Chemical Methods of Organic Ion-Radical Preparation

Electrochemical methods for the generation of anion-radicals consist of potential-controlled electrolysis. The control of a potential allows one to detain rednction just after a one-electron transfer to a depolarizer. The one-electron natnre of the electron transfer is coincidentally inspected by means of coulombometry. One molecnle mnst consnme one electron. If less than one electron is consumed in the framework of the one-electron rednction, it means that the yield of an anion-radical is not quantitative. The electrolysis in a special amponle placed into a resonator of the electron spin resonance (ESR) spectrometer permits one to identify many nnstable anion-radicals. The electrochemical methods of anion-radical generation employ an electrode as an electron donor. 

Another common solvent that contains the oxygen atom easily available for coordination with metal cations is THE. The ability of anion-radicals to remove a proton from the position 2 of THE is sometimes a problem. Dimethyl ether is more stable as a solvent its oxygen atom is also exposed and can coordinate with a metal cation with no steric hindrance from the framing alkyl groups. An added advantage of dimethyl ether is that, because of its low boiling point (-22°C), it can be readily removed after reductive metallation and replaced by the desired solvent. The use of aromatic anion-radicals in dimethyl ether (instead of THE) is well documented (Cohen et al. 2001, references therein). 

Colloidal potassium has recently been proved as a more active reducer than the metal that has been conventionally powdered by shaking it in hot octane (Luche et al. 1984, Chou and You 1987, Wang et al. 1994). To prepare colloidal potassium, a piece of this metal in dry toluene or xylene under an argon atmosphere is submitted to ultrasonic irradiation at ca. 10°C. A silvery blue color rapidly develops, and in a few minutes the metal disappears. A common cleaning bath (e.g., Sono-clean, 35 kHz) filled with water and crushed ice can be used. A very fine suspension of potassium is thus obtained, which settles very slowly on standing. The same method did not work in THF (Luche et al. 1984). Ultrasonic waves interact with the metal by their cavitational effects. These effects are closely related to the physical constants of the medium, such as vapor pressure, viscosity, and surface tension (Sehgal et al. 1982). All of these factors have to be taken into account when one chooses a metal to be ultrasonically dispersed in a given solvent. 

By and large, a hnely divided precipitate of a metal is a very effective one-electron reducer. For example, a hnely divided precipitate of Zr(0) was obtained on mixing naphthalene sodium derivative in THF with ZrCl4. The Zr(0) precipitate dissolved on addition of anthracene or benzophenone to form the corresponding zirconium salts of the anion-radicals (Terekhova et al. 1996). 

Neutral organic molecules can also be one-electron donors. For example, tetracyano-quinodimethane gives rise to anion-radical on reduction with 10-vinylphenothiazine or N,N,N, N -tetramethyl-p-phenylenediamine. Sometimes, alkoxide or phenoxide anions hnd their applications as one-electron donors. There is a certain dependence between carbanion basicity and their ability to be one-electron donors (Bordwell and Clemens 1981). 

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