Benzophenone, photochemical reductive

The benzopinacol obtained by photochemical reduction of benzophenone (p. 8) may be used directly without purification. 

The following discussion begins by presenting an in-depth view of the mechanism for the photochemical reduction of benzophenone by N, iV-dimethyl-aniline. This discussion is followed by a presentation of the theoretical models describing the parameters controlling the dynamics of proton-transfer processes. A survey of our experimental studies is then presented, followed by a discussion of these results within the context of other proton-transfer studies. 

The reaction pathways by which the net transfer of a hydrogen atom from an amine to a photoexcited ketone has been extensively examined in the nanosecond [23, 25-30], picosecond [20, 22, 31-33], and femtosecond [24] time domains. The following mechanism, as it pertains to the photochemical reduction of benzophenone (Bp) by N, A-dimethylaniline (DMA), is derived from these numerous studies. Only an overview of the mechanism will be presented. The details of the studies leading to the mechanism will not be given for specifics, the reader is referred to the original literature. 

The photochemical reduction of benzophenone to benzopinacol in the presence of ferrocene31 apparently relates to a high concentration of ketone so that sufficient unquenched benzophenone triplets are present and proceed to the dimer in the usual fashion. 

For photochemical reduction of benzophenone to benzpinacone by means of isopropyl alcohol, see O.S., XIV., 8. 

Benzohydrol (Coll. Vol. i, 84) In almost quantitative yield by the photochemical reduction of benzophenone in the presence of fio-propyl alcohol and sodium fre-propoxide. Bachmann, J. Am. Chem. Soc. SS, 393 (1933)-... 

Benzophenone can be converted into benzhydrol in nearly quantitative yield by following the procedure outlined above for the preparation of benzopinacol, modified by addition of a very small piece of sodium (5 mg) instead of the acetic acid. The reaction is complete when, after exposure to sunlight, the greenish-blue color disappears. To obtain the benzhydrol the solution is diluted with water, acidified, and evaporated. Benzopinacol is produced as before by photochemical reduction, but it is at once cleaved by the sodium alkoxide. The benzophenone formed by cleavage is converted into more benzopinacol, cleaved, and eventually consumed. 

The pinacol reaction is an example of radical dimerization (Scheme 4.29). Stabihzed free radicals have sufficiently long hfetimes to permit coupling outside solvent cage confinement Scheme 4.30 shows two such coupling reactions. The first is the photochemical reduction of benzophenone to benzopinacol (Scheme 4.30a). The second is an example of the oxidative coupling of phenols, a transformation that is an important step in the biosynthesis of alkaloids (Scheme 4.30b). 

The photoreduction of benzophenone (B) in solution is one of the most extensively investigated photochemical reactions, having been studied using a wide range of reductants, solvents, reactant concentrations, and irradiation conditions. In the presence of H-donating substrates, the triplet excited state of B is able to abstract an H-atom directly (Equation 13.17) or via a sequential electron/H transfer mechanism (Equation 13.18) to produce triplet radical pairs, BHR. 

An assessment of experimental observations concerning the influence of butyl methacrylate on the primary photochemical processes occurring in the reduction of benzophenone by triethylamine suggests that they may point to the participation of ternary exciplexes. Such species may be of the form [donor + acceptor + alkene] and may play an important role in the electron transfer processes. Studies have been reported which characterise the triplet states of m-, and p-hydroxybenzophenones. In non-hydrogen bond... 

Aryl pinacols are readily obtained from the photolysis of substituted benzophenones in isopropyl alcohol. Photochemical coupling of benzophenones in isopropyl alcohol has even been reported to occur in direct sunlight.7 It is known however, that some aryl ketones do not give the pinacol from photolysis in isopropyl alcohol.8 Nevertheless, aryl pinacols have also been prepared from electrochemical reduction,9 hydroxylation of olefins,10 photoreduction of ketones by amines,11 metal reduction,12 and by other routes.13 The phenanthrene ring system has often been... 

The photochemical aspects of carbonyl photochemistry remain important subjects of research. Wagner and Thomas have used CIDNP to elucidate radical formation from a,a,a-trifluoroacetophenone. Irradiation of benzophenone and its derivatives in the presence of molecules with abstractable hydrogen atoms can give rise to intensely fluorescent compounds. This effect may interfere with the observation of nanosecond-domain kinetics.Quantum yields and kinetic isotope effects in nanosecond flash studies of the reduction of benzophenone by aliphatic amines have been measured by Inbar et Rate constant data are given in Tables 13 and 14. Winnik and Maharaj have studied the reaction of benzophenone with n-alkanes through hexane to hexatriacontane is 3.9 0.2kcal for all chain lengths.The effects of substituents on the benzophenone on these reactions have also been examined. The reactions of phenylacetophenone when used as polymerization initiator have been reviewed by Merlin and Fouassier. ... 

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