Hantzsch Ester-Mediated Photochemical Transformations in the Ketone Series: Remote C(sp³)–H Arylation and Cyclopentene Synthesis through Strain Release

Abstract

A metal-free Hantzsch ester-mediated synthesis of cyclopentenylketones as well as γ-hetarylketones starting from ketocyclopropanes under eco-friendly conditions was developed. The versatility of the developed conditions is shown by reacting ketocyclopropanes in both a formal [3 + 2] cycloaddition with terminal alkynes (further investigated using theoretical calculations) and a radical C−C-coupling with cyanopyridines. The newly developed methodologies were later on utilized as a downstream reaction for photogenerated cyclopropanes combining UV and visible light photochemistry. Following this procedure, a UV-driven Norrish−Yang-type reaction induces the ring strain of the intermediates, which serves as activation energy for the subsequent ring transformation.

Vinylcyclopropane [3+2] Cycloaddition with Acetylenic Sulfones Based on Visible Light Photocatalysis**

“…Considering the lack of examples of intermolecular transition metal-catalyzed visible light [3 + 2] cycloadditions involving nonactivated VCPs through EnT, we studied the scope of the same UV-driven strain-promoted/visible light strain-releasing sequence employed in our previous work on cyclopropyl ketones [8] by using meta photocycloadduct bearing a vinylcyclopropane substructure as substrates in a visible lightmediated intermolecular [3 + 2] cycloaddition (Scheme 1). This strategy which has gradually gained attention from the synthetic community, [9] uses the inherent ring-strain of the photogenerated intermediate as a thermodynamic driving force for the subsequent ring expansion.…”

Radical Decyanations of Unactivated Carbon‐CN Bonds: Recent Achievements and Mechanistic Studies

“…[57] Similarly, Opatz and coworkers published a metal-free photoredox synthesis of γ-hetarylated ketones 34 starting from conventionally synthesized ketocyclopropanes 33 by using Hantzsch ester as a photoreductant (Scheme 14b). [58] This gentle and external oxidant-free technique produced a variety of aryl ketones with high efficacy and shown good functional group compatibility and moderate to exceptional yield. To accomplish this radicalmediated ring-opening of cyclopropanols, a protoncoupled electron transfer (PCET) activation approach using a photocatalyst without an external oxidant was also employed.…”

Unusually Chemoselective Photocyclization of 2-(Hydroxyimino)aldehydes to Cyclobutanol Oximes: Synthetic, Stereochemical, and Mechanistic Aspects

“…A renewed interest has been experienced recently in synthetic photochemical processes, thanks to the availability of convenient light sources. − Light can be used to access reaction pathways that are forbidden in the ground state. In many cases, this is achieved in the absence of costly and polluting artificial catalysts, thus abiding by the principles of green chemistry in full.…”