Diastereomer mixture comprising diastereomers of the formulae (I.1) and (I.2) where R2, R4 are each independently selected from —(C1-C12)-alkyl, —(C3-C12)-cycloalkyl, —(C3-C12)-heterocycloalkyl, —(C6-C20)-aryl; the R1, R3 radicals are each a —(C3-C20)-heteroaryl radical. The invention further relates to Pd complex mixtures comprising the diastereomers according to the invention, and to the use thereof in an alkoxycarbonylation process.