Photochemical Reductions of Benzil and Benzoin in the Presence of Triethylamine and TiO2 Photocatalyst

Bulletin of the Korean chemical society

Eun Kyung Kim¹  Joon Woo Park¹  Kwanghee Koh Park¹ 

关键词: Photochemistry;    Reduction;    Benzyl;    Benzoin;    TiO2;   

DOI  :  

学科分类：化学（综合）

来源: Korean Chemical Society

【 摘 要 】

This paper reports the photochemical reduction of benzil 1 to benzoin 2 and the reduction of 2 to hydrobenzoin 4 in deoxygenated solvents in the presence of triethylamine (TEA) and/or TiO₂. Without TEA or TiO₂, the photolysis of 1 resulted in very low yield of 2. The presence of TEA or TiO₂ increased the rate of disappearance of 1 and the yield of 2, which were further increased considerably by the presence of water. The photoreduction of 1 to 2 proceeds through an electron transfer to 1 from TEA or hole-scavenged excited TiO₂ followed by protonation. In the reaction medium of 88 : 7 : 2 : 3 CH₃CN/CH₃OH/H₂O/TEA with 2.5 Х/Б of TiO₂, the yield of 2 was as high as 85 % at 50 % conversion of 1. The photolysis of 2 in homogeneous media resulted in photo-cleavage to benzoyl and hydroxybenzyl radicals, which are mostly converted to benzaldehyde. The reduction product 4 is formed in low yield through the dimerization of hydroxybenzyl radicals. The addition of TEA increased the conversion rate of 2 and the yield of 4 significantly. This was attributed to the scavenging effect of TEA for benzoyl radical to produce N,N-diethylbenzamide and the photoreduction of benzaldehyde in the presence of TEA. The ratio of (�?) and meso isomers of 4 obtained from the photochemical reaction is about 1.1. This ratio is the same as that from the photochemical reduction of benzaldehyde in the presence of TEA. In the TiO₂-sensitized photochemical reduction of 2, meso-4 was obtained in moderate yield. The reduction of 2 to 4 proceeds through two consecutive electron/proton transfer processes on the surface of the photocatalyst without involvement of �?-cleavage. The radical 11 initially formed from 2 by one electron/proton process can also combine with hydroxy methyl radical, which is generated after hole trapping of excited TiO₂ by methanol, to produce 1,2-diphenylpropenone after dehydration reaction.

【 授权许可】

Unknown