C-H Bond Activation by [{(Diimine)Pd(μ-OH)}2]2+ Dimers: Mechanism-Guided Catalytic Improvement

Abstract

These conclusions—that the hydroxy-bridged dimer 1b is the most reactive species in the Pd system, considerably more reactive than either 2a or 2b towards C-H bond activation, and that there is an important solvent-assisted component in the rate law—suggest a way to substantially improve the catalytic conversion of cyclohexene into benzene [Eq. (3)]. Our previous studies involved 2b (or mixtures of 1b and 2b) in the noncoordinating solvent dichloroethane. In a typical experiment, after 24 h under 1 atm of O2 with 5 mol% of 2b as the catalyst, 8% of the cyclohexene had been converted into benzene; furthermore, there was an initiation period before any reaction occurred, and competing disproportionation of cyclohexene to benzene and cyclohexane was a major side reaction.[3] In contrast, under the same conditions but using only 1 mol% of pure 1b as the catalyst and TFE as solvent, conversion was 25% after 24 h, with no induction period or competing isomerization.[7]