Anionic Thia-Fries Rearrangements of Electron-Rich Ferrocenes and the Unanticipated Formation of Diferrocenyl Sulfate from 2-(Trimethylsilyl)ferrocenyl Imidazolylsulfonate

Abstract

Upon ortho lithiation ferrocenyl triflate and 1,1′-ferrocenediyl ditriflate undergo anionic thia-Fries rearrangements instead of triflate elimination. To differentiate between an ortho metalation and an ortho deprotonation, 2-(trimethylsilyl)ferrocenyl triflate was shown to undergo an anionic thia-Fries rearrangement to 2-((trifluoromethyl)sulfonyl)ferrocenol (5) in 84% yield upon treatment with tetrabutylammonium fluoride. Metalation of the respective tributylstannyl derivative with butyllithium also led to 5 in 99% yield as the result of the anionic thia-Fries rearrangement. 2-Methoxyferrocenyl triflate also underwent the rearrangement upon ortho deprotonation with lithium diisopropylamide in practically quantitative yield at low temperature. The electron-rich 2-(((trifluoromethyl)sulfonyl)oxy)ferrocenolate was generated from 2-(((trifluoromethyl)sulfonyl)oxy)ferrocenyl acetate. However, ortho deprotonation again afforded the anionic thia-Fries rearrangement product. These results clearly show that even very electron rich ferrocene derivatives undergo an anionic thia-Fries rearrangement instead of a triflate elimination. In an attempt to induce an elimination supported by steric crowding, 2,3,4-trimethylferrocenyl triflate was deprotonated, giving 3,4,5-trimethyl-2-((trifluoromethyl)sulfonyl)ferrocenol in quantitative yield as the result of an anionic thia-Fries rearrangement. As an alternative to the triflates ferrocenyl imidazolylsulfonate was tested as the starting material. While this compound could not be deprotonated, the corresponding 2-trimethylsilyl derivative reacted with tetrabutylammonium fluoride in a very unusual reaction to give diferrocenyl sulfate in almost quantitative yield.