What is the rate of the Csp2-Csp2 reductive elimination step? Revealing an unusually fast Ni-catalyzed Negishi-type oxidative coupling reaction.

For a direct quantitative investigation of the Csp(2)-Csp(2) reductive elimination rate within a catalytic cycle, a novel oxidative coupling system in the presence of a Ni catalyst and desyl chloride as the oxidant is devised. The reaction progress profiles of arylzinc reagents exhibit zero-order kinetic behavior, and a reductive elimination step is confirmed as the rate-determining step. This allows direct measurement of the Csp(2)-Csp(2) reductive elimination rate constant within a catalytic cycle. The rate constants of p-MePhZnCl are obtained in the range 0.23 to 3.5 s(-1) from 0 to -35 degrees C, which are unusually fast reaction rates. According to the Arrhenius equation, the values of DeltaH(double dagger) and DeltaS(double dagger) (DeltaH(double dagger) = 9.7 kcal mol(-1), DeltaS(double dagger) = 35 J mol(-1) K(-1)) are obtained. The small value of DeltaH(double dagger) reveals that the reductive elimination step of Csp(2)-Ni-Csp(2) is an extremely facile process.