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

Abstract

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.

Recent Advances in Catalysis with Transition‐Metal Pincer Compounds

“…For instance, Lei et al. employed a bis(thioamide)pyridine ligand to prepare a Pd II –SNS pincer complex . Its catalytic performance has been studied in the reaction between aryl iodides with alkylzinc derivatives under mild conditions (Scheme ).…”

Pincer Complexes with Thione Sulfur Donors

“…At the low catalyst loading (0.1-0.5 mol%), both catalysts promoted the reactions at room temperature or even at 0 C and provided high turnover numbers up to 6,100,000 [52]. Additional experiments showed that an active catalyst was an alkylated adduct generated from the catalyst precursor and basic metal reagents under reaction conditions (Scheme 14) [53].…”

Carbon–Carbon Bond Forming Reactions Mediated by Organozinc Reagents

“…Yet another alternative tactic to supersede the β-hydride elimination pathway consists of using catalytic systems operating through a mechanism engaging a Pd(II)/Pd(IV) shuttle, as it has been suggested that reductive elimination from (diorgano)Pd(IV) is a fast process. Pincer thioimido-Pd(II) complexes such as 346 can readily promote the cross-coupling between aryl iodides and alkylzincs, with low catalyst loadings and at temperatures as low as −20 • C (Scheme 4.79) [250,251]. When secondary cycloalkylzincs such as 341 are used, levels of selectivity with regard to competing reduction are excellent.…”