Copper(I) iodide and copper(II) oxide are preferred.Bases suitable for the processes (IV)+(V).fwdarw.(VI) and (IV)+(VII).fwdarw.(II) are alkali metal carbonates and bicarbonates, in particular sodium and potassium carbonates, alkali metal hydroxides, in particular sodium hydroxide, or organicamines, in particular tri-(C.sub.1 -C.sub.6)-alkylamines such as, for example, triethylamine.