Mechanism of the Stereoselective α-Alkylation of Aldehydes Driven by the Photochemical Activity of Enamines

Organocatalytic Enantioselective Protonation for Photoreduction of Activated Ketones and Ketimines Induced by Visible Light

The first catalytic asymmetric photoreduction of 1,2-diketones and α-keto ketimines under visible light irradiation is reported. A transition-metal-free synergistic catalysis platform harnessing dicyanopyrazine-derived chromophore (DPZ) as the photoredox catalyst and a non-covalent chiral organocatalyst is effective for these transformations. With the flexible use of a chiral Brønsted acid or base in H⁺ transfer interchange to control the elusive enantioselective protonation, a variety of chiral α-hydroxy ketones and α-amino ketones were obtained with high yields and enantioselectivities.

Direct Asymmetric Alkylation of Ketones: Still Unconquered

The alkylation of ketones is taught at basic undergraduate level. In many cases this transformation leads to the formation of a new stereogenic center. However, the apparent simplicity of the transformation is belied by a number of problems. So much so, that a general method for the direct asymmetric alkylation of ketones remains an unmet target. Despite the advancement of organocatalysis and transition-metal catalysis, neither field has provided an adequate solution. Indeed, even use of an efficient and general stoichiometric chiral reagent has yet to be reported. Herein we describe the state-of-the-art in terms of direct alkylation reactions of some carbonyl groups. We outline the limited progress that has been made with ketones, and potential routes towards ultimately achieving a widely applicable methodology for the asymmetric alkylation of ketones.

Enantioselective Formal α-Methylation and α-Benzylation of Aldehydes by Means of Photo-organocatalysis

Detailed herein is the photochemical organocatalytic enantioselective α-alkylation of aldehydes with (phenylsulfonyl)alkyl iodides. The chemistry relies on the direct photoexcitation of enamines to trigger the formation of reactive carbon-centered radicals from iodosulfones, while the ground-state chiral enamines provide effective stereochemical control over the radical trapping process. The phenylsulfonyl moiety, acting as a redox auxiliary group, facilitates the generation of radicals. In addition, it can eventually be removed under mild reducing conditions to reveal methyl and benzyl groups.

Visible-light-induced oxidative formylation of N-alkyl-N-(prop-2-yn-1-yl)anilines with molecular oxygen in the absence of an external photosensitizer

Visible-light-induced oxidative formylation of N-alkyl-N-(prop-2-yn-1-yl)anilines with molecular oxygen without an external photosensitizer was developed under mild reaction conditions.

Radical Reactions Induced by Visible Light in Dichloromethane Solutions of Hünig's Base: Synthetic Applications and Mechanistic Observations

β-(3-Iodopropoxy)-substituted α,β-unsaturated lactams, lactones, and cycloalkenones (eight examples) underwent reductive radical reactions in a dichloromethane solution of N,N-diisopropylethylamine (Hünig's base) upon irradiation with visible light (λ=419 nm). Apart from plain reduction reactions (hydro-de-iodination), a significant degree of cyclization was observed in three cases. In parallel to the conversion of the substrates, the formation of intensely colored by-products was observed. Based on mass spectrometric evidence and upon comparison with known compounds, the by-products were identified as cyanine dyes. Their formation supports the hypothesis that irradiation of dichloromethane solutions of Hünig's base leads to the formation of radicals, which in turn can either initiate a radical reaction or combine with cyanine precursors. It was shown by deuterium-labelling experiments, that one equivalent of dichloromethane is incorporated into the cyanine dyes and that the reductive quenching of radical intermediates is at least partially due to hydrogen abstraction from the solvent. As a consequence, a reductive cyclization of the starting materials is favored in CD₂ Cl₂ solutions as shown for two β-(3-iodopropoxy)-substituted tetronates, which underwent in dichloromethane almost exclusive reduction, but gave predominantly the cyclization products in CD₂ Cl₂ .

Intramolecular radical non-reductive alkylation of ketones via transient enamines

A five-minute one-pot cyclization of dichloro-acetamide-tethered ketones was achieved under microwave activation through a radical procedure involving a transient enamine.