Photocatalytic Deoxygenative Carboimination towards Functionalized Pyrrolines by Using Unstrained γ,δ‐Unsaturated Oximes

Abstract

Photocatalytic deoxygenative carboimination of unstrained γ,δ‐unsaturated oximes with π‐acceptors including trifluoromethyl alkenes, 1‐trifluoromethyl‐1,3‐butadienes, gem‐difluoroalkenes, acrylates, and styrenes has been achieved which provides an efficient and mild approach to construct structurally diverse and highly functionalized pyrrolines with good functional group tolerance. Importantly, upon utilization of trifluoromethyl alkenes as the coupling partners, the developed protocol not only allows the introduction of the extremely valuable gem‐difluoroalkene functionality and pyrroline moiety into one molecule, but could also further transform the resulting gem‐difluoroalkene‐functionalized pyrrolines to the architecturally intriguing indolizine alkaloids via oxidation/reduction and sequential C−F bond cleavage. The mechanistic studies indicated that a free‐radical and anionic process was probably involved in this photocatalytic deoxygenative transformation.

Oxime‐Derived Iminyl Radicals in Selective Processes of Hydrogen Atom Transfer and Addition to Carbon‐Carbon π‐Bonds

“…The photocatalytic oxidative cyclization of γ,δunsaturated oximes with trifluoromethyl alkenes 147 and trifluoromethyl-1,3-butadienes 150 as coupling partners was proposed as a convenient method for synthesis of fluoro-functionalized pyrrolines 148 and 151 (Scheme 56). [130] The key stages of the proposed mechanism are depicted in Scheme 57. Under blue light irradiation the P(p-tol)Ph 2 is oxidized by excited Ir III to the phosphine radical-cation.…”

“…The βscission of A leads to iminyl radical B, which undergoes 5-exo-trig cyclization with the formation of alkyl radical C. The trapping of C with trifluoromethyl alkene delivers radical D, which is subsequently reduced by Ir II to carbanion E. The fluoride elimination furnishes the final product 148. [130]…”

Recent Advances in C-F Bond Cleavage Enabled by Visible Light Photoredox Catalysis

“…Using triarylphosphine as the deoxygenative reagents, iminyl radicals can be generated from oximes directly via the photocatalytic phosphoranyl radicalmediated N-O bond cleavage. When this method was applied to γ,δ-unsaturated oximes, iminyl radicals trigger further intramolecular radical cyclization to form alkyl radicals bearing the 2H-pryrole motif, which then undergo defluoronative reaction with trifluoromethyl alkenes (Scheme 31a) [77]. While using strained cyclobutanone oxime as the substrates, the iminyl radical generated by deoxygenation underwent C-C bond cleavage to give γ-cyano alkyl radical, which allowed the synthesis of gem-difluoroalkenes bearing a cyano group (Scheme 31b) [78].…”

Photoredox Catalytic Phosphine-Mediated Deoxygenation of Hydroxylamines Enables the Construction of N-Acyliminophosphoranes

“…Taking advantage of the rapidly developed photochemistry, phosphoranyl radical-mediated fragmentation provided an attractive way to access carbon-centered radicals via the deoxygenation of carboxylic acids and alcohols (Scheme B). , However, the activation of N–O bond for the generation of nitrogen-centered radicals is less developed. Recently, Yang and Yi shown, independently, that deoxygenation of oximes was possible via β-scission of the phosphoranyl radical to produce iminyl radicals . Inspired by these pioneering reports, we wondered whether the direct N–O cleavage of N -hydroxyamide derivatives would produce valuable amidyl radicals, which may undergo radical combination with triphenylphosphine radical cation to form N -acyliminophosphorane (Scheme C).…”

“…Recently, Yang and Yi shown, independently, that deoxygenation of oximes was possible via β-scission of the phosphoranyl radical to produce iminyl radicals . Inspired by these pioneering reports, we wondered whether the direct N–O cleavage of N -hydroxyamide derivatives would produce valuable amidyl radicals, which may undergo radical combination with triphenylphosphine radical cation to form N -acyliminophosphorane (Scheme C). As a part of our continued interest in the phosphoranyl radical-participated transformations, we herein report our preliminary results in the construction of aza-ylide derived from amides, carbamates, and ureas.…”