Redox-Photosesitized Reactions. 13. Ru(bpy)32+ -Photosensitized Reactions of an NADH Model, 1-Benzyl-1,4-dihydronicotinamide, with Aromatic Carbonyl Compounds and Comparision with Thermal Reactions

Article abstract of DOI:10.1021/jo00389a027

Photosensitized reactions of 1-benzyl-1,4-dihydronicotinamide (BNAH) with aromatic carbonyl compounds (1a-f) by Ru(bpy)3²⁺ have been investigated.The reduction to the corresponding alcohols occurs with di-2-pyridyl ketone (1a) in a quantitative yield and with methyl benzoylformate (1b) in a 18percent yield.Noteworthy is the efficient formation of 1:1 adducts 4b-f, a new class of 4-alkyl 1,4-dihydronicotamides, in 55-85percent isolated yields; 4b is a single isomer of a condensed bicyclic imide, whereas the other adducts are obtained as diastereoisomeric mixtures.In the case oftrifluoroacetophenone (1c), the diastereoisomeric 6-alkylated 1,6-dihydronicotamides (4c') are formed as minor 1:1 adducts.The structure of each adduct has been determined by spectroscopic and X-ray crystallographic studies with one exception (4d).The mechanism of the photosensitized reactions is discussed in terms of sequential indirect electron-proton transfer from BNAH to 1a-f followed by electron transfer or cross-coupling between radical intermediates.On the other hand thermal reactions of NAH with 1a-e in the dark give neither the adduct nor the half-oxidized dimers of BNAH but the alcohols.It is suggested that electron-transfer mechanisms are not responsible for the thermal reactions.