Benzophenone anion

Ketones - in sharp contrast to enols - are electron acceptors in a wide variety of organic transformations that occur at the carbonyl carbon. For example, the familiar dark-blue benzophenone anion radical is produced via one-electron reduction of benzophenone with sodium in anhydrous THF (equation 21). 

One should be aware, however, that none of the Grignard reactions of benzophenone proceeds through a completely free coupling process of benzophenone anion-radicals with alkyl radicals. For example, the portion of electron-transfer pathway in the Grignard reactions of benzophenone with isomeric C4H5MgCl was estimated to be 65, 61, and 26% for (CH3)3C—, CH3CH2CH(CH3)—, and CH3CH2CH2CH2-, respectively (Lund et al. 1999). 

In this paper, we will limit ourselves to the discussion of two types of anions in alcohol solutions the benzophenone anion [7-9] and the solvated electron. One can consider that these systems are both measurements of the solvation of an anion, if we consider the electron... 

Figure 3 Solvation time in alcohols of the electron and the benzophenone anion, plotted versus the longitudinal relaxation time tj. (The dipole data are from Refs. [5,6].)...

Anion solvation has been studied by observing the shift in the absorption spectrum of the benzophenone anion in various solvents and as a function of temperature. The benzo-phenone anion was formed from the reaction of the benzophenone molecule and a precursor to the solvated electron. Approximately 0.25 M benzophenone is put into the solution so that all the presolvated electrons will react with the benzophenone and virtually none will form the solvated electron. This process occurs much more quickly than the solvation processes that are observed [14,20]. 

Figure 4 Spectra of the benzophenone anion in n-octanol as a function of time after the generation of the anion.

Figure 6 Cartoon of the effect of structure on benzophenone anion solvation.

Simulations were done where the dipole is reversed in the solvent molecule. This is equivalent to making measurements in acetonitrile. The calculations suggested that there would only be weak solvation in acetonitrile, despite the fact that acetonitrile is far more polar than any of the alcohols that we have measured. This fact is indeed borne out experimentally. The spectrum of the benzophenone anion is considerably to the red of the spectrum of the benzophenone anion in any of the alcohols that we have measured. In addition, there is no evidence for any shift of the spectrum in the time scale that we observe. This lack of shift may not be surprising, because experiments in acetonitrile suggest that the solvation time is very fast. Thus, any relaxation that is going to take place will have occurred on the time scale of the present experiments. 

Figure 7 Time profiles of the absorption of the benzophenone anion as a function of wavelength.

Figure 8 (a) Temperature dependence of the solvation of the benzophenone anion in primary and secondary propanol and n-butanol. (b) Temperature dependence of the solvation of the electron in n-propanol and 2-propanol. 

Figure 10 Solvation kinetics of the benzophenone anion in propanol-hexane mixtures measured at 800 nm.

Experiments done by Kenney-Wallace and Jonah showed a fast initial solvation of the electron followed by a shift/increase of the spectrum towards the blue on a time scale that is similar to (but somewhat slower than) the rate of the solvation of the electron in the neat alcohol [16,17]. There was no long-term alteration of the spectrum on the time scale that would be needed to allow an alcohol molecule to diffuse to the solvated electron (a nanosecond or two, as seen in the kinetics for the benzophenone anion solvation). 

Umemoto [36] used external generation, followed by stopped-flow detection, to study the protonation of anthracene, anthraquinone, and benzophenone anions in dimethylformamide-water solutions using an apparatus similar to Figure 29.20c. The measured half-lives were about 1.5 min or more. In the case of anthracene, the decomposition rates agreed with those obtained from polaro-graphic measurements and the following scheme was proposed. 

Incorporation of an ionic component into a donor/acceptor molecule is a very effective way of suppressing electron back-transfer. One interesting example consists of the photo-oxidation of leuko crystal violet (LCV) to crystal violet (CV, the dye) by benzophe-none bearing a quaternary ammonium ion (Tazuke Kitamura 1984). In this case, the cation radical of LCV formed is repulsed by the ammonium positive charge. At the same time, the benzophenone anion radical remains stabilized by the attached cationic atmosphere (Scheme 5-16). As shown in the scheme, two favorable results are achieved the stabilization of an ion radical pair by counterion exchange and the charge separation by coulombic repulsion between the two positive charges. This leads to 100% efficiency of the photo-oxidation. With unsubstituted benzophenone itself, the efficiency does not exceed 20%. 

Fig. 15. Benzophenone anion solvation observed at 800 nm in two ionic liquids by OFSS pulse radiolysis spectroscopy. The traces consist of three 1.5-ns OFSS segments.

This is exemplified by the reaction of benzophenone anion with the tri(tolylamine) cation (TPTA+) in THF [17]. Formation of all triplet and singlet locally excited states for this system is not thermodynamically possible, yet broad, unstructured red emission is observed and was assigned to the exciplex. 

Lagow [82] has utilized sodium benzophenone anion radical to defluorinate and intramolecularly couple perfluorodicyclohexyl ether to afford perfluorod-ibenzofuran in 60% yield. 

Ps and ns laser photolysis applied to the reaction of excited benzophenone with l,4-diazabicyclo[2.2.2] octane in acetonitrile solution shows that the benzophenone anion free radical abstracts a proton from the ground state of the amine . 

Similar fragmentation occurs on irradiation of /1-phenylthioalcohols such as 84a with benzophenone as sensitizer, resulting in the formation of products such as 85a in 40-93% yield [201]. The mechanism for this reaction involves the formation of the corresponding sulfur radical cation by electron transfer from the sulfide to excited state benzophenone (a process which will be discussed in more detail in a subsequent section). The benzophenone anion radical formed in this process then deprotonates the alcohol moiety concomitantly with C=0 bond formation and C-C bond cleavage as indicated in 86. 

Model four-stage electron transfer chains have been observed (8) using either isopropyl alcohol radicals (or ethyl alcohol), aceto and benzophenone, and, in addition, a very low concentration of ferricyanide ion. One alkaline N20-saturated solution contained 0.5M isopropyl alcohol, 2.5 X 10"8M acetophenone, 10"4M benzophenone, and 8 pM ferricyanide ion. Electron transfer from the alcohol radical ion to acetophenone, followed by transfer to benzophenone, was observed, as expected. However, the benzophenone anion spectrum decayed exponentially. The transmission of the solution, over the spectral region of the ferricyanide absorption (4100 A. maximum) increased, indicating the consumption of this solute. The kinetics of ferricyanide decay were similar to those for decay of the benzophenone ketyl absorption. The... 

Zachariasse, 1975 Bard and Park, 1974). This occurs in the reaction of benzophenone anion radical with TPTA cation radical. Although this reaction is favored by low dielectric constant solvents, such as THF, it has also been observed in acetonitrile. Note that eqn (115) represents the reverse reaction of the formation of radical ions from dissociation of a photo-generated excited charge transfer complex. Finally, if may be so small that only ground state products are produced. When AG° for the annihilation reaction becomes positive, the reaction of A and D occurs spontaneously to form the radical ions. Representative CL and ECL reactions of radical ions, selected from the hundreds that have now been observed and investigated, are given in Table 3. 

The 9,10-anthraquinone system is a classic example of an EE mechanism, which includes a synproportionation process. Absorbance versus distance profiles were measured for this reaction and the homogeneous and heterogeneous rate constants were in agreement with those derived from cychc voltammetry [169]. Protonation of the benzophenone anion radical by benzoic add and o-cresol was studied using this technique [170]. A variety of electrode geometries were explored in determining the heterogeneous... 

Adam and Weissman 116, II7) found for a similar reaction occurring in the sodium-benzophenone system that the mean lifetime of the sodium ion in the ion pair is more than a thousand times greater than the mean lifetime of its counterpart in the ion pair complex, the benzophenone anion. For this reason this process has been called atom transfer. ... 

---