Naphthyl radical

In addition to inorganic radicals, which profoundly modify the properties of a paraflSn hydrocarbon residue, there is a whole series of organic groupings which are distinguished by exceptional reactivity, for example, the ethylene and acetylene groupings, and the phenyl and naphthyl radicals. Thus the characterisation of unsaturated hydrocarbons and their derivatives, e.g., the aromatic compounds, becomes possible. 

Bromonaphthalene does not react with benzenethiol (thiophenol) salts. However, if electric current is passed through a solution containing 1-bromonaphthalene, the tetrabutylammonium salt of thiophenol, and DMSO, then l-(phenylthio)naphthalene is produced in 60% yield. When the reaction is conducted in acetonitrile, it leads to naphthalene above all (Pinson and Saveant 1978, Saveant 1980, Amatore et al. 1982). In the electrochemically provoked reaction, it is sufficient to set up the potential difference corresponding to the initial current of the reduction wave to transform 1-bromonaphtahalene into 1-naphthyl radical. The difference in the consumption of electricity is rather remarkable In the absence of thiophenolate, bromonaphthalene is reduced, accepting two electrons per molecule in the presence of thiophenolate, 1-bromonaphthalene is reduced accepting two electrons for every ten molecules. The reaction with the thiophenolate ion is catalyzed by electric current and takes a reaction path shown in Scheme 5.2. 

The reaction in Scheme 5.2 proceeds through the formation of the 1-bromonaphthalene anion-radical, which rapidly converts into the naphthyl radical. Thiophenolate intercepts the naphthyl radical and forms the anion-radical of l-(phenylthio)naphthalene. The reaction takes place in the preelectrode space. It competes with the formation of the unsubstituted naphthalene. The debromi-nation is a result of hydrogen abstraction from the solvent SolH by the naphthyl radical. The unreacted 1-bromonapthalene oxidizes the l-(phenylthio)naphthalene anion-radical formed. This leads to the neutral l-(phenylthio)naphthalene and the anion-radical of 1-bromonaphthalene. The reaction takes place in the bulk solution and is the key-point for the chain propagation. 

Aryl radical additions to anions are generally very fast, with many reactions occurring at or near the diffusion limit. For example, competition studies involving mixtures of nucleophiles competing for the phenyl radical showed that the relative reactivities were within a factor of 10, suggesting encounter control,and absolute rate constants for additions of cyanophenyl and 1-naphthyl radicals to thiophenox-ide, diethyl phosphite anion, and the enolate of acetone are within an order of magnitude of the diffusional rate constant. ... 

The energy balance of photodissociation the importance of stabilization of the free radicals. When chlorobenzene or chloro-Np loses the halogen atom, a phenyl or a naphthyl radical is formed with the odd electron localized in an sp2 orbital which is orthogonal to the aromatic zr orbitals such a radical is not stabilized through resonance, unlike the benzyl- or the methyl-Np radicals for which several resonance structures can be drawn (Figure 4.32). 

In contrast to the observed reactivity of phenoxide and aryl alkoxide ions, arene and heteroarene thiolate ions typically couple with aryl radical to generate C—S bonds. The only exception to this regioselective reaction is the addition of 1-naphthalene thiolate ion to p-anisyl radical to render both C- and S-substitutions in 14% and 65% yields, respectively, while with 1-naphthyl radical, 95% of C—S coupling is obtained. In general, PhS- ions react with Arl in liquid ammonia under photostimulation to afford good yields of ArSPh or heteroaryl-SAr (70-100%). Substitution of the less-reactive ArBr can be achieved under photochemical initiation in DMF, MeCN, or DM SO [1],... 

The a-hydrogen exchange, Reaction 1, and the e-hydrogen exchange, Reaction 2, are interrelated by a 1,2-hydrogen exchange as shown in Reaction 3. Each of these reactions may be more complicated than that shown above, since they probably involve solvent or coal radical intermediates. In reactions 1 and 2, for example, the coal probably abstracts 2H from the naphthalene-ds to form a naphthyl radical, and subsequently the naphthyl radical abstracts lH from a different part of the coal as shown in Reactions 4 and 5 ... 

Carbonium ions produced from the decay of solid tritiated molecules Recently, the reactions initiated by the -decay of solid naphthalene- -t were studied by a technique based on electron paramagnetic resonance (Lloyd etal., 1968). A polycrystalline sample, stored at 77°K, developed an e.s.r. spectrum ascribed to the 1-naphthyl radical, which changed rapidly on warming to room temperature into the nine-line spectrum of the 1-hydronaphthyl radical ... 

The efficiency of photoinduced racemization (S) of (R)-3b, due to recombination of 1-phenylethyEl-naphthyl radical pairs is given by Equation 13.14. ° In this equation, the conversion fraction is the part of 3b that has become photoproducts. The values of S, as calculated from the slopes of the plots in Fig. 13.3, are nearly constant, 0.13-0.16,... 

According to the model proposed, the ratio of naphthol to CH4 would be unity provided oxynaphthyl- and methylradicals are equally reactive In reactions 2 and 3 The experiment shows a ratio of 0.49/0 6, Indeed close to unity. Also the ratio of CO to naphthalenes Is predicted to be unity provided the naphthyl radical preferentially saturates Itself by H abstraction, over recombination with other radicals. The experimental ratio of 0.41/0.42 Is again in agreement with this model. Finally the ratio of naphthalene to naphthol Indicates the rates of the propagation reactions 4, 5 relative to the initiation reaction 1. The experimental value of 0.42/0.49 shows that the methylene-naphthylether radical is formed mainly through reactions 1 and 3 the chain transfer reactions 4 and 5 are slow relative to initiation 1 and to reactions 6, 7 otherwise naphthalene and CO would have been formed in concentrations well over naphthol. 

Table 5. Calculated and observed spectral data for the cyclohexadienyl and a-hydro-naphthyl radicals...

Fig. 7. Enusaion spectra o( the 1-naphthyl radical generated by photolysis of 1< (chloromethyOnaphthelene (a) excited by a 266-did pulse followed by a 355-nm pulse ko ps later (b) using only the 266-nm pulse (c) using only the 3S5-nm beam.

In addition to the formation of neutral aromatic compounds, DDQ is also an effective agent for the preparation of the salts of stable aromatic cations. High yields of tropylium (eq 8) and triph-enylcyclopropenyl (eq 9) cations have been isolated in the presence of acids such as perchloric, phosphoric, and picric acid, and oxonium, thioxonium, and pyridinium salts may be prepared in reasonable yields from appropriate starting materials under essentially similar conditions. The formation of the peri-naphthyl radical has been reported on oxidation of perinaphthalene with DDQ under neutral conditions, although such products are not usually expected. 

The sodium dissolves to form an addition compound and, by transferring an electron, produces the green naphthalene anion radical. Addition of styrene to the system leads to electron transfer from the naphthyl radical to the monomer to form a red styryl radical anion. 

In this section, studies on the reactions of naphthyl radicals with unsaturated aliphatic hydrocarbons toward the production of three fused rings and of aliphatic residues attached to naphthalene are described. This is an effort to examine the process of polycyclic aromatic hydrocarbon (PAH) growth [88-99]. The source of naphthyl radicals for the experimental study was naphthyl iodide, in view of its low C—I bond dissociation energy. It dissociates very fast following the reflected shock heating. The unsaturated aliphatic hydrocarbons that have been studied were ethylene [28] and acetylene [29]. 

The reactions between naphthyl radical and both ethylene and acetylene are attachment reactions. The reaction with ethylene produces naphthyl ethylenyl radical and with acetylene the product is naphthyl acetylenyl, both with almost no barrier [28,29,88,96]. The attachment products are shown in Figure 6.22. 

The question that arises is why naphthyl ethylene is formed in the reaction of naphthyl radicals with ethylene, whereas naphthyl acetylene is not formed at aU in the reaction with acetylene, although the barriers from 1-naphthyl ethylenyl and 1 -naphthyl acetylenyl for their production are very close ( 17 and 19 kcal/mol in the first step, and 30 (9.8 - - 19.7) and 28 (18.8 + 9.1) kcal/mol in the second step) (Figure 6.23). 

The anions derived from phenols are bidentate nucleophiles and can react through of the O- or the C-atoms. For example, the 2-naphthoxide ions 65 react with ArX to give substitution only at C-1 of the naphthalene ring [61]. In this reaction, the intermediated Ar radical couples in C-1 of 65 to yield the radical anion 66, which by ET gives the product 67 that after tautomerization yields the more stable products 68 (Eq. 10.25). The rates of reactions of 2- and 4-anisyl and 2-methoxy-l-naphthyl radicals with 65 were determined (10 -10 M s" ) using an indirect method, a competition of the coupling reaction with the H-atom abstraction from the DMSO [62] ... 

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