A process for the preparation of a compound of formula I wherein R 1 is halogen and R 2 is hydrogen or R 1 is chloro and R 2 is halogen; comprising a) for the preparation of a compound of formula I, wherein R 1 is halogen and R 2 is hydrogen, reacting the compound of formula II in an aprotic organic solvent in the presence of an aprotic polar co-solvent selected from the group consisting of the compounds of formula VI wherein R 8 and R 9 are, independently from each other, C 1 -C 6 alkyl, C 4 -C 7 cycloalkyl or C 1 -C 6 alkoxylalkyl and both R 10 together represent a -CH 2 -CH 2 - or -CH 2 -CH 2 -CH 2 - chain; with a magnesium amide base which is a compound of formula IV complexed with lithium chloride; wherein R 3 and R 4 are, independently from each other, C 1 -C 6 alkyl, C 4 -C 7 cycloalkyl or C 1 -C 6 alkoxylalkyl; and X is halogen followed by a halogenating agent, to the compound of formula I wherein R 1 is halogen and R 2 is hydrogen; and b) for the preparation of a compound of formula I, wherein R 1 is chloro and R 2 is halogen, reacting the compound of formula I, wherein R 1 is chloro and R 2 is hydrogen, in an aprotic organic solvent in the presence of an aprotic polar co-solvent selected from the group consisting of the compounds of formula VI wherein R 8 and R 9 are, independently from each other, C 1 -C 6 alkyl, C 4 -C 7 cycloalkyl or C 1 -C 6 alkoxylalkyl and both R 10 together represent a -CH 2 -CH 2 - or -CH 2 -CH 2 -CH 2 - chain; with a magnesium amide base which is a compound of formula IV complexed with lithium chloride; wherein R 3 and R 4 are, independently from each other, C 1 -C 6 alkyl, C 4 -C 7 cycloalkyl or C 1 -C 6 alkoxylalkyl; and X is halogen followed by a halogenating agent to a compound of formula I, wherein R 1 is chloro and R 2 is halogen. R 1 is halogen and R 2 is hydrogen or R 1 is chloro and R 2 is halogen; comprising a) for the preparation of a compound of formula I, wherein R 1 is halogen and R 2 is hydrogen, reacting the compound of formula II in an aprotic organic solvent in the presence of an aprotic polar co-solvent selected from the group consisting of the compounds of formula VI wherein R 8 and R 9 are, independently from each other, C 1 -C 6 alkyl, C 4 -C 7 cycloalkyl or C 1 -C 6 alkoxylalkyl and both R 10 together represent a -CH 2 -CH 2 - or -CH 2 -CH 2 -CH 2 - chain; with a magnesium amide base which is a compound of formula IV complexed with lithium chloride; wherein R 3 and R 4 are, independently from each other, C 1 -C 6 alkyl, C 4 -C 7 cycloalkyl or C 1 -C 6 alkoxylalkyl; and X is halogen followed by a halogenating agent, to the compound of formula I wherein R 1 is halogen and R 2 is hydrogen; and b) for the preparation of a compound of formula I, wherein R 1 is chloro and R 2 is halogen, reacting the compound of formula I, wherein R 1 is chloro and R 2 is hydrogen, in an aprotic organic solvent in the presence of an aprotic polar co-solvent selected from the group consisting of the compounds of formula VI wherein R 8 and R 9 are, independently from each other, C 1 -C 6 alkyl, C 4 -C 7 cycloalkyl or C 1 -C 6 alkoxylalkyl and both R 10 together represent a -CH 2 -CH 2 - or -CH 2 -CH 2 -CH 2 - chain; with a magnesium amide base which is a compound of formula IV complexed with lithium chloride; wherein R 3 and R 4 are, independently from each other, C 1 -C 6 alkyl, C 4 -C 7 cycloalkyl or C 1 -C 6 alkoxylalkyl; and X is halogen followed by a halogenating agent to a compound of formula I, wherein R 1 is chloro and R 2 is halogen. a) for the preparation of a compound of formula I, wherein R 1 is halogen and R 2 is hydrogen, reacting the compound of formula II in an aprotic organic solvent in the presence of an aprotic polar co-solvent selected from the group consisting of the compounds of formula VI wherein R 8 and R 9 are, independently from each other, C 1 -C 6 alkyl, C 4 -C 7 cycloalkyl or C 1 -C 6 alkoxylalkyl and both R 10 together represent a -CH 2 -CH 2 - or -CH 2 -CH 2 -CH 2 - chain; with a magnesium amide base which is a compound of formula IV complexed with lithium chloride; wherein R 3 and R 4 are, independently from each other, C 1 -C 6 alkyl, C 4 -C 7 cycloalkyl or C 1 -C 6 alkoxylalkyl; and X is halogen followed by a halogenating agent, to the compound of formula I wherein R 1 is halogen and R 2 is hydrogen; and b) for the preparation of a compound of formula I, wherein R 1 is chloro and R 2 is halogen, reacting the compound of formula I, wherein R 1 is chloro and R 2 is hydrogen, in an aprotic organic solvent in the presence of an aprotic polar co-solvent selected from the group consisting of the compounds of formula VI wherein R 8 and R 9 are, independently from each other, C 1 -C 6 alkyl, C 4 -C 7 cycloalkyl or C 1 -C 6 alkoxylalkyl and both R 10 together represent a -CH 2 -CH 2 - or -CH 2 -CH 2 -CH 2 - chain; with a magnesium amide base which is a compound of formula IV complexed with lithium chloride; wherein R 3 and R 4 are, independently from each other, C 1 -C 6 alkyl, C 4 -C 7 cycloalkyl or C 1 -C 6 alkoxylalkyl; and X is halogen followed by a halogenating agent to a compound of formula I, wherein R 1 is chloro and R 2 is halogen.