Light induced and dye accelerated reductions of phenacyl onium salts by 1,4-dihydropyridines

Photoredox catalysis by [Ru(bpy)3]2+ to trigger transformations of organic molecules. Organic synthesis using visible-light photocatalysis and its 20th century roots

“…Addition of SCHEME 2 Dye-accelerated photoreduction of phenacyl sulfonium salt 2 and related substrates (refs 16,17 ) 1 mole % of [Ru(bpy) 3 ]Cl 2 , TPP, or eosin disodium salt led to acceleration of the irradiated reactions resulting in complete conversions within significantly shorter reaction times of 0.3, 1 and 3 h, for the three dyes, respectively. The authors suggested that light-induced single-electron transfer steps are responsible for the observed sulfonium salt reduction and proposed that the large acceleration effect of [Ru(bpy) 3 ]Cl 2 might be due to the involvement of this photoredox catalyst in the single-electron transfers (SET) 16 . Later on, results of a more detailed mechanistic study were disclosed 17 .…”

“…2) by 1,4-dihydropyridine 3 (2 + 3 → 4, Scheme 2). The reaction was found to be induced by irradiation with visible light and greatly accelerated in the presence of [Ru(bpy) 3 ] 2+ and other dyes, such as TPP (mesotetraphenylporphine), or eosin disodium salt 16 . Importantly, the roles of light and dyes were carefully tested.…”

“…Reductions of other substrates such as ammonium, phosphonium salts, and 4-nitrobenzyl halides have been also achieved (5, 6, 7a and 7b, respectively, Scheme 2) 16,17 . In 1985, the same group described reductions of phenacylbromide, bromomalonates, and related substrates (Scheme 3) using 3-methyl-2,3-dihydrobenzothiazoles using [Ru(bpy) 3 ]Cl 2 or rose bengal as photocatalysts 18 .…”

Recent developments in transition-metal photoredox-catalysed reactions of carbonyl derivatives

“…44, 73 While the reaction was very sluggish in the dark or in the absence of a photoredox catalyst, a significant rate enhancement was observed upon addition of a catalytic amount of Ru II (bpy) 3 Cl 2 to the reaction mixture. In addition to having higher rates, the reactions performed in the presence of Ru II (bpy) 3 Cl 2 were characterized by their cleanness and a lack of competing rearrangement of N -substituted 1,4-dihydropyridines.…”

“…39-43 The use of photoredox catalysts, which, upon photoexcitation with visible light, can engage in single electron transfer (SET) processes with organic substrates, obviates the need for radical initiators and a stoichiometric amount of strong reducing agents. Since Ru II (bpy) 3 Cl 2 was first reported to engage in a SET with aromatic carbonyl compounds in the late 1970s, 44 visible light organometallic photoredox catalysis has become widely recognized as an efficient way to generate ketyl 45-62 and α -aminoalkyl anion radicals under mild reaction conditions. 58, 63-65 …”