Photochemistry of halogenated benzene derivatives. Part VIII. Photoformation of 2-methyl-4,5,6,7-tetrachlorobenzoxazole from pentachlorophenol in water–acetonitrile

Abstract

Photolysis of a solution of pentachlorophenol (1) in H2O–CH3CN (2:3 v/v) was conducted on a synthetic scale using wavelengths ≥290 nm. After changing the pH of the photolyzate of 1 to ≥12, the photoproduct 2-methyl-4,5,6,7-tetrachloro-benzoxazole (2) was extracted with n-hexane. The product 2 was crystallized from methanolic solution. Ultraviolet, IR, FT-1H-NMR, proton-decoupled normal FT-13C-NMR, and proton-decoupled DEPT (distortionless enhancement by polarization transfer) sequence FT-13C-NMR spectroscopy were used to identify photoproduct 2 as 2-methyl-4,5,6,7-tetrachlorobenzoxazole. A mechanism for the photoformation of benzoxazole 2 from 1 is proposed. 3,4,5,6-Tetrachlorocyclohexa-3,5-diene-1,2-ketocarbene formed from 2-hydroxy-3,4,5,6-tetrachlorophenyl radical is suggested as key intermediate involved in the generation of oxazole 2 from phenol 1. The chemical yield of 2 on the basis of 1 was ca. 1%. A mechanism is proposed to explain the mass spectral fragmentation of 2 via the losses of Ċl,:C=O, Cl—Ċ=O, CH3—Ċ=O, CH3CN, and C3H3ClNO species from the parent molecular ion.