Light‐Driven Organocatalysis Using Inexpensive, Nontoxic Bi₂O₃ as the Photocatalyst

Abstract

The development of enantioselective catalytic processes that make use of sunlight as the energy source and nontoxic, affordable materials as catalysts represents one of the new and rapidly evolving areas in chemical research. The direct asymmetric a-alkylation of aldehydes with a-bromocarbonyl compounds can be successfully achieved by combining bismuth-based materials as low-band-gap photocatalysts with the second-generation MacMillan imidazolidinone as the chiral catalyst and simulated sunlight as a low-cost and clean energy source. This reaction also proceeded with high efficiency when the reaction vial was exposed to the morning sunlight on a clear September day in Tarragona, Spain.Chemical transformations that are promoted by sunlight are currently the subject of intense research. [1] Photocatalytic methods have shown promising potential for bulk production and are widely accepted and popular as convenient strategies in the area of asymmetric catalysis. [2] In a pioneering effort, MacMillan and co-workers merged organo-and photoredox catalysis to promote the direct asymmetric a-alkylation of aldehydes [3] through a process catalyzed by [Ru(bpy) 3 ] 2+ . [4] Since then, much effort has been devoted to the development of more convenient reaction conditions and to the extension of this combined approach to other key asymmetric organic transformations. [5] In view of the future, substantial use of such reactions, the "Achilles heel" of these methods is their dependence on ruthenium. This is a scarce and expensive metal with rather low production (ca. twelve tons of ruthenium are mined every year) and very limited estimated world reserves (5000 tons). Furthermore, ruthenium compounds are highly toxic and suspected carcinogens. For these reasons, the identification of more convenient visible-light photocatalysts that are suitable for singly occupied molecular orbital (SOMO) activation [6] has become an area of intense

Enhanced p-cresol photodegradation over BiOBr/Bi2O3 in the presence of rhodamine B

“…[27][28][29][30][31] However, its photoactivity is hampered by fast electron-hole pair recombination. To facilitate the separation of the photo-generated electrons and holes, Bi 2 39 To achieve a high degree of cohesion at the interfaces within the composite, solid Bi 2 O 3 was chemically converted to BiOBr by etching with varying amounts of HBr.…”

Screening Commercial Semiconductors for Visible Light Driven Asymmetric Catalysis

“…It should be pointed out that all the semiconductors in Figure are carefully selected. According to the only four published results, TiO 2 , PbBiO 2 Br, Bi 2 O 3 , and Bi 2 S 3 are recognized as the meaningful photocatalysts in asymmetric catalysis with the help of chiral ligand. All the reported semiconductors have a band gap of around 2.5 eV, characteristic of light absorption around 500 nm.…”

Visible Light‐Driven Atom Transfer Radical Addition to Olefins using Bi2O3 as Photocatalyst

“…Recently, we have reported the photoinduced enantioselective a-alkylation of aldehydes with a-bromocarbonyl compounds by merging the commercially available, cheap semiconducting oxide Bi 2 O 3 as a photocatalyst and the second generation MacMillan imidazolidinone as an organocatalyst. [6] From a mechanistic perspective, these results strongly suggest that the readily available photoexcited state of Bi 2 O 3 (this semiconductor is characterized by a low band gap of ca. 1.3 eV) is appropriate to promote the cleavage of reactive C-Br bonds leading to alkyl radicals.…”