oup, R and R' which may be the same or different are an alkyl grouphaving independently from 1 to 8 carbon atoms and n is an integer of from 1 to 8 dissolved or slurried in an organic solvent or mixture of solvents, which are stable and substantially chemically inert to the components of the reaction system, in thepresence of an organic sulfonic acid promoter such as an alkane sulfonic, halogenate