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

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posted on 02.11.2005, 00:00 by Stuart A. Macgregor, D. Christopher Roe, William J. Marshall, Karen M. Bloch, Vladimir I. Bakhmutov, Vladimir V. Grushin
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 Ea = 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.