Benzophenones, photoreduction

In a study of the benzophenone photoreduction, Wagner104 found that with biphenyl, fluorene, or triphenylene as quenchers, the quantum yield of photoreduction depended upon the benzophenone concentration. This indicates that reversible quenching of the reaction, a phenomenon similar to the reversible sensitization observed by Sandros, is appreciable in this system. 

A] is the concentration of acceptor and [CH2] is the solvent concentration. The ke to ku ratio has been measured for benzophenone in isopropyl alcohol, the most commonly used system for sensitized photoreductions. From a study of naphthalene quenching of benzophenone photoreduction, Beckett and Porter107 found that ke = 192fcH[CH2]. 

In a study of the quenching of benzophenone photoreduction with biacetyl, it was observed that the biacetyl was consumed.116 This phenomenon also is readily explained by chemical quenching, transfer of a hydrogen atom from the benzophenone ketyl radical to biacetyl to form the biacetyl ketyl radical and ultimately the pinacol derived from biacetyl. 

Which of the following molecules do you expect would be useful in quenching benzophenone photoreduction Explain. 

Hydrogen abstraction by benzophenone or xantone derivatives is a classical problem that continues to attract research interest in this field. The substituent effects on the kinetics of benzophenone photoreduction by added isopropanol have been examined in acetonitrile. In agreement with expectations, both experimental results and DFT theoretical calculations are consistent with the involvement of n- i excited states. 

CB Roberts, JE Chateaunuef, JF Brennecke. Unique pressure effects on the absolute kinetics of triplet benzophenone photoreduction in supercritical CO2. J Am Chem Soc 114 8455, 1992. 

Weiner, S. A., Behavior of photochemically generated ketyl radicals modified mechanism for benzophenone photoreduction, /. Am. Chem. Soc., 93, 425,1971. 

TTie photoreduction can be quenched by known triplet quenchers. The effecti e quenchers are those which have T] states less than 69kcal/moI above S,. Quenchers with higher triplet energies are ineffective because the benzophenone n-n triplet is then not sufficiently energetic to effect energy transfer. 

The efficiency of reduction of benzophenone derivatives is greatly diminished when an ortho alkyl substituent is present because a new photoreaction, intramolecular hydrogen-atom abstraction, then becomes the dominant process. The abstraction takes place from the benzylic position on the adjacent alkyl chain, giving an unstable enol that can revert to the original benzophenone without photoreduction. This process is known as photoenolization Photoenolization can be detected, even though no net transformation of the reactant occurs, by photolysis in deuterated hydroxylic solvents. The proton of the enolic hydroxyl is rapidly exchanged with solvent, so deuterium is introduced at the benzylic position. Deuterium is also introduced if the enol is protonated at the benzylic carbon by solvent ... 

Thus irradiation of benzophenone in toluene gives the photoreduction product 19, benzpinacol, and bibenzyl [equation (59)] of which only 19... 

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. ... 

To obtain more information on this point, let us examine the data given in Table 3.6<42-47> for some substituted benzophenones. The data in Table 3.6 indicate that benzophenone derivatives having lowest triplet states of n->TT character undergo very efficient photoreduction in isopropyl alcohol. Those derivatives having a lowest it- -it triplet, on the other hand, are only poorly photoreduced, while those having lowest triplets of the charge-transfer type are the least reactive toward photoreduction. In additon, in some cases photoreduction is more efficient in the nonpolar solvent cyclohexane than in isopropanol. This arises from the solvent effect on the transition energies for -> , ir- , and CT transitions discussed in Chapter 1 (see also Table 3.7). 

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... 

Benzophenone has also been found to be photoreduced in the presence of amines as hydrogen donors, although less efficiently than in the presence of benzhydrol or isopropyl alcohol. The photoreduction of ketones in aromatic amines is thought not to go by the same mechanism as the photoreduction in alcohols, for the following reasons ... 

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. 

Let us now discuss some of the characteristics of this quenching with mercaptans and disulfides. Interestingly, both sulfur derivatives are equally effective in inhibiting the photoreduction and are in fact interconverted during the reaction. The same equilibrium mixture of mercaptan and disulfide is obtained regardless of which was initially added to the reaction mixture. Furthermore, there appears to be no appreciable consumption of the sulfur compounds/64 When benzophenone is irradiated in the presence of isopropanol (OD) and mercaptan, isopropanol containing two deuterium atoms is isolated,... 

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...

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