The First Anionic Thia-Fries Rearrangement at the Cobaltocenium Cation

Abstract

While the chemistry of ferrocene is rich and widely explored, this is not the case for the isoelectronic cobaltocenium cation. Here we report the first syntheses of triflate substituted cobaltocenium cations based on respective (cyclopentadienone)(cylopentadienyl)cobalt(I) complexes, which have first been reported by Vollhardt et al. ca. forty years ago. Reaction with triflic anhydride affords the triflate substituted cobaltocenium cations, which upon treatment with lithium diisopropylamide undergo the first anionic thia-Fries rearrangements of cobaltocenium complexes. To our knowledge, this is, in addition, the first case of a deprotonation of a cobaltocenium complex by a lithium base. Alternatively, desilylation of silyl derivatives with cesium fluoride caused anion formation leading to the rearrangement products in higher yields. The 2-(trifluoromethylsulfonyl)(cyclopentadienone) complexes obtained were fully characterized including a crystal structure analysis. As with ferrocene, respective mesylates or tosylates do not undergo the rearrangement.