A process for the preparation of a compound of formula (I) wherein: R1 is CN or CSNH¿2?; X is N or CR?4; R2 and R4¿ are, each, independently hydrogen or chlorine; R3 is halogen, haloalkyl, haloalkoxy or SF¿5?; R?5 and R6¿ are each independently an alkyl group; and n is 0, 1 or 2; which process comprises (a) a first step of reacting a compound of formula (II), wherein the various symbols are as defined above and W is H, with an alkylating agent of formula (III): R6-Y, wherein R6 is as defined above and Y is a leaving group. The process may also have an intermediate step of reacting compound (II) initially with an inorganic salt or an organic base prior to the addition of the alkylating agent. The intermediate compounds are also claimed as novel compounds.