Hydrogen abstraction, ketones quantum efficiency

One of the first examples of 8-hydrogen abstraction in acyclic ketones was the photocyclization of P-alkoxy ketones, in particular of P-ethoxypropiophenone (34), to the corresponding furanol derivatives 37 (Scheme 8.10). It was revealed that formation of enol 36 as well as a reversion to the starting ketone occur by 1,4-hydrogen transfer from the 1,5-biradical 35 causing the lower quantum efficiency for cyclization [12]. 

The Ti species abstracts hydrogen from the alcohol (p. 347), and then dimerizes. The /PrO radical, which is formed by this process, donates H to another molecule of ground-state benzophenone, producing acetone and another molecule of 51. This mechanism predicts that the quantum yield for the disappearance of benzophenone should be 2, since each quantum of light results in the conversion of 2 equivalents of benzophenone to 51. Under favorable experimental conditions, the observed quantum yield does approach 2. Benzophenone abstracts hydrogen with very high efficiency. Other aromatic ketones are dimerized with lower quantum yields, and some (e.g., p-aminobenzophenone, o-methylacetophenone) cannot be dimerized at all in 2-propanol (although p-aminobenzophenone, e.g., can be dimerized in cyclohexane ). The reaction has also been carried out electrochemically. 

It is generally true that almost all the hydroxybiradicals formed by internal hydrogen abstraction partially revert to starting ketone by an internal radical disproportionation reaction in which the carbon-centered radical site abstracts the hydrogen from the hydroxyl group. This process often is the major reaction of the intermediate biradical, so that the overall quantum yield of product formation is low even when hydrogen abstraction by the excited state is 100% efficient Should the efficiency of hydrogen abstraction be low because of electronic or conformational problems, then biradical disproportionation lowers quantum efficiency even further. 

Whitten and co-workers " studied a ketone that is also a surfactant, (B-(p-toluyl)pentadecanoic acid. When prepared as a monolayer film in arachidic acid, this ketone undergoes only a trace of type II reaction. In benzene, it cleaves to p-meffiylacetophenone with a normal d> = 0.20 in an aqueous SDS solution, = 0.80. The enhanced quantum efficiency in the micelle was discussed above it is interesting that the molecule must be looped so that both carbonyls are in the Stem layer. The lack of y-hydrogen abstraction in the monolayer is ascribed to the linear rigidity of the monolayer environment. The monolayer would appear to allow even less molecular flexibility than does the liquid crystal. 

Inasmuch as this chapter covers the cyclization of hydroxy-biradicals, it is important to remember that excited-state hydrogen abstraction forms a biradical with its two radical sites initially very close to each other. Unless some of the connecting bonds rotate very rapidly, disproportionation back to ketone would be aU that happens. One would think that rather simple conformational changes are all that is required for cyclization to take place. In order to predict cyclization quantum efficiency, the challenge is to understand the kinetics of those bond rotations as well as other conformational changes that lead to competing disproportionations and cleavage (only 1,4-biradicals). 

---