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 α-alkylation of aldehydes with α-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.

Trifunctional Organocatalysts: Catalytic Proficiency by Cooperative Activation

“…[29] As a related case to Melchiorre's work, Pericás, Palomares and co-workers reported α-alkylation of aldehydes (33) with α-bromocarbonyl derivatives (34) to yield substituted 1,5-dicarbonyl compounds (35) at room temperature with 20 mol-% loading of MacMillan's catalyst (32) and commercially available Bi 2 O 3 semiconductor (Scheme 5). [30] The reaction proceeded relatively expediently within 1-24 h with excellent yields (56-86 %) and enantioselectivity (82-98 %) for a wider substrate scope (11 examples). The low band gap of Bi 2 O 3 ensures effective photoexcitation and subsequent formation of the alkyl radical to promote 32b and 32c formation via an enamine intermediate (32a).…”

“…Under these S NR 1-controlled conditions the major product shifted from 28 to the benzyl alcohol oxidation product (29, 40 %), identifying the EDA complex as catalytically relevant species. Products resulting from quenching of the benzylic radical by proton abstraction (30) and cross-coupling (31) were also observed. The activation of the aldehyde HOMO, the formation of an ion pair, and SET are enabled by one secondary amine, making this an example of a chemically monofunctional motif for mechanistically trifunctional organocatalysis in a sequential manner.…”

“…Photochemical α-alkylation of aldehydes promoted by concurrent trifunctional and bifunctional organocatalysis with enamine activation. [30] The Melchiorre group also reported an assembled trifunctional organocatalytic (35 + 37) system comprising two catalytic components that cooperate in direct asymmetric γ-alkylation of α-branched enals (38) with bis[4-(dimethylamino)phenyl]methanol (39) (Scheme 6) to yield γ-substituted α-branched enals (40). [31] Cinchona alkaloid 35 and chiral phosphoric acid 37 jointly activate the enal as a nucleophile.…”

Imidazolidinones and Imidazolidine‐2,4‐diones as Antiviral Agents

“…[21][22][23] Conventionally,2 -imidazolidinones and other cyclic ureas are synthesized by the reaction of diamines with several reagents such as carbonyldiimidazole, [24] trichloromethyl chloroformate, [25] organic carbonates, [26,27] dithiocarbonate, [28] carbonyl selenide, [29,30] and carbon monoxide (Scheme 1). [21][22][23] Conventionally,2 -imidazolidinones and other cyclic ureas are synthesized by the reaction of diamines with several reagents such as carbonyldiimidazole, [24] trichloromethyl chloroformate, [25] organic carbonates, [26,27] dithiocarbonate, [28] carbonyl selenide, [29,30] and carbon monoxide (Scheme 1).…”

Photoredox radical conjugate addition of dithiane-2-carboxylate promoted by an iridium(iii) phenyl-tetrazole complex: a formal radical methylation of Michael acceptors

“…3,4 In particular, MacMillan et al investigated 1,4-conjugate addition (Michael reaction) 5 of radicals 6 in connection with a series of electrophilic olefins. In these reactions a photoredox-mediated CO 2 -extrusion mechanism is operative and a broad array of Michael acceptors have been used.…”