Radicals terf-butoxy

According to Scheme 14, terf-butoxy and polymer carboxylate primary radicals are formed by photodissociation. However, the polymer-bound carboxylate radical may firstly lose CO2 giving rise to a polymer-bound phenyl radical which is also able to initiate the polymerization. The tert-butoxy radical may evolve to give acetone and a very reactive methyl radical. 

The extent of H-abstraction was determined by measuring the ratio of tert-butanol (TBA)/acetone produced. Using cumene as a model for PS, a high level of H-abstraction was observed. However, PS showed a very low level of H-abstraction which decreased further as the DP of the PS was increased (Fig. 13). This was explained by the coil configuration of the PS chains restricting access of the terf-butoxy radicals to the labile tertiary benzylic H-atoms on the PS backbone. 

Head addition to styrene by benzoyloxy radicals is remarkable in that it results in the formation of a significant amount of primary alkyl radicals in preference to the resonance-stabilized benzylic radical. In constrast, the reaction of styrene with most other radicals yields almost exclusively tail addition. The key difference in selectivity between benzoyloxy and terf-butoxy radicals is likely due primarily to steric factors [153]. tert-Butoxy radicals are significantly larger than benzoyloxy radicals and therefore would be expected to add to the sterically less hindered tail position. Also, benzyloxy radicals are likely more electroi Uc than tert-butoxy radicals (due to the elo tron withdrawing effect of the carbonyl) making them more reactive and therefore less selective in their addition to the electron rich styrene molecule. 

The formation of considerable amounts of acetone and tcrt-butyl alcohol during the decomposition of 2,2-bis(tcrt-butylperoxy)butane in styrene indicates that the rates of 3-scission and H-abstraction are competitive with addition to the styrene double bond. Niki and Kamiya [167,168] are in agreement that considerable H-abstraction between terf-butoxy radicals and PS takes place especially at high polymerization temperatures (125 °C). However, p-scission rate is in disagreement with Moad. As mentioned earlier, Moad found that the rate of vinyl addition by tert-butoxy radicals is 76 times faster than P-scission... 

The autoxidation of isobutane is now mainly carried out to obtain terf-butyl hydroperoxide [36]. Halogenated metalloporphyrin complexes are reported to be efficient catalysts for the aerobic oxidation of isobutane [18,37]. It was found that the oxidation of isobutane by air (lOatm) catalyzed by NHPI and Co(OAc)2 in benzoni-trile at 100 °C produced tert-butyl alcohol in high yield (81%) along with acetone (14%) (Eq. (6.3)) [38]. 2-Methylbutane was converted into the carbonacetic acid, rather than the alcohols, as prindpal products. These cleaved products seem to be formed via P-sdssion of an alkoxy radical derived from the decomposition of a hydroperoxide by Co ions. The extent of the P Scission is known to depend on the stability of the radicals released from the alkoxy radicals [39]. It is thought that the 3-scission of a terf-butoxy radical to acetone and a methyl radical occurs with more difficulty than that of a 2-methylbutoxy radical to acetone and an ethyl radical. As a result, isobutane produces terf-butyl alcohol as the principal product, while 2-methylbutane affords mainly acetone and acetic acid. 

Sato T, Shimooka S, Seno M, Tanaka H. Kinetic and electron paramagnetic resonance studies on radical polymerization. Radical copolymerization of /)-terf-butoxy-styrene and dibutyl fumarate in benzene. Macromol Chem Phys 1994 195 833-843. 

Based upon the results of mechanism studies, a proposed mechanism containing two catalytic cycles was concluded by the authors. Firstly, benzylic radical is formed by the hydrogen abstraction of the terf-butoxy radical, which is well supported by literatures. The benzylic radical may be oxidized by iodine to give the benzylic cation with the generation of iodide ion. The interaction between iodide ion and TBHP would regenerate the terf-butoxy radical and finish the first catalytic cycle. Secondly, benzylic iodide is formed as a result of the reaction between benzylic cation and iodide ion, which is then converted into benzaldehyde by a DMSO participated Komblum oxidation. Finally, the iodide ion was oxidized back to iodine by TBHP to finish the second catalytic cycle (Scheme 4.17). Benzylic amine or alcohol may be formed from the benzylic cation. As confirmed in the control experiments, they can also lead to the nitrile product under standard condition. 

Hoare and Wellington (22) produced CH3O radicals from the photochemical (50° and 100°C.) and thermal (135°C.) decompositions of di-terf-butyl peroxide in the presence of 02. The initially formed tert-butoxy radicals decomposed to acetone plus methyl radicals, and the methyl radicals oxidized to methoxy radicals. Formaldehyde and CH3OH were products of the reaction the formation of the former was inhibited, and the latter was enhanced as the reaction proceeded. If the sole fate of CH3O were either... 

Alkyl radicals can be obtained by abstraction of a hydrogen atom from an alkyl group by another radical. This method was utilized for the generation of benzyl radicals from toluene with tert-butoxy radical obtained on heating di-terf-butyl peroxide.38 Benzoyl92 and carboxymethyl88 radicals have also been obtained by this method. The reaction gives rise to a complex mixture of products and therefore is of rather limited use. 

The seleetivity and product composition is different from that obtained for direct chlorination. The selectivity of the t-butoxy radical is intermediate between that of ehlorine and bromine atoms. The selectivity is also solvent- and temperature-dependent. A typieal ratio, in ehlorobenzene as solvent, is terf.sec.pri = 60 10 1. ° Scheme 12.4 (p. 704) gives examples of speeiflc halogenation reactions that proceed by radical-chain mechanisms. 

Many types of peroxides (R-O-O-R ) are also utilized, including diacyl peroxides, peroxydicarbonates, peroxyesters, dialkyl peroxides, and inorganic peroxides such as persulfate, the latter being used mainly in water-based systems. The rate of peroxide decomposition as well as the subsequent reaction pathway is greatly affected by the nature of the peroxide chemical structure, as illustrated for fert-butyl peroxyesters in Scheme 4.2. Pathway (a), the formation of an acyloxy and an alkoxy radical via single bond scission, is favored for structures in which the carbon atom in the a-position to the carbonyl group is primary (for example, terf-butyl peroxyace-tate, R = CHg). Pathway (b), concerted two-bond scission, occurs for secondary and tertiary peroxyesters (for example, terf-butyl peroxypivalate, R = C(CH3)3) [1, 2]. The tert-butoxy radical formed in both pathways may decompose to acetone and a methyl radical, or abstract a hydrogen atom to form tert-butanol. 

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