Photoreduction of benzophenone

We have already discussed one of the earliest photoreactions to be studied, that is, the (4w + 4w) photodimerization of anthracene. That the singlet state was involved in this reaction was conclusively shown in the period 1955-1957. The first reaction in which the triplet state of the molecule was shown to be involved was the photoreduction of benzophenone by Hammond and co-workersa) and Backstrom and co-workers<2) 1959-1961. This was the first in a series of many papers from Hammond s laboratory... 

Table 3.2. Comparison of Rate Constants for the Photoreduction of Benzophenone...

So far in this chapter we have discussed only the photoreduction of benzophenone. We will now concern ourselves with the effect of structure on the photoreduction of aromatic ketones in general. 

Thus for the photoreduction of benzophenone in the presence of benzhydrol (RH = < 2CHOH, C—H x 78 kcal/mole) the overall reaction is exothermic by 26 kcal/mole. For the photoreduction of acetophenone the value calculated in this way is —31 kcal/mole, while for fluorenone the abstraction is still exothermic by 10 kcal/mole. Clearly, the energetics of the abstraction reaction is not the reason why benzophenone and acetophenone photoreduce but fluorenone does not, since the values for the overall reaction would indicate that all three compounds should react. However, this conclusion is not necessarily valid if large activation energies are involved. ... 

As seen earlier in this chapter, the primary chemical process in the photoreduction of aromatic ketones is the abstraction of a hydrogen atom from the solvent, as in the case of the photoreduction of benzophenone in... 

In Section 3.1 it was shown that the photoreduction of benzophenone can be quenched by addition of small amounts of triplet quenchers such as oxygen or ferric dipivaloylmethide.<60) In fact this was presented as evidence that the benzophenone triplet was involved in the photoreduction. This reaction can also be quenched by naphthalene. In the presence of naphthalene, light is still absorbed by benzophenone and thus benzophenone triplets are produced. However, photoreduction products are decreased. On examining this reaction with flash photolysis, triplet-triplet absorptions were observed but these absorptions corresponded to those of the naphthalene triplet. Thus the triplet excitation energy originally present in the benzophenone triplet must have been transferred to naphthalene and since little of the photoreduction product was observed, this transfer must have been fast in relation... 

A number of different mechanisms have been proposed to account for the fact that this product is not observed. Recently, however, a report appeared that described the formation of the mixed pinacol from the photoreduction of benzophenone with isopropyl alcohol and the photoreduction of acetone with benzhydrol. The data from this study are presented in Table 3.10.(73)... 

Attempts to sensitize the rearrangement with benzophenone, propiophenone, and chlorobenzene failed, as indicated in Table 8.1. Although the reaction could not be sensitized, triplet energy transfer was taking place inasmuch as compound (1) quenched the photoreduction of benzophenone without the formation of any new products (Table 8.2). 

We have already seen, however, that kd as determined from the experimental data has a maximum value of 1 x 107. Therefore the reactive species in the photoreduction of benzophenone cannot be the singlet state. 

This reaction is the reverse of the initial ketyl radical formation by the benzophenone triplet and is therm Q4ynamically favorable. The experiments using optically active alcohols as source of hydrogen atoms show, however, that under normal conditions this reaction is unimportant. This is probably due to other, more efficient pathways for reaction of the ketyl radicals or perhaps to diffusion rates which separate the radicals before reverse transfer can occur. That this reaction can be important in some cases even without the presence of sulfur compounds was shown by studying the photoreduction of benzophenone in optically active ethers.<68) Although the reaction of benzophenone in methyl 2-octyl ether is only 0.17 times as fast as that in isopropanol, ethers can be used as sources of hydrogen atoms for photoreduction ... 

Table 3.10. Ratio of Mixed to Normal Pinacol in the Photoreduction of Benzophenone with Isopropyl Alcohol and Acetone with Benzhydrol...

Data obtained from the photoreduction of benzophenone in isopropanol indicates that the quantum yield (for the formation of acetone) is nearly constant using irradiation at several different wavelengths between 366 and 254 mp..16 The quantum yield for phosphorescence emission from the carbonyl n,ir triplet state is independent of exciting wavelength.22 Furthermore, the singlet excitation spectrum for the phosphorescence emission has been found to be very similar (if not identical) to the absorption spectrum.23 There are other examples where irradiation in the region of a given transition has ultimately led to a triplet of a different type.24... 

One of the limitations of the photocycloaddition reaction is that the unsaturated system may itself act as a quencher. Conjugated dienes fall within this category since they quench the n,ir triplet of some carbonyl compounds. For the photoreduction of benzophenone in benzhydrol, the ratio, kqlka, for m-piperylene is 750 (Table II), which indicates that this diene is indeed an efficient quencher for the reaction. 

Direct evidence for triplet-triplet transfer has been provided by sensitizing both the isomerization of cis- or ira/w-olefins,28 38 79,80 and the dimerization of some cyclic olefins81-83 with carbonyl compounds. Furthermore, the phosphorescence of some carbonyl compounds can be quenched by olefins (for example, acetone with 2-pentene30 and phenyl-cyclopropyl ketone with 2-methyl-2-butene37). On the other hand, the phosphorescence of benzophenone is not quenched by 2-methyl-2-butene37 nor is the photoreduction of benzophenone quenched efficiently by cyclohexene (Table II). 

In the examples of Figures 18.8 and 18.9 the probe molecule is diphenyl-methanol, and it reacts with ferf-butoxyl radicals as shown in Scheme 18.3. Usual probe concentrations were between 50 and 200 mM. Figure 18.8 shows a representative trace for the formation of the ketyl radical from diphenylmethanol (i.e., the same formed by photoreduction of benzophenone), the only detectable species in the system. Figure 18.8 shows how the value of growth, given by the slope of the plots, changes with substrate (1,7-octadiene) concentration, as predicted by Eq. 18. 

Recent work on the photochemistry of ketimines has shown that they do not undergo reduction unless ketones are present. Thus chemical sensitization is entirely responsible for the photoreduction of benzophenone methylimine (22)105 while intramolecular chemical sensitization has been suggested as the mechanism for reduction of the acylketimine (23).119 In related work,... 

The photoreduction of benzophenone in 2-propanol was one of the earliest reported photochemical reactions331 and perhaps the one most familiar to students of organic chemistry.332 Study of this reaction fostered the rapidly mushrooming interest in photochemical mechanisms which has marked the past decade. 

Since quenching experiments with piperylene and naphthalene showed no effect, it was suggested that the triplet state of the thiatriazole is not involved in the reactions.22 It was later shown that the photoreactions leading to nitrile, isothiocyanate, sulfur, and nitrogen actually take place from a singlet excited state of the thiatriazole.20 Thus the photoreduction of benzophenone in isopropyl alcohol is effectively quenched by addition of 5-phenylthiatriazole, but analysis revealed that all thiatriazole could be recovered from the photolysis mixture, indicating the lack of photoreactivity of the triplet state. 

The photoreduction of benzophenone in hydroxylic solvents is well known (64). ort/io-Benzylbenzoylbenzene (Formula 161), however, does not undergo photoreduction on irradiation in hydroxylic solvents but rather enolizes (65). Irradiation of Formula 161 in methanol-O-D gives rise to incorporation of 1.04 to 1.09 deuterium per molecule (Formula... 

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