Radical cyclization reactions of .alpha.-silyl amine .alpha.,.beta.-unsaturated ketone and ester systems promoted by single electron transfer photosensitization

Abstract

The results of a broad investigation of the preparative and mechanistic aspects of single electron transfer (SET) promoted photocyclization reactions of -silyl amino and amido ,/3-unsaturated esters and ketones are presented. A number of unique and synthetically useful features of these processes, driven by -silyl amine and amide cation radical desilylation and by intramolecular conjugate addition of intermediate -amino and -amido carbon-centered radicals to unsaturated esters and ketones, are described. Comparisons of the SET-sensitized and direct irradiation promoted reactions of these systems have shown how the former method is superior in inducing photocyclization reactions in cases where the ,ß-unsaturated ketone or ester excited states are too reactive to be quenched by SET from the tethered amine donors and where diradicals produced as intermediates in the direct-irradiation reactions undergo fragmentation rather than cyclization. The current efforts have also demonstrated that problems associated with the ready oxidation of intermediate -amino radicals can be avoided by the proper selection of photosensitizer or amine N substituents. Lastly, the synthetic versatility of this chemistry, exemplified by its application to the preparation of a number of N-heterocyclic substances by pathways involving either exo or endo radical cyclization, is presented.

Stereoselective Triplet‐Sensitised Radical Reactions of Furanone Derivatives

“…[10,13] The same kind of products are obtained when a-aminoalkyl radicals are generated by photochemical electron transfer from a tertiary amine to an excited electron-transfer sensitiser. [12,[14][15][16][17][18][19][20][21] In this case the a,b-unsaturated carbonyl compound remains in its ground state.…”

Photoredox‐Initiated α‐Alkylation of Imines through a Three‐Component Radical/Cationic Reaction

“…[13,14] After radical reduction of alkyl halide 2, the resulting alkyl radical 5 reacts with enamide 1 to produce aamidoalkyl radical 6. The oxidative radical-polar crossover reaction [15] then furnishes an N-acyliminium cation 7, which can be trapped by a suitable nucleophile. Alcohol 3 was the nucleophile of choice to generate b-alkylated a-amido ether 4, which is a stable precursor of a-alkylated imines [16, 14f] and can be found in a number of biologically important natural products.…”

“…[15] To illustrate this, 4 a was easily prepared on a 5 mmol scale using 0.5 mol % of iridiumA C H T U N G T R E N N U N G (III) catalyst 9 (4 mg). Its reduction using triethylsilane (TES) and AgOTf proceeded smoothly to afford amine 10 in quantitative yield (Scheme 2).…”

Photo-Organocatalysis, Photo-Redox, and Electro- Organocatalysis Processes

“…In 2005, Bach and co-workers presented an enantioselective photo-induced electron transfer (PET) sensitization with significant turnover and high enantioselectivity [10]. These PET-catalyzed conjugate additions of α-amino alkyl radicals to enones have already been studied non-enantioselectively [11]. For the first time, an electron-accepting chiral organocatalyst was applied, in contrast to conventional complexing reagents.…”