What is described is a process for preparing organic compounds of the general formula (I) R—R′  (I) by converting a corresponding compound of the general formula (II) R—X  (II) in which X is fluorine, chlorine, bromine or iodine to an organomagnesium compound of the general formula (III) [M+]n[RmMgXkY1]  (III) wherein compounds of the formula (III) are reacted with a compound of the general formula (IV) characterized in that the reaction of (III) with (IV) is performed in the presence of a) catalytic amounts of an iron compound, based on the compound of the general formula (II), and optionally in the presence of b) a nitrogen-, oxygen- and/or phosphorus-containing additive in a catalytic or stoichiometric amount, based on the compound of the general formula (II).