Photocatalytic Organic Reactions

Introduction

Upon light irradiation, photocatalysts can lead to efficient charge separation, which can be served to promote oxidation (reacting with photogenerated holes) and reduction (reacting with photogenerated electrons) of organic molecules, providing a novel methodology for the synthesis of fine chemicals. Photocatalytic organic reaction not only can achieve green synthesis of organic intermediates, but also can obtain the compounds that cannot be obtained by other methods, which plays an important role in the synthesis of fine chemicals. Recent years, a series of homogeneous photocatalysts represented by ruthenium complexes and small molecular dyes have greatly improved photocatalytic organic reactions both in the reaction types and reaction efficiency. However, the high cost and toxicity of the noble metal catalysts represented by ruthenium complexes limit their further application. Therefore, it is crucial to develop non-toxic and recyclable heterogeneous photocatalysts, among them, porous materials, especially MOFs and COFs, as organic photocatalysts have attracted extensive attention due to their superior performances.

Photocatalytic Organic Reactions by MOFs and COFs

MOFs and COFs can be used for different types and a wide variety of photocatalytic organic reactions, the details as described below^[1].

• Oxidation reactions: Photocatalytic oxidation reactions by MOFs and COFs are chosen for the synthesis of a large number of molecules such as (1) in the selective oxidation of alcohols to aldehydes or ketones; (2) in the selective oxidation of sulfides to sulfoxides; (3) in the selective oxidative dehydrogenation of secondary amines; (4) in the oxidation of arylboronic acids to phenols.
• Oxidative coupling: There are many examples of oxidative coupling reactions carried out by MOFs and COFs such as (1) oxidative coupling of primary benzylamines; (2) oxidative coupling of amines; (3) C-N and C-C oxidative coupling reactions; (4) oxidative coupling of primary amines to generate imine; (5) oxidative coupling of phenols with anilines to generate imine.
• Cross-Dehydrogenative coupling:The C-H bond cross-dehydrogenative coupling is an important method to generate new C-C or C-Heteroatom bonds under mild conditions. In the field of MOFs and COFs, there are various examples where this reaction has been employed. For examples, Lin and coworkers utilized the UiO-67-doped material which presented high activity towards aza-Henry reaction between N-aryl-THIQs and nitromethane. Wu employed a pristine acylhydrazone-based COFs, TFB-COF, as photocatalyst for CDC reaction between different N-aryl-THIQs and various nucleophiles (i.e., ketones and nitro-derivatives)^[1].
• Dehalogenation: MOFs and COFs as photocatalysis used for dehalogenation through radical-involved reactions, such as photoreductive dehalogenation, where the halogen atom is substituted by a hydrogen atom. Another dehalogenation method consists on the α-alkylation of aldehydes where the formed radical is trapped by an enamine intermediate which after the catalytic cycle and hydrolysis allows the formation of the alkylated product. For examples, the dehalogenation of bromomalonate, aryl halides, phenacyl bromide, bromomalonates and so on.

Mechanism of MOFs and COFs in Photocatalytic Organic Reactions

Different photocatalytic organic reactions have different mechanisms, but in general, the light is absorbed by the photocatalyst, which in the excited state becomes a powerful oxidant or reducing agent or, in some cases, both. Then, electron transfer processes result in the formation of transient radical species that eventually evolve towards the desired products (as shown in Fig.1).

Fig.1 Energy transfer and electron transfer processes in photocatalysis.

Take the oxidation of alcohols to aldehydes as example, the photocatalyst absorbs light, and in its excited state, separation of charges is produced, promoting electrons to the conduction band and generating holes in the valance band. These electrons activate O₂ to generate superoxide radical anion (O₂^-·), which deprotonates benzyl alcohol obtaining hydroperoxide radical species (·OOH). Photogenerated holes are quenched by the benzoxide anion to oxidize it to its radical form, which subsequently reacts with ·OOH, generating benzaldehyde as the final product (as shown in Fig.2).

Fig.2 Generally mechanism for the oxidation of alcohols

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