Photocatalytic Deoxygenation of Sulfoxides Using Visible Light: Mechanistic Investigations and Synthetic Applications

Abstract

The photocatalytic deoxygenation of sulfoxides to generate sulfides facilitated by either Ir­[(dF­(CF3)­ppy)2(dtbbpy)]­PF6 or fac-Ir­(ppy)3 is reported. Mechanistic studies indicate that a radical chain mechanism operates, which proceeds via a phosphoranyl radical generated from a radical/polar crossover process. Initiation of the radical chain was found to proceed via two opposing photocatalytic quenching mechanisms, offering complementary reactivity. The mild nature of the radical deoxygenation process enables the reduction of a wide range of functionalized sulfoxides, including those containing acid-sensitive groups, in typically high isolated yields.

Phosphoranyl Radical Fragmentation Reactions Driven by Photoredox Catalysis

“… 32 Photocatalyst: With judicious choice of the photocatalyst, much like the point above, one can more precisely marry the redox window of the photocatalyst to the phosphine derivative, while also considering other functionality present within the reaction. 42 Radical chain mechanism: With better recognition of the importance of radical chain processes, of the type that are evident in several of the reactions reported in this Perspective, one can more accurately optimize the overall phosphoranyl radical-mediated transformation. The importance of radical chain processes is sometimes overlooked, 44 especially in the field of photoredox catalysis, where closed cycle mechanisms, that rely on the photocatalyst to mediate reactivity throughout, are commonly proposed instead.…”

“…Photocatalyst: With judicious choice of the photocatalyst, much like the point above, one can more precisely marry the redox window of the photocatalyst to the phosphine derivative, while also considering other functionality present within the reaction. 42 …”

“…More recently, in 2020, Rossi-Ashton and co-workers reported a photocatalytic sulfoxide deoxygenation procedure, proceeding via the formation of phosphoranyl radical 180 ( Scheme 21 ). 42 Because of the mild nature of photoredox catalysis, this high-yielding sulfoxide deoxygenation procedure tolerated the reduction of an array of diaryl, dialkyl, and aryl alkyl sulfoxides, including acid-sensitive functionalities that are not compatible with many existing deoxygenation procedures. The authors describe several complementary conditions for this deoxygenation process, each centered around a radical chain mechanism shown in Scheme 21 .…”

Radical reactions promoted by trivalent tertiary phosphines

“…30). 54 Both the photoredox catalyst 165 possessing a high excited-state oxidation potential and the catalyst 199 with a high excited-state reduction potential were able to promote this radical process. Thioether product 200 comprising both aromatic and aliphatic substituents was afforded in good to excellent yields.…”

Electrophilic Chlorine from Chlorosulfonium Salts: A Highly Chemoselective Reduction of Sulfoxides

“…Finally, deoxygenations, (Scheme c), which are promoted by the activation of a sulfoxide with strong electrophiles, need low temperatures and, in most cases, stoichiometric amounts of base. An alternative photocatalytic deoxygenation was described very recently . It is noteworthy that deoxygenations methods produce strong acids, making them traditionally incompatible with labile acid and nucleophilic functional groups.…”