Photochemistry of 2-chloropyrimidine

Abstract

The photochemistry of 2-chloropyrimidine (ClPy) was investigated by means of nanosecond laser flash photolysis, HPLC, mass spectrometry, polarography and absorption spectroscopy. Two major products were formed on low-intensity UV irradiation (λ = 254 nm) of ClPy in anaerobic aqueous solution: 2-hydroxypyrimidine (quantum yield ∼0.01) and a compound identified as 2-chloro-4,2′-bipyrimidine (quantum yield ∼0.005). Only the former of these products was obtained under aerobic conditions. Investigation by nanosecond flash photolysis revealed the occurrence of efficient intersystem crossing to the ClPy triplet state; the deactivation processes from this state were determined. Photosensitised generation of the ClPy triplet state showed that the triplet is involved in the formation of the bipyrimidine. A reaction scheme is proposed comprising two reaction channels: heterolytic rupture of the C–Cl bond in the excited singlet state of ClPy leading to formation of 2-hydroxypyrimidine, and homolytic C–Cl rupture in the triplet state with creation of pyrimidinyl radicals, which react with excess ClPy to give 2-chloro-4,2′-bipyrimidine.