Scheme I respectively. Thus, the addition reaction seems to proceed more effectively at lower reaction temperatures. Concerning the mechanism of this Cu (1)-catalyzed photoaddition reaction, we have proposed that the initial step of the reaction is reductive cleavage of carbon-halogen bonds of halogen compounds by CuCl under photostimulation. 2 It, however, has remained to be solved if CuCl cleaves carbon-halogen bonds by an ...