Ketones, photoreduction

The tris compounds are highly bridged three-dimensional polymers. Photoreduction of aqueous Ti(IV)-containing alcohols or glycols, but not of ethylene glycol, yields Ti(III) and the aldehyde or ketone corresponding to the alcohol (191,192). A possible mechanism is... 

It is important to point out at this point that the rate constant k and the quantum yield for a photochemical reaction are not fundamentally related. Since the quantum yield depends upon relative rates, the reactivity may be very high (large kr), but if other processes are competing with larger rates, the quantum yield efficiency of the reaction will be very small. That there is no direct correlation between the quantum yield and the rate is clearly seen from the data in Table 1.2 for the photoreduction of some substituted aromatic ketones in isopropanol ... 

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. 

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... 

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 ... 

Although the photoreduction of aryl ketones has been extensively investigated, as we have seen in the preceding sections, few detailed reports have been made concerning the photoreduction of alkanones.... 

The photoreduction of cyclobutanone, cyclopentanone, and cyclohexanone by tri-n-butyl tin hydride was reported by Turro and McDaniel.<83c> Quantum yields for the formation of the corresponding alcohols were 0.01, 0.31, and 0.82, respectively. Although the results for cyclopentanone and cyclohexanone quenching were not clear-cut (deviations from linearity of the Stem-Volmer plots were noted at quencher concentrations >0.6 M), all three ketone photoreductions were quenched by 1,3-pentadiene, again indicating that triplets are involved in the photoreduction. 

To conclude our study of the photoreduction of aryl ketones, let us now briefly consider some of the ways that this photoreduction has been applied to synthesis. 

Earlier reviews on the photochemistry of unsatured ketones and amines are available39,40. The photoreactions of a,/i-unsaturated carbonyl compounds in the presence of amines have been reported to yield 1 1 amine adducts32,33 as well as photoreduction... 

Intensive studies concerning the photoreductive cyclization of distinct ketones and aldehydes are made by Cossy et al. [170], They describe how bicyclic tertiary cycloalkanols 173 and 174 can be prepared from, s-un-saturated ketones 172 in good yields, initiated by photoinduced electron transfer from triethylamine in acetonitrile or by photoionization in pure hexamethyl-phosphoric triamide (HMPA) [171, 172], The reaction is stereo-, chemo- and... 

A photoreductive strategy for a short total synthesis of the sesquiterpene ( )isoafricanol was developed by Cossy et al. Thereby, the hydroazulenol skeleton was generated by photoreductive cyclization of an unsaturated ketone employing triethylamine as the electron donor (Scheme 40) [61],... 

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 presence of alcohols, photoreduction occurs more easily. Ketones like acetone can act as a sensitizer in this case (9). Surfactants can be used as protective colloids instead of polymers (10). Irradiation with 60Co y-rays is more effective for the reduction of metal ions (11). 

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 use of ketones with (ir, n) triplet states to sensitize photoreductions will be attended by chemical sensitization for example, with benzophenone in isopropyl alcohol and 0.5M acceptor, 1% of the sensitizer triplet will still abstract from the solvent even if the acceptor quenches at the diffusion controlled rate. Failure to determine that the quantum yield of reduction was greater than 0.01 might lead to the conclusion that triplet sensitization was occurring. 

Measures of the sensitivity were made in two ways, (l) Loss of ketone carbonyl was determined by FTIR on the exposed samples by measuring the relative absorbance A at 1700 cm-1. The ratio (Aq/A))7oo, was adjusted for film thickness using the styrene bands at 1600, 1495, and 1455 cm-1. This value is proportional to the rates of the Norrish type I and photoreduction processes in the copolymer (2). Changes in molecular weight result from scission in the backbone of the polymer chain. A measure, Z, of the sensitivity to main-chain scission can be derived as follows. 

The photoreduction of aryl ketones by amines generally occur via a charge transfer interaction between the triplet state of ketone and the amines, as shown in the following scheme. A ketone radical anion and an amine radical cation are formed in the intermediate stage. 

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