Triplet-state ketone intermediate

Figure 5.33 Benzophenone-4-iodoacetamide reacts with sulfhydryl-containing compounds to give thioether linkages. Subsequent photoactivation of the benzophenone residue gives a highly reactive triplet-state ketone intermediate. The energized electron can insert in active C—H or N—H bonds to give covalent crosslinks.

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. 

One may well ask why the isomerization of alkenes discussed in the preceding section requires a sensitizer. Why cannot the same result be achieved by direct irradiation One reason is that a tt — tt singlet excited state (5,) produced by direct irradiation of an alkene or arene crosses over to the triplet state (Ij) inefficiently (compared to n —> it excitation of ketones). Also, the Si state leads to other reactions beside isomerization which, in the case of 1,2-diphenyl-ethene and other conjugated hydrocarbons, produce cyclic products. For example, cw-l,2-diphenylethene irradiated in the presence of oxygen gives phenanthrene by the sequence of Equation 28-8. The primary photoreaction is cyclization to a dihydrophenanthrene intermediate, 6, which, in the presence of oxygen, is converted to phenanthrene ... 

In the thermal reaction the [4 + 2] or Diels-Alder adduct is the major product, whereas in the photochemical reaction [2 + 2] cycloadditions dominate. Because the photochemical additions are sensitized by a ketone, C6H5-COCH3, these cycloadditions occur through the triplet state of 1,3-butadiene and, as a result, it is not surprising that these cycloadditions are stepwise, nonstereospecific, and involve diradical intermediates. 

Flash photolysis and laser flash photolysis are probably the most versatile of the methods in the above list they have been particularly useful in identifying very short-lived intermediates such as radicals, radical cations and anions, triplet states, carbenium ions and carbanions. They provide a wealth of structural, kinetic and thermodynamic information, and a simplified generic experimental arrangement of a system suitable for studying very fast and ultrafast processes is shown in Fig. 3.8. Examples of applications include the keton-isation of acetophenone enol in aqueous buffer solutions [35], kinetic and thermodynamic characterisation of the aminium radical cation and aminyl radical derived from N-phenyl-glycine [36] and phenylureas [37], and the first direct observation of a radical cation derived from an enol ether [38],... 

In contrast to the above, the more reactive ethoxycarbonyl nitrene 4 is able to attack the carbonyl group of ketones forming three-membered rings such as compound 90 shown in Sch. 24. The ground state for this nitrene is the triplet state, meaning that the cycloaddition reaction occurs in at least two steps. Indeed is has been found that acetone can be attacked by the reactive intermediate 4 primarily at the carbonyl O-atom (see Sch. 24) the dipolar intermediate is able to add a second acetone molecule to yield the dioxazoline (compound 91) [21]. 

The results of Barltrop and Coyle , on the photolysis of aliphatic ketones in solution at 3130 A, support the explanation suggested by Wagner and Hammond for the low quantum yields. The overall quantum yield for disappearance of 2-pentanone, 2-octanone and 5-methyl 2-octanone was found to increase and approach unity in hydroxylic solvents. This increase can be attributed to the solvation of the hydroxy biradical intermediate. Since, however, the solvent effect was not observed for products originating from excited singlet molecules, it is probably only the triplet state which decomposes via biradicals. 

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