Light-triggered oxy-chlorination of cyclohexane by metal chlorides

Abstract This paper discloses that visible light can trigger efficiently the oxy-chlorination of cyclohexane by some metal chlorides at ambient temperature under N 2 atmosphere. Among the metal chlorides examined, only a few metal chlorides with easily changeable valence such as VOCl 3 , CuCl 2 ·2H 2 O and FeCl 3 ·6H 2 O were found to be efficient chlorinating agents for this photoreaction in acetonitrile or acetone, providing mono- and di-chlorinated cyclohexane as main products, with concomitant formation of a small amount of cyclohexene, cyclohexanol and cyclohexanone. CuCl 2 ·2H 2 O was the most active chlorinating agent for this photoreaction, but provided an appreciable amount of the non-chlorinated products. FeCl 3 ·6H 2 O possessed the best selectivity for the chlorinated products. In addition, it was found that a suitable amount of concentrated HCl was capable of realizing the visible light-driven CuCl 2 ·2H 2 O or the UV-driven FeCl 3 ·6H 2 O to catalyze photo-oxy-chlorination of cyclohexane under air, providing a high chlorination efficiency (turnover number (TON), 2.46 for CuCl 2 ·2H 2 O and 4.66 for FeCl 3 ·6H 2 O). This is likely because the HCl can itself be used as a chlorinating agent; on the other hand, it also efficiently promotes the photoredox cycling of CuCl 2 or FeCl 3 , as supported by UV–vis spectra. Based on these findings, a free radical mechanism for the present photo-oxy-chlorination system was proposed.