Mechanistic study of the Sml2/H2O/amine-mediated reduction of alkyl halides:: Amine base strength (pKBH+) dependent rate

The kinetics of the Sml(2)/H2O/amine-mediated reduction of 1-chlorodecane has been studied in detail. The rate of reaction is first order in amine and 1-chlorodecane, second order in Sml(2), and zero order in H2O. Initial rate studies of more than 20 different amines show a correlation between the base strength (pK(BH+)) of the amine and the logarithm of the observed initial rate, in agreement with Bronsted catalysis rate law. To obtain the activation parameters, the rate constant for the reduction was determined at different temperatures (0 to +40 degrees C, Delta H-double dagger = 32.4 +/- 0.8 kJ mol(-1), Delta S-double dagger = -148 +/- 1 J K-1 mol(-1), and Delta G(298K)(double dagger) = 76.4 +/- 1.2 kJ mol(-1)). Additionally, the C-13 kinetic isotope effects (KIE) were determined for the reduction of 1-iododecane and 1-bromodecane. Primary C-13 KIES (k(12)/k(13), 20 degrees C) of 1.037 +/- 0.007 and 1,062 +/- 0.015, respectively, were determined for these reductions. This shows that cleavage of the carbon-halide bond occurs in the rate-determining step. A mechanism of the Sml(2)/H2O/amine-mediated reduction of alkyl halides is proposed on the basis of these results.