Robust and Electron‐Rich cis‐Palladium(II) Complexes with Phosphine and Carbene Ligands as Catalytic Precursors in Suzuki Coupling Reactions

Abstract

A new imidazolinium ligand precursor [L(2)H]Cl (2) was prepared in 86 % yield. Compared with its imidazolium counterpart, [L(1)H]Cl (1), 2 is very sensitive to moisture and can undergo ring-opening reactions very readily. Palladium complexes with the ring-opened products from imidazolinium salts were isolated and characterized by X-ray crystallography. Theoretical studies confirmed that the imidazolinium salt has a higher propensity for the ring-opening reaction than the imidazolium counterpart. New mixed phosphine/carbene palladium complexes, cis-[PdCl(2)(L)(PR(3))] (L=L(1) and L(2); R=Ph, Cy), were successfully prepared. These complexes are highly robust as revealed by variable-temperature NMR spectroscopic studies and thermal gravimetric analysis. The structural and electronic properties of the new complexes on varying the carbene group (imidazol-2-ylidene group (unsaturated carbene) vs. imidazolin-2-ylidene (saturated carbene)) and the phosphine group (PPh(3) vs. PCy(3)) were studied in detail by X-ray crystallography, X-ray photoelectron spectroscopy, and theoretical calculations. The catalytic study reveals that cis-[PdCl(2)(L(2))(PCy(3))] is a competent Pd(II) precatalyst for Suzuki coupling reactions, in which unreactive aryl chlorides can be applied as substrates.

The influence of moisture on deprotonation mode of imidazolinium chlorides with palladacycle acetate dimer

“…This study, involving deproptonation of imidazolinium salts (1) with 2 revealed that the nature of products can be influenced by the presence of water either in the solvent or as hydrate and Nsubstituents on the ring [23,31]. Thus, N-benzyl substituted salt 1a in THF reacts only to give 5a as a result of hydrolysis of the expected NHC.…”

Palladium‐Catalyzed Oxidative Cyclization for the Synthesis of 2‐Alkylimidazo[5,1,2‐cd]indolizines

“…The coupling reaction between 2‐ tert ‐butylimidazo[1,2‐ a ]pyridine and diphenylacetylene was used as the benchmark reaction and our previous conditions of using 2.5 mol‐% of the palladium complex as precatalyst, a reaction temperature of 90 °C, Cu(OAc) 2 as base, a reaction time of 16 h, dimethylformamide (DMF) as solvent, and n ‐tetrabutylammonium bromide (TBAB) as phase‐transfer catalyst were employed Entry 1 clearly shows that the ionic NHC complex 1a was the most effective, which afforded the coupled product in a good yield of 74 %. The neutral dichloropalladium(II) complex 1b bearing the same NHC ligand and tricyclohexylphosphine, reported by us earlier, was much less effective (entry 2). The palladium(II)–NHC complex with an IMes ligand [IMes = 1,3‐bis(2,4,6‐trimethylphenyl)imidazol‐2‐ylidene] has been proven to be highly effective in various cross‐coupling reactions .…”

Neutral Trinuclear Palladium(II) Metallocycles with Mixed Phosphane/Pyridine‐4‐thiolato Ligands and Their Catalytic Tests in Suzuki–Miyaura Coupling Reactions

“…Side products derived from the reduction or homocoupling of the aryl halides were not detected at all or were observed only in trace amounts. The higher activities of the PCy 3 complexes (Entries 3 and 4) relative to the PPh 3 complexes (Entries 1 and 2) can be attributed to the faster de‐coordination of the labile PCy 3 ligand and facile formation of the Pd 0 species 9. However, the close activities of complex 1 and 2 should result from the presence of the PyS group.…”