Key words
photocatalysis - photoredox catalysis - porphyrins - phthalocyanines - corroles
Photocatalysis offers the advantage of using light as an affordable, sustainable and green source of energy to carry out endergonic reactions.[1] It offers the advantage of milder conditions over those of thermal reactions.[2] As visible light is absorbed by sensitizers but not by most organic compounds, it offers an efficient approach to prevent product degradation and side reactions.[3] In photoredox catalysis, the photocatalyst in its excited state differs from that of the ground state by providing a higher electron affinity and a lower ionization potential, thereby making it a better electron donor as well as an acceptor. Versatile applications of photocatalysts are found in CO2 reduction, H2O splitting, proton-coupled electron transfer, photovoltaics and in the development of photo-electrochemical solar cells.[4]
The formation of carbon–carbon and carbon–heteroatom bonds has been a challenge in organic chemistry, which has been efficiently tackled by photocatalysis.[5] Traditionally, metal complexes (such as Ru and Ir polypyridyl complexes) and organic dyes (such as eosin Y) have been employed extensively as photocatalysts.[6] However, the high cost and toxic nature of metal complexes, as well as the pH-sensitive nature of organic dyes have prompted researchers to explore macrocycles such as porphyrins, phthalocyanines and corroles for photocatalysis.[7] These macrocycles have been examined for the catalysis of cyclopropanations, hydroxylations, aziridinations, epoxidations, sulfoxidations, etc.[8]
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[10] Typically in photoredox catalysis, under light irradiation, these photocatalysts may undergo oxidation or reduction at different potentials and participate in SET (single-electron transfer) with the substrates. In photooxidation reactions, upon photoexcitation, such catalysts can switch from singlet to triplet excited states via ISC (intersystem crossing), and during this process, they can generate singlet oxygen via the type II pathway. Their ability to participate in SET depends on the reaction conditions, the nature of the substrate and also on the types of meso-substituents (electron-donating or electron-withdrawing) present on the catalyst, which in turn will govern their efficiency.
This graphical review provides an overview of organic transformations photocatalyzed by porphyrins, phthalocyanines and corroles, along with selected substrate scopes, that have been reported over the last five years (2019 to 2023). As photocatalysis by corroles is relatively less explored, all the examples described since 2005 are included. This graphical review describes photooxidations, epoxidations, sulfoxidations, aziridinations and cyanations of aliphatic and/or aromatic compounds by employing these macrocycles. In addition, C–H arylations of heteroarenes and thiocyanations utilizing porphyrins are discussed. Researchers have also explored hydroxylations, cycloadditions, perfluoroalkylations and phosphonylations by employing phthalocyanines as photocatalysts. Examples of brominations mediated by corroles are also provided. However, reactions involving inorganic transformations, polymerization, photodegradation and heterogenous catalysis are excluded.
Figure 1 Photocatalytic oxidation of aldehydes by porphyrins[11]
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Figure 2 Photocatalytic epoxidation of styrenes, sulfoxidation of thioanisoles and C–H activation of alkenes by porphyrins[18]
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Figure 3 Photocatalytic oxidation of anthracene, benzyl amine coupling, sulfoxidation of thioanisole and oxygenation of hexamethylbenzene by porphyrins[22]
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Figure 4 Photoredox catalysis and C–H bond amination by porphyrins[26]
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Figure 5 Photocatalytic thiocyanation of diketones and indoles and C–H arylation of heteroarenes by porphyrins[28]
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Figure 6 Photocatalytic oxygenation of a furanic compound, amination and aziridination of alkenes by porphyrins[41]
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Figure 7 Photocatalytic hydroxylation of benzene, oxidation of benzylic alcohol and cross-dehydrogenative couplings by phthalocyanines[43]
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Figure 8 Photocatalytic oxidation of nitrophenol, cyanation of amines and cyclization to quinolones by phthalocyanines[48]
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Figure 9 Photocatalytic perfluoroalkylation of aromatics, sulfides and alkenes, cycloaddition and dehalogenation, and oxidation by phthalocyanines[53]
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Figure 10 Photocatalytic chlorotrifluoromethylation of alkenes, oxidation of nitrophenol and phosphonylation of hydrazines by phthalocyanines[57]
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Figure 11 Photocatalytic oxygenation of thioanisole and alkenes, bromination of phenol and toluene, and oxidation of toluene, thioanisole and cyclohexene by corroles[8c]
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Figure 12 Photocatalytic oxygenation of aromatics, benzylamine coupling and bromination of benzene, phenol and toluene by corroles[78]
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