Photoreductive transformation of fluorinated acetophenone derivatives on titanium dioxide: Defluorination vs. reduction of carbonyl group

• The photocatalytic reduction of fluorinated acetophenone (AP) derivatives proceeds at surface defect sites on TiO2.
• 2-Monofluoromethyl AP (MFAP) was stable and existed as keto form in ethanol, whereas 64% of 2,2-difluoromethyl AP (DFAP) transformed into hemiketal form in ethanol.
• The reduction of MFAP afforded only the defluorinated ketones, while the reduction of DFAP provided not only defluorinated ketones but also a hydrogenated alcohol.
• The reaction routes of reductive defluorination and reduction of carbonyl group are discussed.

Photoreductive transformation of mono- and di-fluoromethyl acetophenone (AP) derivatives on the P25 titanium dioxide (TiO2) has been studied in deaerated ethanol solution under UV irradiation. 2-Monofluoromethyl AP (MFAP) was stable in the dark and existed as keto form in ethanol, whereas 64% of 2,2-difluoromethyl AP (DFAP) transformed into hemiketal form (photocatalytically inactive form) under the same condition. Under the UV irradiation with the TiO2 particles, the reduction of MFAP afforded only the defluorinated ketone, while the reduction of DFAP provided not only defluorinated ketones but also a hydrogenated alcohol. The reduction of carbonyl group and defluorination of DFAP concurrently occurred on TiO2, in which the formation of MFAP was observed as an intermediate of the sequential defluorinations. These two parallel reactions were initiated by electron transfer from the surface defect sites (Tisd) to DFAP adsorbed on the TiO2 surface. A possible reaction mechanism for DFAP is proposed and discussed on the basis of thermodynamic data upon the CF bond cleavage of anion radical species generated during the photocatalysis.

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