Photoinduced Hydroarylation and Cyclization of Alkenes with Luminescent Platinum(II) Complexes

Significance 

Among the different methods for alkene functionalization, photoinduced hydroarylation has attracted significant research attention as a promising synthetic strategy. Typically, photoinduced hydroarylation of alkenes comprises various reactions, including the C(sp2) – C(sp3) bond formation reactions mainly used to complement the reductive Heck coupling reactions. On the other hand, the generation of reactive aryl radicals in the presence of photocatalysts also presents a vital step in the photoredox hydroarylation reactions. However, even though aryl radicals are derivatives of pre-functionalized aryl substrates such as diaryliodonium, their practical applications as radical precursors are sparsely reported despite being commercially available. This can be attributed to their relatively high bond dissociation energy and negative reduction potentials. To this end, intramolecular photoinduced hydroarylation of alkenes has been extensively researched via different mechanisms such as energy transfer and single electron transfer radical pathways.

Recently, luminescent platinum (II) complexes have been identified as a promising class of photocatalysts and photosensitizers for use in light-induced organic transformations. Unlike conventional luminescent complexes, platinum (II) complexes exhibit a longer lifetime due to low metal-to-ligand charge transfer characteristics and are suitable for photochemical processes. Additionally, they have good planar coordination geometry that allows for effective inner-sphere substrate binding and facilitation of radical intermediates during bond formation. Therefore, platinum (II) complexes have resulted in more opportunities for new photo-catalysis.

The previous findings revealed the possibility of generating long-lived photo-reductants from phosphorescent tetradentate platinum (II) – NHC complexes for potential use in the photoinduced debromination of aryl bromides. However, these findings need more clarifications for better understanding. To this end, Dr. Hanchao Cheng, Professor Yungen Liu, and Professor Chi-Ming Che from Southern University of Science and Technology, in collaboration with Dr. Tsz-Lung Lam and Dr. Zhou Tang from the University of Hong Kong, investigated the hydroarylation and cyclization of alkenes in the presence of light using luminescent platinum (II) complexes. Their research work is currently published in the journal, Angewandte Chemie.

In their approach, tetradentate [Pt(O^N^C^N)] complexes Pt-1 and Pt-2 were used as photocatalysts to facilitate the aryl halides bond reductive activation and photocyclization of triplet-sensitized acrylanilides. These reactions were carried out at room temperature and in the presence of visible light irradiation of 410 nm. The efficiency of platinum-catalyzed reactions was validated through control experiments involving comparing the obtained results to those obtained while using conventional photocatalysts like [Ir(ppy)3].

Results demonstrated the feasibility of the photocatalysts. In the presence of the catalysts and visible light irradiation, trapping of the aryl radicals generated from the aryl chlorides/bromides using terminal aryl alkenes was made possible. This further resulted in a up to 95% yield of anti-Markovnikov hydroarylated compounds. Consequently, the authors reported the effective intramolecular photocyclization of acrylanilidines to obtain various structurally diverse 3,4-dihydroquinolinones. Furthermore, both Pt-1 and Pt-2 complexes exhibited a strong absorption band characterized by improved emission lifetime. Generally, the [Pt(O^N^C^N)] photocatalysts outperformed the conventional photocatalysts.

In a nutshell, the authors reported high-performance and efficient visible-light [Pt(O^N^C^N)] photocatalysts for effective cyclization and hydroarylation of alkenes. The generation of aryl radicals and their subsequence trapping using an array of internal alkenes as well as the efficient photocyclization of acrylanilides in the presence of the visible light irradiation showcased the feasibility of the photocatalysts. Moreover, the control experiments revealed the importance of both Pt-2 and light irradiation during the cyclization process. In a statement to Advances in Engineering community, Professor Chi-Ming Che stated that the study provided useful insights regarding the significant role of platinum complexes-based photocatalysts in the photoinduced hydroarylation and cyclization of alkenes and would provide more opportunities for future practical applications.

Reference

Cheng, H., Lam, T., Liu, Y., Tang, Z., & Che, C. (2020). Photoinduced Hydroarylation and Cyclization of Alkenes with Luminescent Platinum(II) ComplexesAngewandte Chemie, 133(3), 1403-1409.

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