Radical species naphthalene anion

The naphthalene anion radical spectrum (Figure 2.2) provided several surprises when Samuel Weissman and his associates1 first obtained it in the early 1950s at Washington University in St. Louis. It was a surprise that such an odd-electron species would be stable, but in the absence of air or other oxidants, [CioHg]- is stable virtually indefinitely. A second surprise was the appearance of hyperfine coupling to the two sets of four equivalent protons. The odd electron was presumed (correctly) to occupy a it molecular orbital... 

Several examples of carbenoid ion-radicals are discussed within this book. A silylene anion-radical preparation and properties is exemplified here. Scheme 2.5 shows the path to this species. Tetrakis(di-tert-butytmethylsilyl)disilylene was reduced by lithium or sodium salt of naphthalene anion-radical in THF at 78°C and then 12-crown-4 was added to the resulting reaction mixture. The silylene anion-radical was obtained as the corresponding alkali salt. Red crystals of the salt were isolated and characterized by ESR spectroscopy and x-ray crystallography (Inoue et al. 2007). 

The authors proposed the following picture of the silylene anion-radical formation. Treatment of the starting material by the naphthalene anion-radical salt with lithium or sodium (the metals are denoted here as M) results in two-electron reduction of >Si=Si< bond with the formation of >SiM—MSi< intermediate. The existence of this intermediate was experimentally proven. The crown ether removes the alkali cation, leaving behind the >Si - Si< counterpart. This sharply increases electrostatic repulsion within the silicon-silicon bond and generates the driving force for its dissociation. In a control experiment, with the alkali cation inserted into the crown ether, >Si — Si< species does dissociate into two [>Si ] particles. 

Electron tunneling between organic species was first detected, by direct kinetic experiments, for reactions of the biphenyl anion radical with naphthalene and pyrene [11] and triphenylethylene [12], As is known, upon irradiating vitreous solutions containing biphenyl or pyrene, Py, these acceptors react with electrons to form Ph2 and Py with characteristic optical spectra [13]. Ph2 particles have been found [11] to enter into the electron exchange reactions at 77 K with naphthalene, Nh, and pyrene molecules in mixtures of ethyl alcohol and diethyl ether (2 1). 

Electrochemistry can also be used to induce aromatic nucleophilic substitutions by setting up the electrode potential at the level, which is appropriate to reduce an aromatic substrate. When this electrochemical process is carried out in the presence of a nucleophilic reagent, the or reactions take place. Indeed, halogenated derivatives of benzophenone, benzonitrile, and naphthalene undergo nucleophilic displacement reactions with thiolates, which are able to occur catalytically [76, 77]. The reaction mechanism involves the formation of the anion radical at the electrode and its further decomposition into a neutral radical, which reacts with a nucleophile, thus yielding the anion-radical of the substitution product. In case of the catalytic reaction, oxidation of the anion-radical species may occur by electron transfer with the substrate and/or the electrode (Scheme 17). 

Reduction is defined as acceptance of electrons. Electrons can be supplied by an electrode - cathode - or else by dissolving metals. If a metal goes into solution it forms a cation and gives away electrons. A compound to be reduced, e.g. a ketone, accepts one electron and changes to a radical anion A. Such a radical anion may exist when stabilized by resonance, as in sodium-naphthalene complexes with some ethers [122], In the absence of protons the radical anion may accept another electron and form a dianion B. Such a process is not easy since it requires an encounter of two negative species, an electron and a radical anion, and the two negative sites are close together. It takes place only with compounds which can stabilize the radical anion and the dianion by resonance. 

Since different reactivity is observed for both the stoichiometric and the catalytic version of the arene-promoted lithiation, different species should be involved in the electron-transfer process from the metal to the organic substrate. It has been well-established that in the case of the stoichiometric version an arene-radical anion [lithium naph-thalenide (LiCioHg) or lithium di-ferf-butylbiphenylide (LiDTBB) for using naphthalene or 4,4 -di-ferf-butylbiphenyl (DTBB) as arenes, respectively] is responsible for the reduction of the substrate, for instance for the transformation of an alkyl halide into an alkyllithium . For the catalytic process, using naphthalene as the arene, an arene-dianion 2 has been proposed which is formed by overreduction of the corresponding radical-anion 1 (Scheme 1). Actually, the dianionic species 2 has been prepared by a completely different approach, namely by double deprotonation of 1,4-dihydronaphthalene, and its X-ray structure determined as its complex with two molecules of N,N,N N tetramethylethylenediamine (TMEDA). ... 

The radical-anion proceeds to propagate in the same manner as discussed above for initiation by sodium naphthalene. (Polymerizations in liquid ammonia are very different from those in organic solvents in that free ions probably constitute the major portion of propagating species.)... 

The anionic polymerization of epoxides such as ethylene and propylene oxides can be initiated by metal hydroxides, alkoxides, oxides, and amides as well as metal alkyls and aryls, including radical-anion species such as sodium naphthalene [Boileau, 1989 Dreyfuss and Drefyfuss, 1976 Inoue and Aida, 1984 Ishii and Sakai, 1969]. Thus the polymerization of ethylene oxide by M+A involves initiation... 

Johnson and Willson interpreted the main feature of the observations on solid polyethylene doped with aromatic solutes in terms of an ionic mechanism it was analogous to that proposed for irradiated frozen glassy-alkane-systems in which ionization occurred with G = 3 — 4 [96], The produced charged species, electron and positive hole, were both mobile as indicated by the radiation-induced conductivity. The production of excited states of aromatic solutes was caused mainly by ion-electron neutralization. The ion-ion recombination was relatively slow but it might contribute to the delayed fluorescence observed. On the basis of Debye-Simoluchovski equation, they evaluated the diffusion coefficients of the radical anion of naphthalene and pyrene as approximately 4 x 10 12 and 1 x 10 12 m2 s 1 respectively the values were about three orders of magnitude less than those found in typical liquid systems. 

A radical anion of an aromatic hydrocarbon was implicated as early as 1866, when Berthelot obtained a black dipotassium salt from naphthalene and potassium [41]. This reaction must have proceeded via the naphthalene radical anion as a more or less fleeting intermediate. Again, Schlenk and co-workers captured the essence of such an intermediate. In the case of anthracene they noticed the existence of two different species, a purple dianion and a blue transient species with a banded spectrum [42]. They identified this intermediate as a monosodium addition product which contains trivalent carbon . Further details were revealed only with the advent of electron paramagnetic resonance spectroscopy. 

A substantial body of work also exists on the preparation of so-called diinitiators, species that result in the simultaneous growth of a polymer chain from both ends. There are two basic approaches to this. One is to use the metal in the presence of a conjugated aromatic species such as naphthalene to generate a radical anion capable of transferring an electron to the monomer. Under suitable conditions, the resulting monomer radical anions rapidly dimerize to form a dilithium species that goes on to add monomer in a living fashion... 

The problem is that, because it is extremely electron rich, this species is also extremely air sensitive, and the real quantity which is being used is not known. In addition, it reacts to give neutral naphthalene which is difficult to separate from the desired product. Finally, another problem (encountered, for instance, in cluster chemistry [58]) is that it can over-reduce the substrate and lead to decomposition whereas less powerful reducing agents are not marred by this problem. Addition of pieces of Li, Na, or K to naphthalene in THF or DME under extremely dry reaction conditions gives a green solution of the naphthalene radical anion, and concentrations of 0.1-0.5 M are employed [59]. The formation of the dianion in this reaction has also been proposed, but its formation is uncertain. It is, in any case, presumably present at low levels only, if at all. 

Indirect reductions are also permitted because species (radical anions and/or dianions) formed after homogeneous electron transfer are so basic that they are rapidly protonated by solvent or any acidic impurities. Thus, dienes and trienes like allocimene and dimethyl 2,3-butadiene may afford a redox catalysis processes [42] in dimethylforma-mide (DMF) in the presence of naphthalene as a mediator. 

The styryl radical anion species is much more reactive than the naphthalene radical anion, and rapidly couples to form a dimeric dianion, the source of the red color and the reason for the disappearance of the ESR signal (Eq. 22.40). The dimeric dianion is a double-ended anionic propagating species useful for the initiation of a number of valuable homopolymerizations (Eq. 22.41). 

In general, however, one must be concerned with the possible dominance of chemistry by small amounts of dianions. Although not seen in electrochemistry, the naphthalene dianion has been reported in the literature ll l5°-159 167) and could dictate the results of quench reactions. In the specific case of sodium naphthalene in tetrahydro-furan, kinetic analysis of a water quench directly implicites the radical anion as the chemically dominant species 150 -158-167>. In the case of the larger aromatic molecule, perylene, however, the dianion and not the radical anion is the species quenched167a). 

Cycloproparen-l-ones are very short-lived species and observable only under extreme conditions the corresponding alkylidenecycloproparenes are isolable, however, and some of their reactions have been studied. Electrochemical reduction of l-(diphenylmethylene)cyclo-propa[h]naphthalene gave a stable radical anion 1 = 519 nm) with a half-wave potential... 

Initiation by electron transfer is based on the ability of the alkali metals to supply electrons to the double bonds. This yields an anion radical and a positively charged, alkali-metal counterion. Initiation may be effected (a) by direct attack of the monomer on the alkali metal, or (b) by attack on the metal through an intermediate compound such as naphthalene. Both result in bifunctional initiation, that is, formation of species with two carbanionic ends. 

The bond lengths in the radical anion of naphthalene are probably intermediate between those of the naphthalene dianion and the naphthalene molecule. The lower-energy angular distortions of the molecule from planarity may however be similar in the radical and dianion species. 

Reactions 7 and 8 are somewhat oversimplified presentations. The transfer of a second electron from the naphthalene to the styrene is also possible and produces a monostyrene di-anion rather than the distyrene di-anion shown in-Reaction 8. In addition, the possibility of coupling of naphthalene and styrene radical-anions exists. However, the only species ever isolated (e.g., by hydrolysis to 1, A-dipheny1 butane) from this type of initiation process has been related to the distyrene species of Reaction 8. 

---