A process for preparing a compound or a mixture of compounds of the general formulae I.a or I.b wherein R 1 is selected from C 1 -C 12 -alkyl, C 3 -C 6 -cycloalkyl, -C(=O)OR 2, -PO(OR 2a) 2, aryl and hetaryl, R 2, R 2a are independently of each other selected from the group consisting of hydrogen, C 1 -C 12 -alkyl, C 1 -C 12 -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 C 1 -C 6 -alkoxy and CN, C 3 -C 6 -cycloalkyl, C 3 -C 6 -cycloalkyl-C 1 -C 4 -alkyl, aryl and aryl-C 1 -C 4 -alkyl, where the last four radicals mentioned are unsubstituted or substituted by 1, 2, 3 or 4 radicals selected from the group consisting of C 1 -C 6 -alkyl, C 1 -C 6 -haloalkyl, C 1 -C 6 -alkoxy, C 1 -C 6 -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 selected from compounds of the general formula V in which R 4 and R 5 are independently selected from the group consisting of C 1 -C 6 -alkyl, aryl and heteroaryl, where aryl and heteroaryl are unsubstituted or substituted by 1, 2, 3 or 4 radicals selected from the group consisting of C 1 -C 4 -alkyl, C 1 -C 4 -haloalkyl, C 1 -C 4 -alkoxy and halogen; R 6 is selected from the group consisting of C 1 -C 12 -alkyl, C 1 -C 12 -alkoxy, C 1 -C 6 -haloalkyl, -NR 7 R 8, halogen, aryloxy, aryl and heteroaryl, where aryloxy, aryl and heteroaryl are unsubstituted or substituted by 1, 2, 3 or 4 radicals selected from the group consisting of C 1 -C 4 -alkyl, C 1 -C 4 -alkoxy and halogen; R 7 is selected from the group consisting of hydrogen, C 1 -C 12 -alkyl and aryl; R 8 is selected from the group consisting of C 1 -C 12 -alkyl and aryl; or R 5 together with R 6 and the atoms to which they are bound form a fused, unsaturated N-heterocycle, which may be unsubstituted or substituted by 1, 2 or 3 radicals selected from the group consisting of C 1 -C 4 -alkyl, C 1 -C 4 -haloalkyl, C 1 -C 4 -alkoxy and halogen, and where the N-heterocycle is mono-, bi- or tricyclic, and where the N-heterocycle may have 1, 2 or 3 further heteroatoms as ring members, which are selected from O, S and N, and in the presence of at least one additive having at least one proton donating functional group, where the additive is used in an amount of from 0.005 to 4 mol-%, based on the total amount of compound in the reaction mixture, and where the proton-donating functional group has a pKa-value in water at 25°C in the range of from 4 to 14, to yield a compound of the general formulae I.a or I.b or a mixture thereof. R 1 is selected from C 1 -C 12 -alkyl, C 3 -C 6 -cycloalkyl, -C(=O)OR 2, -PO(OR 2a) 2, aryl and hetaryl, R 2, R 2a are independently of each other selected from the group consisting of hydrogen, C 1 -C 12 -alkyl, C 1 -C 12 -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 C 1 -C 6 -alkoxy and CN, C 3 -C 6 -cycloalkyl, C 3 -C 6 -cycloalkyl-C 1 -C 4 -alkyl, aryl and aryl-C 1 -C 4 -alkyl, where the last four radicals mentioned are unsubstituted or substituted by 1, 2, 3 or 4 radicals selected from the group consisting of C 1 -C 6 -alkyl, C 1 -C 6 -haloalkyl, C 1 -C 6 -alkoxy, C 1 -C 6 -haloalkoxy, -CN, and halogen, R 4 and R 5 are independently selected from the group consisting of C 1 -C 6 -alkyl, aryl and heteroaryl, where aryl and heteroaryl are unsubstituted or substituted by 1, 2, 3 or 4 radicals selected from the group consisting of C 1 -C 4 -alkyl, C 1 -C 4 -haloalkyl, C 1 -C 4 -alkoxy and halogen; R 6 is selected from the group consisting of C 1 -C 12 -alkyl, C 1 -C 12 -alkoxy, C 1 -C 6 -haloalkyl, -NR 7 R 8, halogen, aryloxy, aryl and heteroaryl, where aryloxy, aryl and heteroaryl are unsubstituted or substituted by 1, 2, 3 or 4 radicals selected from the group consisting of C 1 -C 4 -alkyl, C 1 -C 4 -alkoxy and halogen; R 7 is selected from the group consisting of hydrogen, C 1 -C 12 -alkyl and aryl; R 8 is selected from the group consisting of C 1 -C 12 -alkyl and aryl;