Agent for initiating a radical addition reaction and process using it

A method for preparing a compound of formula (I)

• in which
• X is selected from S and Se;
• R1 is selected from H and alkyl groups;
• R2 is selected from H;
• OH;
• NR4R5;
• NHCOR4 and OCOR4;
• where R4 and R5 are identical or different and are selected from H and alkyl groups;
• R3 is selected from OH;
• CH2OH;
• COOH;
• COOR6;
• CN;
• CONR4R5;
• and COZ, wherein R6 is selected from alkyl, R4 and R5 are as defined above, and Z represents a halide;
• the method comprising the reaction of a compound of formula (II)
• in which
• X is selected from S and Se;
• R1′ is selected from:
• alkyl, groups;
• thiol and thioalkyl groups when X is S;
• and
• selenol and selenoalkyl groups when X is Se, and
• R2′ is selected from H and alkyl groups;
• with a compound of formula (III)
• where R2 and R3 are as defined above for the compound of formula (I),
• in the presence of at least one initiation agent for initiating a radical addition reaction comprising a persulfate and one or several transition metals, in elemental form or in oxidized form, said transition metal(s) being selected from Cu, Zn, Fe and their alloys in their elemental form or in an oxidized form selected from sulfates, phosphates, chlorides, carbonates, nitrites, nitrates, acetates, alcoholates, hydrogenphosphates, and hydrogen sulfates,
• wherein the molar ratio of persulfate to the transition metal(s) ranges from 1:1 to 1000:1, and
• wherein the reaction is carried out at a temperature from −20° C. to 40° C.