The transition state for metal-catalyzed dehalogenation

Substituent effects have been used to probe the nature of the transition st ate to catalytic carbon-halogen bond breaking. Kinetics measurements have d etermined the activation energies (E-act) to C-Cl bond breaking on the Pd(1 11) surface and C-I bond breaking on the Pd(111) and Ag(111) surfaces. Thes e barriers have been measured using alkyl halides with varying degrees of f luorine substitution. The activation energies have been correlated with the inductive or field substituent constants (sigma(F)) of the fluorinated alk yl groups in order to determine reaction constants (E-act= E-0 + rho sigma( F)) for the dehalogenation reactions. In all three cases it has been found that the barriers are insensitive to inductive substituent effects and the reaction constants are all relatively small: rho = -0.5 +/- 1.0 kcal/mol fo r C-Cl cleavage on Pd(111), rho = -0.3 +/- 0.8 kcal/mol for C-I cleavage on Pd(111), and rho = -2.9 +/- 0.4 kcal/mol for C-I cleavage on Ag(111). This implies that the transition state for dehalogenation is homolytic and occu rs early in the reaction coordinate. The implications of this result are di scussed for catalytic dehalogenation processes such as hydrodechlorination.