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]
[9]
[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]
[12]
[13]
[14]
[15]
[16]
[17]
Figure 2 Photocatalytic epoxidation of styrenes, sulfoxidation of thioanisoles and C–H activation
of alkenes by porphyrins[18]
[19]
[20]
[21]
Figure 3 Photocatalytic oxidation of anthracene, benzyl amine coupling, sulfoxidation of thioanisole
and oxygenation of hexamethylbenzene by porphyrins[22]
[23]
[24]
[25]
Figure 4 Photoredox catalysis and C–H bond amination by porphyrins[26]
[27]
Figure 5 Photocatalytic thiocyanation of diketones and indoles and C–H arylation of heteroarenes
by porphyrins[28]
[29]
[30]
[31]
[32]
[33]
[34]
[35]
[36]
[37]
[38]
[39]
[40]
Figure 6 Photocatalytic oxygenation of a furanic compound, amination and aziridination of
alkenes by porphyrins[41]
[42]
Figure 7 Photocatalytic hydroxylation of benzene, oxidation of benzylic alcohol and cross-dehydrogenative
couplings by phthalocyanines[43]
[44]
[45]
[46]
[47]
Figure 8 Photocatalytic oxidation of nitrophenol, cyanation of amines and cyclization to quinolones
by phthalocyanines[48]
[49]
[50]
[51]
[52]
Figure 9 Photocatalytic perfluoroalkylation of aromatics, sulfides and alkenes, cycloaddition
and dehalogenation, and oxidation by phthalocyanines[53]
[54]
[55]
[56]
Figure 10 Photocatalytic chlorotrifluoromethylation of alkenes, oxidation of nitrophenol and
phosphonylation of hydrazines by phthalocyanines[57]
[58]
[59]
[60]
[61]
[62]
[63]
[64]
[65]
[66]
[67]
Figure 11 Photocatalytic oxygenation of thioanisole and alkenes, bromination of phenol and
toluene, and oxidation of toluene, thioanisole and cyclohexene by corroles[8c]
,
[68]
[69]
[70]
[71]
[72]
[73]
[74]
[75]
[76]
[77]
Figure 12 Photocatalytic oxygenation of aromatics, benzylamine coupling and bromination of
benzene, phenol and toluene by corroles[78]
[79]
[80]
[81]
