The bright routes to five-membered heterocycles: a photoredox catalytic approach

Synthetic chemists constantly strive to develop new methodologies to access complex molecules more sustainably. The recently developed photocatalytic approach results in a valid and greener alternative to the classical synthetic methods. Here we present three protocols to furnish five-membered rings exploiting photoredox catalysis. We firstly obtained 4,5-dihydrofurans (4,5-DHFs) from readily available olefins and α-haloketones employing fac-Ir(ppy)3 as a photocatalyst under blue-light irradiation (Figure 1, top). This transformation resulted very broad in scope, thanks to its mild conditions and the avoidance of stoichiometric amounts of oxidants or reductants. Moreover, similar conditions could lead to β,γ-unsaturated ketones, or highly substituted tetrahydrofurans (THFs) by carefully differentiating the substitution pattern on the starting materials and properly adjusting the reaction parameters. We then turned our attention to the reactivity of allenamides employing analogous photocatalytic conditions to access 2-aminofurans (Figure 1, bottom). α-Haloketones again provided the radical generated by fac-Ir(ppy)3 under visible-light irradiation, which added to the π-system and furnished the cyclic molecule. The addition of a second molecule of the α-haloketone moiety led to the formation of the final highly functionalized furan, which might be further elaborated to afford more complex products. The two works were both supplied with mechanistic investigations supported by experimental and computational methods. As our last project, we developed a methodology to achieve cypentanonyl-fused N-methylpyrrolidines (Figure 2), exploiting N,N-dimethylamines and carboxylic acids as radical sources. In two separated photocatalytic steps, both functionalities are manipulated through the photoredox catalysis by 4CzIPN to add to an α,β-enone system, furnishing the bicyclic product.