T-Butoxy radical

An example of this reaction is the reaction of cyclohexene with t-butyl perbenzoate, which is mediated by Cu(I). " The initial step is the reductive cleavage of the perester. The t-butoxy radical then abstracts hydrogen from cyclohexene to give an allylic radical. The radical is oxidized by Cu(II) to the carbocation, which captures benzoate ion. The net effect is an allylic oxidation. 

The overall trend of reactivities for t-butoxy radicals with the fluoro-olcfins more closely parallels that for methyl radicals than that for the electrophilic trifluoromethyl or trichloromethyl radicals. 

Dw-butyl pcroxyoxalatc (DBPOX) is a clean, low temperature, source of t-butoxy radicals (Scheme 3.33).136 The decomposition is proposed to take place by concerted 3-bond cleavage to form two alkoxy radicals and two molecules of carbon dioxide. 

Figure 3.5 Relative reactivity of indicated site towards t-butoxy radicals.

The relative amounts of double bond addition, hydrogen abstraction and 13-scission observed are dependent on the reactivity and concentration of the particular monomer(s) employed and the reaction conditions. Higher reaction temperatures are reported to favor abstraction over addition in the reaction of t-butoxy radicals with AMS413 and cyclopentadiene 417 However, the opposite trend is seen with isobutylene.2 1 24... 

Pioneering work by Wallingj94 established that the specificity shown by t-butoxy radical is solvent dependent. Work21 22396 on the reactions of /-butoxy radicals with a series of a-mcthylvinyl monomers has shown that polar and aromatic solvents favor abstraction over addition, and [3-scission over either addition or abstraction. Recently, Weber and Fischer418 and Tsentalovich at a/.410 reported absolute rate constants for [3-scission of r-butoxy radicals in various solvents. These studies indicate that p-scission is strongly solvent dependent while abstraction is relatively insensitive to solvent. 

Grant et a/.397 examined the reactions of hydroxy radicals with a range of vinyl and a-methylvinyl monomers in organic media. Hydroxy radicals on reaction with AMS give significant yields of products from head addition, abstraction and aromatic substitution (Table 3.8) even though resonance and steric factors combine to favor "normal tail addition. However, it is notable that the extents of abstraction (with AMS and MMA) arc less than obtained with t-butoxy radicals and the amounts of head addition (with MMA and S) are no greater than those seen with benzoyloxy radicals under similar conditions. It is clear that there is no direct correlation between reaclion rale and low specificity. 

Large-ring heterocyclic radicals are not particularly well known as a class. Their behavior often resembles that of their alicyclic counterparts, except for transannular reactions, such as the intramolecular cyclization of 1-azacyclononan-l-yl (Scheme 1) (72CJC1167). As is the case with alicyclic ethers, oxepane in the reaction with t-butoxy radical suffers abstraction of a hydrogen atom from the 2-position in the first reaction step (Scheme 2) (76TL439). 

A recent oommtiruo tion by Gritter and Wallace discloses initiation of a study of the free-radical chemistry of epoxides Under the influence of U t-butoxy radicals, formed by thermal decomposition of di-lerf-butyl peroxide, propylene oxide is believed to yield an epoxy radical as shown in Eq. (3). The latter undergoes Isomerization to CHsCOCH - and further reaction with unreaoted propylene oxide or other available substrates, such as 1-octene, toluene, oyolohexene, and ethanol,fl7a as shown in Eq. (3). 

Comparative studies indicate that abstraction of hydrogen atoms from the methyl group of 1-methylpyrrole is easier than from the methyl group of toluene. Subsequent dimerization and polymerization of the 1-pyrrolylmethyl radicals, initiated by the addition of t-butoxy radicals, occurs and low yields (0.5%) of l,2-bis(l-pyrrolyl)ethane and l-methyl-3-(l-pyrrolylmethyl)pyrrole have been isolated from the tars (B-77MI30500). The radical dimerization of pyrrole initiated with r-butoxy radicals produces 2-(2-pyrrolyl)-A1-pyrroline (B-77MI30500). 

Probably the most interesting series of radicals which have not been trapped and identified conclusively in the solid phase by any method are the alkoxy radicals, RO-. We have attempted to prepare the t-butoxy radical by the following reactions ... 

Stabilities of methyl and fluoromethyl radicals have been calculated [114] to be in the order CFj < CHj < CF2H < CFH2 and the relative rates of formation of such radicals, measured in (S-scission reactions of a series of t-butoxy radical derivatives, as shown in Figure 4.45, lend support to this conclusion [115]. 

The radical initially produced by homolytic decomposition of a dialkyl peroxide can undergo further scission. The rate of scission depends on the temperature and the stability of the resulting radical(s). For example, t-butoxy radicals decompose on heating to methyl radicals and acetone. 

Indirect, quantitative information on the behavior of ethylene-propylene based terpolymers in the presence of t-butoxy radicals have been obtained through kinetic studies carried out on model compoiuKls of triene (III), (V), (VQ and on the reference diene ENB. The model con unds denoted (III,a), (V,1), (VI,a), and NB, were dissolved in isooctane which simulated the ethylene-propjdene units present in EPTMs or EPDMs The approach was essentially that described by Szwarc et al. "). 

The kinetic scheme was verified during the course of the same work and the reactivity of t-butoxy radicals toward the saturated substrate (k2) and model compounds (ks, k4) was related to the 0 scission reaction (k i). The results are listed in Table 7 which shows that the H abstraction mechanism from model compound (III,a) with respect to isooctane (k3/k2) is four times more favored than in the case of NB. The addition reaction of t-butoxy radical to unsaturation (k3/k4) is 15 times... 

However, it is worth noting the ability of t-butoxy radicals to attack the saturated molecule of isooctane (k2/k ). This has also been recently confirmed in the case of linear saturated hydrocarbons In the same work it was found that the benzoyloxy radical disproportionation into phenyl radical and CO2 is, at 140 °C, about four times more fevored than the H abstraction from n-pentadecane, while in the ca% of t-butoxy radical the H abstraction is hi er than the scission reaction by a fiictor of eight. 

The model compounds (III,a), (V,f), (VI,a) and NB described in Sect. B.l h) and the kinetic results obtained from the attack of t-butoxy radical on isooctane solutions of these products have been used to clarify some aspects of the mechanism of ter-polymer cross-linking. The reactivity of isooctane, expressed as the hydrogen abstraction reaction by t-butoxy radical relative to the scission rem tion of (CH3)300 (see k2/ki values in Table 7), has bear converted into the rate cmstant for the H abstraction from ethylene-propylene units (kj) Iqy means of Pryor s... 

Table 10. Amount of t-butoxy radicals unde oing p scission in the presence of different ethylene )ropylene-based elastomers...

Another calculation was made in order to distinguish between the amount of t-butoxy radical reacting with ethylene-propylene units and that reacting with the unsaturations of some terpolymers. The results (Table 12) indicate that there is al-wsys a significant amount of t-butoxy radicals which react with the H atoms of the saturated back-bone, while in the case of (III)-EPTM 34% of (CH3)3C0 reacts with flie unsaturation when the termonomer content is only 0.1 M. The sune result k achieved in the case of ENB-EPDM when the termonomer content is four-fold end hence agrees with the experimental data of Fig. 11. However, the formation of abundant amounts of tertiary alkyl type radkals, e.g. CH2-C(CH3)-CH2, when the termonomer concoitration is low, explain the poor yield of grafting... 

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