A suitable compound is the one where the Lewis acid derived portion is based on antimony, especially SbCl3 or SbCl5. An example of such suitable catalyst has the formula [Cl3P=N-(PCl2=N)-PCl3]+[SbCl6]-, wherein s has a value from 1 to 4. Another suitable compound is the one where the Lewis acid derived portion is based on Al,especially AlCl3