Sensitizer ketone

Several mechanisms have been postulated in order to account for ketone-sensitized photodehydrochlorination. Benzophenone and acetophenone have been suggested to act as singlet sensitizers via a collisional deactivation process (13). An alternative mechanism proposed for benzophenone involves abstraction of a methylene hydrogen from PVC by the triplet ketone (Equation 2), followed by 3 scission of a... 

Highly efficient and stereoselective addition of tertiary amines to electron-deficient alkenes is used by Pete et al. for the synthesis of necine bases [26,27], The photoinduced electron transfer of tertiary amines like Af-methylpyrrolidine to aromatic ketone sensitizers yield regiospecifically only one of the possible radical species which then adds diastereospecifically to (5I )-5-menthyloxy-2-(5//)-furanone as an electron-poor alkene. For the synthesis of pyrrazolidine alkaloids in approximately 30% overall yield, the group uses a second PET step for the oxidative demethylation of the pyrrolidine. The resulting secondary amine react spontaneously to the lactam by intramolecular aminolysis of the lactone (Scheme 20) [26,27]. 

Hydrogen abstraction by ketone sensitizers forms the basis of an elegant alkylation procedure for ethers, amides, lactones, amino acids, and peptides developed by Elad and his group.94 Terminal olefins, for example, can be added to y-butyrolactone (15) in good yield.95... 

As a final example we would like to note the reactions of quinone methide 27, which remains unchanged upon irradiation in methanol solution through a Pyrex or Vycor filter.128 Reduction is observed when 27 is irradiated in isopropyl alcohol with benzophenone or acetophenone,129 in methanol with acetophenone, or in methanol with no sensitizer through quartz.128 While the effect of ketone sensitizers is due to chemical sensitization,129 the wavelength effect is much more surprising. Further investigation showed that irradiation of methanol alone in a quartz vessel yielded ethylene glycol. The 185 nm line of the mercury arc, which is absorbed by Vycor and Pyrex, decomposes... 

Other systems for which the results cannot be explained by energy transfer include the ketone sensitized isomerization of / -methylstyrene, 1,2-dichloro-ethylene and 2-butene studied by Caldwell,147,148 deuterium exchange between acetone-d8 and tetramethylethylene reported by Japar, Pomerantz, and Abrahamson,149 and the acetone sensitized isomerization of 1-methoxy-l-butene examined by Turro and Wriede.71... 

All the olefins involved in these studies are simple olefins to which energy transfer from ketones should be endothermic. Although the details of the mechanism of isomerization are still a matter of some debate,5,71 it is generally agreed that isomerization takes place by addition of the sensitizer to the olefin to form a new intermediate which may be of a biradical nature. Thus if energy transfer is not favored, there is another mechanism by which ketone sensitizers can induce olefin isomerization, and the observed quantum yields and photostationary states may differ sharply from those predicted by the energy transfer mechanism. 

Recently Swenton and coworkers204 have discovered singlet energy transfer in the decomposition of 2-azidobiphenyl (47). Aromatic ketone sensitizers gave 49 in about 40% yield and almost no carbazole 48, while... 

In a careful study of cyclopentenone photocycloaddition reaction, DeMayo and coworkers215 have noted that ketone sensitizers of triplet energy less than 71.2 kcal did not sensitize cycloaddition to cyclohexene. Triphenylene (Et = 66.6 kcal) and acenaphthene (ET = 59.3 kcal) were exceptional since they resulted in quantum yields of 0.10 and 0.21, respectively. This behavior, as well as the fact that 0.1 M cyclopentenone quenched acenaphthene fluorescence by 90% in an EPA (ether-isopentane-alcohol) glass at 77°K strongly implicate singlet energy transfer. 

The reaction of formamide with aromatic compounds under ultraviolet irradiation is still unexplored and only preliminary results have so far been obtained. In the cases already studied it has been found that this reaction must be sensitized with a ketonic sensitizer, usually acetone, in order to take place. The mechanism of the photoamidation of aromatic compounds certainly differs from the one of simple olefins. The detailed mechanism still awaits further experimental evidence, and in some cases involves, most probably, radical combinations and not addition of radical to unsaturated systems. Interactions of the excited sensitizer with aromatic compounds, having in some cases triplet energies similar or just a bit higher than those of the sensitizers used, must be brought into consideration. Experimentally it has been shown that the photosensitized amidation of benzene leads to benzamide (11),... 

In other cases the addition of the ketone sensitizer to the cyclopropane moiety has been observed (equation 53). This secondary reaction is probably occurring bwause of the notably stable benzyl radical center that results upon rupture of the cyclopropane. 

In the emulsion-polymerization of methyl methacrylate by carbonyl initiators, intramicellar processes have been found to reduce the conversion rate. On the other hand, the application of weak magnetic fields has been found to increase the conversion of emulsion-polymerized vinyl monomers using triplet ketone sensitizers. Here external magnetic fields will decrease the efficiency of triplet-to-singlet radical-pair intersystem crossing within the micelles and thus increase the function of radical pairs that escape without terminating the polymer chains. 

In the photoaddition of a saturated hydrocarbon to ethyl propiolate (equation 19) it is likely that the excited state of the acetylenic ester initiates reaction by abstracting a hydrogen atom from the hydrocarbon. The addition of cyclic ethers to an alkyne seems similar (equation 20), although a ketone sensitizer is required for addition of tetrahydropyran or dioxan . When reactions of this type involve a conjugated acetylenic ester, the first-formed a,p-unsaturated ester can normally undergo further photochemical reaction to produce the p,y isomer (see equations 19 and 20). 

Recently Betrand et al. have reported the sensitized intermolecular addition of tertiary amines derived from pyrrolidines to (5R)-5-menthyloxy-2[5//]-furanone (328) using ketonic sensitizers (Scheme 73, Table 13) [329]. 

Irick Jr., G., and Pacifici, J. G. (1969). Photochemistry of azo compounds. II. Effect of ketonic sensitizers on photoreduction of 4-dimethylamino-4 -nitroazobenzene. Tetrahedron Lett. 2207-2209. 

DiallyIphthalate, which has been converted under radical conditions to a low polymer, photocrosslinks in the presence of N-phenylmaleimide and Michler s ketone sensitizer (.2 ). Possibly the cross I inking involves copolymerization of unreacted ally I groups with the maleimide. 

Figure 8 shows the T-T absorption spectra of 11 observed on Michler s ketone sensitization, cis- and trans-11 exhibited different T-T absorption spectra which decayed without changing the spectral profile, indicating that the conversion between c and t is inefficient even in the 10 ps time range [74]. 

A similarly detailed study of the polymerization of methyl methacrylate by diphenyliodonium chloride in aq. AN using various aromatic ketone sensitizers has recently been carried out by Timpe s group [103a]. On the one hand the results were not complicated by monomer quenching of excited ketone sensitizer, but were complicated by operation of simultaneous singlet and triplet pathways for the electron-transfer sensitization. In this study the data were analyzed without reference to the possibility of triplet energy transfer sensitization of the iodonium salt photolysis [22], or the possible involvement of charge transfer complexes. 

Ketone-sensitized photoadditions of aldehydes to aj -unsaturated esters gives 4-oxoalkanoic acids in variable but sometimes excellent yields (Cerfontain and van Noort cf. Rosenthal and Chow). Insoluble benzoylated polystyrene is effective as... 

The furocoumarins (49) yield cis,anti,cis-dimers when irradiated directly or upon Michler s ketone-sensitized irradiation.81 However, the dihydrofuro-coumarins (50) are not so selective and yield both the //, -head-to-head and the c/ sjTi.m-head-to-tail dimers on direct irradiation but only the former... 

Aziridines (55) are formed together with the corresponding imines by direct but not by ketone-sensitized irradiation of the aliphatic allylamines (56).53 The cyclization is believed to proceed via a zwitterion, analogous to the intermolecular... 

Cyclopentadiene gives the following substances on irradiation in the presence of a ketone sensitizer ... 

Figure 2. Effect of concentration of cis-6a on the quantum yields of cis- trans isomerization of 6a on Michler s ketone sensitization.

Energy transfer, discussed above, is one example of the direct reaction of the excited triplet state. It is also possible that the excited triplet could react directly through electron or hydrogen-atom transfer. The photochemical reactivity of a series of phenols (methyl and methoxy substituents) in the presence of natural DOM and ketone sensitizers can be best explained by the latter processes. Reactions were carried out at pH 8 where phenols existed primarily (>98%) in the undissociated form, which were shown (p. 225) not to be particularly reactive with singlet oxygen. 

Figure 6.19 Photosensitized transformation of 10 xM 2,4,6-trimethylphenol, TMP, in the presence of humic acid, FHA (15 mg C/L), or a ketone sensitizer, 3 -methoxyacetophenone, 3 MAP (100 pJW). Irradiated with a 320-nm cutoff. [Reproduced with permission from S. Canonica, U. Jans, K. Stemmier, and J. Hoigne, Environ. Sci. Technol. 29, 1822 (1995). Copyright 1995, American Chemical Society.]...

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