Photosensitized Reactions

The previous examples involve direct photochemical reactions, namely, such that a molecule absorbs light and the thus formed excited-state reacts. In many cases, it may turn out to be advantageous to carry out a different experiment, in which another molecule absorbs the photon and then transfers electronic energy to the reagent (sensitization: the sensitizer does not undergo permanent chemical change and is recovered). In a more thorough alternative, the reaction does not proceed from the excited state, but rather the sensitizer activates the reagent by transforming it into a (ground state) highly reactive intermediate, such as a radical (via atom transfer sensitization) or a radical ion (electron transfer sensitization) (Figure 3.6).

A typical atom transfer sensitization process is shown in Figure 3.7: the alkylation of an unsaturated acid via a radical produced from an alcohol by hydrogen abstraction (in a green solvent, water).8 This principle has been applied to a large series of radical alkylation reactions, where the radical precursor is an alcohol, an ether, or even an alkane.9

The hydrogen transfer photosensitization has been applied to the diastereo-selective alkylation of chiral fumaric acid derivatives, where again the mild conditions of the photochemical method are advantageous (Figure 3.8).10

A convenient application uses an inorganic salt (e.g., a polyoxometallate)11 or a semiconductor oxide12 as the sensitizer. Such materials are often chemically more stable with respect to organic molecules, and again can be conveniently used for the generation of a radical from unconventional precursors.

Energy transfer

Energy transfer

Sens

Sens*

Substrate

Sens

Sens*

Substrate

Substrate*

Atom transfer

Substrate*

Atom transfer

Growing Soilless

Growing Soilless

This is an easy-to-follow, step-by-step guide to growing organic, healthy vegetable, herbs and house plants without soil. Clearly illustrated with black and white line drawings, the book covers every aspect of home hydroponic gardening.

Get My Free Ebook


Post a comment