PURPOSE: To prepare the titled substance useful as a preparation intermediate of herbicide, in one step, in high yield, by reacting a compound containing aromatic group with an N-bromohydantoin derivative in the presence of an organic peroxide.

CONSTITUTION: The objective compound of formula III can be prepared by reacting the compound of formula I (Ar is aromatic group; R1 and R2 are 1W5C alkyl or H) with the compound of formula II (R3 and R6 are H, Br or 1W4C alkyl, provided that one of R3 and R6 is Br; R4 and R5 are 1W3C alkyl or phenyl) in the presence of an organic peroxide at 50W55°C. The organic peroxide is selected from those capable of generating radicals easily at low temperature and transportable and handleable safely, i.e. those having a decomposition temperature of ≤55°C corresponding to the half-life time of 10hr, e.g. diisopropyl peroxydicarbonate, etc., and the amount of the peroxide is 0.1W1wt%.