction shows that the alkyl radical approaches from the side opposite to the aryl moiety in an antiperiplanar orientation to the carbonyl and sulfoxide bond. The 2,4,6-triisopropylphenyl group on sulfur plays a critical role, as it effectively shields the olefin face at the P-position by one of the isopropyl groups. This was confirmed by the 1 1 diastereomeric mixture obtained in the reaction of 4-methyl-2-(p-tolylsulfmyl)-2-cyclopentanone with the tert-butyl radical. 

A reported synthesis (13) of tert-biltyldifluoramine capitalized on the equilibrium dissociation of tetrafluorohydrazine into NF2 free radicals (9) by generating tert-butyl radicals via the decomposition of azoisobutane in the presence of tetrafluorohydrazine. Azoisobutane has been synthesized by two methods (4,16). Using the more efficient of these methods (16), which gave us a 30% yield of the intermediate, the overall yield of tert-butyldifluoramine obtained in the two-step reaction sequence was only 6% of theoretical. 

The presence of small amounts of C8 and C12 olefins (telomers of isobutene) among the reaction products attests to the occurrence of disproportionation between tert-butyl radicals (Equation 4). Coupling of f erf-butyl radicals (Equation 5) also occurred to a minor extent, as evidenced by the appearance of traces of tetramethylbutane. 

Few examples of direct comparisons of rates of reaction of different radicals with a common species are in the literature. In one, the (nondelocalized) tert-butyl radical was found to react more rapidly than pivaloyl radical with an electron-deficient partner, acrylonitrile, in 2-propanol. This is not a good analogy to the comparison between 1-phenylethyl and 2-phenylpropanoyl being made here because we suspect that 1-naphthoxy is more electron-rich than acrylonitrile, polyethylene is much less polar than 2-propanol, and the odd-electron in a 1-phenylethyl radical is delocalized, (a) Jent, F. Paul, H. Roduner, E. Heming, M. Fischer, H. Int. J. Chem. Kinet. 1986, 18, 1113. 

Coordination of the oxazolidinone 9 with the zinc complex activated the electrophi-licity of the alkene moiety toward addition of the nucleophilic radicals, but the stereodetermining step was the subsequent addition-fragmentation reaction of the intermediate radical with an allyltin reagent. A transition state XVIII similar to FV was proposed for the bis(oxazoline)-Mg complex-catalyzed Diels-Alder reaction reported by Corey [13], As the conformation of the bound a-amidyl radical formed by reaction with tert-butyl radical is s-cis [29a], the back face of the prostereogenic radical in XVni is shielded by one of the phenyl substituents on the oxazoline rings. So, the addition reaction occurred from the front face to the radical intermediate XVIII to give the (/ ) product from the (R,R) ligand 12. 

On the other hand, bond dissociation energies show that only 88 kcal is needed for formation of allyl radicals from propylene, as compared with 91 kcal for formation of terhouiyi radicals. Relative to the hydrocarbon from which each is formed, the allyl radical contains less energy and is more stable than the tert-butyl radical. 

Notice that tert-butyl radicals add readily to electrophilic alkenes using tert-butyl Grignards or tert-butyllithium as nucleophiles is much more problematic. 

Clerici, A., Minisci, F., Porta, O. Nucleophilic character of alkyl radicals. X. Polar and steric effects in the alkylation of 3-substituted pyridines by tert-butyl radical. Tetrahedron 1974, 30,4201-4203. 

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