Identification of a Highly Efficient Alkylated Pincer Thioimido–Palladium(II) Complex as the Active Catalyst in Negishi Coupling

PdII ate complex: A novel alkylated pincer thioimido–Pd complex generated from a catalyst precursor and basic organometallic reagents (RM) was observed by in situ IR, 1H NMR, and 13C NMR spectroscopies for the first time and proved to be the active catalyst in stoichiometric and catalytic reactions of aryl iodides with RM (see scheme). The catalyst, as an electron‐rich PdII species, promoted the Negishi coupling of aryl iodides and alkylzinc reagents with high efficiency, even at low temperatures (0 or −20 °C). magnified image 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/Bu4NBr (a palladium–nanoparticle system). Tetramer 2 was isolated from the reaction of 1 and organozinc reagents. Dissociation of complex 2 by PPh3 was achieved, and the structure of resultant complex 8 was confirmed by X‐ray diffraction analysis. A novel alkylated pincer thioimido–PdII complex, 7, generated from catalyst precursor 1 and basic organometallic reagents (RM), was observed by in situ IR, 1H NMR, and 13C 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 PdII complex 2. Furthermore, the catalyst, as an electron‐rich PdII species, efficiently promoted the Negishi coupling of aryl iodides and alkylzinc reagents without an induction period, even at low temperatures (0 °C or −20 °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.