The present invention relates to a process for the preparation of compounds or a mixture of compounds of the general formulae I.a or I.b wherein R1 is selected from C1-C12-alkyl, C3-C6-cycloalkyl, —C(═O)OR2, —PO(OR2a)2, aryl and hetaryl, R2, R2a are independently of each other selected from the group consisting of hydrogen, C1-C12-alkyl, C1-C12-alkenyl, where the last two radicals mentioned are unsubstituted, partly or completely halogenated or substituted by 1, 2, 3 or 4 radicals selected from the group consisting of C1-C6-alkoxy and CN, C3-C6-cycloalkyl, C3-C6-cycloalkyl-C1-C4-alkyl, aryl and aryl-C1-C4-alkyl, where the last four radicals mentioned are unsubstituted or substituted by 1, 2, 3 or 4 radicals selected from the group consisting of C1-C6-alkyl, C1-C6-haloalkyl, C1-C6-alkoxy, C1-C6-haloalkoxy, —CN, and halogen, which comprises dimerizing olefin compound of the general formula II, in the presence of at least one N-heterocyclic carbene catalyst.