The reaction takes place in the presence of a secondary amine and preferably in a suitable solvent which may include aliphatic and aromatic hydrocarbons such as petroleum ether, benzene, toluene, xylene, hexane or cyclohexane; halogenated aliphatic and aromatic hydrocarbons such as carbon tetrachloride, methylene chloride, chloroform, chloro- and dichlorobenzene; ethers such as diethyl ether, tetr