Process for preparing at least one compound of the formula (I-A) or (I-B) wherein R 1, R 2, R 3, R 4, R 1 ', R 2 ', R 3 ', R 4 ' are each independently a C 1 -C 6 -alkyl radical, wherein R 9, R 10, R 9 ', R 10 ' are each independently selected from the group consisting of hydrogen, C 1 -C 6 -alkyl radical, wherein R 5, R 6, R 7, R 7 ' are each independently selected from the group consisting of hydrogen, alkyl, cycloalkyl, aryl, aralkyl, heterocyclyl, wherein the radicals alkyl, cycloalkyl, aryl, aralkyl, heterocyclyl may be substituted by one or more radicals selected from the group consisting of alkoxy, hydroxy, carboxyl, carboxyl ester, carboxyl amide, sulfonate, sulfonic acid ester, sulfonamide, halogen, where, in addition, alkyl may be interrupted by one or more divalent radicals not directly adjacent to one another selected from the group consisting of oxygen, imino, and where, in addition, R 5 and R 6 may together form an alkylene group which may be interrupted by one or more divalent radicals not directly adjacent to one another selected from the group consisting of oxygen, imino, wherein R 8 is an (m+1)-valent organic bridging group, which may be substituted by one or more radicals selected from the group consisting of alkyl, hydroxy, nitro, carboxyl, carboxyl ester, carboxyl amide, sulfonate, sulfonic acid ester, sulfonamide, halogen, and which may be interrupted by one or more divalent radicals not directly adjacent to one another selected from the group consisting of oxygen, imino, where m is an integer from 1 to 5, characterized in that gaseous methyl chloride is introduced into a melt S of at least one compound of the formula (II-A) or (II-B) such that methyl chloride and the at least one compound of the formula (II-A) or (II-B) react to give compound (I-A) or (I-B) respectively. wherein R 1, R 2, R 3, R 4, R 1 ', R 2 ', R 3 ', R 4 ' are each independently a C 1 -C 6 -alkyl radical, wherein R 9, R 10, R 9 ', R 10 ' are each independently selected from the group consisting of hydrogen, C 1 -C 6 -alkyl radical, wherein R 5, R 6, R 7, R 7 ' are each independently selected from the group consisting of hydrogen, alkyl, cycloalkyl, aryl, aralkyl, heterocyclyl, wherein the radicals alkyl, cycloalkyl, aryl, aralkyl, heterocyclyl may be substituted by one or more radicals selected from the group consisting of alkoxy, hydroxy, carboxyl, carboxyl ester, carboxyl amide, sulfonate, sulfonic acid ester, sulfonamide, halogen, where, in addition, alkyl may be interrupted by one or more divalent radicals not directly adjacent to one another selected from the group consisting of oxygen, imino, and where, in addition, R 5 and R 6 may together form an alkylene group which may be interrupted by one or more divalent radicals not directly adjacent to one another selected from the group consisting of oxygen, imino, wherein R 8 is an (m+1)-valent organic bridging group, which may be substituted by one or more radicals selected from the group consisting of alkyl, hydroxy, nitro, carboxyl, carboxyl ester, carboxyl amide, sulfonate, sulfonic acid ester, sulfonamide, halogen, and which may be interrupted by one or more divalent radicals not directly adjacent to one another selected from the group consisting of oxygen, imino, where m is an integer from 1 to 5, characterized in that gaseous methyl chloride is introduced into a melt S of at least one compound of the formula (II-A) or (II-B) such that methyl chloride and the at least one compound of the formula (II-A) or (II-B) react to give compound (I-A) or (I-B) respectively.