Verdazyl radicals

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). 

Complexes with nitronyl nitroxide and verdazyl radical ligands 479... 

Radicals have been known for many years to form organic paramagnetic materials with numerous magnetic properties (ferro- or ferri-magnetism, spin Peierls transition, spin frustration, spin ladder systems) (see [51-60] for verdazyl radicals, [61-68] for thiazyl radicals, [69] for nitronyl nitroxide and [70-78] for Tempo radicals) (Fig. 6). When they are in their cationic form, they are valuable candidates for an association with the M(dmit)2 systems they will then provide the magnetic properties thanks to their free electron(s), whereas the M(dmit)2 moieties will provide the electrical properties. 

No photolyses of hexahydro-1,2,4,5-tetrazines have been published. The kinetics of the photodecomposition of 1,3,5-triphenylverdazyl (61) was studied by Soviet authors in various hydrocarbons. The rate depends on the solvent. The excited radical is consumed by H extraction from the solvent and by dissociation at the N—N bond, forming the product 1,3-diphenyl-l,2,4-triazole (138), which catalyzes the decomposition of the verdazyl radical (74IZV2204). The dipole moment of (61) in the excited state is 9 1 D in the ground state it is 2.94 D. 

Hydrogen-bonding motifs have also been observed with verdazyl radical-substituted pyridines, which co-crystallize with hydroquinone <2001JA7154>, and with iV-oxide derivatives of pyridine, such as found in co-crystals of 4,4 -bipyridine-iV-oxide and cyclohexanetricarboxylic acid <2005CE0551, 2005CGD1683>, bipyridine-iV-oxides with amino and nitrobenzoic acids <2005CGD727>, and bipyridine-iV-oxides and diphenols <2005CGD1041>. 

For R = H, the rate constant is e.g. 4 x 103 s, while that for R=CH3 reaches up to 2x 106 s 1 at 298 K. An increase in the nucleophile character of the corresponding radical of the same type enhances the attack on the peroxidic bond rather significantly. The rate constant of the interaction of perfluorotriphenyl verdazyl radical with 0 — 0 bond of dibenzoyl peroxide in benzene is 8 x 10 3 dm3 mol-1 s 1 at 20 CC, whereas the triphenyl verdazyl radicals which are more nucleophilic react with peroxide by 500 times faster [86]. 

Tetrazines, synthesis and properties 84MI18 78HC(33,1)1. s-Tetrazines, cycloaddition reactions of 81 KGS 1462. Tetrazines, synthesis from hydrazines 84H(22)1821. Verdazyl radicals, vibrational spectra of 84UK1959. 

Original 1,2,4,5-tetrazines disubstituted by heterocyclic rings have been prepared and their electrochemical and spectroscopic properties studied <04NJC387>. A bowl-shaped neutral radical with a core annulene system bearing a verdazyl radical 47 has been synthesized in two steps from aldehyde 44 and carbazide derivative 45, as a stable solid in air <04OL1397>. 

Verdazylium salts 13 can be obtained in high yield by reacting the corresponding triphenylformazans with formaldehyde in acid media in a two phase system (CHCV HjO). The reduction of the salts with ascorbic acid in the two phase system gives the verdazyl radicals, whereas in methanol leucoverdazyls are obtained <97S 17>. 

The magnetochemistry of verdazyl radical-based materials 05CCR(249)... 

Obviously, the use of nitroxides as agents to control polymerizations remains interesting, however, their limitations makes one wonder whether there are other stable radicals which may be superior. Unfortunately, there are a limited number of stable radicals available, and those that have been studied have not been particularly successful. Earlier studies with galvi-noxyl radicals have been followed recently with the use of triazolinyl radicals and verdazyl radicals 1. However, in the only reported use of verdazyl radicals to mediate styrene polymerizations no control was observed with 2, an adduct of the 1,3,5-triphenylverdazyl radical and the 2-(2-cyano-2-propyl) radical derived from 2,2 -azobisisobutyronitrile (AIBN), at reaction temperatures between 80 °C and 120 In the case of the triazolinyl radicals, a spirotriazolinyl radical controlled the polymerization of styrene reasonably weU, but was only moderately effective for methyl methacrylate ... 

Various synthetic approaches to the verdazyl radicals have been reported. All... 

Despite these results the verdazyl radicals piqued our interest, primarily because they could be synthesized with a variety of substituents, each with the potential to affect the stabUity of the radical and its steric interactions with a propagating polymer chain. First reported in 1963,... 

The described syntheses offer easy access to a variety of structurally different verdazyl radicals providing the opportunity to extend the initial work of Yamada and coworkers to determine if these stable radicals have the potential to solve some of the problems associated with nitroxides as mediating reagents for living-radical polymerizations. 

CC126). X-ray crystal structures of the derivatives 160a,b were obtained, and using cyclic voltammetry it was shown that they can be reduced to borataverdazyl radical anions, the first-ever boron-containing verdazyl radicals. 

Another commonly used stable radical is 1,3,5-triphenylverdazyl 19 (446, 447). It is less thermally stable than TEMPO. Both TEMPO and the verdazyl radical do not react with oxygen-centered radicals or oxygen. If an initiator generates an oxygen-centered radical the nitroxide will capture the carbon-centered radical that is generated via the first addition step involving a monomer. Galvinoxyl 20 and l,3-bisdiphenylene-2-phenylallyl (or Koelsch s) radical 21 can also be used as inhibitors. Diphenylpicrylhydrazyl 22 is used much less frequently because of its complicated reaction mode of inhibition (448). 

In conclusion, it appears that only a relatively small number of spin-labelled moieties can be introduced into the polyacetylene skeleton through modification reactions. the magnetic (para, ferro) properties of the modified polyacetylenes were therefore very poor, but in all cases there was an interaction between the localized moments of spin labels and the conduction electrons (holes) in iodine-doped, modified polyacetylene [39]. New routes for obtaining ferromagnetic and antiferromagnetic materials from verdazyl radicals have since been developed [39a]. 

The cycloaddition proceeds regioselectively (and stereoselectively with stereodefined olefins) and is interpreted as a 1,3-dipolar cycloaddition via the azomethine yUde 31, which results from disproportionation of the verdazyl radical 28 to give the 1,3-dipole 31 and leukoverdazyl 30, which is readily reoxidized to 28 by aerial O2. 

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