Kinetics and mechanism of halide exchange in reactions of CpRu(PPh3)2Cl with alkyl halides: evidence for radical pairs†
*ab
and
Rein U.
Kirss
*a
Abstract
Halide exchange reactions between CpRu(PPh3)2Cl (1) and CBr4, CHBr3, CBr3CO2Et or CHBr2CO2Et yielding CpRu(PPh3)2Br (3) are facile and appear to proceed by a radical pair intermediate in a second order reaction. The relative reactivity of the bromocarbon compounds follows the order: CBr3CO2Et > CBr4> CHBr2CO2Et > CHBr3 ≫ C4H9Br. Activation parameters for the reaction of 1 with CHBr3 are measured as ΔH† = 65 ± 5 kJ mol−1 and ΔS† = −157 ± 17 J mol−1 K−1. Substitution of CH3C5H4Ru(PPh3)2Cl (4) for 1 in reactions with CBr4 increases the rate of halide exchange. The observation of ATRA reactivity of CBr4, CHBr3, CBr3CO2Et and CHBr2CO2Et with styrene when catalyzed by 1 and AIBN supports the formation of d5 RuIII intermediates. Density functional theory calculations on potential intermediates suggests that a pathway leading to CpRu(PPh3)(Cl)(Br)˙ is more likely than oxidative addition, which is preferred in reactions of 1 with bromoethane.
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