Isotope Effects in Arene C-H Bond Activation by

Article abstract of DOI:10.1021/ja00276a019

The isotope effects involved in the activation of arene C-H bonds by the intermediate <(C5Me5)Rh(PMe3)> have been investigated.The ratio k_H/k_D for coordination to a double bond in benzene (the rate-determining step of arene activation) is found to be 1.05(6), whereas k_H/k_D=1.4 for the second step in which the C-H bond of the coordinated arene undergoes oxidative addition.The isotope effect K_H/D for the equilibrium between the more stable phenyl hydride complex (C5Me5)Rh(PMe3)(C6D5)H and the complex containing hydrogen in the ortho position of the phenyl ring (C5Me5)Rh(PMe3)(o-C6D4H)D shows a preference for hydrogen (vs. deuterium) on the metal of 2.7.The kinetic isotope effect for reductive elimination and dissociation of m-xylene from (C5Me5)Rh(PMe3)(3,5-C6H3Me2)H vs. (C5Me5)Rh(PMe3)(3,5-C6H3Me2)D is found to be inverse, with k_H/k_D=o.51.Analysis of the data for the deuterated benzene derivatives confirms that the kinetic isotope for the reductive elimination step is inverse.Attemps to prepare the complex (C5Me5)Rh(PMe3)(CH3)D by reduction of <(C5Me5)Rh(PMe3)(CH3)(THF)>⁺ with ^- resulted in the formation of both (C5Me5)Rh(PMe3)(CH3)D and (C5Me5)Rh(PMe3)(CH2D)H.