Galvinoxyls

Transformation products of stabilizers formed during melt processing may exert either or both anti- and/ or pro-oxidant effects. For example, in the case of BHT, peroxydienones, PxD (reactions 9b, b") lead to pro-oxidant effects, due to the presence of the labile peroxide bonds, whereas quinonoid oxidation products, BQ, SQ, and G- (reaction 9 b, c, d) are antioxidants and are more effective than BHT as melt stabilizers for PP [29], The quinones are effective CB—A antioxidants and those which are stable in their oxidized and reduced forms (e.g., galvinoxyl, G-, and its reduced form, hydrogalvi-noxyl, HG) may deactivate both alkyl (CB—A mecha-... 

Addition of radical inhibitors (e.g. duroquinone, galvinoxyl), which will slow up any pathway involving radicals. 

Most radicals are transient species. They (e.%. 1-10) decay by self-reaction with rates at or close to the diffusion-controlled limit (Section 1.4). This situation also pertains in conventional radical polymerization. Certain radicals, however, have thermodynamic stability, kinetic stability (persistence) or both that is conferred by appropriate substitution. Some well-known examples of stable radicals are diphenylpicrylhydrazyl (DPPH), nitroxides such as 2,2,6,6-tetramethylpiperidin-A -oxyl (TEMPO), triphenylniethyl radical (13) and galvinoxyl (14). Some examples of carbon-centered radicals which are persistent but which do not have intrinsic thermodynamic stability are shown in Section 1.4.3.2. These radicals (DPPH, TEMPO, 13, 14) are comparatively stable in isolation as solids or in solution and either do not react or react very slowly with compounds usually thought of as substrates for radical reactions. They may, nonetheless, react with less stable radicals at close to diffusion controlled rates. In polymer synthesis these species find use as inhibitors (to stabilize monomers against polymerization or to quench radical reactions - Section 5,3.1) and as reversible termination agents (in living radical polymerization - Section 9.3). 

Examples of radicals which are reported to meet these criteria are diphenylpicrylhydrazyl [DPPH, (22)], Koelsch radical (26), nitroxides [e.g. TEMPO (23), Fremy s Salt (24)], triphenylmethyl (25), galvinoxyl (27), and verdazyl radicals [e.g. triphenylverdazyl (28)]. These reagents have seen practical application in a number of contexts. They have been widely utilized in the determination of initiator efficiency (Section 3.3.1.1.3) and in mechanistic investigations (Section 3.5.2). 

The fact that these reactions are catalyzed by free-radical initiators and inhibited by galvinoxyl (a free-radical inhibitor) " indicates that free-radical mechanisms are involved. 

Although these reactions are formulated as ionic reactions via 947 and 949, because of the apparent partial formation of polymers and inhibition of the fluoride-catalyzed reaction of pyridine N-oxide 860 with aUyl 82 or benzyltrimethylsilane 83 by sulfur or galvinoxyl yet not by Tempo, a radical mechanism caimot be excluded [61, 62]. The closely related additions of allyltrimethylsilane 82 (cf. Section 7.3) to nitrones 976 are catalyzed by TMSOTf 20 to give, via 977, either o-unsatu-rated hydroxylamines 978 or isoxazoHdines 979 (cf also the additions of 965 to 962a and 969 in schemes 7.20 and 7.21). 

DPPH- has an intense absorption maximum around 520 run (Yordanov and Christova, 1997), and antioxidant capacity and activity measured by the reduction of DPPH- are easily quantified by VIS-spectroscopy (Brand-Williams et al, 1995 Bondet et al, 1997, Espin et al, 2000). The stable radicals Fremy s salt (potassium nitrosodisulphonate) and galvinoxyl (2,6-di-tert-butyl-a-(3,5-di-tert-butyl-4-oxo-2,5-cyclohexadien-l-ylidene)-p-tolyloxy radical) have been used in a similar manner but with ESR detection, which can be used with samples that are not optically transparent (Gardner et al, 1998). 

Since this reaction is not affected by hydroquinone and galvinoxyl and does not initiate polymerization of styrene, it obviously occurs without the formation of free radicals. The kinetic parameters of the reactions of three hydroperoxides with triphenyl phosphite in different solvents are given in Table 17.2 [21]. 

Bu3Sn-SiMe3 CH2C02SnBu3 Pd(PPh3)4(2) Galvinoxyl, THF, reflux, 6 h 80 220,221... 

Biaryl synthesisThis reagent promotes coupling of aryl Grignard reagents to symmetrical biaryls in 70-95% yield with formation of allene and 3-aryl-2-chloropropene as co-products. The reaction is retarded by galvinoxyl and evidently involves an electron-transfer from the Grignard reagent to the dichloropropene. 

The reaction of trialkylboranes with 1,4-benzoquinones to give in quantitative yield 2-alkylhydroquinones was the first reaction of this type occurring without the assistance of a metal mediator [81,82], An ionic mechanism was originally proposed but rapidly refuted since the reaction is inhibited by radical scavengers such as galvinoxyl and iodine [83]. This procedure is in many cases superior to the more widely use organometallic additions. For instance, when primary and secondary alkyl radicals have been used and afford the addition products in high yield (Scheme 33) [84],... 

Figure 14.4 Four examples of the phenolic compounds (ArOH) involved in the equilibrium studies of reaction 14.32. Compound 5 is a-tocopherol, a very important antioxidant. The reactant (Ar O) is the galvinoxyl radical, produced from 1.

The stable free radical diphenylpicrylhydrazinyl (39) was first prepared by Goldschmidt in 1922,and this " and galvinoxyl (40) have found application as radical scavengers in kinetic studies. " Davies and Roberts used galvinoxyl as a radical inhibitor to show the radical nature of the autoxidation of 1-phenylethylboronic acid. " ... 

Dr. Steelink. Although the proposed radical species were written with the unpaired electrons on oxygen (as above and in the ionic state these electrons are extensively delocalized, and a number of canonical forms can be written with the unpaired electrons on carbon. The electron on oxygen is favored only because of its resemblance to well established phenoxy radicals such as galvinoxyl. 

Since 2,4,6-trisubstituted phenols are readily oxidized to fairly stable phenoxy radicals, this system has been used to construct various biradical species. The bisphenol 6 below loses three hydrogens to form the very stable crystalline radical galvinoxyl.86... 

For benzoyl peroxide, which decomposes at convenient rates in the neighborhood of 80°G, addition of iodine57 or the stable radical galvinoxyl (13),58 to the reaction reduces the yield of carbon dioxide.59 One interpretation of this... 

An intriguing problem still remained could a simple, solid, stable free radical be prepared, whose structure would be consistent with lignin models Our attention was directed to the compound galvinoxyl (VII),... 

Figure 8. First derivative EPR spectra of A) syrinoxyl in CH2Cl2, g = 2.0060 arrow) and (B) galvinoxyl in CH2Cl2i g = 2.0044 arrow). Distance between dark lines is 18 gauss in both spectra.

Further to its ability to perform allylic and benzylic oxidations,149 /-butylpcroxy-iodane (6) effects radical oxidation of 4-alkylphenols to give 2,5-cyclohexadien-l-ones under mild conditions in good yields.150 o,o-Coupling dimers as side products and inhibition of the reaction by added galvinoxyl radical scavenger support a radical oxidation mechanism. 

Negative reaction constants p1 for the oxidation of sulfides by [10-1-3]—(r-butylperoxy)iodanes are consistent with a mechanism involving rate-limiting formation of a sulfonium species by nucleophilic attack of sulfide on the iodine(III) atom followed by attack of water to give sulfoxide.151 However, in dichloromethane, inhibition by galvinoxyl implicates a free radical mechanism perhaps by homolytic cleavage of the weak iodine(III)-peroxy bond. 

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