process according to the invention homogeneous catalysts are preferred suitable homogeneous catalysts are the salts of palladium with for example nitric acid sulfuric acid or particularly alkanoic acids among the alkanoic acids those having not more than 12 carbon atoms per molecule are preferred very good results have been obtained with palladium acetate moreover palladium complexes may be used for example palladium acetylacetonate tetrakistriphenylphosphinepalladium bis tri o tolylphosphinepalladium acetate or bistriphenylphosphinepalladium sulfate palladium bonded to an ion exchanger for instance an ion exchanger comprising sulfonic acid groups is an example of a suitable heterogeneous catalyst suitable organic phosphines which may be used in the process according to the invention include those of the general formula iii str2 in which r 1 represents an optionally substituted aryl group and r 2 and r 3 each an optionally substituted alkyl optionally substituted cycloalkyl or optionally substituted aryl group or r 2 and r 3 together represent an optionally substituted alkylene or phosphacycloalkylene group preferably the alkyl group has up to 20 carbon atoms the cycloalkyl group up to 5 to 7 carbon atoms in the ring and the aryl group up to 18 carbon atoms in the ring the aryl group may be an anthryl naphthyl or preferably a phenyl group phosphines of the general formula i in which r 1 and r 2 each represent an optionally substituted phenyl group are a preferred group of phosphines the phosphines of this group in which r 3 also represents an optionally substituted phenyl.