Identification of a highly efficient alkylated pincer thioimido-palladium(II) complex as the active catalyst in Negishi coupling.

The induction period of Negishi coupling catalyzed by pincer thioamide-palladium complex 1 was investigated. A heterogeneous mechanism was excluded by kinetic studies and comparison with Negishi coupling reactions promoted by Pd(OAc)(2)/Bu(4)NBr (a palladium-nanoparticle system). Tetramer 2 was isolated from the reaction of 1 and organozinc reagents. Dissociation of complex 2 by PPh(3) was achieved, and the structure of resultant complex 8 was confirmed by X-ray diffraction analysis. A novel alkylated pincer thioimido-Pd(II) complex, 7, generated from catalyst precursor 1 and basic organometallic reagents (RM), was observed by in situ IR, (1)H NMR, and (13)C NMR spectroscopy for the first time. The reaction of 7 with methyl 2-iodobenzoate afforded 74% of the cross-coupled product, methyl 2-methylbenzoate, together with 60% of Pd(II) complex 2. Furthermore, the catalyst, as an electron-rich Pd(II) species, efficiently promoted the Negishi coupling of aryl iodides and alkylzinc reagents without an induction period, even at low temperatures (0 degrees C or -20 degrees C). To evaluate the influence of the catalyst structure upon the induction period, complex 9 was prepared, in which the nBu groups of 1 were displaced by more bulky 1,3,5-trimethylphenyl groups. Trimer 10 was isolated from the reaction of complex 9 and basic organometallic reagents such as CyZnCl or CyMgCl (Cy: cyclohexyl); this is consistent with the result obtained with complex 1. The rate in the induction period of the model reaction catalyzed by 9 was faster than that with 1. Plausible catalytic cycles for the reaction, based upon the experimental results, are discussed.