Xylene radical anion

The / -xylene anion radical is easiest. We expect to have four equivalent ring protons and six equivalent methyl protons. Thus we expect to see a 1 4 6 4 1 quintet of 1 6 15 20 15 6 1 septets. Spectrum (b) shows the quintet clearly, but the CH3 proton coupling is poorly resolved. No other isomer has a quintet, so we can assign (b) to the p-xylene radical anion. 

Figure 2.10 ESR spectra of o-, m-, and p-xylene radical anions (see text for assignment of spectra). Spectrum (a) was simulated with permission using hyperfine parameters from Ref. 17b, copyright (1964) American Institute of Physics spectra (b) and (c) were simulated with permission using hyperfine parameters from ref. 17a, copyright (1961) Taylor and Francis (www.tandk. co.uk).

The sonochemistry of the other alkali metals is less explored. The use of ultrasound to produce colloidal Na has early origins and was found to greatly facilitate the production of the radical anion salt of 5,6-benzo-quinoline (225) and to give higher yields with greater control in the synthesis of phenylsodium (226). In addition, the use of an ultrasonic cleaning bath to promote the formation of other aromatic radical anions from chunk Na in undried solvents has been reported (227). Luche has recently studied the ultrasonic dispersion of potassium in toluene or xylene and its use for the cyclization of a, o-difunctionalized alkanes and for other reactions (228). 

Alkyl alkanoates are reduced only at very negative potentials so that preparative scale experiments at mercury or lead cathodes are not successful. Phenyl alkanoates afford 30-36% yields of the alkan-l-ol under acid conditions [148]. Preparative scale reduction of methyl alkanoates is best achieved at a magnesium cathode in tetrahydrofuran containing tm-butanol as proton donor. The reaction is carried out in an undivided cell with a sacrificial magnesium anode and affords the alkan-l-ol in good yields [151]. In the absence of a proton donor and in the presence of chlorotrimethylsilane, acyloin derivatives 30 arc formed in a process related to the acyloin condensation of esters using sodium in xylene [152], Radical-anions formed initially can be trapped by intramolecular addition to an alkene function in substrates such as 31 to give aiicyclic products [151]. 

The so-called acyloin condensation consists of the reduction of esters—and the reduction of diesters in particular—with sodium in xylene. The reaction mechanism of this condensation is shown in rows 2-4 of Figure 14.51. Only the first of these intermediates, radical anion C, occurs as an intermediate in the Bouveault-Blanc reduction as well. In xylene, of course, the radical anion C cannot be protonated. As a consequence, it persists until the second ester also has taken up an electron while forming the bis(radical anion) F. The two radical centers of F combine in the next step to give the sodium glycolate G. Compound G, the dianion of a bis(hemiacetal), is converted into the 1,2-diketone J by elimination of two equivalents of sodium alkoxide. This diketone is converted by two successive electron transfer reactions into the enediolate I, which is stable in xylene until it is converted into the enediol H during acidic aqueous workup. This enediol tautomerizes subsequently to furnish the a-hydroxyketone—or... 

Generation of the radical cation of aromatic substrates in the presence of sodium boron hydride offers another path for reduction, alternative to that via the radical anion seen in Sect. 2.1.2, and, as one may expect in view of the different mechanism, with a different regiochemistry [174-175], Thus, e.g. irradiation of the xylenes in the presence of m-dicyanobenzene and NaBH4 yields the corresponding 1,4-dihydro derivatives rather than the 2,5-dihydro derivatives obtained with dissolved metals [175]. 

The naphthalimide (227) imdergoes a SET process with arylalkenes such as p-xylene. The initial process yields a radical cation/radical anion pair within... 

The crystal structure of bis(NN-di-isobutyldithiocarbamato)nickel(ii). [Ni(S2-CNBu 2)2], shows that nickel is approximately square planar and co-ordinated by two symmetric bidentate ligands (Ni—S = 2.20 A) the ligand symmetry approximates to 2- The reduction mechanism of a series of nickel(ii) dithiocarbamates has been investigated in DMSO at the mercury electrode it is claimed to involve a dissociation to a nickel species which is more easily reduced than the nickel(ii) dithiocarbamate. An e.p.r. study of the reversible electrochemical reduction of nickel(ii) diethyldithio-carbamates in the presence of 2,2 -bipyridyl show that a bipy radical anion is formed initially. Ligand alkylation occurs when ao -dibromo-o-xylene is added to bis-(NiV-diethyldithiocarbamato)nickel(ii). The electron-transfer properties of 16 nickel(ii) dithiocarbamate complexes have been studied in acetone at a platinum electrode. Their oxidation is difficult and irreversible the overall process is ... 

Emission spectra of anthraquinone radical anions,354 p-xylene-type free radicals,355 and of aromatic hydrocarbon mono- and di-negative ions 356 have been recorded. Fluorescence spectra of a charge-transfer complex have been presented.357 A method using single-mode laser-induced resonance fluorescence to obtain the rotational structure of large molecules has been outlined.358... 

Macrocyclic bis-thionolactones have been prepared with LR. These were converted by reduction with sodium naphthalenide into the radical anions, which gave bicyclic systems through radical dimerization and subsequent methylation (eq 14). This method was successfully applied by Nicolaou et al. in the total synthesis of hemibrevetoxin B. One of the crucial steps of the synthesis was the preparation of the bis-thiono ester (11), which was achieved by using LR together with tetramethylthiourea in xylene at 175 °CP... 

The PET reactions of isobutylene(2-methylpropene) (21) in the absence of methanol have yielded a novel photochemical nucleophile-alkene combination aromatic substitution reaction. The solvent, acetonitrile, was found to act as a nucleophile and add to the alkene radical cation to give a distonic radical cation that subsequently adds to the 1,4-dicyanobenzene radical anion. Cyclization to the ortho position of the phenyl group leads to product formation. Two-colour, two-laser techniques have been used to study the photolysis of a, a -dichloro-o- or -p-xylene to yield xylylenes. ... 

We can apply these rules to the three spectra shown in Figure 2.10. The radicals in the figure are anion radicals of the three isomers of xylene (dimethylben-zene).17 Let us see if we can figure out which is which. 

Table 2.2 gives the coupling constants for the three xylene anion radical isomers. 

Give the number of lines in the ESR spectrum of the anion radical of each of the following molecules. (Assume all lines are resolved.) (a) Naphthalene (b) anthracene (c) pentacene (d) azulene (e) o-xylene (f) w-xylene (g) p-xylene (h) nitrobenzene (i) />-fluoronitrobenzene. 

The free-radicals are generated by discharge of proton, peroxides and easily reducible compounds at the cathode according to Eq. (1—4). The radial-anion of monomer is obtained by both direct and indirect electron transfer process [Eq. (5—6)]. The indirect process means that the active oxidizing species is formed from the electrolytes by electrode reaction, followed by interaction with the monomer. An unstable monomer like a,a -2-trichloro-p-xylene is formed and polymerizes instantaneously [Eq. (7)]. The regeneration of ferrous ion from the pool of used up ferric ion in a redox system is electrolytically successful [Eq. (8)] and an... 

A transformation method can introduce some functional groups at the junction as in B-63, which bear a fluorescent dye between the polyisoprene and polystyrene segments.375 The preparation is based on quenching the living anionic polymerization of isoprene with l-(9-phenanthryl)-l-phenylethylene followed by addition of excess a,a -dibromo-/>xylene, which affords a C—Br terminal effective for the copper-catalyzed radical polymerization of styrene. 

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