Quantum yield cyclic ketones

The photorearrangement of 7V-aryl lactams 34 to the ortho-position offers an interesting possibility for the preparation of cyclic benzo-aza-ketones of type 35, whereas the rearrangement to the para-position should lead to para-cyclophane 144 formation. 7-, 8-, and 13-Membered ring jV-aryl lactams (34, n = 5, 6, and 11) were successfully rearranged in ethanol to 35 in chemical yields of 60, 83, and 80%,13 and in quantum yields of 0.071, 0.11, and 0.082, respectively.14... 

The values of the primary quantum yields, found in the photolysis of ketones with y-H atoms, were explained by Brunet and Noyes on the basis of steric effects that diminish the probability of the formation of the cyclic complex. Following the original suggestion of Whiteway and Masson, Martin and Pitts proposed the internal conversion of the cyclic structure to be responsible for the low primary quantum yields observed in the photolysis of ketones capable of forming such a structure. In contrast to other interpretations. Wagner and Hammond explained the low quantum yields by an elementary chemical process, suggesting that the y-H atom transfer is reversible, i.e. that the biradical, after vibrational relaxation, may convert back into the ground state ketone molecule, viz. 

It is to be noted that the low quantum yields may also be explained, by similar reasoning, on the basis of the concerted mechanism. In this case, it should be assumed that the cyclic complex (or complexes) may convert back into a ground state ketone molecule. 

On irradiating cyclic ketones in the vapour phase by uv light, only fluorescence was observed. The influence of oxygen on the product quantum yields is negligible it is identical with that exerted by inert ... 

A pressure increase, brought about by an increase in the concentration of the ketone or by the addition of an inert gas, enhances the formation of the unsaturated aldehyde as compared to that of CO. The value of awehyde increases at the expense of 0co> thus, the ketone consumption yield is independent of pressure. This seems to be generally valid in the photolysis of the cyclic ketones it was confirmed, for instance, for cyclopentanone °, cyclohexanone 2-methyl cyclohexanone and 2,6-dimethyl cyclohexanone . An increase in wavelength also favours the formation of the aldehyde as compared to decarbonylation in the photolysis of cyclobutanone , cyclopentanone and cyclohexanone . At 3130 A, the decrease in temperature has a similar effect on the product distribution in the photolysis of cyclopentanone , cyclohexanone , 2-methyl cyclohexanone , and 2,6-dimethyl cyclohexanone to that caused by the increase in wavelength or pressure. However, at shorter wavelengths, the quantum yields seem to be independent of temperature . ... 

There is significant disagreement between some of the results obtained in different laboratories. The conclusions given above are mainly based on only one kind of information, i.e. on the pressure-dependence of the product quantum yields, for which more than one equally reasonable explanation often exists. Since phosphorescence is not observed with cyclic ketones, the method of phosphorescence quenching—which is one of the main sources of information about the role of the triplet state—cannot be used here. These facts clearly indicate that further studies are required before a reliable scheme for the photo-physical processes can be formulated. 

Cyclopentanone emits both fluorescence and phosphorescence in the liquid phase ° The results of Dunion and Trumbore show that the quantum yield of 4-pentenal formation, in the photolysis of cyclopentanone, is decreased by O2 or piperylene . Since both substances are known to be triplet state quenchers, it may be suggested that the triplet state participates in the photolytic decomposition of cyclic ketones in the liquid phase and in solution. 

The most extensively used source of trifluoromethyl radicals is the photolysis of hexafluoroacetone. The primary process has been investigated in some detail by Kutschke and co-workers. Two mechanisms of photolysis were observed, one from thermally excited singlet and the other from the lowest triplet state ( ). The quantum yield is temperature dependent, the activation energy for the decomposition of the triplet being estimated at 16 kcal mol". There is evidence that the excited singlet hexafluoroacetone can undergo cyclic addition with perfluoroolefins to form the corresponding oxetane (4). This reaction involves addition of the triplet state of the ketone to the electron deficient double bond of the fluoroolefin( ). 

For copolymers of styrene containing minor amounts of phenyl vinyl ketone, photolysed in the solid phase, the quantum yields are low at temperatures below 100°C they rise abruptly to the value obtained in solution at temperatures above 100°C [510]. This temperature corresponds to the glass transition temperature (Tg) for the copolymer. It is characteristic of the polymer structure, and represents the temperature at which there is sufficient free volume in the polymer so that extensive motion may occur. For amorphous polymers, the macromolecules motion is sufficient to facilitate the formation of the cyclic intermediate required for the Type II process to occur, and for the separation of radical species by the Type I process. Above Tg, molecular mobility is sufficiently great that the rate of formation of the... 

Silyloxyproline 25 (10 mol%) was able to catalyzed the reaction of several cyclic and acychc ketones with dibenzylic azodicarboxylate (18b). In order to achieve high yields and enantioselectivities, nine equivalents of water were added to the reaction mixture, although this addition retards the reaction [33]. When these conditions were applied to the reaction with cyclopentanone, the major product was the a,a -diaminated cyclopentanone in 46% yield and 99% ee. In this process, contrary to that observed for the case of aldehydes, a linear correlation between the enantiomeric excess of the product versus the catalyst was found. Quantum mechanical calculations suggested that the most stable transition state was that in which the anti-conflgured enamine reacts with the azodicarboxylate in the trans configuration. 

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