Porphyrins are natural functional dyes which are utilized by Nature as prosthetic groups of various enzymes not per se because of their coordination characteristic, but due to the combination of other properties with absorption of light (which translates to color) being the most important.

Efficient C−C bond formation in a green, nontoxic, and inexpensive way has always been a challenge and development of light-induced methodologies is one of the means to achieve such a goal.¹ Photoredox catalysis is based on a photoinduced electron transfer process (PET) between a substrate and a photoredox catalyst, commonly Ru- or Ir-complexes.² Though organic dyes have been well-known for their ability to participate in photoinduced electron transfer processes, their use as catalysts in such reactions has been less explored as the replacement of Ir- and Ru-complexes with known organic dyes is not always possible.³ We wondered whether simple porphyrins could be used as photocatalysts.

Herein, we demonstrate a successful application of these compounds as efficient photoredox catalysts for C-C bond forming reactions involving the reductive or oxidative quenching. Employing dual catalytic system – photocatalysis merged with enamine-iminium catalysis alkylation of aldehydes at the a position was accomplished (Scheme 1a).⁴

We have also found that porphyrins with tuned physicochemical properties, via tailoring various substituents at the periphery of the macrocycle, are also effective in catalyzing light-induced direct arylation of heteroarenes and cumarins with diazonium salts (Scheme 1b).⁵ The reaction afforded arylated products in good yields and tolerates a wide range of functional groups.

Scheme 1. a) Alkylation of aldehydes at the a-position b) Arylation of heteroarenes

References:

1. a) M. Fintecave, Angew. Chem. Int. Ed. 2015, 54, 6946. b) Bucker, J. W.; Stephenson, C. R. J. J. Org. Chem. 2012, 77, 1617. C) Skubi, K. L.; Blum, T. R.; Yoon, T. P. Chem. Rev. 2016, 116,10035.
2. Prier, C. K.; Rankic, D. A.; MacMillan, D. W. C.Chem. Rev. 2013, 113, 5322.
3. Nicewicz, D. A.; Nguyen, T. M. ACS Catal. 2014, 4, 355.
4. K. Rybicka-Jasińska, W. Shan, K. Zawada, K. M. Kadish, D. Gryko J. Am. Chem. Soc. 2016, 138, 15451-15458.
5. K. Rybicka-Jasińska, B. Konig, D. Gryko, Eur. J. Org. Chem. - submitted