Photocatalyzed and Electrocatalyzed Reduction of vic-Dibromides and Activated Ketones Using Ruthenium(I) Tris(bipyridine) as Electron-Transfer Mediator

Article abstract of DOI:10.1021/jo00296a023

Effective photosensitized reduction of vic-dibromo and keto compounds occurs in acetonitrile solutions that include ruthenium(II) tris(bipyridine) (Ru(bpy)3²⁺) as photosensitizer and triethylamine (TEA) as electron donor. (R,S)-1,1'-(1,2-Dibromo-1,2-ethanediyl)bis(benzene) (1) and (R,R:S,S)-1,1'-(1,2-dibromo-1,2-ethanediyl)bis(benzene) (2) are debrominated to a mixture of (E)- (5) and (Z)-1,1'-(1,2-ethenediyl)bis(benzene) (6); Φ = 0.34 for 1 and Φ = 0.24 for 2.Ethyl α,β-dibromobenzenepropionate (3) and (α,β-dibromo-α,β-dihydro-1-propenyl)benzene (4) are debrominated to (E)-ethyl α,β-didehydrobenzenepropionic acid (7), Φ = 1.87, and (E)-1-propenylbenzene (8), Φ = 0.08.The mixture of 5 and 6 formed upon debromination of 1 and 2 undergoes isomerization in the reaction medium to 6 through an energy-transfer mechanism.Debromination of the substrates occurs through the photosensitized formation of Ru(bpy)3⁺ (E^O = -1.33 V vs SCE), which acts as an electron-transfer mediator in the process.The rates of the debromination processes are controlled by the primary reduction of the dibromo compounds by Ru(bpy)3⁺, excect for 3 as substrate, where a radical chain mechanism is suggested.Cyclic voltammetry measurements reveal that Ru(bpy)3⁺ acts as catalyst in the debromination processes.Similarly activated ketones, i.e., diphenylethanedione (9) and ethyl α-oxobenzeneacetate (10) are reduced by photogenerated Ru(bpy)3⁺ to 2-hydroxy-1,2-diphenylethanone (11), Φ = 0.025, and ethyl α-hydroxybenzeneacetate (12), Φ = 0.049.