Figure 3.12 Examples of alkylation reactions via radicals generated through electron transfer sensitization.

silylated or by oxygenated precursors, such as alcohols, ethers, and acetals, as well as by alkylaromatics and alkylalkenes.5,14

The preceding examples give a hint of the synthetic versatility obtained in radical addition reactions by producing the key species through a photosensitized reaction. Several other classes of highly active reagents can be smoothly generated by a similar method, for example, carbenes or nitrenes, and the photochemical alternative is usually more versatile than the thermal counterpart, since the photochemically induced cleavage of the precursor is very fast and thus less affected by the experimental conditions. In other cases, photochemistry offers a convenient entry into a class of reactive intermediates otherwise inaccessible for all practical purposes, as recently illustrated by the case of aryl cations: once a photochemical method for their production has been found, a distinctive chemistry (selective addition to p nucleophiles) has been discovered.15

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