PHOTOCATALYZED AND ELECTROCATALYZED REDUCTION OF VIC-DIBROMIDES AND ACTIVATED KETONES USING RUTHENIUM(I) TRIS(BIPYRIDINE) AS ELECTRON-TRANSFER MEDIATOR

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. CR,S)-l'-(l,2-Dibromo-l,2-ethanediyl)bis(benzene) (1) and (R,R:S,S)-l'-(l,2-dibromo-l,2-ethanediyl)bis(benzene) (2) are debrominated to a mixture of (E)- (5) and (Z)-1,1'-(l,2-ethenediyl)bis(benzene) (6); ɸ = 0.34 for 1 and ɸ = 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), ɸ = 1.87, and (-E)-1-propenylbenzene (8), ɸ = 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° = -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+, except 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-l,2-diphenylethanone (11), ɸ = 0.025, and ethyl α-hydroxybenzeneacetate (12), ɸ = 0.049. © 1990, American Chemical Society. All rights reserved.