The F/Ph Rearrangement Reaction of [(Ph3P)3RhF], the Fluoride Congener of Wilkinson's Catalyst

Abstract

The fluoride congener of Wilkinson's catalyst, [(Ph₃P)₃RhF] (1), has been synthesized and fully characterized. Unlike Wilkinson's catalyst, 1 easily activates the inert C−Cl bond of ArCl (Ar = Ph, p-tolyl) under mild conditions (3 h at 80 °C) to produce trans-[(Ph₃P)₂Rh(Ph₂PF)(Cl)] (2) and ArPh as a result of C−Cl, Rh−F, and P−C bond cleavage and C−C, Rh−Cl, and P−F bond formation. In benzene (2−3 h at 80 °C), 1 decomposes to a 1:1 mixture of trans-[(Ph₃P)₂Rh(Ph₂PF)(F)] (3) and the cyclometalated complex [(Ph₃P)₂Rh(Ph₂PC₆H₄)] (4). Both the chloroarene activation and the thermal decomposition reactions have been shown to occur via the facile and reversible F/Ph rearrangement reaction of 1 to cis-[(Ph₃P)₂Rh(Ph)(Ph₂PF)] (5), which has been isolated and fully characterized. Kinetic studies of the F/Ph rearrangement, an intramolecular process not influenced by extra phosphine, have led to the determination of E_a = 22.7 ± 1.2 kcal mol^-1, ΔH ^⧧ = 22.0 ± 1.2 kcal mol^-1, and ΔS ^⧧ = −10.0 ± 3.7 eu. Theoretical studies of F/Ph exchange with the [(PH₃)₂(PH₂Ph)RhF] model system pointed to two possible mechanisms: (i) Ph transfer to Rh followed by F transfer to P (formally oxidative addition followed by reductive elimination, pathway 1) and (ii) F transfer to produce a metallophosphorane with subsequent Ph transfer to Rh (pathway 2). Although pathway 1 cannot be ruled out completely, the metallophosphorane mechanism finds more support from both our own and previously reported observations. Possible involvement of metallophosphorane intermediates in various P−F, P−O, and P−C bond-forming reactions at a metal center is discussed.