Lithium-mediated Ferration of Fluoroarenes

Abstract

While fluoroaryl fragments are ubiquitous in many pharmaceuticals, the deprotonation of fluoroarenes using organolithium bases constitutes an important challenge in polar organometallic chemistry. This has been widely attributed to the low stability of the in situ generated aryl lithium intermediates that even at –78 °C can undergo unwanted side reactions. Herein, pairing lithium amide LiHMDS (HMDS = N{SiMe₃}₂) with Fe^II(HMDS)₂ enables the selective deprotonation at room temperature of pentafluorobenzene and 1,3,5-trifluorobenzene via the mixed-metal base [(dioxane)LiFe(HMDS)₃] (1) (dioxane = 1,4-dioxane). Structural elucidation of the organometallic intermediates [(dioxane)Li(HMDS)₂Fe(ArF)] (ArF = C₆F₅, 2; 1,3,5-F₃-C₆H₂, 3) prior electrophilic interception demonstrates that these deprotonations are actually ferrations, with Fe occupying the position previously filled by a hydrogen atom. Notwithstanding, the presence of lithium is essential for the reactions to take place as Fe^II(HMDS)₂ on its own is completely inert towards the metallation of these substrates. Interestingly 2 and 3 are thermally stable and they do not undergo benzyne formation via LiF elimination.