Ferrocenyl triflate (1) and 1,1′-ferrocenediyl ditriflate (5) undergo single and double anionic thia-Fries rearrangements at low temperatures in high yields with the latter forming exclusively the respective meso product. The detailed mechanisms of the anionic thia-Fries rearrangement of 1, as well as that of the double anionic thia-Fries rearrangement of 5, are examined with the aid of DFT calculations. Functionals, basis set and pseudopotentials applied were selected in accordance with a benchmark using the crystal structure analysis of the prime product 3 of the reaction of 1 before hydrolysis for comparison, which has so far not been reported. The unprecedented meso diastereoselectivity of the double anionic thia-Fries rearrangement of 5 is shown to be a result of a distinctive degree of chelation arising from two diastereomeric transition states
