Kinetics and Thermodynamics of Intra- and Intermolecular Carbon-Hydrogen Bond Activation

Article abstract of DOI:10.1021/ja00289a014

The preference for intra- and intermolecular C-H bond activation has been determined by equilibration of the complex (C5Me5)Rh(PMe2CH2C6H5)(C6H5)H and its cyclometalated analogue in neat benzene at 51.2 deg C (K_eq=36.7, ΔG⁰=-2.32 kcal/mol).By monitoring the approach to equilibrium over a 40 deg C temperature range, the difference between the activation parameters for intra- and intermolecular activation by the 16-electron intermediate <(C5Me5)Rh(PMe2CH2C6H5)> can be obtained (intra-inter): ΔΔH<*>=1.7+/-0.8 kcal/mol; ΔΔS<*>=4.5+/-2.5 eu.At 25 deg C, this corresponds to a 1.86:1 kinetic preference for intermolecular activation of the neat benzene solvent by the coordinatively unsaturated intermediate <(C5Me5)Rh(PMe2CH2C6H5)> over intramolecular cycloaddition.The effect of solvent concentration on activation selectivity is dicussed.A comparison with intra- and intermolecular alkane activation is made by equilibrating the complex with benzene and by examining the kinetics of cyclometalation vs. alkane activation.These studies reveal the same general trend with regard to thermodynamic and kinetic selectivity in alkanes and arenes: while there is little kinetic selectivity between intra- and intermolecular reactions involving neat solvent, there is a moderate thermodynamic preference for the intramolecular activation.