Acetophenone ketyl radical

H Lutz, E Breheret, and L Lindqvist. Effects of Solvent and Substituents on the Absorption Spectra of Triplet Acetophenone and the Acetophenone Ketyl Radical Studied by Nanosecond Laser Photolysis. 7. Phys. Chem. 77 1758-1762, 1973. 

Side-chain aromatic radicals of the type Ph—CR—OH cannot be produced efficiently by reaction of OH with the alcohol because OH tends to add to the aromatic ring more efficiently. They are, therefore, produced by addition of an electron to the carbonyl compound. The pRwalues for the acetophenone and benzophenone ketyl radicals are 9-5 and they are affected by substituents on the aromatic ring as well as by those directly at the radical site. ... 

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

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]. 

Several examples have appeared of diversion of the normal photochemistry of ortho-alkyl aromatic ketones due to apparent interception of the intermediate biradical by an unsaturated group present elsewhere in the molecule.Thus the vinyl aryl ketones (376) are converted to (377) in reasonable yields upon irradiation with ultra-violet light,while irradiation of the ortho-alkynyl acetophenone (378) in methanol yields diastereomers of (379)The latter reaction is thought to proceed by coupling of the initially formed ketyl radical onto the alkyne to give a carbene (380). The photochemistry of the jS-diketone (381) gives a mixture of the benzocyclobutenol (382) and the tetralone (383), and the proportions depend on the identity of the R-substituent in (381). The formation of (383) can be rationalised in terms of interception of the ketyl radical in the initially formed 1,4-biradical by the remote carbonyl group. 

This powerful system was also applied for the trapping of ketyl radicals, which are very difficult intermediates to be detected and quantified with traditional techniques (i.e. EPR). Ketyl radicals were initially produced using photochemical reactions of acetophenone, whose excited triplet state is able to abstract hydrogen from an H donor... 

A novel method for generating semidione radicals has recently been reported by Monroe, Weiner, and Hammond 178) who found that the quantum yield for photoreduction of camphorquinone in 2-propanol was markedly enhanced when benzophenone was added and the solution irradiated at wavelength 3660 A where most of the light was absorbed by benzophenone. Instead of benzpinacol formation, the dione underwent photoreduction. Similar enhancement was not observed with jw-methoxy-acetophenone which does not abstract hydrogen from 2-propanol. The conclusion was that the ketyl radical, formed in the efficient H-abstraction reaction of benzophenone, transferred a hydrogen atom to camphorquinone to generate the semidione radical. It was suggested that this phenomenon be called "chemical sensitization . 

Other Mechanisms Other three-component systems involve different mechanisms (see Refs [li,m] and references therein). For example, in the dye (crystal violet, phenosaffanine, methylene blue, thiopyronine)/amine/ketone (acetophenone, benzo-phenone, thioxanthone, 4,4 -bis-dimethylamino benzophenone) combi-nation, the mechanism is quite complex. In the camphorquinone CQ/amino-benzophenone ABP two-component system, both CQ and ABP absorbs the light an aminoalkyl radical on ABP and a ketyl radical on CQ are formed. 

Recently, enantioselective additions of samarium diiodide-generated ketyl radicals to olefins have been demonstrated [15]. As illustrated in Eq. (10), the reductive coupling of acetophenone with methyl acrylate (31a) in the presence of chiral phosphine ligand 29 (i -BINAPO) gives somewhat low yields (mostly under 50%) but moderate to good levels of enantioselectivity (60-70% ee) for the y-butyrolactone products 34. 

Acetophenone promoted the oxidative coupling of aniline to azobenzene, perhaps by such a mechanism (Anjaneyulu and Mallavadhani, 1988), but also perhaps by a ketyl radical process. 

In the case of monomerization, the form of the current-potential curve and the values of peak current and potential as a function of sweep rate provide a useful means of characterizing this mechanism. In the case of the dimerization type of reaction, however, the i-E curves are not appreciably affected by the chemical reaction. Also, unless the dimerization is relatively slow, the detection of an intermediate in the anodic direction corresponding to a reversed scan is difficult. With acetophenone reduction, for example, reoxidation of the ketyl radical intermediate formed in the cathodic direction of sweep in acid solutions (CHs-COH -c -h e CH3 COH < ) is not observed. 

Similarly, methyl thiophenecarboxylate reacts at the C5 position with a series of substituted benzaldehydes and acetophenone in the presence of the SmI2/THF-HMPA system to give a samarium enediolate which can be trapped by a second electrophile [59] (Scheme 23). The samarium-bonded ketyl anion radical does not trap the hydrogen atom or undergo acyloin coupling, presumably due to the hindrance of the ligated HMPA molecules. 

Electron transfer to acetophenone from the isopropyl radical ion, an a-hydroxy radical, has been discussed above and is responsible for the ketyl spectrum in N20-saturated solutions of isopropyl alcohol. Hence, transfer from the tert-butyl radical ion, a /3-hydroxy radical, either does not take place or is too slow to be observed under the given experimental conditions. 

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... 

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