The reaction is preferably carried out in the presence of a solvent and at a temperature within the range of -70??? to 30??? C. preferably -30??? to 10??? C. Suitable solvents for the reaction include aprotic solvents such as hydrocarbons, e.g. hexane or cyclohexane, acetonitrile or ethers such as tertiary butyl methyl ether, dioxan or tetrahydrofuran, or protic solvents such as an alkanol e.g. me