Acetophenone radical anions

TABLE 2. Rate constants for /J-cleavage of several a-substituted acetophenone radical anions 1... 

Therefore, a molecule in the TICT state can be regarded as a rigidly linked radical anion-radical cation pair. Experimental proof for this expectation can be gained from transient absorption spectroscopy. In the simplest case, the absorption spectrum of the TICT excited state is expected to be the sum of the individual ion spectra. This was indeed found in a few cases, with some perturbations which can be explained by the interaction of the closely-spaced radical ions. Thus, the transient absorption spectra of DMABN [135], of DMABK (or DMABA) [137] and of BA [58] resemble the spectrum of benzonitrile and acetophenone radical anion (the absorption of the dimethyl-amino radical cation is expected to be situated in the UV region and could not be observed) and to the sum of anthracene anion and cation absorption spectra, respectively. 

In 1967, the first three-stage electron-transfer process examined by pulse radiolysis was reported [67], Such a cascade process is of relevance to electron transport in biological systems. By irradiating an aqueous solution containing acetone (0.82 mol dm ), acetophenone (3.34 mmol dm ), and benzophenone (72 pmol dm ) at pH 13, Adams et al. [67] were able to observe, at 2 ps after the pulse, the spectrum of the acetophenone radical anion (2max 445 nm, max 260 m mol ) [68] originating from the reduction of acetophenone by the (CH3)2CO radical. In the following 50 ps, the acetophenone radical anion reacted with benzophenone k — 7.8 X 10 dm mol" s ) so that the absorption band at 445 nm disappeared... 

Based on the known photoreduction chemistry of Rose Bengal [275], one would anticipate that electron transfer would reduce the xanthene skeleton of RBAX and that the radical anion thence formed might decay by the elimination of an acetyl radical. Acetyl is totally analogous to benzoyl, the radical that initiates chains in the case of most Norrish type I UV photoinitiators, that is, benzoin ethers or acetophenone acetals. The putative scheme is shown in Scheme 7. 

Disubstitution products are obtained when dihalobenzenes (Cl, Br, I) react with aliphatic ketone enolate anions. Conversely, the reactions of o-iodohalobenzenes (X = I, Br, Cl) with the enolate anions of aromatic ketones, such as acetophenone, propiophenone and 2-naphthyl methyl ketone in DMSO yield mainly monosubstitution with the retention of one halogen (Scheme 10.7). The extent of dehalogenation is explained in terms of the energetics of the intramolecular ET from the ArCO-7t-system to the C—X bond in the monosubstituted radical anions proposed as intermediates [19]. 

The response of the peroxybenzoates to the CIEEL activators depends remarkably on the nature of the substituent. The unsubstituted peroxybenzoate [29a] behaves in a fashion nearly identical to that observed for the peroxyacetate [28]. The p-methoxy-substituted peroxybenzoate [29b] behaves quite like the parent, but the nitro-substituted compounds [29d] and [29e] and the p-dimethylamino-substituted peroxybenzoate [29c] behave quite differently. The properties of these compounds, however, can be understood within the CIEEL mechanism. For example, the nitro-substituted peroxybenzoates [29d] and [29e] with DMAC exhibit a value of k2 approximately ten times greater than for the unsubstituted peroxide. Yet the yield of excited singlet DMAC generated by [29d] and [29e] is 700 times less than from 29a. This seeming inconsistency can be easily understood. In the postulated CIEEL path, the reduction of the peroxide results in its fragmentation to acetophenone and an acid. One of these species must be a radical anion. For the peroxyacetate and all of the substituted peroxybenzoates examined, with the exception of the nitro-substituted examples, the more easily reduced species of this pair is... 

These dimerizations are analogous to those of the radical anions R2C 0 which are intermediates in the reduction of ketones to pinacols. Indeed, in the presence of magnesium amalgam, pyridine condenses with acetophenone to give alcohol 396 by oxidation of the intermediate dihydropyridine. In a similar reaction type, pyridine with zinc and acetic anhydride or ethyl chloroformate yields (397 R = Me or OEt, respectively). 

This reaction gives high product yields and works with a methylbenzyl-cyanide, 9-methylfluorenide anions as well as with the anion of a-meth-ylphenylacetic ester or propiophenone. No reaction is however found with the non methylated anions of phenylacetonitrile, fluorene, phenylacetic ester or acetophenone. The postulated mechanism implies an hydrogen abstraction from an anion leading to a radical anion intermediate ... 

Several aromatic ketyl radical anions such as those from acetophenone and henzophenone have been prepared in solution and their e.s.r. and... 

Presumably a layer of tetraalkylammonium ions is formed at the mercury surface from which the radical anion of acetophenone is quickly expelled into the bulk solution, where pinacolization can take place. An increased bulk of the alkyl chain of the ketone caused a lower selectivity reduction of isobutyrophe-none afforded, besides 68% pinacol, 17% of the corresponding alcohol. Reduction of isobutyrophenone under protic conditions gave predominantly alcohol and only 13% of the pinacol. Aryl alkyl ketones also undergo pinacolization in excellent yields when lithium ions are added to the aprotic medium. However,... 

Figure 10 shows a comparison of the simulated and experimental cyclic voltammograms it encompasses the waves of both the starting compound, 3-chloroacetophenone, and the acetophenone formed during the scan. While the first wave is described well theoretically, the second wave of acetophenone deviates somewhat. The fit to the second wave may be improved by introduction of new processes involving the radical anion of acetophenone besides the dimerization and cross reactions. However, since reproducible values of ka and 2kcyclic voltammetric wave for the different substrate concentrations and sweep rates employed, these reactions were not considered in greater detail. 

Kinetic studies have revealed that aliphatic ketyl radical anions are very shortlived compared with aromatic (half life of acetone " in aqueous 2-propanol is 72 ps, whereas that for acetophenone " is 1.5 ms) [253]. The reductive dimerization of simple aromatic aldehydes has been studied in aprotic solvents, with the second order rate constant being larger in acetonitrile than in DMF, because of ion-pair effects [254]. Electron-withdrawing substituents reduce the speed of dimerization (benzaldehyde " k = 2.4x 10 m" s", p-cyanobenzaldehyde k = 5 M s" ) [255], whereas protic solvents lead to protonation before dimerization [256]. 

The detailed studies of the surface of CdS nanocrystallites prepared in N,N-dimethylformamide (CdS-DMF) by means of emission measurements, in-situ Cd K-edge EXAFS analysis, and theoretical MO calculations reveal the correlation of the photocatalysis of CdS-DMF and the formation of sulfur vacancies on its surface. It has been experimentally proved that CO2 interacts with the sulfur vacancies and is converted into its radical anion under irradiation as an intermediate in the photocatalysis. The knowledge on the photocatalysis obtained above has led to the achievement of the photofixation of CO2 into benzophenone, acetophenone and benzyl halides under visible light irradiation in the presence of TEA as an electron donor. 

Electronic excitation of l,2-bis[4-(A, A -dimethylamino)phenyl]ethane-l,2-diol and 2,3-bis[4-(A, A -dimethylamino)phenyl]butane-2,3-diol in aerated chloroform induces photoelectron transfer to give the chloroform radical anion followed by its subsequent dechlorination, and also a retro-pinacol reaction in the substrates. This latter process leads to 4-(A, 7V-dimethylamino)-benzaldehyde and 4-(A, iV-dimethylamino)acetophenone. 

Beckett, A., A. D. Osborne, and G. Porter Primary Photochemical Processes in Aromatic Molecules. Part II. Radicals and Radical Anions Derived from Benz-aldehyde. Acetophenone and Benzil. Trans. Faraday Soc. 60, 873 (1964). 

Figure 9 shows internal reflection spectra taken during the electrolysis of p-benzoquinone on a Ge prism electrode at potentials < - 0.35 V vs. NHE showing the formation of the p-benzoquinone radical anion in DMSO, as reported by Tallant and Evans in 1969 [25 and references cited therein]. They also detected the anion radical of Benzil, as well as unassigned reduction products of acetophenone and benzophenone. Because of the relatively low sensitivity of the technique, rather high concentrations of reactants had to be used in order to be able to detect any products (> lOmM). An additional problem was the relatively long settling times before steady-state concentra-... 

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