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 ... 

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 ... 

Figure 10.1 (a) Comparison of the potential energies of the propyl radical (+H ) and the isopropyl radical (+H ) relative to propane. The isopropyl radical — a 2° radical — is more stable than the 1° radical by 15 kJ mole-1, (b) Comparison of the potential energies of the tert-butyl radical (+H ) and the isobutyl radical (+H ) relative to isobutane. The 3° radical is more stable than the 1° radical by 29 kJ mole-1. 

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. 

The photochemistry of di-terf-butyl nitroxide was studied149. When di-tert-butylnitroxide (DTBN) is excited at 254 nm to the rnt state in pentane solution, it is cleaved to terf-butyl radical and 2-methyl-2-nitrosopropane (with quantum yield of 0.21). The tert-butyl radical is scavenged by DTBN to give di-ferf-butyl-terf-butoxyamine150 (equation 129). 

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... 

NA), 9-tert-butyl-10-methylanthracene (BMA), and methylanthracene (MA) with the ratio of 1 1.5 2.7 forMA BMA NA, whereas APin 100 in aqueous solution gives the ratio 1 0 3.4. The result demonstrates that inclusion of AP in 100 modifies the photoreactivity of AP, suppressing the reaction tert-butyl radical with the 10-position of the anthryl ring. 

In the polar reaction, a proton in HBr first adds to the terminal sp2 carbon in isobutene to produce a stable tert-butyl cation (8), and then it reacts with the counter bromide anion to form tert-butyl bromide. Thus, the proton in HBr adds to the less substituted sp2 carbon in alkene to produce a more stable carbocation. This is based on the Markovnikov rule. In radical reactions, the hydrogen atom of HBr is abstracted first by the initiator, PhCO (or Ph ) derived from (PhC02)2, and the formed bromine atom then adds to the terminal sp2 carbon in isobutene to form the stable (3-bromo tert-butyl radical (9), and then it reacts with HBr to produce /so-butyl bromide and a bromine atom. This bromine atom again... 

Figure 1.4 Inductive effect in tert-butyl cation and tert-butyl radical...

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. 

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