Photolysis of tris(4-tert-butoxycarbonyloxyphenyl) sulphonium salts. A mechanistic study

Abstract

Tris(4-tert-butoxycarbonyloxyphenyl)sulphonium salts of the structure Ar₃S⁺X^–(X^–: CF₃SO₃^– or AsF₆^–) were irradiated with UV light in acetonitrile or dimethyl sulphoxide solution. Product analysis by ion pair chromatography (IPC), ¹H NMR measurements and IR spectroscopy revealed that simultaneously bis(4-tert-butoxycarbonyloxyphenyl) sulphide, Ar₂S, and a protonic acid (CF₃SO₃H or HAsF₆) were formed. The latter catalyses the decomposition of the tert-butylcarbonate groups in the absence of water at room temperature. In the presence of water the acids dissociate, and H₃O⁺ ions, thus formed, only catalyse the decomposition significantly at elevated temperatures (T > 70 °C). Therefore, photolysis products formed in the absence of water consist, apart from (4-HO–C₆H₄)₂S, essentially of a mixture of salts of the ions: Ar₂(4-HO–C₆H₄)S⁺, Ar(4-HO–C₆H₄)₂S⁺ and (4-HO–C₆H₄)₃S⁺. In addition to these products, (Bu^tOCO₂-C₆H₄)₂S and an as yet unidentified product were formed when the irradiation was performed in the presence of water. Quantum yields (in the absence of water) determined by IPC are: φ[^–salt]= 0.2 and φ[(4-HO–C₆H₄)₂S]= 0.1.