Electron and hydrogen transfer in organic photochemical reactions

Abstract

Electron and hydrogen transfers are basic steps in many chemical reactions. Photochemical or electronic excitation significantly influences the reactivity of chemical compounds and thus also these transfer processes. Furthermore, the formation of typical intermediates has a decisive impact on the outcome of the transformations. Based on these properties of photochemical processes, efficient homogeneous and heterogeneous photocatalytic transformations for application to organic synthesis have been developed. Efficient reactions without any chemical activation but induced by photochemical electron transfer can also be carried out. The photon acts as a traceless reagent. In the context of photochemical hydrogen transfer, two processes are frequently encountered. The two particles the electron and the proton are transferred either simultaneously or in two steps. In the latter case, the electron is transferred first and the proton follows. Both processes are applied to organic synthesis. Copyright © 2014 John Wiley & Sons, Ltd.

Photocatalysis with TiO2 Applied to Organic Synthesis

“…In this context, it should be mentioned that a variety of simple tertiary amines that are not further functionalized have been transformed under these conditions. [87,88] Avoiding activating groups contributes to atom economy in chemical reactions. The formation of a-aminoalkyl radicals and the addition to electrondeficient olefins was performed in a very efficient way using heterogeneous photocatalysis with TiO 2 and similar inorganic semiconductors.…”

“…It has been shown that the formation of these products is linked to the formation of compounds 125a,b. [88,91,94] The following mechanism has been suggested (Scheme 18).…”

“…Using isotopic labelling, it was proved that the hydrogen was transferred in part to the unsaturated lactone (131), leading to the side products 126 and 127. [88,91] In the case of the present reaction using heterogeneous photocatalysis, deuterium was transferred from the methyl group in the corresponding starting compound 132 into the a-position of the saturated lactone 133. The following detailed mechanism has been discussed (Scheme 18).…”

“…[32] For these reasons, structure 136 with a high charge density in the b-position has been proposed. [88] After protonation in this position, the adsorbed oxoallyl radical 137 is formed. This electrophilic radical reacts with deuterated N,N-dimethylaniline 132, leading to compound 133 possessing a deuterium atom in the a-position.…”

Selective photooxidation of ortho-substituted benzyl alcohols and the catalytic role of ortho-methoxybenzaldehyde

“…Homogeneous photocatalysis has been also deeply investigated during the last decades for carrying out reactions such as water splitting, degradation of harmful contaminants and organic syntheses [14][15][16][17][18][19][20]. On the last field, however, a limited number of studies on the selective oxidation of alcohols has been done, although this photoreaction has been deeply investigated in heterogeneous systems containing TiO 2 -based catalysts [13,[21][22][23].…”