Energy and molecules from photochemical/photocatalytic reactions. An overview

Abstract

Photocatalytic reactions have been defined as those processes that require both a (not consumed) catalyst and light. A previous definition was whether such reactions brought a system towards or away from the (thermal) equilibrium. This consideration brings in the question whether a part of the photon energy is incorporated into the photochemical reaction products. Data are provided for representative organic reactions involving or not molecular catalysts and show that energy storage occurs only when a heavily strained structure is generated, and in that case only a minor part of photon energy is actually stored (ΔG up to 25 kcal·mol-1). The green role of photochemistry/photocatalysis is rather that of forming highly reactive intermediates under mild conditions.

Scheme 1

The effect of light on a chemical equilibrium and the historical definitions adopted.

Scheme 2

Hydrosilylation of olefins catalyzed by a (supported) Pt complex photochemically generated in situ [10].

Scheme 3

Organocatalytic (asymmetric) α-benzylation of aldehydes via an intermediate EDA (Electron Donor-Acceptor) complex under visible light irradiation [11].

Scheme 4

Photocatalyzed conjugated radical addition to electron-poor olefins promoted by excited ketones and polyoxometalates [15].

Scheme 5

Photocatalyzed cyclobutane ring formation exploiting a tertiary amine as a sacrificial electron donor [17].

Scheme 6

Definition of the different light-induced processes examined in Table 1.

Figure 1

The energetic profile of the steps involved in the reaction reported in Table 1, entry 3. Adapted from Ref. [49].

Figure 2

Chain vs. non-chain, photocatalytic process. The energy change associated to each step is considered. In the thermal chain process [50], both steps a and b involve the formation of a bond that is more energetic than the one that is cleaved, and the overall process involves relatively low energies. On the contrary, steps a' and b' in the photocatalytic process [15] involve the cleavage of a strong bond and the endergonic nature of the latter step precludes a chain process. Adapted from Ref. [49].